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Benjamin G. M. Webber, Adrian J. Matthews, P. N. Vinayachandran, C. P. Neema, Alejandra Sanchez-Franks, V. Vijith, P. Amol, and Dariusz B. Baranowski


The strong stratification of the Bay of Bengal (BoB) causes rapid variations in sea surface temperature (SST) that influence the development of monsoon rainfall systems. This stratification is driven by the salinity difference between the fresh surface waters of the northern bay and the supply of warm, salty water by the Southwest Monsoon Current (SMC). Despite the influence of the SMC on monsoon dynamics, observations of this current during the monsoon are sparse. Using data from high-resolution in situ measurements along an east–west section at 8°N in the southern BoB, we calculate that the northward transport during July 2016 was between 16.7 and 24.5 Sv (1 Sv ≡ 106 m3 s−1), although up to ⅔ of this transport is associated with persistent recirculating eddies, including the Sri Lanka Dome. Comparison with climatology suggests the SMC in early July was close to the average annual maximum strength. The NEMO 1/12° ocean model with data assimilation is found to faithfully represent the variability of the SMC and associated water masses. We show how the variability in SMC strength and position is driven by the complex interplay between local forcing (wind stress curl over the Sri Lanka Dome) and remote forcing (Kelvin and Rossby wave propagation). Thus, various modes of climatic variability will influence SMC strength and location on time scales from weeks to years. Idealized one-dimensional ocean model experiments show that subsurface water masses advected by the SMC significantly alter the evolution of SST and salinity, potentially impacting Indian monsoon rainfall.

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Jenson V. George, P. N. Vinayachandran, V. Vijith, V. Thushara, Anoop A. Nayak, Shrikant M. Pargaonkar, P. Amol, K. Vijaykumar, and Adrian J. Matthews


During the Bay of Bengal (BoB) Boundary Layer Experiment (BoBBLE) in the southern BoB, time series of microstructure measurements were obtained at 8°N, 89°E from 4 to 14 July 2016. These observations captured events of barrier layer (BL) erosion and reformation. Initially, a three-layer structure was observed: a fresh surface mixed layer (ML) of thickness 10–20 m; a BL below of 30–40-m thickness with similar temperature but higher salinity; and a high salinity core layer, associated with the Summer Monsoon Current. Each of these three layers was in relative motion to the others, leading to regions of high shear at the interfaces. However, the destabilizing influence of the shear regions was not enough to overcome the haline stratification, and the three-layer structure was preserved. A salinity budget using in situ observations suggested that during the BL erosion, differential advection brought high salinity surface waters (34.5 psu) with weak stratification to the time series location and replaced the three-layer structure with a deep ML (~60 m). The resulting weakened stratification at the time series location then allowed atmospheric wind forcing to penetrate deeper. The turbulent kinetic energy dissipation rate and eddy diffusivity showed elevated values above 10−7 W kg−1 and 10−4 m2 s−1, respectively, in the upper 60 m. Later, the surface salinity decreased again (33.8 psu) through differential horizontal advection, stratification became stronger and elevated mixing rates were confined to the upper 20 m, and the BL reformed. A 1D model experiment suggested that in the study region, differential advection of temperature–salinity characteristics is essential for the maintenance of BL and to the extent to which mixing penetrates the water column.

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