Climate Change in the Southern Hemisphere Extratropics

南大洋区域对全球气候很重要，因为据信有很大一部分人为二氧化碳进入该区域的海洋，而且南极冰盖的生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，南大洋对温室强迫的平衡敏感性很高，由于与深海的强烈耦合，达到平衡的速度很慢，以及与南环状模式相关的异常强烈的内部变率。在先前的支持下，Hall的团队开发了理解北半球陆地地区雪反射率反馈的机制和限制其强度的技术。在这里，他们将使用类似的方法来分析全球气候模型(GCM)所代表的南大洋气候反馈。相关的反馈是那些作用于短波辐射预算的反馈，即云和海冰反照率的反馈，那些作用于长波辐射的反馈，以及与地球其他地方的气候变化有关的反馈。冰-反照率反馈可以进一步分解为与冰面积和冰厚度变化相关的影响。后者具有一个有趣的特性，即寒冷季节冰层厚度的变化会导致融化季节表面温度的变化。他们将计算该地区的能量收支，用于一些气候模型，将目前的模拟与未来变暖的气候模拟进行比较。结果将被用来计算各种反馈，并将这些反馈的强度与不同模式的气候敏感性差异联系起来。这些反馈涉及的机制将通过分析海冰反照率对气候变化的反应以及与海冰变化相关的云量变化引起的行星反照率变化来确定。云的行为将与从卫星云气候学获得的观测结果进行比较。模型中地表反照率反馈的有效性将通过比较季节性周期的观测到的和模拟的冰反照率反馈来确定。此外，他们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试图确定该区域最近的气候趋势是否与内部变化一致。在对南大洋区域气候变化的分析中，将综合平衡敏感度和内部变率的研究结果。将确定不同模式显示出明显的外部强迫变化出现的时间，并将研究这些出现时间中模式差异的原因。这项工作的广泛影响包括缩小气候变化模型预测之间的差异，从而增加对海洋二氧化碳吸收和南极冰盖显著融化风险的估计的信心。霍尔将在新生研讨会上讲授“南极冰盖的命运”。该项目将支持一名博士后研究员和一名研究生的培训。

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.