Special Seminar: The ocean's role in polar climate change
Abstract: The retreat of Arctic sea ice has been one of the most striking features of climate change in recent decades, with the summer sea ice cover decreasing by about 40% since the 1980s. Accompanying this sea ice loss has been amplified warming in the Arctic, at a rate several time faster than the global mean. In stark contrast, the Antarctic sea ice cover has been steadily increasing over this period, consistent with an overall cooling of the Southern Ocean sea surface from the sea ice edge out to the Antarctic Circumpolar Current (the ACC, where substantial warming has occurred). Here I argue that interhemispheric asymmetries in the large-scale ocean circulation play a fundamental role in driving such distinct polar climate trends under greenhouse gas forcing: wind-driven upwelling around Antarctica acts to delay warming at the sea surface, while Arctic warming is accelerated by enhanced poleward ocean heat transport. I further discuss: (i) the extent to which these changes are a consequence of the background ocean circulation versus changes in circulation driven by the active nature of ocean heat uptake; (ii) a possible mechanism for the observed warming within the ACC region; and (iii) the role of internal climate variability in the observed Southern Ocean cooling and sea ice expansion.