What's happening over the poles?

In recent decades, the Arctic has been warming and sea ice disappearing. By contrast, the Southern Ocean around Antarctica has been (mainly) cooling and sea-ice extent growing. We argue here that inter-hemispheric asymmetries in the mean ocean circulation, with sinking in the northern North Atlantic and upwelling around Antarctica, strongly influence the sea-surface temperature (SST) response to anthropogenic greenhouse gas (GHG) forcing, accelerating warming in the Arctic while delaying it in the Antarctic. Furthermore, while the amplitude of GHG forcing has been similar at the poles, significant ozone depletion only occurs over Antarctica contributing to enhanced surface westerly wind trends around Antarctica. Through analysis of a hierarchy of models and observations, we suggest that the initial response of SST around Antarctica to westerly wind trends is one of cooling and sea-ice expansion. However, this could be followed on decadal timescales by upwelling of relatively warm water from depth, under the seasonal sea-ice zone, leading to sea-ice loss.