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 is altered through nudging, while maintaining a quasi-fixed rainfall climatology. Results show that nudging 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.

中文翻译：

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关于潮湿开尔文波和 MJO 的区域性：背景纬向流的关键作用

具有超参数化物理的全局模型用于阐明对流耦合开尔文波和马登-朱利安振荡 (MJO) 的观测区域性。对带状均匀海面温度进行了一系列水行星模拟，其中背景带状流的轴对称结构通过轻推改变，同时保持准固定降雨气候。结果表明，轻推在赤道匹配典型的印度洋-太平洋或东太平洋扇区的剖面，产生这些扇区典型的向东移动的热带雨谱。除了多普勒频移效应外，还确定了两种不同的机械途径是造成这种平均流量依赖性的原因。第一个是通过副热带上层对流层罗斯比波临界线的移动，影响赤道开尔文模式环流的横向强迫，由向东和向赤道传播的涡流从高纬度冲击热带。第二种是通过热带对流层低层平均气旋切变强度的变化影响开尔文模式纬向风的季节内波动对高频赤道罗斯比型涡旋活动的调节程度。