Aspects of sea level variability in the southwest Indian Ocean and the east coast of Africa - (latitude 0-35°S and from the coast to 60°E)
Analysis of tide gauge sea level observations of varying durations in the southwest Indian Ocean and the East coast of Africa (Lamu, Mombasa, Zanzibar, Durban, Port La Rue and Port Louis) show variability which are related to global, regional time scales, local weather and climatic changes, oceanographic and hydrological forcing that manifest in both short and long time scales. The investigations on the tide gauge sea level observations are conducted through the separation of the total sea level measurements into the contributing components (tides and residuals) using a Matlab in built software (t-tide). Short time scale sea level variability in the southwest Indian Ocean is due to the effects of tides which exhibit tidal range variations with latitude and shelf width, storm surges resulting from tropical cyclones passage especially in the mid-latitude region, atmospheric pressure fluctuations over the surface of the sea and local wind fields. Sea surface temperature variations during summer and winter result in differential heating of the ocean surface and contribute to the observed sea level variability at seasonal time scale especially in the region 25°S and southwards where the temperature differences are large. The equatorial region is characterized by a near constant sea surface temperature that sustains thermal expansion of the upper layer of the ocean water throughout the year. Monsoon periods show significant and variable wind speeds that impact on sea level variability in the southwest Indian Ocean and the East coast of Africa and are greatest during the summer monsoon (from June to August). On longer time scales (Interannual and decadal), sea level variations in this region is mostly influenced by the El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). During the 1997/98 El Nino event, the sea levels are significantly higher than normal at the coast and the islands. During the 2000/2001 La Nina, the sea levels are significantly lower than normal at the coasts in the southwest Indian Ocean. Indian Ocean Dipole effects are significant in the southwest Indian Ocean during the period 2006 through to 2008 and are more enhanced in 2007. The annual highest sea levels in this region are influenced by the year to year changes in weather pattern and the perigean cycle of the tides on a 4.4 year period but their secular trends are not statistically significant.
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