Wavelet isolation of meridionally moving geopotential height perturbations near the subtropics of eastern Africa and their relationship with the Madden–Julian Oscillation

Abstract

AbstractPrior to the onset of convection over the western Indian Ocean for many Madden–Julian Oscillation (MJO) events, equatorward‐propagating upper tropospheric negative geopotential height anomalies are observed along the east coast of Africa. To identify these signals independently from the MJO, we calculate partial meridional‐temporal wavelet transforms averaged between 35 and 45°E and centred in the subtropics. During times when the amplitudes of MJO indices are strong and leading up to convective onset over the western Indian Ocean, increased wavelet power relative to the mean state is observed in the equatorward direction at intraseasonal time‐scales from both hemispheres. Regressions against groups of equatorward intraseasonal wavelet transform time series centred on negative anomalies in the subtropics show extratropical Rossby wave trains extending to the west across the Atlantic similar to signals previously observed leading up to the onset of Indian basin convection in composite MJO events. Compared to globally averaged mean‐state power spectra in the subtropics, increased equatorward propagation of upper tropospheric geopotential height anomalies occurs regionally near eastern Africa on intraseasonal time‐scales, which suggests that local mean‐state conditions, in addition to the MJO, may influence meridional wave propagation. Since these anomalies may in turn influence the onset of MJO convection over the western Indian Ocean and have higher local power, mean‐state conditions near the subtropics of eastern Africa may change the timing of the MJO.