Institute of Oceanography

University of Hamburg

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Institute of Oceanography
University of Hamburg
Bundesstraße 53
D-20146 Hamburg
Tel.: +49 40 42838-2605 / -5449
Fax: +49 40 42838-7488
E-Mail:  waltraut.domke-sommer(at)zmaw.de

Sea Ice Volume Flux

Gunnar Spreen, Stefan Kern, and Detlef Stammer

Introduction

The sea ice export through Fram Strait into the Greenland Sea is one key component of the mass balance of the Arctic Ocean. The annual export amounts about 15% of the total sea ice mass of the Arctic Ocean and is extremely variable. It forms the largest single component of the freshwater balance of the Greenland Sea, and has a large impact on oceanic deep convection in the Greenland Sea and further downstream. Up to date this export is observed using a combination of the ice area flux (satellites), and point-wise ice draft (ULS) and in-situ ice thickness (drilling) measurements. Here we demonstrate how satellite observations of the ice freeboard height can be used to derive the ice volume flux through the Fram Strait by combining the ice area flux with the ice thickness distribution across the Fram Strait.

Data & Technique

State-of-the-art estimates of the ice concentration and the ice drift are used to derive the ice area flux. Ice concentrations are obtained from AMSR-E 89 GHz data using the ARTIST Sea Ice (ASI) algorithm. For the figures shown here the ice drift is obtained from maximum cross-correlation analysis of merged vertically and horizontally polarized AMSR-E 89 GHz data using a 2-day time lag. Additionally we use QuikSCAT ice drift estimates for comparison. Elevation measurements of the GLAS sensor on board ICESat are used to estimate the ice freeboard height in several steps. These involve filtering, exclusion of open water (zero ASI ice con-centration), derivation of the residual elevation (high-pass), estimation of the sea surface height (SSH) from the lowest 2% of the residual elevations, and calculation of the freeboard height with a linear model for the SSH.

 

Data Flow

Sea Ice Concentration



Sea Ice Drift

Fig. 1: Schematic flow diagram of quantities involved in the estimation of the sea ice volume flux. In the lower row the satellite sensors used are given. Fig. 2: Sea ice concentration on 26 February 2003 in the Fram Strait region obtained from AMSR-E 89 GHz data (ASI algorithm). Grid spacing is 6.25 km. The corresponding sea ice drift is shown
in Figure 3.
Fig. 3: Ice drift in the Fram Strait region obtained from merged AMSR-E 89 GHz data (both polarizations), February 26-28, 2003; grid spacing is 25 km.

ICESat’s Sea Ice Measurement Principle


Sea Ice Thickness

Sea Ice Volume Flux

Fig. 4: Schematic diagram showing the interrelation of ice freeboard, F, snow depth, S, and sea ice thickness, I (left), and an artist’s view of ICESat’s measurement principle including the involved surfaces (right & equation at the top). Fig. 5: Ice thickness distribution in the Fram Strait region for two  ICESat measurement periods in Feb./Mar. 2005 (left) and Feb./Mar. 2008 (right). Grey areas denote missing ICESat data or land areas. The ice thickness is calculated from the freeboard height F assuming isostatic balance. Two ice density classes for multiyear and first-year ice fraction obtained from QuikSCAT data and snow depth from combined in-situ and climatological data is used. Fig. 6: (a, b) Sea ice volume flux maps for ICESat periods Oct./Nov. 2006 and Mar./Apr. 2007. Color-coded and
vectors (every third 25km grid cell): absolute volume flux; gray: missing data (around land, south of 74° N, north of
86° N) or open ocean without ice; white lines: transects for c); white crosses: locations of ULS’s. (c) Time series of the
total sea ice volume export through Fram Strait across 80° N (black line with squares) and 76° N (blue triangles) between
February 2003 and March 2008 for eleven ICESat periods. Error bars denote error estimates. Red bars show the difference
between the two transects..

Monthly Fram Strait Sea Ice Volume Export

Fig. 7: a) Seasonal cycle of monthly Fram Strait sea ice volume flux from this study (years 2003–2008, black), from Vinje et al. [1998] (VNK98, years 1990–1996, red), and from Kwok et al. [2004] (KCP04, years 1991–1999, green). Error bars denote ± one standard deviation. b) Time series 2003 to 2008 of monthly (black squares) and 30-days running mean (green line) winter sea ice volume export through Fram Strait at 80° N. c) Extended Fram Strait sea ice export time series of combined VNK98, KCP04 data and this study. Green and orange lines show their average monthly Oct.–Apr. ice volume export, respectively. Grey bars denote summer months (May–Sep.).

 



Results

By combining sea ice concentration, drift and thickness derived from satellite data the spatial distribution of the sea ice volume flux in the Fram Strait region was obtained for eleven about one month long ICESat measurement periods. From these we estimated on a monthly basis winter Fram Strait sea ice volume export between January 2003 and April 2008 of 217 km³/month, varying between 92 and 420 km³/month.
Our results suggest that the Oct.–Apr. sea ice volume export through Fram Strait during 2003 to 2008 has not changed significantly compared to the 1990s.

Publications:

G. Spreen, S. Kern, D. Stammer and E. Hansen (2009), Fram Strait Sea Ice Volume Export Estimated Between 2003 and 2008 From Satellite Data, Geophys. Res. Lett., accepted.

G. Spreen (2008), Satellite-based estimates of sea ice volume flux: Applications to the Fram Strait region, Ph.D. thesis, University of Hamburg, Institute of Oceanography, www.sub.uni-hamburg.de/opus/volltexte/2008/3776/.

G. Spreen, S. Kern, D. Stammer, R. Forsberg, and J. Haarpaintner (2006), Satellite-based estimates of sea ice volume flux through Fram Strait, Ann. Glaciol., 44, 321–328.


The authors acknowledge data provision by the National Snow & Ice Data Center, Boulder, CO, USA, by IFREMER/CERSAT, Brest, France, Jörg Haarpaintner (Norut IT, Norway), and by the Danish National Space Center, Copenhagen, Denmark. This work was supported by the German Science Foundation (DFG) under project SFB512-TP-E1.