Seasonal variations in the equatorial Indian Ocean and their impact on the Lombok throughflow

Abstract

In order to identify the origin of the nonequilibrated pressure field which introduces semiannual variations to the Lombok throughflow, one of the major components of the Indonesian throughflow, seasonal sea level records along the eastern boundary of the Indian Ocean and subsurface temperature records in the equatorial Indian Ocean are compared with the results from ocean general circulation models forced by climatological winds. It is shown, as discussed by Clarke and Liu [1993] using a simple analytical model, that the annual component of the sea level is excited locally by monsoonal winds, whereas the semiannual component is excited remotely by zonal winds in the central equatorial Indian Ocean during the monsoon transition periods. The eastward semiannual propagation observed both in the model and in the 20°C isotherm depth anomaly along the equatorial Indian Ocean is interpreted in terms of the second mode equatorial Kelvin wave excited by the zonal winds during the monsoon transition periods. The propagation direction is westward in the off‐equatorial region along 4°N, suggesting that the equatorial Kelvin wave reflects as an equatorial Rossby wave twice a year at the coast of Sumatra. Since the seasonal winds responsible for those semiannual oceanic variations suffer short‐term climate variabilities such as El Niño‐Southern Oscillation events, they may introduce interannual modulations to the seasonal cycle of the Lombok throughflow. As a corollary, we suggest an ageostrophic limitation due to a nonequilibrated pressure field on the use of Godfrey's [1989] island rule in estimating the throughflow variations.