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Proceedings

Hamburg 24-26 September 2012

Agenda: download

Minutes of the meeting: download

List of attendance: download

Photos of the meeting: download

Project management and reporting

The presentation summarizes the main "To do's" on the list for the final reporting period and for the closure of the project.

Chiara Bearzotti (UHAM):Project management and reporting duties download

CT Reports

The following presentations report on the activities of the last period of the project and the plan for the reporting to the Commission.

CT 1 presentation of Johann Jungclaus and Kikki Kleiven: download

CT 2 presentation of Steffen M. Olsen: download

CT 3 presentation of Svein Oesterhus: download

CT 4 presentation of Wilco Hazeleger: download

CT 5.1 presentation of Martin Visbeck: download

CT5.2 presentation of Detlef Stammer: download

Scientific talks

Andrea Tilche, European research on Climate Change and oceans: from FP7 to Horizon 2020 (download PDF)

[European Commission, Head of Unit I4 – Climate change and natural hazards, DG Research & Innovation]

 

TOPIC 1-Observation systems

Martin Visbeck, In Situ Sustained Ocean Observations in support of MOC research and ocean indices (download PDF)

[Helmholtz Centre for Ocean Research Kiel]

Juergen Fischer, Intra-seasonal variability of the DWBC in the western subpolar North Atlantic (download PDF)

[Helmholtz Centre for Ocean Research Kiel]

Abstract

The Deep western Boundary Current (DWBC) along the western margin of the subpolar North Atlantic represents an important component of the deep part of the Meridional Overturning near its northern origin. Moored Arrays from the Denmark Strait to the Tail of the Grand Banks have been installed to observe currents and transports of North Atlantic Deep Water on time scales from days to decades. The dominant variability in all of the moored efforts is in the week to month periods, and this intraseasonal variability may well mask possible interannual to decadal variability that is an order of magnitude smaller than the intraseasonal fluctuations. Herein we quantify the impact of the intraseasonal variability on the estimates of long term fluctuations at key positions along the DWBC in both, observations and high resolution model (VIKING20) data. A contribution of the EU-FP7 THOR group and the BMBF North Atlantic programme and NOCS scientists.

Bogi Hansen, Interactions between the Atlantic inflow branches to the Nordic Seas (download PDF)

[Faroe Marine Research Institute]

Abstract

Three current branches transport warm and saline water from the Atlantic across the Greenland-Scotland Ridge and into the Nordic Seas. Long-term monitoring efforts have provided time series of volume transport from the late 1990s, which allow us to investigate relationships between the branches, especially potential correlations between volume transport of the individual branches from time scales of months up to a decade. The observational results are compared with runs from a numerical ocean model. The Atlantic water in the Faroe Current, passing between Iceland and Faroes, bifurcates at the eastern corner of the Faroe Plateau with one branch flowing into the Faroe-Shetland Channel. Although this has been known for more than a century, there is still no consensus on the downstream fate of this branch within the Faroese channels. Does most of it recirculate within the Faroe-Shetland Channel or is there a considerable clockwise circulation around the Faroe Plateau over the slope region and, if so, how does this flow vary in time? These questions will be addressed utilizing several data sources.

Christian Mertens, Observations of circulation and transport variability in the Newfoundland Basin (download PDF)

[Univ. Bremen]

Abstract

In the southwestern part of the subpolar North Atlantic warm and saline waters enter the subpolar gyre within the North Atlantic Current (NAC) while cold and fresh waters leave within the Deep Western Boundary Current (DWBC). The circulation in this area was intensely studied during 2003-2012 by hydrographic sections and moored time series. The ship surveys were carried out along 47°N between Flemish Cap and the Mid- Atlantic Ridge (MAR) and along a section following the MAR to the northwest along its western margin. Current meter measurements were carried out in the DWBC and inverted echo sounders (IES) were deployed along the MAR. The circulation along 47°N east of Flemish Cap is characterized by strong meridional currents: the southward flowing deep western boundary current (DWBC), the northward flowing North Atlantic Current (NAC), and an adjacent southward return flow. The current meter measurements show a mean southward transport of deep water along the continental slope of 20 Sv. The mean transport of the NAC constructed from shipboard data amounts to 90 Sv and the southward recirculation to 60 Sv. The residual of 30 Sv NAC transport corresponds to the results from IES transport estimates across the MAR. A contribution of the Nordatlantik/RACE group (AP2.1/TP1.3).

Johanna Baehr, AMOC state and variability from existing and planned mooring arrays in the Atlantic (download PDF)

[Univ. Hamburg, Institute of Oceanography]

Abstract

We look at the AMOC state and variability by reviewing several existing trans basin mooring arrays, and testing additional trans basin mooring arrays in numerical models. Specifically, we look at the RAPID 26N line, and the 41N AMOC estimate. We also look at the combined effect of new observing systems such as the OSNAP array. The review of the existing time series will be accompanied by model based observing system simulations conducted in a consistent modeling framework.

Uwe Send, Comparison of the Atlantic Meridional Overturning Circulation (AMOC) at two latitudes - 16°N and 24°N (download PDF)

[San Diego Scripps Institution of Oceanography]

Abstract

The AMOC is responsible for the meridional transport of heat on climate relevant timescales. Numerical model studies suggest that the AMOC varies on broad range of periods: Wind forcing dominates local and intraseasonal variability; buoyancy forcing is important at longer periods and sets – at least the mean strength – of a meridionally coherent AMOC. We shall present contemporaneous observations targeting AMOC fluctuations at 16°N and 26°N together with numerical model simulations. Continuous mooring-based density observations of the deep southward transport of North Atlantic Deep Water in the western basin are available since the year 2000 at 16°N in the framework of the MOVE experiment (operated by SIO, San Diego, USA). From 2006 the western basin transports are extended to the eastern basin using data from the TENATSO mooring near Cape Verde. This enables us to compute NADW transports integrated across almost the entire zonal width of the Atlantic at 16°N. From 2004 the RAPID / MOCHA array (NOC, UK / RSMAS & AOML, USA) has been monitoring the strength of the AMOC (both the northward transport of warm water and cold NADW return flow) at 26°N. We present evidence for the occurrence of coherent changes of the AMOC between 16°N and 26°N, suggesting that AMOC anomalies with meridional scales larger than 1000 km are observed. We find evidence for periods of pronounced divergence in NADW transports between 16°N and 26°N. Isolating different transport contributions of the AMOC signal at both latitudes we trace the origin of coherent and divergent events. A series of global ocean/sea-ice model simulations are used to further interpret the observations. The model experiments span hindcasts of the past 60 years at eddy-permitting (1/4°) to high (1/12°) resolutions, completed by sensitivity experiments to isolate wind-driven from buoyancy effects. Specific focus will be given on what parts of the observational record are meridionally coherent between 16° and 26°N.

Shane Elipot , Observed basin-wide response of the North Atlantic Meridional Overturning Circulation to wind stress forcing (download PDF)

[National Oceanography Centre]

Abstract

The dynamics and response to wind stress forcing of the North Atlantic Meridional Overturning Circulation (AMOC) are investigated using four time series of overturning transports below 1000 m spanning 3.7 years. These time series are derived from four moored arrays located on the western boundary of basin: the RAPID WAVE array (42N), the Woods Hole Oceanographic Institution Line W array (39N), the RAPID MOC array (26N), and the MOVE array (16N). Mode decomposition of the cross-covariance between these time series and scaterrometer wind stress over the North Atlantic suggests that basin-scale changes in atmospheric forcing significantly affect the AMOC on relatively short time scales. First, the phasing of the transport time series at semiannual and annual time scales is shown to be statistically linked to basin-wide seasonal patterns of wind stress and wind stress curl. This predominant mode of covariability is interpreted in terms of rapid basin-scale adjustments in the form of two counter-rotating meridional overturning Ekman cells centered on the tropics and the subtropical gyre. A second mode of co-variability is found which is associated with anomalous patterns of wind stress and wind stress curl correlated with the North Atlantic Oscillation. This mode acts to modulate the horizontal gyre circulations, and to reinforce and weaken at times an overturning cell at the inter-gyre boundary.

Gerard McCarthy,  The Seasonal Cycle of the AMOC at 26.5N - Local or Global? (download PDF)

[National Oceanography Centre]

Abstract

The RAPID array at 26ºN has been providing continuous estimates of the Atlantic Meridional Overturning Circulation (AMOC) since 2004. The longer the timeseries the more it is possible to understand seasonal variability. The major component in the seasonal cycle comes from the eastern boundary where seasonal variations in wind stress curl drive density anomalies visible to below 1000m. This is out of phase with thermocline upwelling but can be linked to the cycle of seaonally reversing intermediate water flow in this region. Since 2008, we have been running ocean glider missions to the region. The gliders supplement mooring data in the crucial depth range around 1000m near to the coast. These data reinforce the link between the intermediate circulation and the seasonal variability of the AMOC. With that link established we address questions about the meridional extent of this intermediate circulation and the causes of the seasonally asymmetric transports that we observe.

TOPIC 2-Natural variability

Jim Hurrell, Sources of Uncertainty in Climate Predictions: The Role of Natural Variability (download PDF)

[University Corporation for Atmospheric Research]

Mojib Latif, Southern Ocean Centennial Climate Variability: Origin, AMOC Impact, and Implications for Global Warming (download PDF)

[Helmholtz Centre for Ocean Research Kiel]

Abstract

Sea surface temperature observations of the last 160 years and climate model simulations suggest that the Southern Ocean exhibits an internal centennial mode, which is characterized by fluctuations in Weddell Sea deep convection. The mode is of global significance, as it projects on globally averaged surface air temperature. It is driven by the competing roles of heat accumulation at mid-depth and surface salinity on the stability of the water column. The recent decadal trends observed in both the ocean and the atmosphere in the Southern Ocean Sector can be understood as part of the centennial cycle. In the climate model, the Atlantic Meridional Overturning Circulation (AMOC) is strongly affected by the Southern Ocean centennial variability. This is reflected, for instance, in North Atlantic sea level which also features strong centennial changes in the model. Finally, the Southern Ocean centennial mode, which is currently in its cold phase, due to its long timescale may continue to retard global warming during the next decades.

Johann Jungclaus,  Arctic climate and sea-ice changes over the last millennium: what is the role of the Atlantic Meridional Overturning Circulation?

(download PDF)

[Max-Planck Institute for Meteorology]

Abstract

Arctic climate is characterized by pronounced variations on multidecadal to centennial time scales. Model simulations demonstrate that both internal variability and the response to external forcing show aspects of Arctic amplification. Using reconstructions for pan-arctic temperature and sea-ice changes we assess the ability of recent model simulations, carried out in the framework of the Coupled Model Intercomparison Project, to reproduce observed changes and to identify mechanisms driving variations on interdecadal to centennial time-scales. The low-frequency character of the variability suggests a role of ocean dynamics and, in particular, changes in the overturning circulation. We explore the role of ocean heat advection and atmosphere-ocean-sea ice coupling in driving such multidecadal variations in the Max Planck Institute Earth System Model.

