Is the bipolar ocean seesaw broken?
Joel Pedro (Centre for Ice and Climate, Niels Bohr Institute, University of Copenahgen)
The bipolar ocean seesaw hypothesis is the prevailing explanation for the interhemispheric coupling of abrupt climate change. Stocker and Johnsen  provide the thermodynamic basis for the hypothesis with their suggestion that the northern (Dansgaard-Oeschger) and southern (Antarctic Isotope Maxima) climate anomalies are most simply explained by a bi-stability in northward heat transport in the Atlantic, that is modulated at southern high latitudes by a large heat reservoir. Here we use palaeoclimate data and GCM experiments to critically evaluate the seesaw hypothesis. We identify four key challenges: (1) Changes in Atlantic heat transport are largely compensated by opposing anomalies in heat transport in the global atmosphere and Pacific Ocean. (2) The speed and amplitude at which temperature anomalies develop in the South Atlantic is too great to be explained by the advective heat transport processes invoked by the seesaw. (3) Ocean signal propagation to the Southern Ocean is restricted by the steeply outcropping isopycnals of the Antarctic Circumpolar Current and by the lack of boundaries to support wave propagation. (4) Changes in atmospheric heat transport are needed to explain Antarctic warming. We conclude with a revised model of inter-hemispheric coupling that takes account of ocean and atmospheric heat transports and the energy balance of the Southern Ocean climate system.