Zonal Propagation of the Indian Basin MJO Across Varying Background Wind and Seasonal Background Wind States

The Madden–Julian Oscillation (MJO) varies seasonally. Both moist and dry dynamical processes would contribute to this seasonality. Previous results have suggested strong dependence of MJO phase speed on planetary-scale upper tropospheric Kelvin waves interacting with the mean flow. Composites and phase speed spectra assess the association between the Indian Basin MJO circulation and convection with variations in equatorial upper tropospheric background wind patterns, including seasonal variability. Results show that the fastest eastward propagation over the Indian Ocean (>10 ms−1) tends to occur during northern spring when background upper tropospheric easterlies are weakest. Northern winter signals typically advance eastward between 4 and 10 ms−1. Strong easterly background wind conditions during northern summer usually prevent propagation eastward along the equator from the Western Indian Ocean. Results also show relative amplitude variations between the MJO’s upper and lower tropospheric zonal wind signals, with the upper tropospheric circulation signals being disproportionately stronger than the lower tropospheric ones over the Western Hemisphere to East Africa. The upper tropospheric easterly wind anomalies grow over the Western Indian Ocean first, as specific humidity increases in lower tropospheric easterly wind to the east. Then, lower tropospheric westerly wind emerges west of the emerging convection, suggesting that lower tropospheric wind change depends more directly on moist processes than the upper tropospheric wind.