Matthew Menary, Solar forcing of the AMOC since 1859 (download PDF)

[Met Office Hadley Centre]

Abstract

In this presentation, it is shown that, over the last 150 years, natural forcing factors alone have been enough to temporarily significantly strengthen the Atlantic Meridional Overturning Circulation (AMOC) in a new state-of-the-art general circulation model: HadGEM2-ES. Specifically, it is shown that the most important of these natural forcings was solar rather than volcanic forcing, despite the strong volcanism of the 1880s. The AMOC strengthening occurs through a densification of the North Atlantic, driven by anomalous surface freshwater fluxes due to increased evaporation. These are the result of the 11 year solar magnetic cycle interacting with stratospheric ozone concentrations which affects both the shortwave radiation incident at the surface as well as the atmospheric circulation via the North Atlantic Oscillation. Discussion is made of large changes linked to the strengthening of the AMOC, such as a northward shift of the intertropical convergence zone (ITCZ), anomalous Amazonian rainfall, and intensification of El-Nino Southern Oscillation.

Stephen Outten, Compensation and the multi-decadal variability of heat Transports (download PDF)

[Nansen Environmental and Remote Sensing Center Bjerknes]

Abstract

The meridional transport of heat, through both the ocean and atmosphere, is a basic component for maintaining Earth’s climate. Understanding the decadal to multi-decadal changes in these transports provides insight into the natural variability of the climate system. In 1964, Jacob Bjerknes proposed that large anomalies in the oceanic heat transport should be balanced by opposing variations in the atmospheric heat transport; a process later named Bjerknes Compensation. Bjerknes compensation has been identified in the 600 year control run of the Bergen Climate Model by examining the anomalies of the implied meridional heat transports in both the ocean and atmosphere. These anomalies show strong anti-correlation (r=-0.81, p≤0.05). They also show a multi decadal variability with a period of approximately 70 years. The variability in the Atlantic region will be discussed, along with the identification of Bjerknes Compensation in proxy data.

Guillaume Gastineau, Atmospheric response to the North Atlantic Ocean variability on seasonal to decadal time scales (download PDF)

[Université Pierre et Marie Curie- LOCEAN/IPSL]

Abstract

The NCEP 20th century reanalysis and a 500-year control simulation with the IPSL-CM5 climate model are used to assess the influence of ocean-atmosphere coupling in the North Atlantic region at seasonal to decadal time scales. At the seasonal scale, the air-sea interaction patterns are similar in the model and observations. In both, a statistically significant summer sea surface temperature (SST) anomaly with a horseshoe shape leads an atmospheric signal that resembles the North Atlantic Oscillation (NAO) during the winter. The air-sea interactions in the model thus seem realistic, although the amplitude of the atmospheric signal is half that observed, and it is detected throughout the cold season, while it is significant only in late fall and early winter in the observations. In both model and observations, the North Atlantic horseshoe SST anomaly pattern is in part generated by the spring and summer internal atmospheric variability. In the model, the influence of the ocean dynamics can be assessed and is found to contribute to the SST anomaly, in particular at the decadal scale. Indeed, the North Atlantic SST anomalies that follow an intensification of the Atlantic meridional overturning circulation (AMOC) by about 9 years, or an intensification of a clockwise intergyre gyre in the Atlantic Ocean by 6 years, resemble the horseshoe pattern, and are also similar to the model Atlantic Multidecadal Oscillation (AMO). As the AMOC is shown to have a significant impact on the winter NAO, most strongly when it leads by 9 years, the decadal interactions in the model are consistent with the seasonal analysis. In the observations, there is also a strong correlation between theAMO and the SST horseshoe pattern that influences the NAO. The analogy with the coupled model suggests that the natural variability of the AMOC and the gyre circulation might influence the climate of the North Atlantic region at the decadal scale.

Sybren Drijfhout, An abrupt cold event in the pre-industrial control run of ECEARTH induced by the AMOC (download PDF)

[Royal Netherlands Meteorological Institute]

Abstract

During an 1150-year pre-industrial control run, an abrupt colling episode occurred after 450 years, lasting for about a century. The signal is clearly seen in the Atlantic Multidecadal Oscillation index, featuring an anomaly of -0.8 for about 100 years. The temperature-anomaly over the European continent was about 0.5 degree, but over Scandinavia and the British Isles the average cooling surpassed more than a degree. The maximum signal is over the Labrador Sea where temperature dropped with 3 degrees and salinity with 1.5 psu. The halocline stopped deep convection there and was caused by anomalous export of sea-ice from east of Greenland. There, the coupled system tipped into an anomalous state in which sea-increase induced a thermal high that caused north-easterly winds to further increase sea-ice cover. This positive oceanatmosphere- sea-ice feedback was halted when sea-ice melt in the Labrador Sea caused an even larger thermal anomaly and high-pressure field in the Labrador Sea. Analysis with a special toolkit for critical transitions shows clear signs of a noise-driven transition from one state to another in many ocean variables, and early warning signals in net downward shortwave radiation over the Labrador and Greenland Seas.

 

TOPIC 3-Key processes

Antonio Busalacchi, Physical Processes Key to Advancing our Understanding of North Atlantic Climate Variability (download PDF)

[Earth System Science Interdisciplinary Center]

Juliette Mignot, Volcanic impact on the Atlantic Meridional Overturning Circulation in two generations of the IPSL model (download PDF)

[IRD-LOCEAN]

Abstract

The oceanic response to volcanic eruptions over the last 1000 years is investigated with a focus on the North Atlantic Ocean, using two versions of the fully coupled AOGCM developped at IPSL forced by a realistic time series of volcanic eruptions, total solar irradiance (TSI) and atmospheric greenhouse gases concentration. One major difference between the two versions of the model lies in the vertical resolution, which is enhanced from 19 vertical levels and 39 vertical levels, allowing a much finer resolution of the stratosphere. The module implementing volcanic eruptions was also improved in the last version of the model with a more realistic spreading of sulfate aerosol optical properties over the first layers of the stratosphere and relevant spectral intervals. The tropospheric natural aerosols are also taken into accounts following a prescribed pre-industrial climatology. The earliest version of the IPSL model had shown a different response of the AMOC to eruptions occurring after 1400 A.D. and to the stronger eruptions occurring between 1100 and 1300. This study tests the robustness of the result in order to gain understanding in the diversity of AMOC response to volcanic eruptions in climate models.

Chris Roberts, Are SSTs useful for detecting changes in the Atlantic meridional overturning circulation? (download PDF)

[Met Office Hadley Centre]

Abstract

It is likely to take several decades to detect an anthropogenic weakening of the Atlantic meridional overturning circulation (AMOC) using transport observations alone. Previous work has shown that detection times may be improved by making use of targeted hydrographic observations that are physically linked to changes in the AMOC (Roberts and Palmer, 2012). In addition, several modelling studies have demonstrated significant relationships between the AMOC and large-scale patterns of North Atlantic sea-surface temperatures (SSTs). However, it is not clear if patterns identified using model control simulations can be reliably used to estimate AMOC changes in the presence of external climate forcings. We explore the relationship between SSTs and the AMOC in several simulations within a state-of-the-art earth system model (HadGEM2-ES) and compare the utility of SST-based fingerprints with those derived using other ocean variables.

Claus Boening, Decadal changes in subpolar North Atlantic heat and freshwater content orchestrated by the NAO (download PDF)

[Helmholtz Centre for Ocean Research Kiel]

Abstract

During the last five decades striking changes occurred in the freshwater and heat content of the subpolar North Atlantic (SNA). The deep and abyssal waters that originate in this region to feed the lower limb of the AMOC experienced a remarkable decrease in salinities after the late 1960s, linked to a progressive dilution of the system of overflows from the Nordic Seas. A rapid reversal of the freshening trend occurred in the mid-1990s, with a pronounced salinification especially in the eastern SNA; at the same time, the freshening of the overflows slowed to a stop. An intriguing feature of the observational record is the tight co-variability of the freshwater and heat content of the SNA: the long-term freshening was accompanied by a progressive decrease in heat content, and both trends reversed simultaneously after 1995. Here we use a sequence of experiments with high-resolution (1/4°, 1/12°, and 1/20°) oceansea ice models to identify the dynamical causes of these decadal changes. Hindcast simulations of the oceanic response to the atmospheric variability 1948-2007 (as given by the “CORE” reanalysis) capture the decadal variability in the integral properties of the SNA as reconstructed from historical salinity and temperature data, and reproduce pertinent observational indices of mid-latitude circulation variability. Analysis of the freshwater and heat budgets shows that the largest contribution to the SNA property changes has been due to variations in the inflow of warm, saline water with the subtropical North Atlantic, with only a minor contribution due to variations in the inflow of cold, fresh waters from the northern basins. The subtropical-subpolar flux variability is not directly related to the AMOC; it can conceptually be understood in terms of the “intergyre-gyre” response to the mid-latitude westerlies associated with the NAO.

Eleanor Frajka-Williams, Causes and impact of the recent slowdown of the MOC at 26N on heat content in the North Atlantic (download PDF)

[National Oceanography Centre]

Abstract

The RAPID-MOC observing system has been estimating the strength of the Atlantic Meridional Overturning Circulation (AMOC) at 26°N in the Atlantic since April 2004, with the latest transbasin time series complete through April 2011. During these 7 years of measurement, surprising sources of variability have been identified--from weekly to interannual timescales, due to winds, Rossby waves and eddies. In the 2009/10 and 2010/11 winters, a negative NAO index associated with a reversal in winds at 26N resulted in anomalously low MOC, which subsequently recovered. What did not recover, however, is the upper-ocean heat content in the North Atlantic. In particular, ocean heat content in the mid-latitude North Atlantic from 2010 is significantly colder than previously. Combining heat transport estimates at 26N (Johns et al, 2011) with an estimate of heat transport at 41N (Willis et al, 2011) and surface air-sea heat fluxes from reanalysis, we can show that the changes in heat transport due to horizontal advection dominate the observed changes in heat content in the North Atlantic. This has ramifications for the ocean modulation of atmospheric circulation via the redistribution of ocean heat content.

