Extreme sea levels around the coast of Southern Africa
Tide gauge data from ten ports around the coast of Southern Africa are used to study the nature and behaviour of extreme high sea levels with a view towards predicting the likelihood of extremes occurring in the future. A recorded sea level height can be thought of as a combination of an astronomical tide and a weather determined component. In Southern Africa tides are typically 2 to 2.5 metres in range and the non-tidal residual accounts for up to 50 cm. Sea level is governed by local tides and local meteorology and there is great similarity in the magnitudes and timing at all ports. However tides are found to be the dominant contribution to extreme levels, hence the long term character of tidal variations is important in Southern African extremes. The fortnightly, equinoctial and 4.4 year tidal cycles determine the expected sea level extremes. A prediction technique developed here makes use of the tidal dominance by calculating the likelihood of exceedance for a specific month in a particular year. For any month the highest tide is known and an extreme will depend on the necessary surge occurring. Probability is derived from the surge distribution for that month, carried out for each month in a year, and the results presented as an exceedance chart.
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