On the Regionality of Moist Kelvin Waves and the MJO: The Critical Role of the Background Zonal Flow
On the Regionality of Moist Kelvin Waves and the MJO: The Critical Role of the Background Zonal Flow
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DOI：
10.1029/2021ms002528
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发表时间：
2021-08
影响因子：
6.8
通讯作者：
S. Tulich;G. Kiladis
中科院分区：
文献类型：
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作者：
S. Tulich;G. Kiladis
使用具有超参数化物理的全球模型来阐明对流耦合Kelvin波和Madden-Julian振荡（MJO）的观测区域性。在纬向均匀海面温度下进行了一系列水行星模拟，其中背景纬向流[u <$]的轴对称结构通过轻推而改变，同时保持准固定的降雨气候学。结果表明，在赤道上轻推[u <$]以匹配印度太平洋或东太平洋地区典型的廓线，产生了这些地区典型的东移热带雨谱。除了多普勒频移效应之外，还确定了两种不同的机制途径负责这种平均流量依赖性。第一种是通过副热带对流层上部Rossby波临界线的移动，影响赤道开尔文模式环流的横向强迫，这是由从高纬度向东和向赤道传播的涡旋撞击热带造成的。第二种是通过热带对流层低层平均气旋性切变强度的变化，影响开尔文模式纬向风季节内波动对高频赤道Rossby型涡旋活动的调节程度。在平均低层气旋性切变增强的情况下，这种调制的强度，被称为“切变诱导的涡旋调制”或SIEM，也被认为是增强的，使得MJO样的变率模式变得不稳定或接近中性，这取决于切变的强度。
A global model with superparameterized physics is used to shed light on the observed regionality of convectively coupled Kelvin waves and the Madden‐Julian Oscillation (MJO). A series of aquaplanet simulations over zonally uniform sea‐surface temperatures is performed, in which the axisymmetric structure of the background zonal flow [u¯] is altered through nudging, while maintaining a quasi‐fixed rainfall climatology. Results show that nudging [u¯] at the equator to match profiles typical of the Indo‐Pacific or eastern Pacific sectors yields eastward‐moving tropical rain spectra typical of those sectors. Two different mechanistic pathways are identified as being responsible for this mean‐flow dependence, in addition to Doppler shifting effects. The first is through shifts of the Rossby wave critical line in the subtropical upper troposphere that affect the lateral forcing of Kelvin‐mode circulations at the equator by eastward and equatorward‐propagating eddies impinging on the tropics from higher latitudes. The second is through changes in the strength of the mean cyclonic shear in the lower tropical troposphere that affect the degree to which intraseasonal fluctuations in Kelvin‐mode zonal winds modulate the activity of higher‐frequency equatorial Rossby‐type eddies. In cases where the mean low‐level cyclonic shear is enhanced, the strength of this modulation, referred to as “shear‐induced eddy modulation” or SIEM, is also seen to be enhanced, such that MJO‐like modes of variability are rendered either unstable or near neutral, depending on the strength of the shear.