The Equatorial Upper-Tropospheric MJO Signal Is Created by Kelvin Wave Dynamics Interacting with the Mean Flow

Abstract Fields of equatorial upper-tropospheric circulation data are lag regressed against wavelet-filtered indices of upper-tropospheric zonal wind anomalies over a range of phase speeds at 50-day periods to study the propagation mechanisms of the upper-tropospheric circulation signal of the Madden–Julian oscillation (MJO) over the Indian Ocean. Results show that the MJO upper-tropospheric zonal wind is accelerated by the geopotential gradient force in quadrature with the existing wind anomaly, yielding its eastward propagation. This effect is offset by Doppler advection of the MJO wind by the easterly background wind. Divergence of mass by the zonal wind propagates the associated geopotential height anomalies in concert with the winds. Results confirm that advection of background wind by the MJO wind amplifies MJO wind anomalies in phase with those anomalies where the background wind is zonally confluent and breaks it down in regions of diffluent background wind. Coincidence of the mass source driven by moist convection and zonally diffluent zonal wind anomalies with falling geopotential height relative to the regions east and west is consistent with planetary-scale Kelvin wave zonal wind signals providing favorable conditions for convection. Circulation data at 100 hPa confirm approximately dry Kelvin wave dynamics driving MJO-associated equatorial circulations eastward.