Monthly Variations in Sea Level at the Island of Zanzibar

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.