Observations and Simulations of the Mixed Layer Characteristics off Bombay, West coast of India during the summer monsoon of 1988

Abstract

The observed thermal and current structures in the coastal waters off Bombay during summer monsoon are presented. The thermal structure was characterized by a sharp thermocline (2·5 °C in 10 m) sandwiched between upper and bottom isothermal layers. Pre-monsoonal heating in the surface layers and strong monsoonal vertical mixing over the remnants of the winter water caused the formation of the three layer structure. Residual flow within the upper isothermal layer was 90° to the right of the prevailing southwesterly surface wind; whereas it was easterly within and below the thermocline. Currents at different levels exhibited rotary nature and the change in the direction of flow between all levels was in phase. FFT analysis revealed the dominance of semi-diurnal tidal frequency over the diurnal frequency. The mixed layer characteristics were simulated using the one dimensional numerical models of Miller (1976) and Niiler and Kraus (1977). Minimum departure between observed and simulated mixed layer depth (r.m.s. error ± 5 m) and mixed layer temperature (r.m.s. error ± 0·1 °C) was obtained with the model of Niiler and Kraus (1977). The cooler and deeper mixed layer simulated by the model of Miller (1976) was attributed to improper parameterizations of convective efficiency and solar radiation absorption.