Agulhas Current (Global Warming)

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 south-westward 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 x 106 m.3/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 (1,200 m.), with a maximum velocity of about 0.2 m./s and carrying about 4 x 106 m.3/s equatorward. 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 degrees C to 22 degrees C) from summer to winter.

Using its flow characteristics, the Agulhas Current can be clearly 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 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 retro-flects, 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 itself 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.

Having formed, most rings dissipate in the mixing cauldron of the Cape Basin northwest of Cape Town. Here, Agulhas rings have to contend with previously-formed rings, as well as Cape Basin cyclones. The few that survive this interaction move northwestward across the South Atlantic Ocean, slowly shedding their anomalous heat, salt, and vorticity to the ambient waters. The inter-ocean fluxes brought about by the ring-shedding process have been estimated to lie between 5 and 20 x 106 m.3/s. By this process and others, the Agulhas Current becomes a profound factor in local weather and in global climate.

It has been demonstrated that the presence of the Agulhas Current enhances coastal rainfall along South Africa’s east coast. Wherever the current is further offshore, this enhancement is reduced. The enormous ocean-to-atmosphere heat fluxes at the Agul-has retroflection and from Agulhas rings and Agulhas eddies; the latter shed across the Subtropical Convergence by the Agulhas Return Current, directly affect the overlying atmosphere and, thus, local weather.

Even more important climatologically, is the role the Agulhas Current plays in the global thermo-haline circulation. The westward heat and salt flux south of Africa is a crucial link in this chain of currents and fluxes. The Atlantic Ocean is the only ocean basin where there is a net heat flux northward across the equator; this has been assumed by some to be the result of the net westward heat flux south of Africa. Modelling studies have shown that a forced decrease in this flux is linearly related to a decrease in the meridional overturning of the whole Atlantic Ocean. Studies of temporal changes of inter-ocean fluxes, such as changes in the bottom sediments, have shown substantial variation in the penetration of Agulhas Current water into the South Atlantic over the past 500,000 years, which has never shut off completely. Furthermore, analyses of bottom sediments have demonstrated conclusively big surges of Agulhas Current water into the South Atlantic at the end of each glacial period.

Understanding significant changes in the inter-ocean exchanges requires a look at how the Indian Ocean controls Agulhas ring shedding. The main factor controlling the horizontal circulation system in the ocean gyres to either side of South Africa is the wind stress pattern. The succession of trade winds and westerlies drives a subtropical ocean gyre system both in the South Indian and Atlantic Oceans. This subtropical circulation is connected south of Africa, largely by the Agulhas rings. Meridional shifts of the wind pattern or variations in their strength lead to variations in this connection. Modelling studies suggest that a large southward shift opens the Agulhas valve and results in a supergyre that flows unobstructed through both oceans. Similarly, the wind patterns shifting northward, reduces the flux from the Indian to Atlantic Ocean. Data from the paleo record indicate that such fluctuations of the inter-ocean exchange have occurred.

Less dramatic variations of the exchange have been observed over the past decades. They seem to be controlled by the tropical and subtropical climate modes over the Indian Ocean. Associated wind variations force anomalies over the central and eastern Indian Ocean that propagate as planetary waves from east to west across the Indian Ocean. On encountering the island of Madagascar, the anomalies interact with the background current system, involving swift boundary currents along the east coast of Madagascar and jets that separate from both the northern and southern tips of the island. This leads to instabilities in the flow, with mesoscale eddies formed both in the Mozambique Channel and south of Madagascar. The resulting eddy trains propagate to the African coast where they either trigger the formation of Natal Pulses or move into the Agulhas retroflection region. In both cases, they seem to regulate the frequency of Agulhas ring shedding. The rate of eddy formation around Madagascar has been observed to vary with the appearance and strength of the Indian Ocean (sub)tropical climate modes. The whole chain of events takes several years to propagate from the tropical and subtropical region to the Agulhas retroflection.

Agulhas rings that reach the South Atlantic Ocean and shed their heat, salt, and vorticity to the ambient waters are part of what makes the Agulhas Current a profound factor in both local weather and the global climate.

Agulhas rings that reach the South Atlantic Ocean and shed their heat, salt, and vorticity to the ambient waters are part of what makes the Agulhas Current a profound factor in both local weather and the global climate.

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