Variability of the oceanic boundary layer characteristics in the northern Bay of Bengal during MONTBLEX-90

Abstract

Variability of the ocean surface boundary layer characteristics on daily time-scale is btucied utilizing the 3-hourly hydrographie data collected at a stationary location (20°X, S9°E)in thsBay of Bengal during August (13th - 31st land September (9th - 19ih). 1990 under MONTBLEX-90 field programme. The daily variations of tenperature, salinity, mixed layer tniekness, stability, heat contain and rate of change of-ncat content in the upper 100 m are discussed in relation to prevailing weather (depressions) and hydrographie conditions (influx of fresh water, presence of eddies). The mixed layer thickness is examined through temperauireand T-based criteria considering also the surface salirhu in the latter. The T-baseri mixed layer thickness is. always higher than that of σ-based thickness. The rate of change of heat content is also computed up ;o the depth of20°C and 14°C isotherms which tikes -into account the vertical motion and hence divergence. With the development of a low into a deep depression close to the study area, intense upweihng of subsurface cold waters is noticed from 100 m to the bottom of the surface mixed layer (20 m) from 18th to 20th August. The upweiling is weakened by 21 it August when the depression moved away from the stud) location. This variation of upweiling is supported by the variation of surface mixed layer thickness, static stability at 50 m ulepiti, heat content in the upper 100m and the heat con tent up to the depth of 20°C isotherm from 18th to 21 st August. The rate of change of heat content in the upper 100 m and up to the depths cf 20°C and 14°C isotherms leads to net heat storage during August and to net heat depletion during September. This together with the net surface heat gam lead to an import (197 Wm-1) and export (233 Wm-2) of heat during August and September respectively through horizontal advective processes. These advectiye processes are attributed to the presence and movement of a warm core eddy through the study location. © Printed in India.