Observations of Trapped Waves in the Atmosphere and Ocean Along the Coast of Southern Africa

Abstract

The coastally trapped low pressure cell and associated shelf wave which travel eastward along the southern edge of the African continent are studied, Composite analysis suggests a continuous wave-like propagation in sea levels in phase with a sharp reversal in alongshore winds set up by transient coastal lows. The shelf wave amplitude grows and decays during its lifespan and gradually slows on the east coast to fall out of phase with its atmosphen ccounterpart. The time-height evolution of the coastal low is examined for a number of cases. Offshore flow a head of the system is found to be an important forcing mechanism, while the symmetric adjustment of an inversion layer, the east/west wind switch below it, and a continuous propagation a re symptoms consistent with Kelvin wave theory. Stratified and sheared conditions in the pre-low phase are replaced by a well mixed boundary layer following coastal low passage. Composite and case study analyses confirm a geographically anchored acceleration of the coastal low, and its synoptic forcing over the Agulhas Current.