Internal Southern Ocean Centennial Variability: Dynamics, Impacts and Implications for Global Warming

Abstract

It is well established that centennial climate variability can be externally forced by, e.g., quasi-oscillatory fluctuations of the solar constant or slowly varying atmospheric aerosol concentrations in association with changes of volcanic activity. Climate models recently suggested that substantial centennial variability can be also produced internally, and different competing mechanisms have been proposed. This paper deals with the internal centennial variability originating in the Southern Ocean Sector simulated by the Kiel Climate Model (KCM). In that model, the Southern Ocean centennial variability (SOCV) is linked to Weddell Sea deep convection activity and drives regional as well as global climate variations, as witnessed, for example, by coherent changes in Antarctic sea ice extent and globally averaged surface air temperature (SAT). Furthermore, the SOCV is associated with changes in deep Southern Ocean temperature in the KCM. Interestingly, a warming of the abyssal Southern Ocean has been observed during the recent decades, suggesting a contribution from SOCV. Another important impact of the SOCV in the model concerns the Atlantic Meridional Overturning Circulation (AMOC). The AMOC strengthens and deepens after the cessation of Weddell Sea deep convection and Antarctic Bottom Water (AABW) formation with a time delay of several decades to a century. Internal North Atlantic sea level variations can be as large as ± 15cm/century in the model with a strong contribution from the SOCV. Such regional sea level variations are of the same order of magnitude as the observed globally averaged 20th century sea level rise amounting to about 15-20cm. Finally, the KCM simulation suggests that the SOCV may have contributed to the current hiatus in global warming through an enhanced deep ocean heat uptake.