Ice-atmosphere feedbacks dominate the response of the climate system to Drake Passage closure

Abstract

he response of the global climate system to Drake Passage (DP) closure is examined using a fully coupled ocean-atmosphere-ice model. Unlike most previous studies, a full three-dimensional atmospheric general circulation model is included with a complete hydrological cycle and a freely evolving wind field, as well as a coupled dynamic-thermodynamic sea-ice module. Upon DP closure the initial response is found to be consistent with previous ocean-only and intermediate complexity climate model studies, with an expansion and invigoration of the Antarctic meridional overturning, along with a slowdown in North Atlantic Deep Water (NADW) production. This results in a dominance of Southern Ocean poleward geostrophic flow and Antarctic sinking when DP is closed. However, within just a decade of DP closure, the increased southward heat transport has melted back a substantial fraction of Antarctic sea-ice. At the same time the polar oceans warm by 4-6 °C on the zonal mean, and the maximum strength of the Southern Hemisphere westerlies weaken by ?10%. These effects, not captured in models without ice and atmosphere feedbacks, combine to force Antarctic Bottom Water (AABW) to warm and freshen, to the point that this water mass becomes less dense than NADW. This leads to a marked contraction of the Antarctic overturning, allowing NADW to ventilate the abyssal ocean once more. Poleward heat transport settles back to very similar values as seen in the unperturbed DP open case. Yet remarkably, the equilibrium climate in the closed DP configuration retains a strong Southern Hemisphere warming, similar to past studies with no dynamic atmosphere. However, here it is ocean-atmosphere-ice feedbacks, primarily the ice-albedo feedback and partly the weakened mid-latitude jet, not a vigorous southern sinking, that maintain the warm polar oceans. This demonstrates that DP closure can drive a hemisphere-scale warming with polar amplification, without the presence of any vigorous Southern Hemisphere overturning circulation. Indeed DP closure leads to warming that is sufficient over the West Antarctic Ice Sheet region to inhibit ice sheet growth. This highlights the importance of the DP gap, Antarctic sea-ice and the associated ice-albedo feedback in maintaining the present-day glacial state over Antarctica.