Mechanisms of barrier layer formation and erosion from in situ observations in the Bay of Bengal
Date
2019Author
George, J.V.
Vinayachandran, P.N.
Vijith, V.
Thushara, V.
Nayak, A.A.
Pargaonkar, S.M.
Amol, P.
Vijaykumar, K.
Matthews, A.J.
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During the Bay of Bengal (BoB) Boundary Layer Experiment (BoBBLE) in the southern BoB, time series
of microstructure measurements were obtained at 88N, 898E 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 tem-
perature 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 in-
terfaces. 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 sug-
gested 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 1027 W kg21 and 1024 m2 s21 , 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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