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Western Indian Ocean Journal of Marine Science

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Monthly Variations in Sea Level at the Island of Zanzibar

SB Mahongo, J Francis

Abstract


Meteorological and tide gauge data were used to analyze correlations between climatic parameters and variations in mean sea level at Zanzibar for the period 1985-2004. This involved spectral and multiple regression analysis of the monthly variables, as well as harmonic analysis of hourly sea level. Air pressure and rainfall remained relatively constant during the 20-year study period, but there were trends in sea level, northeast winds, southeast winds and air temperature. Monthly variations in mean sea level, composed predominantly of semi-annual, annual and 4-year oscillations, were represented by the steric effect proxies of rainfall and air temperature (45%), southeast and northeast monsoon winds (41%), and air pressure (5%). The trend in sea level (9%) appeared to be mainly correlated with northeast winds. The annual cycle in sea level (36%) was represented to a certain degree by rainfall (11%), air temperature (10%), southeast winds (8%) and northeast winds (7%). The semi-annual component (28%) was best represented by southeast winds (15%), with the remaining 13% of the variability being equally represented by rainfall, northeast winds and air pressure. The 4-year oscillations, which accounted for about 27% of the variation in sea level, were mainly represented by air temperature (12%), rainfall (8%) and southeast winds (6%). There is a strong likelihood that physical processes other than meteorology and tides influenced the observed variations in sea level, especially in the 4-year cycle.

Keywords: Zanzibar, sea level variations, climate, spectral analysis, multiple regression.






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