Three-dimensional Structure of Mass-weighted Isentropic Time Mean Meridional Circulations
Three-dimensional Structure of Mass-weighted Isentropic Time Mean Meridional Circulations
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质量加权等熵时间平均经向环流的三维结构
DOI：
10.1175/jas-d-17-0154.1
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发表时间：
2018
期刊：
影响因子：
--
通讯作者：
and T. Iwasaki
中科院分区：
文献类型：
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作者：
Kanno;Y.;and T. Iwasaki
本文建立了一个研究质量加权等熵时间平均(T-MIM)经向环流三维(3D)结构的诊断框架，并对冬季半球进行了初步分析。T-MIM的经向速度可以在纬向上展开时间平均的二维(2D)质量加权等熵纬向平均。此外，T-MIM速度可以分解为未加权等熵时间平均(UTM)速度和与时间相关的涡旋相关传输速度，即所谓的团注速度。团流速度对对流层中的2D温带直接环流和平流层中的Breyer-Dobson环流有很大贡献。三维团流速度似乎反映了斜压不稳定波活动的地理分布。在北半球冬季，北太平洋和北大西洋风暴路径周围既有低层赤道气流，也有高层极地气流。在南半球冬季，低层赤道气流横跨中纬度地区。在副热带地区，三维团流速度在Hadley环流的上层分支中是重要的。建立了一个纬向动量方程，考察了T-MIM框架下经向环流的三维动量平衡。在温带外，UTM和BOUS经向速度与3DEliassen-Palm(EP)通量散度的定常分量和瞬变分量分别处于地转平衡状态。瞬变斜压不稳定波的气压梯度力与风暴轴上团流速度的低层赤道流动相平衡。
The present study develops a diagnostic framework for investigating the three-dimensional (3D) structure of mass-weighted isentropic time-mean (T-MIM) meridional circulations and conducts a preliminary analysis of the winter hemispheres. The T-MIM meridional velocity can unfold, in the zonal direction, time-averaged two-dimensional (2D) mass-weighted isentropic zonal means. Furthermore, the T-MIM velocity can be decomposed into the unweighted isentropic time-mean (uTM) velocity and the temporal eddy-correlated transport velocity, the so-called bolus velocity. The bolus velocity greatly contributes to the 2D extratropical direct circulation in the troposphere and to the Brewer–Dobson circulation in the stratosphere. The 3D bolus velocity seems to reflect the geographical distributions of baroclinic instability wave activity. In the boreal winter, both low-level equatorward flows and upper-level poleward flows are located around the North Pacific and North Atlantic storm tracks. In the austral winter, low-level equatorward flows extend zonally across the midlatitudes. In the subtropics, the 3D bolus velocity is found to be significant in the upper branch of the Hadley circulation. A zonal momentum equation is formulated to examine the 3D momentum balance of the meridional circulation in the T-MIM framework. In the extratropics, the uTM and bolus meridional velocities are in geostrophic balance with the stationary and transient components of the 3D Eliassen–Palm (EP) flux divergence, respectively. The pressure gradient force of transient baroclinic instability waves balances with the low-level equatorward flows of the bolus velocity in the storm tracks.