Altimetric observations of sea level cycles near the Strait of Bab al Mandab

Abstract

The Strait of Bab al Mandab connects the Red Sea with the Gulf of Aden and Indian Ocean. Flow through the Strait is dominated by monsoonal forcing with surface water inflow in the winter and outflow in the summer. Analysis of TOPEX/POSEIDON altimeter sea surface height residuals at subsatellite locations in the Red Sea, the Strait and Gulf of Aden indicates a dominant annual cycle and a secondary semi-annual cycle at all locations. A combined annual and semi-annual model is then fitted to sea surface height anomalies at each of the locations. The combined model explains most of the variance in the data: from 74 per cent variance in the Gulf of Aden to 92 per cent variance in the southern Red Sea. The amplitude of the annual cycle is 18cm in the Red Sea and 13 cm in the Gulf of Aden. An analysis of coherence between altimeterderived wind stress and sea surface height residuals shows that the annual cycle is probably related to wind forcing. A weaker semi-annual cycle of 4-8 cm is probably related to the cycle of evaporation. A two-layer model of flow through the Strait shows that the dynamic sea surface height signature is only 2cm. This signature of exchange is too small to be resolved by current satellite altimeter measurements