Nature Geosci. http://doi.org/bjnp (2016)

The polar regions are undergoing contrasting changes — the Arctic is warming rapidly, whereas the Southern Ocean has warmed less than the global average. However, in the palaeorecord and model simulations the Southern Hemisphere shows polar amplification, and warming on par with the Arctic, emerging after several centuries. This delay in warming has been attributed to heat storage in deep mixed layers of the ocean, but the deepest of these layers are not located where warming is delayed.

Kyle Armour of the University of Washington, Seattle, USA and co-authors analyse ocean observations and model simulations to show that the Southern Ocean delayed warming is a result of the meridional overturning circulation of the region. The circumpolar wind-driven upwelling of old deep waters counters atmospheric warming around the Antarctic continent, and the equator-ward movement of surface water balances heat uptake of those waters with heat transport north. That is, the ocean currents are delaying the warming of the region.

These results indicate that warming will be delayed in the Southern Ocean until the deep waters, originating in the North Atlantic, are warmed, and this should be taken into account when interpreting warming trends in the region.