ALBERT

Description: The southern hemisphere westerly winds are projected to increase in the future, altering local ocean hydrography and dynamics. While anomalies of southern hemisphere origin were shown to affect the Atlantic Meridional Overturning Circulation, many details of this connection remain unexplained. Most existing studies are limited by the application of forced ocean models that prevent an atmospheric response to the anomalies, coarse resolution ocean models not able to explicitly simulate mesoscale variability, or a short runtime of experiments. Here we apply a coupled, nested model configuration covering the entire Atlantic at an eddying resolution of 1/10°, to study the inter-hemispheric propagation of anomalies generated by a 30% increase in southern hemisphere wind stress. To obtain statistically more robust results, a 5-member ensemble experiment is conducted with a runtime of 120 years of each member. The South Atlantic response is dominated by a sustained increase of Agulhas Leakage by 3 Sv. Based on a combination of Lagrangian and Eulerian analysis, we show that the propagation of anomalies generated by an increased inflow of Indian Ocean water into the South Atlantic, is a complicated interplay of advection, mixing and local atmospheric responses and thus not simply follows the advective pathways of the leakage water. The North Atlantic overturning responds with a strengthening of 1.5 Sv after approximately 80 years. Anomalous water mass transformation associated with the increased AMOC mainly occurs at the entry into the Nordic Seas, while enhanced sinking is seen along the boundary of the subpolar gyre.

Description: Agulhas leakage, the transport of warm and salty waters from the Indian Ocean into the South Atlantic, represents a choke point for the surface branch of the global overturning circulation. Previous studies suggest that Agulhas leakage has been increasing under anthropogenic climate change as a response to strengthening Southern Hemisphere westerly winds, and that the resulting enhanced salt transport into the South Atlantic may counteract the projected weakening of the Atlantic meridional overturning circulation (AMOC) through warming and ice melting. However, due to its turbulent and intermittent nature, estimates for the past and future evolution of Agulhas leakage are sparse and individual estimates are associated with considerable uncertainties. Here we present an analysis of already established as well as new observation- and model-based estimates for Agulhas leakage variability to robustly quantify it’s (sub-)decadal evolution since the 1960s. We find that Agulhas leakage very likely increased in the 1960s through the 1980s, in agreement with strengthening Southern Hemisphere winds, while it appears unlikely that Agulhas leakage substantially increased since the 1990s, despite continuously strengthening winds. Our models further suggest that the increase in leakage coincided with a strengthening of the AMOC in the South Atlantic, which propagated into the North Atlantic within one to two decades. Hence, the South Atlantic may not only be important for future AMOC changes but may already have modulated basin-wide AMOC variability over the past decades. This underlines the importance of sustained efforts to monitor the AMOC in the South Atlantic, e.g., across the SAMBA array.