Jin-Song von Storch, An estimate of Lorenz energy cycle for the world ocean based on the one tenth degree STORM/NCEP simulation (download PDF)

[Max-Planck Institute for Meteorology]

Abstract

This paper presents an estimate of the oceanic Lorenz energy cycle derived from a 1/10 degree simulation forced by 6-hourly fluxes obtained from NCAR/NCEP reanalysis-1. The total rate of energy generation amounts to 6.6 TW, of which 1.9 TW are generated by the time-mean winds and 2.2 TW by the time-varying winds. The dissipation of kinetic energy amounts to 4.4 TW, of which 3 TW originate from dissipation of eddy kinetic energy. The energy exchange between reservoirs is dominated by the baroclinic pathway and the pathway that distributes the energy generated by the time-mean winds. The former converts 0.7 to 0.8 TW mean available potential energy to eddy available potential energy and finally to eddy kinetic energy, whereas the latter converts 0.5 TW mean kinetic energy to mean available potential energy. This energy cycle differs from the atmospheric one in two aspects. First, the generation of the mean kinetic and mean available potential energy ieach, to a first approximation, balanced by the dissipation. The interaction of the oceanic general circulation with mesoscale eddies is hence less crucial than the corresponding interaction in the atmosphere. Second, the baroclinic pathway in the ocean is facilitated not only by the surface buoyancy flux but also by the winds through a convertion of 0.5TW mean kinetic energy to mean available potential energy. In the atmosphere, the respective conversion is almost absent and the baroclinic energy pathway is driven solely by the differential heating. It is because of this conversion that the ocean behaves not as a 'heat engine' but instead as a ‘wind mill’.

TOPIC 4-Predictability

Tim Palmer, On the reliability of multi-year forecasts of climate (download PDF)

[University of Oxford]

Wilco Hazeleger, Multimodel decadal predictions of the North Atlantic Ocean (download PDF)

[Royal Netherlands Meteorological Institute]

Abstract

We present the multimodel THOR decadal predictions of the subpolar Atlantic. The aim is to predict the Atlantic meridional overturning circulation. Because the overturning itself is not observed at decadal time scales, we focus on predicting oceanographic phenomena that are thought to be associated with the AMOC and that can be verified against long term observations. The multimodel predictions show good skill in prediction low frequency SST variability associated with the Atlantic Multidecadal Oscillation. Also, subsurface characteristics in the subpolar gyre are well predicted up to 9 years in advance. We could not find clear predictability of Great Salinity Anomalies, but the Labrador Sea fresh water and heat content up to 700 m is remarkably well predictable. This predictability is much higher than in standard historical climate model simulations. Since the sea level pressure predictions do not show skill, this must be associated with oceanic processes.

Susanna  Corti, Impact of initial conditions with respect to external forcing in the decadal predictions: a sensitivity experiment (download PDF)

[European Centre for Medium-Range Weather Forecasts]

Abstract

To isolate the impact of the initial conditions from the impacts of greenhouse gas a sensitivity experiment has been performed. Two initial dates (1965 and 1995) have been taken from distinct decades and four 10-year integrations have been carried out: two control integrations initialised with estimates of the 1965 and 1995 climate states and external forcing such as, aerosols, solar activity and green house gases of 1965-1975 and 1995-2005 decades; two sensitivity hindcasts initialised in 1965 and 1995, but using the external forcing of 1995-2005 and 1965-1975 respectively. By comparing control and sensitivity integrations we estimate the impact of external forcing versus initial conditions on the skill of decadal predictions. Our results suggest that the impact of the forcing is overall dominant. However the memory of initial conditions can persist for more than one year in some specific regions.

Juergen Kroeger, Decadal Climate Prediction with ECHAM5-MPIOM: Sensitivity  to Initialization (download PDF)

[Max-Planck Institute for Meteorology]

Abstract

We study the impact of three ocean state estimates (GECCO, SODA, [ECMWF]-ORAS3) on decadal predictability in one particular forecast system, an updated version of the CMIP3 climate model system from the Max Planck Institute for Meteorology in Hamburg (ECHAM5-MPIOM). The forecast procedure follows two steps. First, anomalies of temperature and salinity of the observational estimates are assimilated into our coupled model. Second, the assimilation runs are then used to initialize 10-year-long hindcasts/forecasts starting from each year between 1960 and 2001. The impact of the individual ocean state estimates is evaluated both by the extent to which climate variations from the ocean state estimates are adopted by the forecast system (`fidelity') and by the prediction skill of the corresponding hindcast experiments. The evaluation focuses on North Atlantic (NA) sea surface temperature (SST), upperlevel (0-700~m) NA ocean heat content (OHC) and the Atlantic meridional overturning circulation (MOC). The ORA-S3 reanalysis gives the best results for our forecast system as measured by both overall fidelity of the assimilation procedure and predictions of upper-level OHC in the North Atlantic. As a next step, the ORA-S3 assimilation is used to investigate the impact of initial perturbation methods on decadal predictions where the initial state of either the ocean or the atmosphere or of both ocean and atmosphere are perturbed.

Nick Dunstone (Doug Smith), Results from multi-model idealised observing system experiments being carried out for THOR WP4.2 Impact of Ocean Observations on THC predictions (download PDF)

[Met Office Hadley Centre]

Abstract

We present analysis of the THOR (WP4.2) multi-model experimental effort to study the impact of a ocean data coverage on our ability to initialise key climate variables (e.g. the AMOC) . The experiments are performed in the idealised model world framework and so allow robust estimation of the success or otherwise of both the assimilation methods used and of different observational coverage. We consider both an ARGO like sub-surface data distribution (coverage available today) against pre-ARGO (1990's type) coverage. At the time of abstract submission, the different institutions are still running these experiments.

Daniela Matei, Towards multiyear climate predictions of the North Atlantic/European climate (download PDF)

[Max-Planck Institute for Meteorology]

Abstract

The latest set of the MPI-M Earth System Model (MPI-ESM) decadal predictions provided for the 5th coupled model intercomparison project (CMIP5) is examined with a focus on multiyear predictability of the North Atlantic climate and oceanic fluctuations. The experiments include suites of decadal hindcast experiments performed with two different set-ups of the MPI-ESM coupled model (MPI-ESM-LR and MPI-ESM-MR). The ocean initializations assimilate three-dimensional temperature and salinity anomalies from MPIOM ocean experiments forced with the NCEP-NCAR atmospheric reanalysis. The hindcast procedures take into account yearly and 5-yearly initialisation. We asses the skill of the initialized decadal hindcast experiments against the benchmark prediction skill of the non-initialized hindcast simulations and statistical forecasts. The results show that variations in sea surface temperature in the North Atlantic region, especially over the subpolar gyre, and the Mediterranean can be skilfully predicted up to a decade ahead and with greater skill than by both uninitialized simulations and persistence forecasts. Multiyear predictability of a number of ocean dynamical quantities, such as North Atlantic Ocean volume and meridional heat transport variations is also presented, together with a discussion on the potential impact of AMOC on North Atlantic climate predictability.

Ed Hawkins, Statistical decadal predictions for sea surface temperatures: a benchmark for dynamical GCM predictions (download PDF)

[NCAS-Climate, University of Reading]

Abstract

Accurate decadal climate predictions could be used to inform adaptation actions to a changing climate. The skill of such predictions from initialised dynamical global climate models (GCMs) may be assessed by comparing with predictions from statistical models which are based solely on historical observations. This paper presents two benchmark statistical models for predicting both the radiatively forced trend and internal variability of annual mean sea surface temperatures (SSTs) on a decadal timescale based on the gridded observation data set HadISST. For both statistical models, the trend related to radiative forcing is modelled using a linear regression of SST time series at each grid box on the time series of equivalent global mean atmospheric CO2 concentration. The residual internal variability is then modelled by (1) a first-order autoregressive model (AR1) and (2) a constructed analogue model (CA). The verification of 46 retrospective forecasts with start years from 1960 to 2005 shows that the anomaly correlation coefficient for AR1 is greater than 0.7 over parts of extratropical North Atlantic, the Indian Ocean and western Pacific. This is primarily related to the prediction of the forced trend. More importantly, both CA and AR1 give skillful predictions of the internal variability of SSTs in the subpolar gyre region over the far North Atlantic for lead time of 2 to 5 years, with the correlation coefficient greater than 0.5. For the subpolar gyre and parts of the South Atlantic, CA is superior to AR1 for lead time of 6 to 9 years. These statistical forecasts are also compared with ensemble mean retrospective forecasts by DePreSys, an initialised GCM. DePreSys is found to outperform the statistical models over large parts of the North Atlantic for lead times of 2 to 5 years and 6 to 9 years, however AR1 is generally superior to DePreSys in the North Atlantic Current region, while CA is superior to DePreSys in parts of South Atlantic for lead time of 6 to 9 years. These findings encourage further development of benchmark statistical decadal prediction models, and methods to combine different predictions.

Thomas Martin, Uncertainties of North Atlantic MOC and atmosphere-ocean interaction in future climate projections (download PDF)

[Helmholtz Centre for Ocean Research Kiel]

Abstract

Future changes in the Atlantic meridional overturning circulation (MOC) will result from processes both internal and external to the climate system. Global warming leads to changes in the vertical salinity and temperature profiles. The meridional structure of the changes diminishes the meridional oceanic density contrast. Consequently, climate models predict a weakening of the MOC during the twenty-first century when forced by increasing levels of greenhouse gas concentrations. However, large uncertainty exists, even under identical forcing. Some models suggest that MOC changes are strongly related to large-scale salinity anomalies and therefore probably to changes in the surface freshwater fluxes and transport. We therefore derived the general relationship between the MOC and freshwater budget of the Northern Hemisphere analyzing the CMIP3 20th century simulations and the A1B scenario runs. A quantification of the different sources of uncertainty (scenario, internal and model uncertainties) indicates the model error as the largest component, even at lead times of any decades. The internal variability is significant during the first few decades, while scenario uncertainty is almost negligible at all lead times. The different contributions to model uncertainty like surface wind, density, salinity, and temperature have been also analyzed. Overall, the strongest MOC changes have been projected around 40°N, whereas the strongest signal-tonoise ratio is located south of 40°N. Uncertainties in meridional ocean density profiles are dominated by model uncertainties in the salinity changes. The local signal-to-noise ratio of the ocean freshwater flux is low in the Arctic and sub-polar region. First analyses of the CMIP5 historical simulations and the RCP45 scenario basically confirm the pervious results from CMIP3.

TOPIC 5-Marginal seas and coastal impacts

Hans von Storch, Coastal seas: resources and risks (download PDF)

[Helmholtz-Zentrum Geesthacht Centre for Materials and Coastal Research]

Abstract

Coastal seas are marginal parts of the global oceans, which are strongly and directly influenced by land. Coastal land is mostly inhabited, and often massively so. Thus, the land influence is modified by humans, e.g., by discharge of nutrients and pollutants, fixing coastlines, modifying sediment loads, extracting oil, gas, water and fish, introducing new species and more indirectly: climate change. Coastal environments present risks, such as high waves, strong winds, surges, algea blooms and erosion; they also provide opportunities of human exploitations, i.e. represent a resource. The coastal seas are subject to geophysical and ecological dynamics, but also to societal dynamics. Challenges of coastal sea research concern the ability to detect and predict short-term hazards, possibly related to accidents, the detection of ongoing long-term changes, attributing plausible causes for such changes as well as anticipating future changes. The catalogue of changes comprises climate variabilty and change, modifications of landand seascape, usage of resources or release of substances, to mention some. The scientific approaches CoastDat, COSYNA and CoastMap are briefly sketched, as well as the issue of embedding into a societal context.

