Climate Change in the Southern Hemisphere Extratropics

南大洋区域对全球气候很重要，因为据信很大一部分人为二氧化碳在该区域进入海洋，而且南极冰盖的增长或消融敏感地取决于周围海洋的气候。南大洋的气候动力学的特点是，由于海冰的反馈，对温室强迫的平衡敏感性很高，由于与深海的强烈耦合，接近平衡的速度很慢，以及与南半球环形模态有关的异常强烈的内部变率。霍尔的研究小组开发了一些技术，用于理解北方陆地地区雪反馈的机制和限制其强度。在这里，他们将应用类似的方法来分析全球气候模型（GCM）中所代表的南大洋的气候反馈。相关的反馈是那些对短波辐射收支起作用的反馈，即云和海冰的对流反馈，那些对长波辐射起作用的反馈，以及那些与地球上其他地方的气候变化有关的反馈。冰-覆冰反馈可以进一步分解为与冰面积和冰厚度的变化相关的影响。后者有一个有趣的特性，即寒冷季节冰厚度的变化会导致融化季节表面温度的变化。他们将计算该地区许多气候模型的能量收支，并将目前的模拟结果与未来更温暖气候的模拟结果进行比较。结果将用于计算各种反馈，并将这些反馈的强度与不同模型的气候敏感性差异联系起来。这些反馈所涉及的机制将通过分析海冰冰量对气候变化的反应以及与海冰变化有关的云量变化引起的行星冰量变化来确定。云的行为将与从卫星云气候学获得的观测结果进行比较。将通过比较观测到的和模拟的季节性循环的冰-冰-冰反馈来确定模型中表面冰-冰反馈的有效性。此外，他们还将分析南大洋区域内部变化的性质和机制，并将利用这些结果来确定该区域最近的气候趋势是否与内部变化一致。平衡敏感性研究结果和内部变率研究结果将综合用于分析南大洋区域的瞬时气候变化。将确定不同模型出现明显的外部强迫变化的时间，并研究这些出现时间模型差异的原因。研究结果将用于提出改进模型的建议，以便对南大洋未来气候作出更可靠的预测，这项工作的更广泛影响包括有可能缩小气候变化模型预测之间的差距，从而提高对海洋吸收二氧化碳和南极冰盖大幅度融化风险估计的信心。霍尔将教授一个关于“南极冰盖的命运”的新生研讨会。“该项目将支持培训一名博士后研究员和一名研究生。

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.