Ocean circulation
Atlantic Meridional Overturning Circulation (AMOC)
Our eddying ocean simulations contribute to understanding the complex structure and interaction of the AMOC in the climate system.
The northward heat transport associated with the AMOC is crucial for the climate of the North Atlantic region and hence its potential weakening under global warming is part of an intense debate. Research of the Ocean Dynamics unit contributes to a deeper understanding of the AMOC by analysing its variability on seasonal to decadal timescales and its potential to mask long-term trends. This includes impacts by surface heat fluxes and freshwater variations. Our high-resolution simulations are used for studies in combination with observations and Lagrangian techniques and contribute to multi-national model intercomparisons.
Subpolar North Atlantic
Sustained heat loss to the atmosphere over the subpolar North Atlantic, in particular in winters, drive the densification of the warm, saline subtropical waters transported northward with the Gulf Stream and North Atlantic Current. This densification leads to the actual overturning, sinking of the water masses to mid-depth. Regional freshening by local air-sea fluxes, Arctic Ocean export and Greenland ice sheet melting can weaken this overturning. Our recent work has focused on the water mass transformation in the Irminger and Labrador Seas, the role of air-sea fluxes in forcing AMOC trends and potential impacts by enhanced Greenland melting.
Agulhas region
[Arne Biastoch, René Schubert, Tobias Schulzki, Leon-Cornelius Mock]
Warm and saline inflow from the Indian Ocean around South Africa introduces an important density anomaly to the Atlantic with the potential to stabilize the AMOC. This so-called Agulhas leakage finds its way to the north on decadal to multi-decadal timescales, thereby strengthening the AMOC. This is of particular interest since Agulhas leakage is projected to increase significantly under global warming in an eddy-rich coupled model with atmospheric chemistry that includes both CO2 and ozone, due to changes in the Southern Hemisphere westerlies. These processes emphasize the importance of the South Atlantic that mediates between the Indian and Southern Oceans and the North Atlantic.
Baltic Sea
The Baltic Sea is particular sensitive to climate change and global warming due to its small volume and limited exchange with the world ocean. One of the Baltic’s most remarkable features is its surface salinity gradient that is horizontally decreasing from the saline North Sea to the near fresh Bothnian Sea in the north, and Gulf of Finland in the east. Additionally, a vertical gradient and strong stratification separate between less saline surface water and deep saline water. These salinity features are mainly driven by a combination of river runoff, net precipitation, wind conditions, limited mixing, and geographic features that lead to restricted and irregular inflow of saltwater into the Baltic Sea. Recent changes in salinity are less clear due to a high variability, but overall surface salinity seems to decrease with a simultaneous increase in the deeper water layers. In a recent study, SMOS (Soil Moisture Ocean Salinity) satellite data have been assimilated with a hydrodynamic regional model of the Baltic Sea to improve the understanding of salinity dynamics and circulation of the Baltic Sea.
