Abrupt ice-age shifts in southern westerly winds and Antarctic climate forced from the north.

Abstract

The mid-latitude westerly winds of the Southern Hemisphere play a central role in the global climate system via Southern Ocean upwelling¹, carbon exchange with the deep ocean², Agulhas leakage (transport of Indian Ocean waters into the Atlantic)³ and possibly Antarctic ice-sheet stability⁴. Meridional shifts of the Southern Hemisphere westerly winds have been hypothesized to occur^5,6 in parallel with the well-documented shifts of the intertropical convergence zone⁷ in response to Dansgaard-Oeschger (DO) events- abrupt North Atlantic climate change events of the last ice age. Shifting moisture pathways to West Antarctica⁸ are consistent with this view but may represent a Pacific teleconnection pattern forced from the tropics⁹. The full response of the Southern Hemisphere atmospheric circulation to the DO cycle and its impact on Antarctic temperature remain unclear¹⁰. Here we use five ice cores synchronized via volcanic markers to show that the Antarctic temperature response to the DO cycle can be understood as the superposition of two modes: a spatially homogeneous oceanic 'bipolar seesaw' mode that lags behind Northern Hemisphere climate by about 200 years, and a spatially heterogeneous atmospheric mode that is synchronous with abrupt events in the Northern Hemisphere. Temperature anomalies of the atmospheric mode are similar to those associated with present-day Southern Annular Mode variability, rather than the Pacific-South American pattern. Moreover, deuterium-excess records suggest a zonally coherent migration of the Southern Hemisphere westerly winds over all ocean basins in phase with Northern Hemisphere climate. Our work provides a simple conceptual framework for understanding circum-Antarctic temperature variations forced by abrupt Northern Hemisphere climate change. We provide observational evidence of abrupt shifts in the Southern Hemisphere westerly winds, which have previously documented^1-3 ramifications for global ocean circulation and atmospheric carbon dioxide. These coupled changes highlight the necessity of a global, rather than a purely North Atlantic, perspective on the DO cycle.