Evolution of Upper Tropospheric Zonal Momentum over the Western Indian Ocean Preceding Madden-Julian Oscillation Deep Convective Onset

Madden-Julian Oscillation (MJO) deep convection often initially forms over the Western Indian Ocean. Recent research suggests a large-scale dynamical reduction in subsidence is necessary for the onset of MJO deep convection. Circulation signals that usually precede MJO convective onset include upper tropospheric eastward circumnavigating easterly winds over the Western Hemisphere and cyclonic Rossby gyres in the subtropics near the Western Indian Ocean. To better understand the evolution of circulatory precursors leading up to MJO convective onset, we use a regionally based index to construct the 200hPa zonal momentum budget over the Western Indian Ocean.

Along the equator, the acceleration of easterly momentum is in phase with mid-tropospheric upward vertical motion, suggesting that the easterly tendency may provide upper tropospheric divergence which acts to decrease large-scale subsidence over the region. For most of the cases, the pressure gradient force is the leading source of easterly acceleration over the equatorial western Indian Ocean. However, the spatial evolution of the term suggests that the pressure gradient force does not propagate eastwards over the region, which is inconsistent with Kelvin type wave dynamics. Rather, equatorward propagation of the signal from the extratropics is associated with intrusions of mid-tropospheric cold anomalies and upper-tropospheric low-pressure anomalies likely related to extratropical Rossby wavebreaking. Thus, results demonstrate a mechanism by which extratropical signals may act to initiate MJO convection.