Dmitry Sein, Modeling anthropogenic Climate Change of the northwest European Shelves and the northeast Atlantic (download PDF)

[Max-Planck Institute for Meteorology]

Abstract

The global general circulation models involved in IPCC simulations are usually too coarse to reproduce many regional processes, which could have an impact on the future climate change in regions such as the North Sea and Baltic Sea. We present a novel approach to downscale climate change scenarios and to investigate the interactions between the North Atlantic Ocean and the European shelves. A global ocean - sea ice - marine biogeochemistry model with regionally high horizontal resolution is coupled to an atmospheric regional model and global terrestrial hydrology model. The model approach and the results of downscaled A1B scenario for the North Atlantic and North European shelves are presented. Numerical experiments covering the period 1920-2100 were carried out. Future changes in ocean and atmospheric circulation focusing on different regions of North Atlantic and North European shelves were analyzed. In addition to the climate warming, other processes like northward shift of the Gulf Stream position, Atlantic MOC weakening, decrease of biological production in North Sea region, regional sea level rise, extreme floods and changes in amplitude and phase of the seasonal cycle of river runoff were estimated.

Tor Eldevik, Sources and propagation of anomalous freshwater content in the Nordic Seas  (download PDF)

[Geophysical Institute and Bjerknes Centre for Climate Research]

Abstract

The freshwater content of the Nordic Seas is expected to increase due to for example melting of sea ice and Greenland glaciers and an amplified hydrological cycle under global warming. Hence anomalous freshwater content in the Nordic Seas, the gateway between the Arctic Ocean and Atlantic proper, is an indicator for climate change. There are two oceanic sources for freshwater anomalies in the Nordic Seas: The Arctic Ocean to the north, and the North Atlantic to the south. We use a most comprehensive dataset of hydrographic observations to determine sources and propagation of anomalous freshwater content in the Nordic Seas over the last 60 years. Complimentary we use a state-of-the-art 600-year climate model simulation for a closed and quantitative assessment. We conclude that the Atlantic Inflow determines most of the variability of the Nordic Seas freshwater content, and that anomalies propagate from the Atlantic Inflow through the Norwegian Sea into the Greenland- and later on Iceland Sea.

Roshin Pappukutty Raj, Atlantic Water temperature variability in the northeastern Nordic Seas (download PDF)

[Nansen Environmental and Remote Sensing Center]

Abstract

The Atlantic Water entering the Norwegian Sea and associated changes in its temperature has large importance for the processes relevant for the Atlantic Meridional Overturning Circulation. The variability in the Atlantic Water temperature in the north eastern Nordic Seas (10o-20o E and 70o-80oN) during a period of 60 years (1948-2007) is investigated using a synoptic forced, 13 km resolution, regional version of the Miami Isopycnic Coordinate Ocean Model (MICOM). The model successfully simulates the spatial distribution of the Atlantic Water temperature in the Nordic Seas. In addition, the inter-annual variability in simulated Atlantic Water (75-350 m) temperature of the north eastern Nordic Seas agrees very well with the existing observations (r=0.62; detrended). In this study, the possible mechanism, which impacts the temperature variability of the Atlantic Water is investigated.

Héðinn Valdimarsson, Observations of currents and hydrography in the eastern Iceland Sea (download PDF)

[Marine Research Institute Iceland]

Abstract

The Iceland Sea is an area of great climatic variability. Atlantic Water enters this area from the south and east in variable quantities as the East Greenland Current and the East Icelandic Current bring the Polar Water from the north. The Kolbeinsey Ridge divides between the western and eastern parts of this sea area. MRI has been involved in several studies of hydrography and currents that have been done in various locations in the eastern part of the Iceland Sea in the last decade. The aim of these studies has been different such as hydrobiological variability in connection with research on fish stocks, direct current measurements and hydrographic observations connected to oil exploration as well as clarification of the North Icelandic Jet. In THOR a mooring was deployed to estimate the flow of fresh water in the East Icelandic Current. A review is given of these different studies and the status of knowledge of the circulation.

Michael J. Karcher, Arctic ocean circulation variability – causes and Consequences (download PDF)

[Alfred Wegener Institute for Polar and Marine Research]

Abstract

The presentation will provide an update on recent modifications in the understanding of the Arctic ocean circulation and its communication with the adjacent oceans. Results derive from the application of the coupled ocean sea-ice model NAOSIM and from observed data. A strong link has been found e.g.between the intensity of the Beaufort Gyre circulation, the freshwater content of the gyre and the intensity of the cyclonic circulation of the intermediate Atlantic Water Layer. The driving force for this coupled fluctuation of the large Arctic gyres at the different depth levels is Ekman Pumping caused by the surface stress curl in the Amerasian Basin. In addition, large scale atmospheric sea level pressure patterns like the Arctic Oscillation modify the distribution and the pathways of Arctic Ocean water masses. Our results shed new light on the circulation of the mid depth Atlantic water layer in the Arctic, which appears to be much less stable than anticipated. We will discuss how the corresponding changes of circulation modify the characteristics of the exports from the Arctic to sub-polar latitudes at the surface and at overflow depths.

Ursula Schauer , Change of heat in the Eurasian Basin of the Arctic Ocean in the last two decades (download PDF)

[Alfred Wegener Institute for Polar and Marine Research   ]

Abstract

The early 1990s featured warming of intermediate waters in the Arctic Ocean as a consequence of warmer water advection from the North Atlantic. The 2000s showed again increased temperatures at the Atlantic entrance of the Arctic Ocean with respect to the late 1990s. This warming was associated with an increase in oceanic heat transport through Fram Strait by 30%. Here we attempt to quantify the change in the heat content of the Eurasian Basin between the 1990s and the 2000s related to the respective inflow variability of the Atlantic Water. Despite an increase in large-scale cruises to the central Arctic and the deployment of ice-tethered CTD-profilers in recent years, we are limited by the scarcity of repeated observations in the central Arctic. As a consequence of the different regional data coverage spatial variation may conceal temporal variation. To take into account a possible spatial bias, data from both periods are referred to the EWG Atlas which comprises Arctic-wide data between 1950 and 1980. Based on this method, we find that the Atlantic layer in the Eurasian Arctic was much warmer in the two recent decades than in the EWG period, but there is no significant difference in the Atlantic layer heat content between the 1990s and the 2000s. Recent warming was only found in the layer below 1000 m, and this warming is not likely attributed to heat input from the North Atlantic/Nordic Seas.

Posters

Erik Behrens, Simulating the response of the subpolar North Atlantic increasing Greenland meltwater fluxes: impact of mesoscale current phenomena (download PDF)

[Helmholtz Centre for Ocean Research Kiel]

Abstract

A sequence of global ocean/sea-ice models with increasing resolution is used to study the impact of the narrow boundary currents and mesoscale eddies on the response of the subpolar North Atlantic to changes in atmospheric forcing and Greenland meltwater run-off. Building on model developments in the European DRAKKAR initiative, we examine the effects of a successive refinement in horizontal grid sizes, from a non-eddying ½°-grid (~30 km mesh size at 60°N), to an “eddy-permitting” ¼°-grid (~15 km), up to an “eddy-resolving” 1/20°-grid (~3 km); the latter realized via a two-way nesting methodology (AGRIF) in which a 1/20°- North Atlantic (from 30° to 80°N, VIKING20) is embedded in the global ¼°-domain. The first set of experiments examined the ocean variability forced by atmospheric fluxes during the last decades, as given by the CORE reanalysis product for 1948-2007, allowing a critical assessment of key features of the subarctic circulation, including shallow and deep boundary current structures and transports, deep winter convection, and eddy statistics, as well as indices of multi-decadal transport variability. Subsequent sets of experiments investigated the response to prescribed perturbations in meltwater runoff from Greenland, considering both the classical idealization of a constant and equally-distributed 100mSv-flux, and a more realistic, inhomogeneous and gradually increasing flux distribution based on recent glaciological estimates. The model sequence demonstrates the critical role of the cross-shelf flux of freshwater in the ocean’s response behaviour: compared to the coarser models the export of freshwater from the shelf areas to the interior basins is significantly retarded in the 1/20°- simulation. Consequences include an enhanced initial accumulation of freshwater in Baffin Bay, and a concentration of exports to the subarctic basin interiors to areas of high eddy activity, such as near Cape Desolation off southwestern Greenland. Implications for changes in the intensity of deep winter convectionand the dynamical response of the subpolar gyre and AMOC will be discussed.

Simon Blessing, Testing variational calibration of an Earth System Model (download PDF)

[FastOpt]

Abstract

The MITogcm coupled as oceanic component to the PlanetSimulator PlaSim constitutes an Earth System Model. Both, coupled initialisation and calibration for this model are explored using variational techniques. The required derivative information information is efficiently provided by the model's adjoint, which is generated by the automatic differentiation tool TAF from the ~ 400,000 lines of Fortran source code of the coupled model. The studied resolution is 4° in the ocean and 5.6° (T21) in the atmosphere and land surface components. To extend the assimilation window non-smooth processes such as precipitation in the atmosphere are switched off for the experiments. In identical twin experiments, a combination of process parameters and initial conditions are recovered to high precision from observations generated with the coupled model.

Michael Brüdgam, Impact of natural and antropogen generated variabilityof the thermohaline circulation on the carbon uptake (not available)

[Institute of Oceanography, University of Hamburg]   

 

Abstract

For an estimate of the ocean state, many different observations are assimilated into numerical model. The assimilation products show rather significant differences in the meridional overturning circulation (MOC) important for the carbon cycle. For this reason, we investigate if decadal-scale MOC variability generated by artificial surface heat flux variability can be reproduced and which observations are the most appropriate. Improved assimilation products should lead to enhanced predictions about the carbon uptake and its budget.

Aurélie Duchez, Decadal Variability Of The Atlantic Meridional OverturningCirculation In Rapid Observations And Nemo Simulations (download PDF)

[National Oceanography Centre/RAPID]

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) has received considerable attention, motivated by its major role in the global climate system. Observations of AMOC strength at 26°N made by RAPID-WATCH (which is a combination of Gulf Stream transport, zonally integrated Ekman transport and mid-ocean transport), provide our best current estimate of the state of the AMOC. The period 2004-2012 where RAPID AMOC is available is too short to assess the decadal variability of the AMOC. Here, we define a new AMOC index at 26°N that combines the Gulf Stream transport, the Ekmantransport and the southward geostrophic Sverdrup transport. This index is expected to reflect variations in the AMOC at interannual/ decadal time scales.This estimate of the surface branch of the AMOC can be constructed back in time for the period when reliable measurements are available for the Gulf Stream and the wind stress. Our observationbased AMOC index suggests a trend toward decreasing AMOC strength since 1980 due to an increased Sverdrup transport of the recirculating thermocline waters. To test the reliability of the index on interannual and longer timescales we use three different NEMO simulations: a forced, a coupled and a climatic simulation. Using these simulations the index can capture a substantial fraction of the AMOC variability and is in a good agreement with the AMOC transport at 26°N both at interannual and decadal timescales. These results indicate that it might be possible to extend an observation-based AMOC index at 26°N back to the 1980s.

