On the Regionality of Moist Kelvin Waves and the MJO: The Critical Role of the Background Zonal Flow

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 over bar ] is altered through nudging, while maintaining a quasi-fixed rainfall climatology. Results show that nudging [u over bar ] 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.