Agulhas Current

The Agulhas Current is the major western boundary current of the Southern Hemisphere. It completes the anti-cyclonic gyre of the South Indian Ocean, and because the African continent terminates at a relatively modest latitude, it becomes a mechanism for the climatologically important inter-ocean exchange between the Indian and Atlantic oceans. The southwestward flowing Agulhas Current only becomes fully constituted along the east coast of southern Africa at a latitude somewhere between Durban (South Africa) and Maputo (Mozambique). It increases in speed and volume flux downstream. On average, its volume flux is 70 × 106 m3/s, with only small temporal changes. Its depth, by contrast, can vary from 6561 ft. (2,000 m) to the sea floor at 9,842 ft. (3,000 m) over a period of months. It is underlain by an opposing undercurrent at a depth of 3,937 ft. (1,200 m), with a maximum velocity of about 0.2 m/s and carrying about 4 × 106 m3/s equator-ward. An offshore profile of the surface speed of the current shows a peak of about 1.5 m/s close inshore, slowly tapering off to about 0.2 m/s at a distance of roughly 62 mi. (100 km) offshore. The temperature of its surface waters is about 11 degrees F (6 degrees C) higher than ambient waters and decreases from 80 to 71 degrees F (27 to 22 degrees C) from summer to winter.

Using its flow characteristics, the Agulhas Current can be divided into northern and a southern parts. The northern part follows the continental shelf edge very closely, meandering less than 9 mi. (15 km) to either side. Downstream of Port Elizabeth, in its southern part, it starts meandering with increasing distances to either side of its mean trajectory, producing large shear edge eddies and plumes on its landward side in the process. The path stability of the northern Agulhas Current is interrupted at irregular intervals by a major perturbation, called the Natal Pulse, which is triggered [Page 21]just upstream of Durban. Triggering is thought to come about when offshore eddies interact with the current. This singular meander grows as it travels downstream at a rate of about 12 mi. (20 km) per day. It has an embedded cyclone on its landward side. Natal Pulses have been perceived to play an important role in inter-ocean exchange.

Once the southern Agulhas Current overshoots the tip of the continental shelf south of Africa, it retroflects, with most of its water heading zonally eastward as the Agulhas Return Current, more or less parallel to the Subtropical Convergence. The Agulhas Return Current is largely steered by the bathymetry and exhibits large meridional meanders. The retroflection loop is unstable, and at irregular intervals it occludes, forming a large ring 155 mi. (or 250 km diameter) of warm Agulhas Current water. These rings may extend to the sea floor. After such an event, the newly formed retroflection loop starts prograding into the South Atlantic Ocean once more. There is evidence that nearly all ring shedding events are set off by the arrival of a Natal Pulse from farther upstream.

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