Tor Eldevik, Northern constraints on the Atlantic thermohaline Circulation (not available)

[Geophysical institute, University of Bergen] 

Abstract

The Atlantic Ocean's thermohaline circulation is an important modulator of global climate. The inflow of warm and saline Atlantic water to the Nordic Seas extends to the cold Arctic, a region that appears to be particularly influenced by climate change. We present an analytical model, rooted in observations and corroborated by a multi-century climate model simulation that constrains the strength and structure of the Arctic/Atlantic thermohaline circulation. The analytical model suggests, maybe surprisingly, that the strength of the Atlantic inflow is relatively insensitive to anomalous freshwater input; it mainly reflects changes in the northern heat budget. Freshwater anomalies are predominantly balanced by the inflow's partition into estuarine and overturning circulation with southward polar outflow in the surface and dense overflow at depth, respectively. Our study thus provides a simple framework for constraining the thermohaline circulation's response to observed or estimated past and future change in the northern seas. We find in particular that present trends of Arctic freshwater input and sea ice retreat, associated with heat loss, are consistent with increased Atlantic inflow toward the Arctic.

Matthias Fischer, Observing system simulations for a Line W based AMOC (download PDF)

[Institute of Oceanography, University of Hamburg]

Abstarct

We test whether the Atlantic meridional overturning circulation (AMOC) can be observed based on the existing Line W western boundary array. We deploy a potential trans-basin observing system for the AMOC into the numerical ocean circulation model ORCA025. The simulated observing system combines observations of the western boundary current (WBC) transport at Line W with a mid-ocean transport estimate obtained from thermal wind “measurements” and Ekman transport to the total AMOC. First, we close Line W zonally at 32°N supplementing the western boundary array with several “moorings” in the basin (Line W-32°N). Second, we supplement the western boundary array with a combination of observations at Bermuda and the eastern part of the RAPID array at 26°N (Line W-B-RAPID). Both, a small number of density profiles across the basin, and also only sampling the eastern and western bound- ary captures the variability of the AMOC at Line W-32°N and Line W-B-RAPID. The successful representation of the AMOC variability at both Line W-32°N and Line W-B-RAPID is based on Hirschi et al. (2003). We find that the AMOC variability is dominated by the WBC at Line W-32°N and Line W-B-RAPID, but we do not find a significant relation between the variability of both Line W sections and 26°N. Away from the western boundary, the interior transport (east of Bermuda) shows no significant relation between the two Line W-based sections and 26°N. Suggesting that a Line W based AMOC estimate could yield an estimate of the meridional transport that is independent of the 26°N RAPID estimate, the model-based observing system simulations presented here provide support for the use of Line Was a cornerstone for a trans-basin AMOC observing system.

Anna Friedrichs, An analysis of the velocities in the Faroe-Shetland Channel in VIKING20 (download PDF)

[Institute of Oceanography, University of Hamburg]

Abstarct

The circulation in the Faroe-Shetland Channel (FSC) in VIKING20, a general ocean circulation model with 3 km x 3 km resolution, is compared to two observational data sets: one from an ADCP-mooring array deployed since 1994, the other from a vessel-mounted ADCP operated for 4 years on the M/F Norröna, a regular ferry between Iceland and Denmark. Both observational data sets show a strong current to the southwest at the Faroe side of the FSC. We investigate this jet in the VIKING20 model. The FSC is part of the Greenland-Scotland Ridge (GSR), which separates the Nordic Seas from the North Atlantic. It is one of the main inflow branches of warm water to the Nordic Seas. The east side of the FSC is occupied by this northeastward inflow of Atlantic Water, while the deep part of the channel is filled with dense water flowing southwestward. These two opposing flows have mean velocities of approximately 0.3 m/s. In the surrounding water the mean velocity is below 0.05 m/s, except for a recently observed jet at the shelf edge of the Faroe Islands. The jet is seen in both observational data sets. It occupies the top 350 m and flows southwestward with up to 0.15 m/s. It is suspected that the jet may be recirculated Atlantic Water or water which circulates clockwise around the Faroe Islands, but this hypothesis has not been confirmed yet. The net transport across the FSC is to the Northeast in VIKING20 and the vessel-mounted-ADCP, but very small in the moored-ADCP estimate. The southwestward jet on the western flank of the FSC party compensates the Atlantic Inflow and significantly reduces the net transport obtained from the moored-ADCP data. An improved estimate of the recirculation in the jet is important for accurately quantifying volume and heat fluxes across the FSC.

Yongqi Gao, North Atlantic ventilation using chlorofluorocarbons and idealized—tracer simulations (download PDF)

[Nansen Environmental and Remote Sensing Center]

Abstract

The simulated chlorofluorocarbon CFC-11 and 29 geographically defined CFC-11 tracers as well as 29 geographically defined idealized tracers are used to quantify the regional contribution to the ventilation of the North Atlantic Ocean in a global version of Miami Isopycnal Coordinate Ocean Model (MICOM). Age tracers attached to 29 idealized tracers are also used to estimate the timescales for the water masses transports. The modeled overturning circulation matches the observed data for both intensity and variability, and the simulated distribution of CFC--11 in the subtropical North Atlantic Ocean is in good agreement with the observations, particularly above 800 m in depth. The simulated results show that the sandwich--like distribution of CFC--11 in the subtropical North Atlantic in both the observations and simulations was caused by subduction from western and eastern subpolar North Atlantic, but the contribution of the former (56%) is almost four times larger than that of the latter (15.7%). We show that the source function and the ocean dynamics determine the trend of passive tracer concentrations and demonstrate that the concentrations of both dynamically active tracers (such as water temperature and salinity) and passive tracers (such as CFC-11 and idealized tracer) in the deep North Atlantic are highly correlated on annual and interannual timescales, implying the dominant role of ocean dynamics in the variability of tracer concentrations.

Jeremy Grist, Surface Estimates of the Atlantic Overturning in Density Space in an Eddy-Permitting Ocean Model (download PDF)

[National Oceanography Centre]

Abstract

A method to estimate the variability of the Atlantic Meridional Overturning Circulation (AMOC) from surface observations is investigated using an eddy-permitting ocean-only model (ORCA-025). The approach is based on the estimate of dense water formation from surface density fluxes. Analysis using 78 years of two repeat forcing model runs reveals that the surface forcing based estimate accounts for over 60 % of the interannual AMOC variability in density co-ordinates between 37 ºN and 51 ºN. The analysis provides correlations between surface-forced and actual overturning that exceed those obtained in an earlier analysis of a coarser resolution coupled model. Our results indicate, that in accordance with theoretical considerations behind the method, it provides a better estimate of the overturning in density coordinates than in z coordinates in sub-polar latitudes. The inclusion of the anomalous Ekman Transport increases the amount of variance explained by an average 16 % throughout the North Atlantic and provides the greatest potential for estimating the variability of the AMOC in density space between 33 ºN and 54 ºN. In that latitude range, 70-84 % of the variance is explained and the RMS difference is less than 1 Sv. Results from the method can be compared with other emerging estimates and proxies of mid-latitude AMOC variability.

Matthias Gröger, The potential collapse of the NW European shelf pump, productivity, and CO2 uptake caused by the global climate warming during the 21st Century (download PDF)

[Max-Planck Institute for Meteorology]

Abstract

The North Sea absorbs ~ 35 Mio. tons of CO2 per year. The driving mechanism is the high carbon fixation by biological production and the subsequent export of carbon rich waters to the open Atlantic. In turn, biological production is extremely sensitive to the availability of dissolved nutrients. The main nutrient source for the North Sea is the adjacent Atlantic. Besides this, the North Sea receives nutrients from rivers. On the yearly average, the North Sea is under-supplied by nutrients. Therefore, the CO2 - absorption is highly vulnerable to changing nutrient imports. Here, a global ocean and biogeochemistry model with increased resolution in the North Sea and over the North Atlantic is used to downscale the IPCC AR 4 A1B scenario results from the MPI-M IPCC model.

Alonso Hernandez-Guerra, Observations of the North Atlantic Meridional Overturning Circulation (download PDF)

[University of Las Palmas de Gran Canaria variability]

Abstract

Using a variety of oceanographic data, including direct volume transports in the  Florida Strait and Argo float profiles and drift velocities at 24 and 36N in the North Atlantic, inverse calculations are presented in which the net meridional transport, down to a depth of approximately 1600 m, is estimated at both latitudes for a five year period 2003-2007. The upper ocean is divided into 7 layers using neutral density, and mass conservation constraints have been applied to a closed box bounded by these latitudes, including the Florida Strait. Ekman layer transports have been included in the top-most layer, and the inverse calculation has solved for changes from the initial reference velocities, Ekman and Florida Strait transports, given a priori estimates on the accuracy of each of thesequantities. Solutions with and without transformations due to Mediterranean Water (MW) formation are made. Temperature data from the Argo network are also compared with previous oceanographic sections at 24.5N. Our results indicate that 1) Comparison of the total integrated interior mass transports in the thermocline waters for the period 2003-2007 with the previous estimates based on trans-ocean hydrographic sections shows that the Meridional Overturning Circulation has not significantly changed since 1957; 2) A mean warming of 0.26 ºC ocurred in the upper ocean (600-1800 dbar range) during the 1998-1957 period; 3) A mean cooling of -0.15 ºC of the upper ocean is found for the 2005-1998 period and; 4) This cooling significantly decreases the maximum upper-ocean warming found in 1998 to only 0.11 ºC for the period 2005-1957, less than half that found during the 1998-1957 period.

Marie-Noelle Houssais, Dynamics of the Norwegian Atlantic Current from high resolution modeling and observations (download PDF)

[CNRS- LOCEAN/IPSL] 

Abstract

The Norwegian Atlantic Current carries northward large amounts of heat and salt to the northern latitudes. These transports are largely dependent upon the dynamics of the current. Instability and interaction with the bottom topography may lead to generation of eddies which divert part of the heat and salt to the basin interior while modulating the large-scale properties of the current. The dynamics of the Norwegian Current is investigated based on a hindcast simulation with a high resolution (1/16°) ice-ocean model of the Nordics Seas-overflow region forced by the ERA-interim atmospheric reanalysis. A particular attention is paid to the distribution of the Atlantic water core properties (temperature, salinity and velocity) and their northward evolution along the path of the current in the Nordic Seas, based on a comparison of model hydrographic fields with observations on different key sections. The mesoscale activity in the Atlantic Current is characterized and compared to available surface observations. Instability criteria are formed to determine the main regions of current instability. The impact of the mesoscale activity on the large scale distribution of current properties is evaluated by comparing the high resolution simulation with a similar simulation differing only by the resolution (1/4 degree). The eddy contribution to the heat and fresh water transports is evaluated.

