Subtropical and Extratropical Precursors to Western Indian Ocean MJO Convective Onset

Leading up to the organization of Madden-Julian Oscillation deep convection over the western Indian Ocean, intraseasonal enhanced convective signals typically form over the subtropics of Africa and Southwest Asia with suspected subsequent tropical intrusions. Since any meridional incursions would be particularly regionally focused, typical global MJO indices may be unable to distinguish specific structures. To better resolve signals, composites are based on a time-extended latitudinal EOF index of outgoing longwave radiation for the region; results show probable interactions between the extratropics, subtropics, and tropics which may influence the ensuing MJO convective initiation. Specifically, upper-tropospheric cold anomalies and their associated circulation signals move equatorwards on the tails of eastward moving zonally extended signals over the subtropics just before the time of convective onset, which may encourage large-scale upward motion. Moreover, the regional index is able to depict a continuous globally propagating MJO signal, of particular note since eastern Africa and the western Indian Ocean are not locations with the strongest deep convection. Potential prompting for the subtropical signals likely results from either Rossby gyres responding to MJO suppressed convection over the eastern Indian Ocean or upstream extratropical wavetrains. Further analysis using this index will help to illuminate these regionally specific details and their relationship to MJO convective initiation.