Processes Guiding Monsoon Intraseasonal Oscillations in the Bay of Bengal

The southwest monsoon is an inherently multi-scale system with interannual (IA), intraseasonal (IS), synoptic, and diurnal and smaller scales - all interacting in complex manner. The quality of life in the southeastern Asian subcontinent, where nearly 80% of the annual rainfall is produced during the southwest monsoon period, heavily relies on accurate prediction of the spatial-temporal distribution of monsoon rainfall. Active-break cycles of these subseasonal (30-60 day) episodes are referred to as Monsoon Intraseasonal Oscillations (MISOs), a characteristic feature of the SWM, and they modulate the spatial structure, frequency, and duration of active and break spells. In this dissertation, observational datasets covering a wide range of spatial and temporal scales are analyzed and interpreted to unravel the dynamical processes affecting the development and advancement of MISOs. Considering that monsoons are a strongly coupled ocean-atmosphere phenomenon, a better understanding of the interplay between the different air-sea salient modes is needed for improving monsoons forecasting skills. This prompted analysis using a novel spectral approach where MISO structure and evolution along the BOB was examined using admissible terms (state, stress and flux terms) regarding air-sea coupling processes. Results show northward propagation for all variables during active and break MISO spells. Heat flux into the ocean favored the active MISO phase by ∼ 2 days and warm SSTs by ∼ 8 days, validated against traditional band-filtered methodology. Results suggests the development of a new, more robust, multivariate MISO index for the purpose of real-time monitoring and forecasting. A major component of this dissertation was a study of monsoon intraseasonal oscillations in the Bay of Bengal during boreal summers 2018-19, dubbed MISO-BOB. Notwithstanding that the overall MISO structure shows some year-to-year resemblance, the spatial-temporal characteristics of summer precipitation are complex, scattered, and fairly variable. Such variability is a result of a combination of systems across scales of motion from planetary scale disturbances to the distribution of mesoscale weather systems. The extended coverage provided from MISO-BOB field experiments allowed to explore equatorial motions and their relationship with MISO. Results suggests strength and extent of MISO in BOB is controlled by the amount of moisture from (strength of) eastward propagating convective equatorial signals in the equatorial Indian ocean. Similar results are found for stationary cruise observations where periods of intense precipitations, one of them reaching up to 120 mm, were commonly found to be product of aggregating active northwestward propagating (MISO) mode and equatorial mode(s). The granular or local observations of weather passing events during stationary cruises gave insight on the role of air-sea interactions. Break conditions in BOB allowed for intense warming of the ocean surface and accumulation of convective potential energy. The transition to a convective state was observed to be accompanied sharp strengthening of westerly winds and subsequent rapid advance of precipitation, with ocean cooling at rates of ∼100W/m2 . These results suggest that BOB thermodynamic preconditioning together with moisture brought from the Equatorial Indian Ocean (EIO) is essential for the development and propagation of convective cells, hence MISO.