Laura Jackson,Chris Roberts, Exploring the stability of the AMOC in HadCM3 (download PDF)

[Met Office Hadley Centre]

Abstract

The fresh water advection into the Atlantic ocean by the overturning circulation (Fov) has been suggested as an indicator for the stability of the Atlantic Meridional Overturning Circulation (AMOC). This hypothesis is explored in a global climate model with and without flux adjustments. Flux adjustments alter the model near surface salinity, changing Fov from a net importer, to a net exporter of fresh water. The AMOC recovers in strength from a collapsed state, however that in the experiment with flux adjustments recovers later and more slowly than that without flux adjustments. The different behaviour is traced back to the sign of Fov, confirming the indicator’s importance for predicting the stability of the AMOC and suggesting that model biases affecting Fov need to be addressed in order to assess the likelihood of irreversible changes in the AMOC.

Steingrímur Jónsson, Variability, forcing and fate of the inflow of Atlantic Water to the north Icelandic shelf (download PDF)

[University of Akureyri and Marine Research Institute, Iceland]

Abstract

One of the tasks in THOR was to measure the flux of water, heat and salt with the North Icelandic Irminger Current to the north Icelandic shelf and the Nordic Seas during the field phase. To determine this, the Marine Research Institute has been monitoring the flow with current meters on a section north of Iceland since 1994. The flow until 2010 consisted on the average of 68% of Atlantic Water with its transport being 0.88 Sv and the associated heat transport was estimated to be 24 TW. Since 1994 there is a positive trend in both the transport of Atlantic Water and the heat transport. The increase in the heat transport is mainly due to an increase in the temperature of the Atlantic Water. The variability of both the transport and the water mass composition of the current are closely correlated with the NCEP wind over the area. The transport responds very quickly to changes in the wind whereas the water mass composition takes longer time to adjust. Thus the local wind is a primary force driving at least the shorter term variability of the two parameters. However the direct wind forcing does not explain the mean flow of Atlantic Water to the north Icelandic shelf. Some of the Atlantic Water is deflected into the deeper parts of the Iceland Sea along the Kolbeinsey Ridge. Partly this water recirculates back to the west whereas the rest becomes entrained into an anticyclonic gyre east of the ridge.

Johannes Karstensen, Deep-sea data telemetry systems development in THOR (download PDF)

[Helmholtz Centre for Ocean Research Kiel (GEOMAR)]

Abstract

Moored time series over extended time periods are one of the keystones of the global ocean observing system. Multiyear long deployment periods are desirable to save costs and also to minimize variations in the experimental setup. However, the timely and safe data retrieval from such moored instrumentation is important for scientific investigations and for monitoring the marine environment. In As part of WP5 of the THOR project, two deep-sea subsurface data telemetry systems, suitable for long-term routine installation in conventional mooring lines, are presented. Both systems utilize combinations of data collection and data messaging systems that have not been used before on deepseamoorings. One system, the Bergen system” combines a central data collection unit with an underwater acoustics modem to deliver the data “on demand” to a passing-by ship or other platform. The other system, the “Kiel system” uses expendable data capsules, so-called PopUp buoys, deployed at a safe depth and populated with data collected from a central unit via serial connection and inductive link technology. The PopUp buoys are released on predefined dates, rise to the ocean surface and transmit the data to shore via satellite communication.

Dagmar Kieke, Formation and spreading of Labrador Sea Water derived from transient tracers (not available)

[Institute for Environmental Physics, University of Bremen]

Abstract

For two decades measurements of anthropogenic transient tracers such as chlorofluorocarbons are available for the subpolar North Atlantic. These serve as valuable tools to study the formation and spreading of Labrador Sea Water (LSW), one of the key water masses contributing to the deep and cold limb of the Atlantic Meridional Overturning Circulation (AMOC). The data set allows investigating the penetration of tracers into the deep ocean as a result of water mass formation processes happening in the Labrador Sea. From changes in the large-scale tracer inventories the formation history is derived in terms of estimating changes in the formation rate. Furthermore, by establishing property time series in different sub-basins the spreading of LSW across the subpolar gyre is analyzed, and resulting time series are compared to changes observed in the formation region. Particular focus is put on spreading in the Deep Western Boundary Current, from the formation region into the tropical North Atlantic Ocean, as this is the fastest track for transporting young deep water components out of the subpolar gyre into the tropics.

Armin Koehl, Causes of decadal changes of freshwater content of the Arctic Ocean (download PDF)

[Institute of Oceanography, University of Hamburg]

Abstract

Decadal changes of the freshwater content in the Arctic Ocean are of importance because changes related to the export may have an impact on the deep water formation in the North Atlantic and the global thermohaline circulation. We use adjoint sensitivities to determine causes of the freshwater export across 80N to forcing changes and circulation changes in the Nordic Seas and the Arctic Ocean. Sensitivities show that freshwater volume changes in the Norwegian North Atlantic Current north of the Lofoten and a salinity maxima in the Fram Strait and the Canadian Archipelago lead to an enhanced northward transport of freshwater. On time scales of 4 years, the influential regions move into the center of the Arctic and the area of the Greenland Scotland ridge, respectively. Dynamical sensitivities indicate that stronger freshwater export from the Arctic is related to an enhanced cyclonic circulation around Greenland with an enhanced import through Fram Strait and export through the Canadian Archipelago. Associated with this circulation is a cyclonic circulation in the Arctic. Cyclonic wind stress anomalies are optimal to force these changes. Results from the sensitivities obtained with a low resolution model are in agreement with the mechanisms implied by a simulation over the period 1948 to 2010 of a 1/12 degree OGCM of the Atlantic and Arctic Ocean. The simulation is agreement with observations of salinity anomalies at Fylla Bank. Changes of the liquid freshwater content are mainly related to changes of export into the North Atlantic. On inter-annual to decadal timescales changes in volume transport are more important than changes in salinity of the exported water. A large fraction of the changes due to changed salinity is associated  with the recirculation in the Fram Strait and therefore compensated by the associated volume transport changes. Applying Godfrey's Island Rule to the circulation around Greenland demonstrates that most of the changes in freshwater content can be explained by wind stress changes.

Young-Oh Kwon, Multi-decadal Variability of Atlantic Meridional Overturning Circulation in the Community Climate System Model version3 (download PDF)

[Physical Oceanography Department Woods Hole Oceanographic Institution]  

Abstract

Multi-decadal variability of Atlantic meridional overturning circulation (AMOC) is examined from a 700- year present-day control integration of the NCAR Community Climate System Model version 3 (CCSM3). AMOC variability in CCSM3 exhibit two distinct regimes, i.e. periods with very regular and strong decadal (~20-years) variability versus irregular red-noise-like variability with a persistence longer than 10 years,with an abrupt transition between them. The red noise-like multi-decadal AMOC variability in the last 250 years of the 700 year-long integration is primarily forced by the surface fluxes associated with stochastic changes in the North Atlantic Oscillation (NAO) that intensify and shift northward the deep convection in the Labrador Sea. However, the persistence of the AMOC and the associated oceanic anomalies that are directly forced by the NAO forcing does not exceed about 5 years. The additional persistence originates from anomalous horizontal advection and vertical mixing, which generate density anomalies on the continental shelf along the eastern boundary of the subpolar gyre. These anomalies are subsequently advected by the mean boundary current into the northern part of the Labrador Sea convection region, reinforcing the density changes directly forced by the NAO. On the other hand, the slightly contracted and stronger subpolar gyre in the preceding ~300 years results in the strong oscillatory AMOC by advecting opposite signed density anomalies from the North Atlantic Current region to the convection site. As a weak atmospheric response is found only in the red-noise regime which reinforces the AMOC variability, the both regimes of the multi-decadal AMOC variability in CCSM3 are suggested to be an ocean-only response to stochastic NAO forcing with opposite delayed feedback caused by the slightly different mean horizontal ocean circulation.

Helene R. Langehaug, Water mass transformation and the North Atlantic Current in three multi-century climate model simulations (download PDF)

[Nansen Environmental and Remote Sensing Center] 

Abstract

The simulated chlorouorocarbon CFC-11 and 29 geographically defined CFC-11 tracers as well as 29 geographically defined idealized tracers are used to quantify the regional contribution to the ventilation of the North Atlantic Ocean in a global version of Miami Isopycnal Coordinate Ocean Model (MICOM) driven by the daily NCEP/NCAR forcing. Age tracers attached to 29 idealized tracers are also used to estimate the timescales for the water masses transports. Our results show that the simulated overturning circulation matches the available observed data for both in tensity and variability, and the simulated distribution of CFC-11 concentration in the subtropical North Atlantic Ocean is in good agreement with the observations, particularly above 800 m in depth. We found that the sandwich-like distribution of CFC-11 concentration in the subtropical North Atlantic in both the observations and simulations is mainly caused by subduction from western and eastern subpolar North Atlantic, but the contribution of the former (56.0%) is almost four times larger than that of the latter (15.7%). We demonstrated that the ocean dynamics, instead of the source function, determine the annual and interannual variability in both dynamically active tracer (such as water temperature and salinity) and passive tracer (such as CFC-11 and idealized tracer) concentrations in the deep North Atlantic. The `apparent age' distribution shows that the surface water in the western subpolar North Atlantic takes about 12 years to reach the Nordic Seas and takes 20 years from the Nordic Seas to the subtropical deep North Atlantic. The transit time derived by `optimum time lag' approach shows a 9.3 to 13.6 year lag for the signals propagating from the western subpolar North Atlantic to the subtropical North Atlantic, which is generally consistent with that of about 10 years derived from the `apparent age'. The study suggests that geographically defined tracers can be used as an efficient tool to investigate the source and spreading pathway of water, and to forecast the spreading and spreading time of environmental accidents such as the radioactive waste in the world oceans.

