Coastal-trapped waves off the coast of South Africa : Generation, propagation and current structures

Coastal sea level variations from six sites around South Africa are used to establish the characteristics of coastal-trapped wave (CTW) propagation. Substantial amplitudes (> 50 cm) are found along the south coast, but further propagation on the east coast against the Agulhas Current is inhibited. Current measurements over the shelf in this region show barotropic reversals during the passage of the peak of a CTW, with essentially geostrophic flow occurring. Comparisons are made with theoretically calculated first-mode CTW characteristics along various sections of the coast. These calculated speeds fall within the range of speeds determined from observation, though an admixture of a mode 2 CTW is possible. It is found that the speeds of wind systems moving along the coast also fall in the same range, probably leading to a resonance condition, and an explanation for the large CTW amplitudes.