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  • Author or Editor: Cornelia Köberle x
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Cornelia Köberle
and
Rüdiger Gerdes

Abstract

The Arctic Ocean freshwater balance over the period 1948–2001 is examined using results from a hindcast simulation with an ocean–sea ice model of the Atlantic and Arctic Oceans. Atmospheric forcing is taken from the NCEP–NCAR reanalysis and different terrestrial freshwater sources as well as the Bering Strait throughflow are specified as constant seasonal cycles. The long-term variability of the Arctic Ocean liquid freshwater content is determined by the variability of lateral exchanges with the subpolar seas. Surface freshwater flux variability is dominated by the thermodynamic growth of sea ice. This component of the freshwater balance has larger variability at interannual frequencies. The Arctic Ocean liquid freshwater content was at a maximum in the middle of the 1960s. Extremely low liquid freshwater export through Fram Strait caused this maximum in the freshwater content. The low export rate was related to weak volume transports in the East Greenland Current. Low volume transports were forced by a reduction in sea surface height across Fram Strait, triggered by anomalous meltwater from Barents Sea ice export that was carried toward Fram Strait with the West Spitzbergen Current. After the 1960s maximum liquid freshwater content, the Arctic Ocean gradually returned to an equilibrium between export through the passages toward the Atlantic and the freshwater sources.

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Cornelia Köberle
and
Rüdiger Gerdes

Abstract

In an ocean–sea ice model of the Arctic and the northern North Atlantic driven with 50-yr NCEP–NCAR reanalysis data, no appreciable trend in sea ice volume is found for the period 1948–98. However, rather long subperiods, for example, 1965–95, exhibit a large decline in Arctic sea ice volume. These results and the current data situation make connecting “global warming” to Arctic ice thinning very difficult because the large decadal and multidecadal variability masks any trend. Thermal and wind effects linearly contribute to the total sea ice volume variability. Wind stress forcing significantly contributes to the decadal variability in the Arctic ice volume, affecting both thermodynamic growth and the ice export rate. Ice export events are triggered by enhanced cyclonic wind stress over the eastern Arctic. However, large ice export events depend to a similar degree on the presence of thick ice that is generated in a preceding accumulation phase and do not depend on the local wind conditions around Fram Strait.

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Rüdiger Gerdes
and
Cornelia Köberle

Abstract

Two numerical experiments regarding the North Atlantic circulation are compared. The two experiments are initialized with climatological temperatures and salinities and are integrated for ten years with different surface boundary conditions in the lceland Sea. One case climatological wind stress, SST, and surface salinity to force the model. It reproduces the Atlantic circulation found in similar studies. A change in surface boundary values In the Iceland Sea in order to improve the Denmark Strait Overflow water properties in the second case results in major changes in circulation and hydrography. The meridional overturning more than doubles and horizontal gyres are intensified. Recirculation cells occur north and south of the Gulf Stream. The Gulf Stream separates from the North American coast in a more realistic manner, and its further course is in better agreement with observations than in the reference run. The subsurface temperature distribution differs by several kelvin in the subpolar and subtropical gyres between the experiments. The authors examine mechanisms and timescales for the changes in the density field that result from the different buoyancy forcing and investigate how the large-scale circulation reacts. The adjustment takes place in two stages and is completed within a period of ten years. This result underlines the importance of variations in high-latitude buoyancy forcing for decadal variability in the Atlantic.

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Aike Beckmann
,
Claus W. Böning
,
Cornelia Köberle
, and
Jürgen Willebrand

Abstract

Global mean and eddy fields from a four-year experiment with a 1/6° × 1/5° horizontal resolution implementation of the CME North Atlantic model are presented. The time-averaged wind-driven and thermohaline circulation in the model is compared to the results of a 1/3° × 2/5° model run in very similar configuration. In general, the higher resolution results are found to confirm that the resolution of previous CME experiments is sufficient to describe many features of the large-scale circulation and water mass distribution quite well. While the increased resolution does not lead to large changes in the mean flow patterns, the variability in the model is enhanced significantly. On the other hand, however, not all aspects of the circulation have improved with resolution. The Azores Current Frontal Zone with its variability in the eastern basin is still represented very poorly. Particular attention is also directed toward the unrealistic stationary anticyclones north of Cape Hatteras and in the Gulf of Mexico.

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