Katrin Latarius, One decade of Argo float measurements in the Nordic Seas provide insight into the water mass transformation within the deep basins of the area (download PDF)

[Institute of Oceanography, University of Hamburg]

Abstract

Warm and salty Atlantic water masses flow into the Arctic Mediterranean from the south across the Greenland-Scotland-Ridge. These waters circumnavigate the area cyclonically in the form of a narrow edge current along the continental slopes. Also loop currents, containing Atlantic Water, have been observed to follow the submarine ridges in the Arctic Ocean and Nordic Seas. On the way the water masses are transformed into two main components. (1) Mixing of the Atlantic water with freshwater from precipitation, river run-off and ice-melt creates the light, low salinity Polar Water. (2) The negative buoyancy flux at the sea surface associated with cooling and brine rejection during sea ice formation enhances the surface layer density and in turn leads to strong vertical mixing, creating dense intermediate and deep waters. These deeper water masses eventually exit into the Subpolar North Atlantic via the overflows across the Greenland-Scotland- Ridge. This vertical circulation loop is the northern limb of the Atlantic Meridional Overturning Circulation, a major constituent of the global Conveyer Belt Circulation. The Nordic Seas, the region of interest in this study, directly border on the Atlantic Ocean to the north. They are separated by submarine ridges into four major basins, the Norwegian and Lofoten Basins, the Greenland Sea Basin and the somewhat shallower Icelandic Plateau. Water mass transformation takes place in the surrounding edge currents but also in the interior basins, where the residence time of the waters is long and the surface area is large compared to those of the edge currents. As the export of the waters takes places within the edge currents some exchange between the two regimes is required. Ten years of Argo-float measurements in the Nordic Seas provide 4000 hydrographic profiles of the upper 2000 m of the water column mainly from the deep basins. These are used to detect changes in temperature and salinity on time scales from seasonal to interannual. Mean wintertime mixed-layer depths differ regionally between 250 m (Iceland Plateau) and 1400 m (Greenland Sea basin), as do the heat loss to the atmosphere which is most extensive in the Lofoten Basin in the eastern part of the area. Heat and freshwater budgets for all basins are constructed from the combination of the information about the temperature and salinity development from float measurements with the information about the atmosphere-ocean exchange from model reanalyses. These budgets reveal a similar transformation loop in all four basins, where net heat loss to the atmosphere is compensated by heat gain through lateral import of Atlantic water masses. The exchange between basins and edge currents effects about 15% of the total transformation in the Arctic Mediterranean from Atlantic into overflow water masses and is dominated by the eastern basins. It accounts for 50% of the whole Nordic Seas transformation although the basins capture only 25% of the total area and for 19% of the total transformation in the Arctic Mediterranean although they capture only 4% of the total area of the Arctic Mediterranean.

Michaela Markovic, Transport variability along the North Sea shelf: Analysis of exchanges processes with the North Atlantic in MPI-OM 20 century run     (download PDF)

[Federal Maritime and Hydrographic Agency]

Abstract

The North Sea as a shallow shelf sea is strongly influenced by atmospheric forcing but inflow of salty water from the Atlantic and low-salinity water from the Baltic provide sources of oceanic variability as well. The poster presents an analysis of climatic change in the North Sea from a simulation with the coupled climate model MPIOM/REMO/ECHAM5 for the A1B scenario. The model has been validated against observations using hindcast simulation.Focus of the analysis is on the variability in the atmospheric forcing and the northern boundary region to the Atlantic and subsequent changes in the North Sea proper.

Charlotte Mielke, Observed and simulated variability of the AMOC at 26N and 41N (download PDF)

[Institute of Oceanography, University of Hamburg]

Abstract

Timeseries of the Atlantic Meridional Overturning Circulation (AMOC) have recently become available, but so far no meridional coherence has been documented in these timeseries. Here, we analyze the variability of the 26◦ N RAPID and the 41◦ N ARGO-based AMOC observations on seasonal timescales, and we compare them to a high-resolution ocean model simulation with MPIOM, forced by NCEP reanalysis data (STORM project). In our analysis of the observed timeseries, we find an inverse seasonal cycle between 26◦ N and 41◦ N. While nthe AMOC at 26◦ N shows a maximum in autumn and a minimum in spring, the AMOC at 41◦ N shows a maximum in spring and a minimum in autumn. The two timeseries with the mean seasonal cycle removed at each latitude show a non-stationary covariability. The meridionally coherent cycle is also apparent in the Ekman and Sverdrup transports. Our results suggest a potentially meridionally coherent seasonal cycle in the AMOC, although the phasing differs in the analyzed model.

Tor Lien Mjell, Reconstructing ocean circulation and climate based on the “Gardar Drift” (download PDF)

[Department of Earth Science and Bjerknes Centre for Climate Research]

Abstract

The Gardar Sediment Drift is a large sediment package in the Iceland Basin that consists of accumulated sediments transported and deposited by Iceland-Scotland Overflow Water (ISOW), a main component of the Atlantic meridional overturning circulation (AMOC). Inspired by the sortable silt record from the Gardar Drift that reveals variability in ISOW vigor on multidecadal-to-centennial timescales during the past 600 years, we suggest a consistent framework for interpreting Gardar variability based on a multi-century simulation with the Bergen Climate Model (BCM). We find in particular that BCM’s “Gardar Drift” is the downstream location most representative of ISOW variability, with about 20% variance explained on interannual timescales. The suggested framework is corroborated by the facts that BCM broadly reproduces the variability of the silt record, and that its Arctic/Atlantic exchanges compare favorably with the instrumental record.

Nikesh Narayan, The variability of thermodynamics and sea level of North Sea under recent climate variations (1950-2000) (download PDF)

[Federal Maritime and Hydrographic Agency]

Abstract

Variability of stratification and sea level of the North Sea was studied using a simulation performed with HAMSOM (HAMburg Shelf Ocean Model, 1951- 2000). The atmospheric forcing was obtained from NCEP and the open boundary values from a larger scale model (GECCO). The model results were validated using both insitu and satellite derived datasets. We compared the simulated temperature and salinity distribution in a vertical section, taken in the Northern North Sea with observational data, the results of which indicated that the model was able to capture the general characteristics of thermodynamics and stratification in the North Sea. The sea level trends showed a steady increase in the whole North Sea, with the strongest increase in the German Bight. The response of the model derived sea level to the North Atlantic Oscillation is nonuniform over the North Sea, as indicated by very high positive correlation along the eastern North Sea (especially German Bight), and weak to negative correlation along the English coast, which is comparable to the results gained from satellite derived SSH for a later period (1992-2008). The surface temperature is also found to be influenced by NAO, with warm waters penetrating east of 6°E, during the positive phase of NAO.

Vadim Paka, Microstructure measurements in the Danish Strait overflow (download PDF)

[Shirshov Instite of Oceanology, Russian Academy of Sciences]

Abstract

Direct measurements of the turbulent microstructure were made during the rv Maria Merian 21- 1b cruise in the Denmark Strait overflow plume by means of the experimental tethered quasi free falling sounde Baklan 3500. Free falling mode began at 350 m above the bottom and frequently lasted to the bottom. The sounde utilized the Rosette CTD/LADCP system both for delivery to the layer of interest and for current velocity data. 17 casts were made mostly in two sites with depths 1450m and 1760m. Entrainment velocity,ωe , was calculated from measured TKE dissipation rate,ε ,overflow plume thickness, H, buoyancy jump, B, and average plume velocity. U. The mean values of the entrainment ratio, we /U and the Froude number Fr=U/(BH)1/2 are E= 5.0к 10-5, Fr= 0.89 (the 1450-m site) and E= 3.7к 10-4, Fr= 0.71 (the 1760- m site). These data are plotted along with other data obtained by different ways [Wells et al. 2010]. Our data show lower entrainment than those related to the DS overflow which were obtained from the along channel water density changes. Our explanation for such difference is that direct measurements of the entrainment probably miss events of extremely strong mixing which lead to strong sudden water changes.

Matthew Palmer, Model simulations of Atlantic vertical heat transport during surface warming "hiatus" decades (download PDF)

[Met Office Hadley Centre]

Abstract

Control simulations from three generations of Met Office Hadley Centre climate model are used to investigate the potential role of vertical ocean heat transport during surface warming “hiatus” decades. These model results suggest that perhaps 10-30% of Earth's top-of-atmosphere radiation imbalance could be offset through vertical heat export across 700 m for a decadal time scale. Spatial patterns associated with this heat export highlight the importance of the North Atlantic and Southern Ocean sectors.

Iuliia Polkova, Impact of initialization procedures on the predictive skill of a coupled ocean-atmosphere model (download PDF)

[Max-Planck Institute for Meteorology]

Abstract

The sensitivity of the forecast skill of decadal climate predictions to the three different initialization approaches: full state initialization (FSI), anomaly initialization (AI) and full state initialization employing flux correction (FC) is investigated. For this purpose, a coupled ocean-atmosphere climate model (MITgcm/UCLA CGCM) is used. For each initialization approach the assimilation run and a series of hindcasts are carried out. The assimilation runs produce atmospheric initial conditions for the 10-year hindcasts. The ocean initial conditions are sampled from the GECCO state (the German contribution to Estimating the Circulation and Climate of the Ocean project). The ensembles of hindcasts are performed at constant intervals every 5 years over the period 1961-2010. The predictive skill of initialized decadal hindcasts is evaluated with respect to the GECCO fields in terms of the anomaly correlation coefficient and the mean squared skill score. The initialized hindcasts are compared against uninitialized hindcasts (externally forced) and a low-skill forecasting method such as persistence forecast. The skill estimation focuses on identifying regions and duration of high predictability of the sea surface temperature and the Atlantic meridional overturning circulation in the context of the three initialization approaches. The correlation of the sea surface temperature is significant in the first lead year for all three initialization schemes over wide areas of the ocean, particularly in the northern and the eastern North Atlantic, the tropical South Pacific, the western North Pacific, the Mid-Indian Basin and the extratropical Southern Hemisphere. Compared to FSI and AI, FC demonstrates very persistent skill, up to a decade, in the eastern North Atlantic and the extratropical Southern Hemisphere. The predictability of the Atlantic meridional overturning circulation at 26.5N using FSI and AI are for time scales of up to 5 years superior to FC. The results for the North Atlantic sea surface temperature and the North Atlantic meridional overturning circulation show an advantage of the three initialized hindcasts over the externally forced hindcasts and persistence forecasts.

Benjamin Rabe, Arctic Ocean freshwater content variability from interannual to decadal timescales (download PDF)

[Alfred Wegener Institute for Polar and Marine Research]

Abstract

The upper Arctic Ocean has experienced significant freshening from the 11 1990s to late 2000s. A very strong Beaufort Gyre and a freshened Transpolar Drift raise the question how much longer the Arctic Ocean can accumulate freshwater. Since 2006, autonomous CTD profilers have allowed to estimate upper ocean properties in the Arctic through all seasons. In combination with observations from other platforms, these data show a continuous increase in liquid freshwater into the second decade of the 21st century. Excellent agreement between these observational estimates and results from the ice-ocean coupled general circulation model NAOSIM allows to view this increase in context of multidecadal freshwater variability. The model shows that a freshwater minimum in the mid-1990s was preceded by a maximum near the end of the 1960s. The level in 2011 is similar to the one around 1980, and it remains to be seen if liquid freshwater levels will supersede that of the end 1960s. The observed increase in liquid freshwater is likely to influence the vertical exchange of heat and freshwater in the Arctic Ocean, and, hence, the modification of the circulating Atlantic Water. Furthermore, the succession of maxima and minima in the NAOSIM freshwater time series raises the question when and where this additional freshwater will be released from the Arctic Ocean to the regions of deep-water formation in the North Atlantic in future years.

