Low-frequency sea-level fluctuations along the coasts of Namibia and South Africa

Abstract

Summary. Sea-level records at nine ports along the coasts of Namibia and South Africa are used to establish the existence of coastal trapped disturbances in sea-level as a response to the passage of synoptic weather systems. Using spectral analysis the characteristics and spatial variability of the sea-level fluctuations are identified. The results of cross-correlation analyses performed on sea-level data at adjacent ports for two periods during 1982 are discussed in detail to examine the propagation of coastal trapped waves round the coast. Sea-level records from coastal areas in southern Africa exhibit extra-tidal fluctuations on several different time-scales associated with local or remotely forced atmospheric phenomena. These scales include the synoptic or event scale, of period 2-20 day, the seasonal scale, of period 3-6 month, and the interannual or long-term scale, with a period of a few years. The seasonal and long-term components of sea-level variability along the west coast of southern Africa have been discussed by Brundrit (1984). He concluded that there is a clear difference between the seasonal response at the northern and southern ports associated with the difference in winds affecting these two regions. The long-term component was found to be similar at all ports suggesting large-scale, non-local forcing. The theoretical response of sea-level to pressure changes at the low frequencies associated with the passage of weather systems is that of an inverted baromei.er (Robinson 1964). Records should show a decrease in sea-level of 1.01 cm for an increase in atmospheric pressure of 1 mb. This assumes an ocean such that the total pressure at any fxed point on the bottom is constant. In coastal areas, over continental shelves, this is rarely valid. Changes in atmospheric pressure are accompanied by changes in the velocity and direction of the wind and hence wind stress and these also contribute to fluctuations in sea-level. Robinson