Climate Change in the Southern Hemisphere Extratropics

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The region of the Southern Ocean is important for the global climate, because a large fraction of anthropogenic carbon dioxide is believed to enter the ocean in this region and because the growth or ablation of Antarctic ice sheets depends sensitively on the climate of the surrounding ocean. The climate dynamics of the Southern Ocean are distinguished by a high equilibrium sensitivity to greenhouse forcing, because of sea-ice feedbacks, a slow approach to equilibrium, because of the strong coupling to the deep ocean, and exceptionally strong internal variability associated with the Southern annular mode.Under prior support, Hall's group developed techniques for understanding the mechanisms for and constraining the strength of snow-albedo feedback in Northern Hemisphere land areas. Here they will apply similar methods to analyzing the climate feedbacks in the Southern Ocean as represented in global climate models (GCMs). The relevant feedbacks are those acting on the shortwave radiation budget, namely cloud and sea-ice albedo feedbacks, those acting on the long-wave radiation, and those associated with climate changes elsewhere on the planet. The ice-albedo feedback can be further decomposed into effects associated with changes in ice area and in ice thickness. The latter has the interesting property that a change in ice thickness during the cold season produces a change in surface temperature during the melting season.They will calculate the energy budgets over this region for a number of climate models, comparing present-day simulations with those of a future warmer climate. The results will be used to compute the various feedbacks and to relate the strengths of these feedbacks to differences in the climate sensitivity of the different models. The mechanisms involved in these feedbacks will be determined by analyzing the responses of sea-ice albedo to climate change as well as the changes in planetary albedo, due to changes in cloud cover, associated with changes in sea ice. Cloud behavior will be compared with observations obtained from satellite cloud climatologies. The validity of surface albedo feedbacks in the models will be determined by comparing observed and simulated ice-albedo feedbacks for the seasonal cycle. Additionally they will analyze the nature and mechanisms for internal variability in the Southern Ocean region and will use these results to attempt to determine whether recent climate trends in this region are consistent with internal variability. The results of the studies of equilibrium sensitivity and those of internal variability will be synthesized in an analysis of the transient climate change in the Southern Ocean region. The times at which different models exhibit the emergence of a clearly externally forced change will be determined, and the reasons for model differences in these emergence times will be studied. The results will be used to suggest the model improvements that are needed to produce more reliable projections of future climate in the Southern Ocean.Broader impacts of this work include its potential to narrow the spread among model projections of climate change and, therefore, to increase confidence in estimates of the ocean uptake of carbon dioxide and the risk of significant melting of the Antarctic ice sheets. Hall will teach a freshman seminar on the "Fate of the Antarctic Ice Sheet." The project will support the training of a postdoctoral fellow and a graduate student.