Midlatitude circulation trends and the efficiency of poleward heat transport in the upper troposphere

对流层高层中纬度环流趋势和向极地热传输效率

• 批准号: 2440392
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2020
• 资助国家: 英国
• 起止时间: 2020 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2440392
• 关键词: Midlatitude; circulation; trends; efficiency; poleward

The jet streams are bands of intense zonal winds in the Earth's troposphere and are a dominant feature of climate in the midlatitudes. In each hemisphere, the jet's latitude is in thermal wind balance withthe meridional equator-to-pole temperature gradient, @T=@y. Climate models predict differently signed changes in the zonal-mean @T=@y at different altitudes in response to anthropogenic forcing. As theclimate warms, models suggest a \tug-of-war" [1] between rapid polar surface warming { which acts to reduce @T=@y at low levels and push the jet equatorward { and enhanced warming of the tropical uppertroposphere (UT), with an increased @T=@y at high levels associated with a poleward shift of the jet. State-of-the-art climate models differ in their projections of jet shifts under anthropogenic forcing,reflecting uncertainty in the outcome of this tug-of-war. Most models predict a small poleward shift of the jets by the end of the century [2]. However, reanalysis data covering the last 40 years suggest largerpoleward trends in the jets than are found in model simulations of the same period [3]. Additionally, while the amplification of polar surface warming is well observed in reanalysis data [4], trends in thetropical upper troposphere are more uncertain [5], with some observational datasets showing smaller warming trends than in models [6].Recent work has drawn attention to anomalous trends in poleward eddy heat and momentum fluxes in the upper troposphere [3] and has proposed that, in the zonal mean, the anomalous eddy heat flux[v*T*] is linked to the meridional temperature gradient by a simple flux-gradient relationship[v*T*] = D @T/@yHere, [.] denotes a zonal mean and D is some diffusivity coefficient. The hypothesis is that the real atmosphere exports heat and momentum out of the tropics more efficiently than in models, consistentwith both the stronger trends in the eddy-driven jets and weaker warming trends in the tropical UT. In this framework, a circulation that is more sensitive to tropical heating corresponds to a higher valueof the diffusivity D.

急流是地球对流层中强烈的纬向风带，是中纬度气候的主要特征。在每一半球，急流的纬度与赤道-极点温度梯度呈纬向热成风平衡，@T=@y。气候模式预测在不同高度的纬向平均值@T=@y响应于人为强迫的不同符号的变化。随着气候变暖，模式显示极地表面快速变暖（其作用是降低低层的@T=@y并将急流推向赤道）与热带对流层上层（UT）变暖增强之间的“拔河”[1]，高层的@T=@y增加与急流向极地移动有关。最先进的气候模式在人为强迫下对急流转变的预测各不相同，反映了这场拉锯战结果的不确定性。大多数模型预测到世纪末喷流会有一个小的向极移动[2]。然而，覆盖过去40年的再分析数据表明，喷流的向极趋势比同期模式模拟中发现的要大[3]。此外，虽然在再分析数据中很好地观察到极地表面变暖的放大[4]，但热带对流层上部的趋势更加不确定[5]，一些观测数据集显示的变暖趋势比模式中的要小[6]。最近的工作引起了对对流层上部极向涡动热量和动量通量异常趋势的关注[3]，并提出，在纬向平均中，反常涡动热通量[v*T*]与涡旋温度梯度之间存在一个简单的通量-梯度关系[v*T*] = D @T/@ y。表示分区平均值，D是某个扩散系数。该假设是，真实的大气出口的热量和动量的热带更有效地比在模型中，一致的强趋势的涡驱动的射流和较弱的变暖趋势在热带UT。在这个框架下，对热带加热更敏感的环流对应于更高的扩散系数D值。