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

急流是地球对流层中强烈的纬向风带，是中纬度地区气候的主要特征。在每个半球，喷流的纬度与经向赤道到极点的温度梯度@T=@y处于热风平衡状态。气候模式预测不同海拔地区的纬向平均值@T=@y随人为强迫的变化有不同的标志。随着气候变暖，模式显示极地表面快速变暖{使低层的@T=@y减少并推动急流向赤道移动}与热带对流层上层（UT）的升温增强之间存在拔河”[1]，高层的@T=@y增加与急流向极地移动有关。最先进的气候模式对人为强迫下的喷流变化的预估存在差异，反映了这种拉锯战结果的不确定性。大多数模型预测，到本世纪末，喷流将小幅向极地移动。然而，覆盖过去40年的再分析数据表明，与同期的模式模拟相比，喷流的极向趋势更大。此外，虽然在再分析资料[4]中可以很好地观察到极地表面变暖的放大，但热带对流层上层的趋势更不确定[5]，一些观测资料集显示的变暖趋势比模式[6]要小。最近的工作引起了对流层上层[3]向极地涡旋热通量和动量通量的异常趋势，并提出在纬向平均值上，涡旋热通量[v*T*]与经向温度梯度之间存在简单的通量-梯度关系[v*T*] = D @T/@yHere,[。]]为分区平均值，D为某扩散系数。假设是真实的大气输出热量和动量比模型更有效，这与涡旋驱动喷流的强趋势和热带UT变暖的弱趋势相一致。在这个框架中，对热带加热更敏感的环流对应于更高的扩散系数D值。

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.