Using drifter observations to unearth the mysteries of Monsoons in the Bay of Bengal

This study uses drifters and minimet drifters to characterize investigate the onset of summer monsoons and their active-break periods in the Bay of Bengal. The Indian subcontinent receives abundant seasonal rainfall as a result of Monsoon Intra-Seasonal Oscillations (MISO). MISO propagation in the Bay of Bengal (BoB) leads to active phases (intervals of relatively high rainfall) and break phases (intervals of little or no rainfall). There are considerable differences in the air-sea interactions in the two phases. The onset and duration of the active and break periods are not well predicted, and errors in simulating and predicting these affects the weather forecasting across the globe. Monsoon Intra-Seasonal Oscillations in the Bay of Bengal (MISOBOB) program from USA and Ocean Mixing and Monsoons (OMM) program from India brought in teams of scientists to intensively observe air-sea interaction in the context of Monsoons. As a part of MISOBOB, 30 drifters and 5 minimet drifters were deployed in the central BoB in July 2019. Here we investigate whether the different phases associated with the MISOs can be detected using surface measurements from these autonomous instruments. With observations from multiple drifters, it is also possible to compare the spatial gradients in the diel cycles of SST over the length scale of an order of 100km or less (i.e. within (sub)mesoscale length scales) during both phases. These spatial statistics during active and break phases are not well known in the BoB from in-situ measurements. Similarly, observations from minimet drifters could be used to compare spatial gradients in wind speeds. These gradients over length scales of an order of 100 km or lesser can lead to horizontal gradients in diurnal warm layer properties and thus provide a mechanism for submesoscale and mesoscale horizontal mixing of surface waters.