Some good news in the world of climate research: the Agulhas Current off the coast of South Africa, is said to stimulate North-South ocean circulation in the Atlantic. This "conveyor belt", which redistributes and controls heat around the globe, is threatening to slow down due to melting ice. As has been shown in a recent study however, published in Nature Climate Change (1), based on satellite altimeter measurements, this famous current is accelerating.
Located at the interface between the Indian and Atlantic Oceans, this current multiplies injections of warm and, above all, highly salt water into the Atlantic from the Indian Ocean. The phenomenon, also caused by global warming, could in return be offsetting the effects of glacial melting on thermohaline circulation(2) and the global climate. More locally, it will considerably change the climate in southern Africa.
Melting of Arctic ice due to global warming could slow down worldwide ocean currents*, responsible for controlling the global climate of the Earth. This destabilising effect could be affected by the famous Agulhas Current off the coast of South Africa, as shown by a research team from the IRD and their South-African partners in the journal Nature Climate Change.
The Agulhas Current is located at the interface between the Indian and Atlantic Oceans and is a key element in the planet's "weather machine." Generated by winds above the Indian Ocean, it originates in the Mozambique Channel, between Madagascar and Africa. It runs along the South-African coasts, before spectacularly turning back on itself off the Cape of Good Hope. This phenomenon is known as the "Agulhas Retroflection." However, part of the flow does not return to the Indian Ocean, but escapes to the west. This "Agulhas leak" injects warm and very salty water from the Indian Ocean discontinuously into the Atlantic. The surface water moves in the form of gigantic eddies that can reach up to 500 km in diameter, some of the largest in the world, known as the "Agulhas eddies." They feed worldwide thermohaline(2) circulation and the arrival of warm, salty water from the north of Europe.
The current is accelerating
Using satellite observations of sea surface topography measured from 1993 to 2009, scientists have revealed that the Agulhas Current has been accelerating over the last 17 years. This confirms the results of previous work carried out using digital simulations combined with other satellite observations(3) that have modelled this intensification since the 1980's.
As shown in this new study, the acceleration is due to changes in the winds above the Indian Ocean. The legendary westerly winds of the "roaring forties"(4) and trade winds in the region are strengthening due to global warming. The rise in surface water temperatures and increased rainfall at the equator in the Indian Ocean are accentuating atmospheric circulation above this ocean. Stronger winds are thus reinforcing the Agulhas current.
Ocean circulation maintained
The increased power of the current amplifies the "Agulhas leak" and increases the number and intensity of ocean eddies caused in the Atlantic. The flow of saltier water towards the north is thus helping to maintain worldwide thermohaline circulation. The phenomenon could be partly offsetting the effects of melt ice, and ultimately help to regulate global warming.
In the past, the Agulhas current has already proven its influence on world climate. Paleo-oceanographic research has demonstrated its major role in the extent and duration of glacial periods.
What consequences for fishing?
The vast ocean eddies also play a role in the productivity of the seas. They structure the whole food chain, from plankton to birds, including fish and crustaceans. Their intensification could also have an impact on fishing resources south of Africa. The MESOBIO(5) project, conducted by the IRD and its South-African partners, aims at studying their influence on fishing in the Mozambique Channel, important fishing grounds for tuna and swordfish in particular. The programme is attempting to describe the effect of the eddies via a multidisciplinary approach, from physics and modelling to sea observation and the management of exploited resources.
The acceleration of the Agulhas current observed by the research team, itself due to anthropogenic global change, in return has a stabilising effect on the planet's climate. This is good news for the future of the planet and its inhabitants, faced with irreversible global warming.
(1) This work was conducted by scientists at the University of Cape Town and the Nansen/Tutu Center in South Africa, in partnership with IRD scientists and the mixed international IRD laboratory ICEMASA.
(2) Very slow circulation of the ocean on a global scale linked to differences in temperature and salinity between water masses. At the higher latitudes, particularly in the North Atlantic, denser cold and salty water drops to the ocean floor and feeds this circulation.
(3) Rouault M., Penven Pierrick, Pohl B., Warming in the Agulhas Current system since the 1980's, Geophysical Research Letters, 2009, 36, L12602.
(4) Sailors have given this name to the latitudes located between the 40th and 50th southern parallels, due to the strongly westerly winds that blow there.
(5) Project "Influence of meso-scale dynamics on biological productivity at various trophic levels in the Mozambique Channel" in partnership with the universities of La Réunion, Cape Town and Rhodes, the DEA/Oceans and Coasts and the Bayworld Centre for Research and Education in South Africa the Institut d'Halieutique et des Sciences Marines in Madagascar and the Instituto de Investigação Pesquira in Mozambique.
* According to a "disaster" climate scenario, desalination of the North Atlantic due to melting of the Arctic icecap is reducing the density of ocean surface water. In a few decades, it will no longer be able to feed worldwide thermohaline circulation.
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