Darren Rayner, Stuart Cunningham, The RAPID-MOC/MOCHA Atlantic mooring array at 26°N (download PDF)

[National Oceanography Centre]

Abstract

A pre-operational array of moorings has been deployed across the Atlantic at 26°N since 2004 to monitor the strength and structure of the Atlantic Meridional Overturning Circulation (AMOC). The AMOC is composed of three components that are measured separately: (1) the Gulf Stream transport through the Florida Straits that is measured through a disused submarine telephone cable, (2) the near-surface wind-driven Ekman layer derived from satellite based measurements (more recently reanalysis products) of the wind stress and (3) the mid-ocean flow between the Bahamas and Africa which is measured by the mooring array. The mid-ocean flow can be further separated into: the internal geostrophic flow calculated from end-point density moorings; the meridional transport over the continental shelf between the westernmost density mooring and the Bahamas, derived from direct current meter measurements; and a hypsometric compensation applied to conserve mass balance. The mooring array also includes a number of moorings at the mid-Atlantic ridge to allow separation of the AMOC contribution from the eastern and western basins. This observation system has recently been independently reviewed and recommendations for the future operation of the array after 2015 will be discussed.

Jon Robson, Predictability of the rapid warming of the North Atlantic in the mid 1990s and its climate impacts (download PDF)

[University of Reading/ NCAS Climate, VALOR]

Abstract

In the mid 1990s the North Atlantic subpolar gyre (SPG) warmed rapidly, with sea surface temperatures (SST) increasing by 1C in just a few years. By examining initialized hindcasts made with the UK Met Office Decadal Prediction System (DePreSys), it is shown that the warming could have been predicted. Conversely, hindcasts that only consider changes in radiative forcings are not able to capture the rapid warming. Heat budget analysis shows that the success of the DePreSys hindcasts is due to the initialisation of anomalously strong northward ocean heat transport. Furthermore, it is found that initializing a strong Atlantic circulation, and in particular a strong Atlantic Meridional Overturning Circulation, is key for successful predictions. Finally, we show that DePreSys is able to predict significant changes in SST and other surface climate variables related to the North Atlantic warming.

Christian Rodehacke, How does Greenland respond and interact in near-future projects: Fully coupled ice sheet-earth system simulations (download PDF)

[Max-Planck Institute for Meteorology]

Abstract

As ice sheets belong to the slowest climate components, they are usually not interactively coupled in current climate models. Therefore, long-term climate projections are incomplete and only the consideration of ice sheet interactions allows tackling fundamental questions, such as how do ice sheets modify the reaction of the climate systems under a strong CO2 forcing? The earth system model MPI-ESM, with the atmosphere model ECHAM6 and ocean model MPIOM, is coupled to the modified ice sheet model PISM. The ice sheet model represents the Greenlandic ice sheet at a horizontal resolution of 10 km. The coupling is performed by calculating the surface mass balance based on 6-hourly atmospheric data to determine the boundary condition for the ice sheet model. The response of the ice sheet to this forcing, which includes orographic changes and fresh water fluxes, are passed back to the ESM. In contrast to commonly used strategies, we use a mass conserving scheme and do therefore neither apply flux corrections nor utilize anomaly coupling. Under a strong CO2 forcing a disintegrating Greenlandic ice sheet contributes to a rising sea level and has the potential to alter the formation of deep water masses in the adjacent formation sites Labrador Sea and Nordic Seas. We will present results for selected scenarios and for idealized forcings, such as a growing atmospheric CO2 concentration that rises by 1% per year until four-times the pre-industrial level has been reached. We will discuss the reaction of the ice sheet and immediate responses of the ocean to ice loss.

Linn Schneider, Export of Labrador Sea Water through Flemish Pass (download PDF)

[Institute for Environmental Physics, University of Bremen]

Abstract

In the Labrador Sea of the North Atlantic the Labrador Sea Water (LSW) is formed at times of deep convection events. LSW propagates southward as part of the Deep Western Boundary Current (DWBC) which constitutes the cold return flow of the climate relevant Atlantic meridional overturning circulation (AMOC). Changes in the AMOC strength modulate climate variability; such changes are thought to be linked to variations in LSW formation. At the southern exit of the Labrador Sea the underwater plateau Flemish Cap is a topographic obstacle which splits the DWBC into an upper branch carrying LSW through Flemish Pass (~1200m sill depth) and a branch carrying all DWBC components along the continental slope around Flemish Cap. Up to now, transports of LSW through Flemish Pass and their contribution to the AMOC are still uncertain, the importance of the pass for the export of LSW and its associated variability is yet unknown. In this project we investigate the magnitude of LSW transports and its associated variability through Flemish Pass as well as the processes which drive the variability. For this reason two moorings were deployed in Flemish Pass in summer 2011 and will be recovered and redeployed in summer 2012. Additionally, ship-based measurements of hydrography and current velocity are analyzed. The results will be compared to a model analysis. Here, first results are presented.

Nuno Serra, North Atlantic key processes investigated with a high-resolution data assimilation system (download PDF)

[Institute of Oceanography, University of Hamburg]

Abstract

A dynamically consistent description of the changing North Atlantic circulation over the past decade (2000-2011) and associated surface forcing fields was obtained through model-data synthesis using the adjoint method at the eddy-permitting/resolving resolution of 16 km. The assimilated observational data were temperature and salinity profiles from monthly averages of CTDs, XBTs and ARGO floats, daily values of alongtrack sea surface height anomalies from altimetric missions and monthly averages of microwave-sensor sea surface temperatures. The variables modified (on a daily basis) to bring the model into consistency with the observations were the 2-m air temperature and specific humidity, the precipitation rate and the 10-m wind. The ocean synthesis was analyzed for changes between the optimized and the unoptimized model integrations and the impact of the optimization procedure on different key physical processes in the Atlantic was studied. Results for the mean and variability of 1) the meridional overturning circulation at 27°N and its components, 2) the light and dense volume transports through the Greenland-Iceland-Scotland Ridge and 3) the Labrador Sea Water production and export are discussed.

Raquel Somavilla Cabrillo and Ursula Schauer, Towards a warmer and saltier deep Greenland Sea (download PDF)

[Alfred Wegener Institute for Polar and Marine Research]

Abstract

Deep convection is known to provide the coldest and freshest waters to the deep Greenland Sea, which properties are balanced with the advection of warmer and saltier waters from the deep Arctic Ocean. The halt of deep convection in the last two decades combined with the continuous advection of Arctic Ocean Deep Waters has resulted in a warming and saltening of the deep Greenland Sea. Below 2000 m depth, in the last 18 years, the temperature and salinity have increased at a mean rate of 0.136°C decade-1 and 0.008 decade-1, respectively. This new scenario enables us the estimation of the necessary transports from the deep Arctic to explain the observed changes. A transport of 0.1+/-0.018 Sv, over time periods of complete absence and convection activity, is obtained. On the other hand, warming and saltening of the central Greenland Sea is presently observed in the entire water column. Since 2000, the fresh upper waters of Arctic origin have been substituted by saltier and warmer Atlantic waters. This has ledto the filling of the central Greenland Sea with lighter waters above the Deep Waters derived from the Arctic Ocean (Canadian Basin Deep Waterand Eurasian Basin Deep Water) since the beginning of our time-series in 1994, and hence also caused a continuous deepening of the isopycnals characteristic of these water masses. The Canadian Basin Deep Water remained above the depth of the Jan Mayen Ridge with constant thermohaline properties until 2008. Then, it reached the depth of this ridge coinciding with increases in temperature and salinity, which could also be related with a strong winter mixing episode in 2008. The relation of these changes in the Greenland Sea with those in the North Atlantic and Arctic Ocean states will provide important information about future re-adjustments of this system.

Didier Swingedouw, Decadal fingerprints of fresh water discharge around Greenland in multi-model ensembles (download PDF)

[Institut Pierre Simon Laplace IPSL]

Abstract

In order to investigate the impact of a large Greenland ice sheet (GrIS) melting on the ocean and climate, we use six models: one ocean-only model and five state-of-the art AOGCMs. We utilize an idealized framework where we put 0.1 Sv around Greenland for the historical era 1965-2004. We found similar fingerprints after 4 decades of hosing in the different models and a general weakening of Atlantic Meridional Overturning Circulation (AMOC) and of the gyres. The fresh water spreads along the main currents. A large part of the negative salinity anomalies can be found along the Canary Current after 4 decades, which we call the fresh water leakage. This leakage is actually an escape path for the fresh water anomalies from the North Atlantic. As a consequence, we show that the AMOC weakening is smaller if the leakage is large. We relate this leakage with the gyre asymmetry among the models. When compared with observations, we notice that this asymmetry is in better agreement with models showing a limited leakage and the largest AMOC weakening, indicating a potential larger than previously thought AMOC weakening in response to GrIS melting.

Bente Tiedje, North Atlantic heat transport and AMOC: Latitude dependence of potential predictability (download PDF)

[Institute of Oceanography, University of Hamburg]

Abstract

The relation between the latitude-dependent inter-annual potential predictability of the North Atlantic's meridional heat transport (MHT) and the AMOC is analyzed with hindcast ensembles based on an oceanic state estimate. We analyze the prognostic potential predictability (PPP), using the GECCO synthesis as the initial conditions and the boundary conditions. The PPP of both the MHT and the AMOC varies with latitude, but the PPP structure of the MHT cannot be directly related to the PPP structure of the AMOC. The PPP of the AMOC is only similar to the PPP of the MHT where the overturning component controls the PPP of the MHT (subtropical gyre), while the PPP of the AMOC is not similar to the PPP of the MHT where the gyre component controls the PPP of the MHT (subpolar gyre). For both quantities the buoyancy-driven transport is estimated by the subtraction of the Ekman variability and by using the basin wide zonal density gradients (thermal wind velocity field). The resulting PPP structures of the MHT and the AMOC are similar for both approaches, indicating a common dynamical origin in the variability of the boundary densities. To understand the roles of the eastern and western density boundary variability we conduct experiments by holding the density boundary values temporally constant at one side of the basin, while keeping the full signal at the other side prior to the potential predictability analysis. We find that potential predictability in the MHT and the AMOC result from the western boundary variability in the density field, rather than eastern boundary variability. Ultimately, our findings open the prospectto use density boundary values in combination with wind field data for predicting the AMOC. For the MHT this is somewhat more complicated due to the additional influence of the temperature field and the gyre circulation.

Eirini Varotsou, The transport variability of the Labrador Sea Water through Flemish Pass in relation tochanges at 53°N (download PDF)

[Institute of Oceanography, University of Hamburg]

Abstract

The Labrador Sea is characterized by a cyclonic boundary current surrounding one of the most active areas of water mass transformation. Labrador Sea Water (LSW), formed in the Labrador Sea from the deep convection events, is exported to the subtropical gyre at the western boundary, through Flemish Pass or by the Deep Western Boundary Current (DWBC) around Flemish Cap. In this study, data is used from the Hamburg MIT/gcm Model with a horizontal resolution of 1/120 and variable atmospheric forcing (NCEP/NCAR and ERA interim); a coupled ice and ocean model covering the Atlantic from 33°S to the Arctic Ocean. The data consists of the mean monthly values for the time period January of 1958 to December of 2009. The modeling data will be validated with the observational data of the Flemish Pass area. The focus of this study is to analyze the interannual and seasonal variability of LSW transports through Flemish Pass and in the DWBC during the study period, and to relate the variability to upstream fluctuations at 53°N.

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