Genesis of meteorological disturbances and thermohaline variability of the upper layers in the head of the Bay of Bengal during MONsoon Trough Boundary Layer EXperiment (MONTBLEX-90)

Abstract

Time-series measurements of surface meteorological elements and near-surface thermohaline profiles made at a station at the head of the Bay of Bengal during the MONsoon Trough Boundary Layer EXperiment (MONTBLEX-90; August–September 1990) were utilized to describe and explain the observed short-term variability in the thermohaline structure during active and inactive regimes of the summer monsoon. The observed near-surface thermal structure showed the occurrence of distinct isolated mini-warm pockets with temperatures in excess of 29°C extending from the surface to about 30 m depth, with a temporal correspondence to the formation of meteorological disturbances over the Bay's head. Accumulation of heat in the upper layers in excess of 0.4 × 108 J m−2 (with respect to the 28.7°C isotherm) appeared to have triggered the formation of meteorological disturbances that extracted this surplus energy to lower the surface temperature to around 28.7°C. The depth-time sections of thermohaline fields suggest the signature of a transient eddy or propagating wave. The small-scale structure in the thermohaline profiles was more prominent during September 1990 compared to August 1990. Two simple one-dimensional numerical models following Kraus and Turner (1967, Tellus, 19, 98–106), Denman (1973, Journal of Physical Oceanography, 3, 173–189), Miller (1976, Journal of Physical Oceanography, 6, 29–35) and Niiler (1975, Journal of Marine Research, 33, 405–422), Niiler and Kraus (1977, Modelling and prediction of the upper layers of the ocean, Pergamon, pp. 143–172) were evaluated to assess the relative importance of the local atmospheric forcing to account the observed variability in the near-surface mixed layer characteristics.