The Role of Oceanic Processes in the Initiation of Indian Summer Monsoon Intraseasonal Oscillations Over the Indian Ocean

Observational analyses and a hierarchy of ocean general circulation model (OGCM) experiments were performed to understand the influence of oceanic processes on the warm sea surface temperature anomalies (SSTAs) prior to the convection initiation of Indian summer monsoon intraseasonal oscillations (MISOs) in the equatorial Indian Ocean. Satellite observations revealed 41 strong MISO events that initiated over the central equatorial Indian Ocean and propagated northward to the Indian subcontinent and/or eastward to the Maritime Continent during the May-October season of the 2001-2012 period. Eight of those events were preceded by SSTAs that were strongly influenced by wind stress-driven oceanic processes. Composite analyses for the eight events showed that more than one-third of the preconvection warming in the initiation region resulted from oceanic processes, including advection and entrainment. Case studies of two strong MISO events associated with ocean-dynamically induced SSTAs showed unmistakable evidence for the preconvection SSTAs and sea surface height anomalies (SSHAs) associated with eastern boundary-reflected (in June/July 2004) and directly forced (in June 2006) intraseasonal oceanic Rossby waves. In those two case studies, a mixed-layer heat budget analysis of the OGCM results showed that advection played a much larger role than entrainment in elevating SSTAs. Further analysis showed that intraseasonal currents associated with the Rossby waves advected high seasonal mean SSTs into the initiation region. As current forecasting skills for the onset of the MISO are low, these results underscore the importance of thoroughly understanding the ocean's role in affecting SSTAs, which contribute to MISO initiation.