Sources of the Agulhas current in the spring of 1964

Abstract

Charts of water properties and acceleration potentials on selected sigma- t surfaces, constructed of data from quasi-synoptic cruises in the southwest Indian Ocean, showed the waters of the Agulhas Current off Durban were composed of two main types. One is derived from the Mo¢ambique Current; the other comes from three sources, the South Equatorial Current, Agulhas Return Current Water recycled west of the Madagascar Ridge, and water recycled in a local vortex. These two main types formed a deep front extending from between Durban and Port St. Johns to the northern end of the Mo¢ambique Channel. Mixing took place in anticyclonic vortices along this front, but it was incomplete and large isolated pockets of water contributed to the complexity of the system. Three of the vortices were of the order of 200 miles in diameter and transported 10–20 × 10 6m 3/sec. The Witte-Margules equation was used to calculate the transport distribution with sigma- t for certain key sections. These were used to compare the predicted transport (the sum of the tributaries), with those observed in the section. Transport calculations showed that along two meridional lines east of Madagascar (occupied earlier in 1964) there was an intensification of flow of the South Equatorial Current which between 20°S–23°S in 55°E amounted to nearly 40 × 10 6 m 3/sec. That portion of the South Equatorial Current which was inferred to pass south of Madagascar constituted between a third and a half of the Agulhas Current's 67 × 10 6m 3/sec. The remainder was contributed by, (a) the Mo¢ambique Current flowing through the Narrows of the Channel—10 7 m 3/sec, (b) local recycling in a vortex off Durban—12 × 10 6m 3/sec, and (c) recycled Agulhas Return Current 15 × 10 6m 3/sec. Because of the inadequacy of the station coverage, the shortcomings of the computational method, and the difficulty of resolving currents from vortices, these estimates are indicative rather than definitive.