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  • Author or Editor: P.N. Vinayachandran x
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P. N. Vinayachandran and Toshio Yamagata

Abstract

Results from an ocean general circulation model are used to study the response of the oceanic region surrounding Sri Lanka to monsoonal winds. East of Sri Lanka, a cold dome (Sri Lanka dome, SLD) develops during the southwest monsoon (SWM) in response to cyclonic curl in the local wind field. The dome decays after September due to the arrival of a long Rossby wave, associated with the reflection of the spring Wyrtki jet at the eastern boundary of the ocean. East of the SLD an anticyclonic eddy exists that is in intermediate geostrophic (IG) balance. North of Sri Lanka a cold dome (Bay of Bengal dome) develops after the SWM associated with a cyclonic gyre forced by Ekman pumping. The source of cold water of the Bay of Bengal dome is traced back to the SLD and upwelling zone along the east coast of India. South of Sri Lanka a major part of the Southwest Monsoon Current (SMC) turns northeastward and flows into the Bay of Bengal. The part that flows eastward terminates at progressively western longitudes as the season progresses. This termination and the shallowness of the SMC is due to a Rossby wave generated near the eastern boundary by weakening of the spring Wyrtki jet and anticyclonic wind stress curl. This Rossby wave follows the one associated with the spring Wyrtki jet and has dominant velocities toward southwest. A large anticyclonic vortex, embedded in the SMC, results from the geostrophic adjustment process for the surface water converged by the long Rossby wave and the eastward zonal current. Energy analysis of this anticyclonic vortex as well as the IG eddy east of the SLD shows direct conversion from mean kinetic energy to eddy kinetic energy suggesting that barotropic instability is the mechanism that leads to eddy generation.

This study suggests two links that allow exchange between the Bay of Bengal and the rest of the Indian Ocean: The first is the SMC, which is an open ocean current, and the second is the equatorward East India Coastal Current during November–January, which is closely attached to the coast.

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Jenson V. George, P. N. Vinayachandran, and Anoop A. Nayak

Abstract

The inflow of high-saline water from the Arabian Sea (AS) into the Bay of Bengal (BoB) and its subsequent mixing with the relatively fresh BoB water is vital for the north Indian Ocean salt budget. During June–September, the Summer Monsoon Current carries high-salinity water from the AS to the BoB. A time series of microstructure and hydrographic data collected from 4 to 14 July 2016 in the southern BoB (8°N, 89°E) showed the presence of a subsurface (60–150 m) high-salinity core. The high-salinity core was composed of relatively warm and saline AS water overlying the relatively cold and fresh BoB water. The lower part of the high-salinity core showed double-diffusive salt fingering instability. Salt fingering staircases with varying thickness (up to 10 m) in the temperature and salinity profiles were also observed at the base of a high-salinity core at approximately 75–150-m depth. The average downward diapycnal salt flux out of the high-salinity core due to the effect of salt fingering was 2.8 × 10−7 kg m−2 s−1, approximately one order of magnitude higher than the flux if salt fingering was neglected.

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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

Abstract

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

Abstract

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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