Extratropical and Circumnavigating Circulation Precursors to the Onset of Madden–Julian Oscillation Deep Convection over the Western Indian Ocean

The onset of deep convection in the Madden Julian Oscillation (MJO) often occurs over the western Indian Ocean. Convective initiation is typically preceded by upper tropospheric circulation signals consisting of tropically circumnavigating easterly winds and a pair of cyclonic Rossby gyres over the subtropics of Africa and Southwest Asia. While the gyres may be forced by a Gill type response to suppressed convection over the eastern Indian Ocean, they also phase with intraseasonal extratropical wavetrains and may have subsequent influence on MJO activity. To better understand these circulation precursors, a budget analyses of momentum at 200hPa is implemented using a regional index based on the temporal evolution of the meridional structure of outgoing longwave radiation (OLR) over eastern Africa and the western Indian Ocean.

Results show that the circumnavigating time tendency of easterly wind may provide dynamical forcing for convective initiation as it phases with mid-tropospheric upward vertical velocity. However, the terms that contribute to the momentum tendency are not continuous, differing zonally between the Western Hemisphere and warm pool regions with a transition zone proximate to Africa. Both the horizontal structure of easterly acceleration due to the pressure gradient force and the phasing of meridional advection terms suggest a role for forcing from the extratropics near the western Indian Ocean.

Likewise, negative OLR anomalies associated with the cyclonic Rossby gyres shift from the subtropics to the tropics near the western Indian Ocean around the time of convective onset, concurrent with equatorward intraseasonal wave activity flux. Thus, results suggest that large-scale dynamical forcings, which potentially trigger MJO convection, may have extratropical origins.