Shear Flow Instabilities and Unstable Events Over the North Bay of Bengal

A year‐long mooring data are used to study the upper ocean unstable events and instabilities at 18°N 89°E, which is a climatologically important region in the North Bay of Bengal. Near‐surface stability is studied from the context of the buoyancy frequency normalized shear (Vz/N) and reduced shear (S2−4N2) which are convenient measures to quantify flow stability, compared to the more widely used Richardson number (Ri). The analysis is carried out across three contrasting time periods, the monsoon, postmonsoon, and the winter of year 2012. Although it is well known that the flow stability changes from stable to unstable at Ri = Ricr=0.25, the relative importance of the perturbations of shear and buoyancy frequency in driving the unstable events is not well studied over the open oceans and more particularly over the Bay of Bengal. At 18°N, 89°E both higher than average shear and lower than average buoyancy frequency perturbations are crucial in driving the unstable events during the summer and premonsoon period. However, at increasing depths, the influence of shear perturbations becomes more dominant. Invoking the Miles‐Howard criteria for flow instability, it is seen that during the postmonsoon period, the buoyancy frequency perturbations are more critical than shear perturbations in driving the unstable events. In winter, the unstable events are influenced by both the buoyancy frequency and shear perturbations. Plain Language Summary This study is about upper ocean mixing in the Bay of Bengal. The Bay of Bengal is one of the least explored when compared with other oceans and seas. It is peculiar in the sense that it is landlocked from three directions. The Indian subcontinent is the home for more than a billion people and the economy crucially depends on the monsoons. Understanding near‐surface air‐sea interaction processes would help give fundamental insight into the weather over the subcontinent. The last decade was witness to a number of buoys that were deployed in the Bay of Bengal to measure key meteorological and ocean parameters. The Bay of Bengal is characterized by a strong and shallow stratification. This inhibits mixing. Lack of mixing has strong ramifications. Some of the mechanisms that inhibit or drive the near‐surface mixing are studied. This study is of fundamental importance and has relevance to weather modeling and air‐sea interaction across a broad scales of motion. Key Points Upper ocean stratification and shear instabilities are critic