Extratropical Influence on 200-hPa Easterly Acceleration over the Western Indian Ocean Preceding Madden–Julian Oscillation Convective Onset

Abstract

The onset of Madden?Julian oscillation (MJO) deep convection often occurs over the western Indian Ocean and has upper-tropospheric circulation precursors that consist of eastward-circumnavigating tropical easterlies and subtropical cyclonic Rossby gyres near eastern Africa. Moreover, the evolution of the large-scale circulation and its ability to reduce subsidence may be necessary for the initial development of organized deep convection. To better understand the evolution of the circulation precursors and their interaction with convective onset, this paper analyzes the upper-tropospheric zonal momentum budget using a regional index based on the temporal progression of the meridional structure of intraseasonal outgoing longwave radiation anomalies over eastern Africa and the western Indian Ocean. The circumnavigating intraseasonal easterly acceleration produces upper-level divergence when it reaches the western extent of a region of intraseasonal westerlies and may provide a forcing for the in-phase midtropospheric upward vertical motion. For about three-quarters of the identified cases, the easterly acceleration over the western Indian Ocean is a response to the zonal pressure gradient over the region. In the composite, the negative pressure gradient force may be initially induced by the injection of negative geopotential height anomalies from the extratropics of both hemispheres to the tropics over eastern Africa, though tropically circumnavigating and local signals may also contribute to the easterly acceleration, especially in the days following convective onset.