Ice-atmosphere feedbacks dominate the response of the climate system to drake passage closure

• Published in: Journal of Climate
• Main Authors: England, MH, Hutchinson, DK, Santoso, A, Sijp, WP
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Meteorological Society 2017
• Subjects: 13 Climate Action; 14 Life Below Water; anzsrc-for: 0401 Atmospheric Sciences; anzsrc-for: 0405 Oceanography; anzsrc-for: 0909 Geomatic Engineering; Antarctic; Southern Ocean; The Antarctic; Drake Passage; Antarc*; NADW; North Atlantic Deep Water; North Atlantic; Sea ice
• Online Access: http://hdl.handle.net/1959.4/unsworks_45705; https://doi.org/10.1175/JCLI-D-15-0554.1

The 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 weakens 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 midlatitude jet, not a vigorous southern sinking, which 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 ...