Theoretical Oceanography
Research objectives:
Understanding the large-scale ocean circulation
Describing and understanding the large-scale ocean circulation is the primary topic of physical oceanography. Prominent features of the ocean circulation are the western boundary currents (link western boundary currents) in the gyre circulation of the closed ocean basins - like the Gulf Stream in the North Atlantic Ocean - and the Antarctic Circumpolar Current in the zonally unbounded Southern Ocean.
- A schematic view of the near-surface ocean circulation (after Schmitz 1996). Subtropical gyres are represented with red, subpolar and polar gyre with blue, the equatorial gyres with magenta lines. The Antarctic Circumpolar Current is also blue. The green lines represent exchange between basins and gyres.
The role of the large-scale ocean circulation in the climate system
The large-scale ocean circulation is an important part of the climate system. On the global scale, the ocean transports heat polewards, in a comparable amount as in the atmosphere. It also takes up currently about 50% of the anthropogenically released carbon dioxide, which might, however, change in the future. An efficient mean to transport heat and trace gases as carbon dioxide is the meridional overturning circulation of the ocean.
- Two popular schematics of the overturning circulation: a) was compiled by Schmitz (1996) based on many observational studies and shows a complicated web of connected circulation cells in which the major water masses of the ocean are interleaved, all converging within the Antarctic Circumpolar Current (ACC) in the Southern Ocean which connects all oceans. b) shows one only large global overturning circulation cell and thus simplifies the spreading of the major water masses, but highlights the global aspect of the circulation.
Understanding and parameterizing meso-scale and small-scale turbulence in the ocean
Meso-scale variability has time and space scales much smaller than the basin-wide large-scale ocean circulation, but can act as a strong control on the ocean. Prominant example of such a control is the meridional overturning circulation in the Southern Ocean. Small-scale variability, on the other hand, ranges on even smaller time and space scales but also acts as an important driver of the large-scale ocean circulation.
- Space-time diagram showing the relevant dynamical regimes in the ocean as red ellipses, linear wave solutions (dispersion relations) as solid black lines and numerical ocean models as grey rectangular areas. Shown are (from large to small frequencies) surface and internal gravity waves, barotropic and baroclinic Rossby waves for different vertical wavenumbers (or vertical mode number). Also shown are important frequencies as horizontal blue dashed lines - the Coriolis parameter f and the stability frequency N � and space scales as vertical blue dashed lines � the barotropic and first internal Rossby radius, Ro and Ri. Also shown is the expected increase in ocean model resolution in the next couple of years as dashed black lines. The grey area to the right of the thick blue line is not included in hydrostatic ocean general circulation models, and it becomes clear that it will also not be possible to include it in the next future. Therefore, the effect of the small-scale turbulence and internal wave field on the large-scale ocean circulation needs still to be parameterized in ocean models. In coarse resolution ocean climate models, the effect of meso-scale turbulence also needs to be parameterized.
The role of biogeochemical cycles in the ocean
The large-scale ocean circulation also acts as an important driver of biogeochemical cycles in the ocean, by supplying nutrients to the sunlit surface ocean, where the subsequent primary production leads to a significant part of the biomass on earth. The ocean circulation also distributes the continuous trickling of dead and excreted organic material from the surface ocean in the abyssal ocean and eventually resupplies it to the surface ocean after bacterial remineralization. The understanding and quantification of the role of the ocean circulation with respect to its mean and its long-term variability for the biogeochemical cycles in the ocean is a further topic of the research group.
- Simulated deep chlorophyll maximum (DCM) at the ESTOC station (29°N, 15.5°W) in the North Atlantic. Such a summer chlorophyll maximum below the surface is due to nutrient depletion in the upper water column, sufficient light intensities in greater depths promoted by clear water and increasing nutrient concentrations beyond the DCM. Such areas of primary production are not recognized by satellites; they can only be detected by continuous measurements or by model simulations.
The role of shelf seas in the climate system
Climate changes at the shelfs of the oceans are directly effecting society. The regionalization of the large-scale climate change scenarios as projected by ocean/atmosphere climate models in selected key areas the global shelf is therefore an important topic of physical oceanography.
- a) and b) shows shows a projection of the surface currents and the sea surface elevation for March 2080 in the North Sea based on the IPCC scenario A1B, using a regional shelf model (HAMSOM) forced by data from the regional atmospheric model (REMO). Both the regional atmosphere and the ocean shelf model are forced at the lateral open boundaries by the climate projection of a coupled climate model. c) shows the expected division of the Indonesian throughflow into its western and eastern branches in a regional shelf model (HAMSOM), for which a ocean climate model (MPI-OM) provides the lateral boundary values. The figure shows the horizontal distribution of volume transport within the upper 700 m for Oct 1st, 2004, in Sv (10^6 m^3/s). The shading indicates magnitudes, the arrows the direction. (MC: Mindanao Current, NECC: North Equatorial Counter Current, NGCC: New Guinea Coastal Current).
Ecosystem Modelling
For biogeochemical modelling the shelf areas can be seen as a burning glass or a giant mesocosm experiment in double meaning: On the one hand, the effects of anthropogenic activities and the reaction of the marine ecosystem are more pronounced in shelf seas than in the open sea. On the other hand, because of the shallowness, climate-change driven alterations like an increase in temperature and a decrease in pH are often more pronounced than in the open ocean. Beside severe biological impacts of such changes a further important aspect is the decline of CO2 uptake capacity of shelf-sea water due to its warming and acidification.
- a) Simulated deviations from monthly mean sea surface temperature in the North Sea (°C) from 1970 to 2006. We see a general warming trend, especially in the last decade and several short-term anomalies like the cold first half of 1996 and the hot second half of 2006. b) Annual mean air-flux of CO2. In the last decade the values are decreasing and very low uptake of atmospheric CO2 could be found in 1997 and 2006
