Planetary-type Waves in the South West Indian Ocean

Abstract

SOUTH of the latitude of the Cape of Good Hope, the Agulhas Current Stream (or at least most of it) turns back into the Indian Ocean and forms a zonal current called the Agulhas Return Current, whose mean axis of flow is approximately 38°–39°S. Near the start of its eastward flow, the Agulhas Return Current has been shown to undergo sharp changes of direction (Fig. 1a). These deflections are apparent in the analysis made by Dietrich¹ and Le Pichon². The latter shows them occurring down to the depth of the salinity maximum and draws attention to the important influence of bottom topography. They are also very evident in the study made by Darbyshire³ of data from the cruises of SAS Natal and RS Africana II, occurring in three out of the four seasons examined. The deflections all occur near a prominent topographical feature, the Agulhas Plateau, and their recurrence suggests that they are a quasi-stationary feature and possibly planetary waves caused by the rapid changes in water depth. An investigation into this possibility has been made by comparing the flow patterns deduced from hydrological data with those predicted by theory.