Southern Ocean Centennial Variability Impact on North Atlantic Sea Level

A quasi-oscillatory multi-centennial mode of open ocean deep convection in the Atlantic Sector of the Southern Ocean is found in a control integration of the Kiel Climate Model. The centennial variability affects global climate and consists of a strong built-up of heat at mid-depth (~1000-3000 m) in the Atlantic-Indian Ocean Basin and its subsequent release to the atmosphere during open ocean deep convection in the Weddell Sea. The heat is provided by the lower branch of the Atlantic Meridional Overturning Circulation (AMOC). Both instrumental sea surface temperature (SST) and paleo-proxy records support the existence of such centennial variability. The latter impacts North Atlantic sea level in the subpolar gyre region, along the path of the North Atlantic current and its extension, and along the coast of Western Europe in the model. In our simulation the sea level in the North Atlantic drops several decades after deep convection occurred in the Weddell Sea and vice versa. The variations can amount to about ±30cm/century in the center of the subpolar gyre, which is of the same order of magnitude as the current rise in global average sea level of 3mm/year. Clearly, internal centennial variability in sea level cannot be neglected when assessing regional sea level rise in the North Atlantic during the 21st century. The shutdown of deep convection in the Weddell Sea is associated with a decrease in the Antarctic Bottom Water (AABW) formation rate, enabling a strengthening of the AMOC with a time delay of several decades. This is expressed, for instance, by an enhanced outflow of North Atlantic Deep Water (NADW) at 30°S. The shutdown of the deep convection also appears as a pronounced reduction in sea surface salinity in the Weddell Sea region. This freshening signal slowly propagates northward in the Atlantic and reaches the subpolar North Atlantic deep convection region after about 80 years, thereby weakening NADW formation, and thus connecting deep convection in the Northern Hemisphere to that in the Southern Hemisphere. It takes about 130 years for the AMOC strength and North Atlantic sea level to fully adjust to their previous values that prevailed prior to the deep convection shutdown in the Weddell Sea.