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A yearlong record from moored current, temperature, conductivity, and four mixing meters (chi pods) in the northernmost international waters of the Bay of Bengal quantifies upper-ocean turbulent diffusivity of heat (K-t) and its response to the Indian monsoon. Data indicate (1) pronounced intermittency in turbulence at semidiurnal, diurnal, and near-inertial timescales, (2) strong turbulence above 25-m depth during the SW (summer) and NE (winter) monsoon relative to the transition periods (compare K-t > 10 (-4) m(2)/s to K-t similar to 10(-5) m(2)/s, and (3) persistent suppression of turbulence (K-t < 10(-5) m(2)/s) for 3 to 5 months in the latter half of the SW monsoon coincident with enhanced near-surface stratification postarrival of low-salinity water from the Brahmaputra-Ganga-Meghna delta and monsoonal precipitation. This suppression promotes maintenance of the low-salinity surface waters within the interior of the bay preconditioning the upper northern Indian Ocean for the next year's monsoon.

Plain Language Summary Fluctuations in the intensity of the Indian monsoon system propagate northward from the equator toward the Indian subcontinent, bringing intervals of relatively wet and dry conditions. Rains both feed many rivers that discharge into the northern Bay of Bengal and provide close to 2 m of rainfall over the basin. The freshwater persists as a shallow layer in the bay for 3-5 months starting around July (the latter half of the summer monsoon). This shallow, freshwater layer adjusts quickly to changes in air-sea heat fluxes but also limits atmospheric forcing of the ocean below. Our yearlong data set quantifies upper-ocean turbulent mixing in the northern bay. Above 25 m, we find that (1) mixing is very strong during the summer monsoon (June-September) due to the strong winds, (2) mixing is reduced from summer monsoon values during the winter monsoon (November-January), and (3) mixing is lowest during transition periods between the two. The ocean responds differently below 25 m. The freshwater layer on top acts as a barrier to the winds, and mixing is suppressed for several months. Below 60 m, even an intense cyclone could not generate appreciable ocean mixing when the freshwater layer was present.