The climate response to a hemispheric asymmetric change in polar ice cover

A remarkable feature of the mid-Pliocene climate is the asymmetry in polar ice cover: Antarctica was well on its way towards full glaciation, whereas the northern high latitudes was still relatively warm and ice-free. The question we ask is: given that we are interested in the fast-acting components of the climate (the atmosphere and surface ocean), what sort of climate is consistent with such a hemispheric asymmetric ice configuration? We investigate this question by modeling the climate response to the removal of northern hemisphere sea ice using the Community Climate model version 3 coupled to a slab ocean model. In addition to showing the expected strong warming in the northern hemisphere, the remarkable aspect of the simulation is a pronounced hemispheric asymmetric change to the tropical climate, characterized by northward shift of the marine ITCZ. Furthermore, humidity increases throughout the northern hemisphere and decreases throughout the southern hemisphere, indicative a changed Hadley cell regime. The observed low-latitude and southern hemisphere changes are not reproduced in a control study using fixed-SSTs, indicating that an interactive surface ocean is crucial to obtaining the tropical climate behavior. . Our model results suggest that an altered tropical climate may be a strong feature during the unipolar glaciated conditions in the late Cenozoic, in sympathy with a previously proposed hypothesis for hemispheric asymmetric climate change (Flohn 1980). To test this idea, we are analyzing various published and synthesized paleoclimate proxy data during this period. Our study may also offer some insight into global warming conditions with a much reduced if not absent Arctic sea ice cover.