Computer models reconstruct ocean currents

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21.12.2015 07:10
Kategorie: News

 

Researchers map out ocean currents at Cape of Good Hope

'METEOR' in heavy seas off South Africa © A. Biastoch
"METEOR" in heavy seas off
South Africa © A. Biastoch

The Agulhas Current around the Cape of Good Hope in southern Africa plays a key function in the interplay of global ocean currents. By combining several computer models and the observations of oceanographers, an international team of researchers are able to reconstruct the behaviour of the Agulhas Current since 1870 for the first time.

Similar to a global conveyor belt, ocean currents transport heat energy throughout the world's oceans. Part of this circulation is the Gulf Stream system, and this is responsible for the mild climate in northern Europe. However, this “conveyor belt” is not a uniform system – it consists of many individual inter-related components that are constantly changing. A key area is the southern tip of Africa, where the Agulhas Current meets the Atlantic Ocean. Now, the researchers from the GEOMAR Helmholtz Centre for Ocean Research Kiel, together with their colleagues from the US and Great Britain, have succeeded in reconstructing the movements of the ocean currents since 1870.

"Such long-term studies are important to distinguish human-induced changes in ocean currents from natural variation," said Prof. Dr. Arne Biastoch from GEOMAR, lead author of the study, which was published in the Nature Communications journal.

The Agulhas Current is one of the world's most powerful ocean currents. It transports up to 70 million cubic metres of warm, saline water per second to the south, moving along the South African coast in the Indian Ocean. "That's a tremendous amount. By comparison, all the rivers in the world carry along only one million cubic metres of water per second,” explained Professor Biastoch.

At the south of Africa, the oceanic flow bends abruptly and changes direction into the Indian Ocean. As a result, some portions of the water mass detaches and forms large vortices that drift westward and into the Atlantic. These are the “Agulhas Rings” or “Agulhas leakage”. These vortices can be as huge as 200 kilometres in diameter and reach down to more than a kilometre deep. Previous studies have shown them to be an important source of warm, salty water in the Atlantic Ocean.

Snapshot of the temperature (in ° C shading) and the velocities at 250-400 m depth in a high-resolution ocean model. Source: GEOMAR
Snapshot of the temperature (in ° C shading) and the
velocities at 250-400 m depth in a high-resolution
ocean model. Source: GEOMAR

Conducting site surveys is difficult, as the Agulhas Current is dynamic and the eddies are not a permanent feature. As such, there is no long-term data for the Agulhas Current or the Agulhas Rings. At best, it is only the surface temperatures that have been measured since the 19th century, initially using buckets to manually collect water from the sea surface, or now, with the help of satellites. Hence, the researchers have now combined the simulations of the oceans and several atmospheric computer models with existing measurements.

The computational effort of these modern models is so high that even the most powerful supercomputers, including those in Kiel and Stuttgart, run for several months, said Dr Jonathan Durgadoo, co-author of the study.

All the models showed a similar relationship between the temperature of the ocean surface and the amount of warm water entering the Atlantic via the Agulhas Rings. It also showed that the movement of the water masses into the Atlantic is affected by the wind systems over the Southern Ocean.

From other studies, we already know that these systems could be modified with climate change,” said Biastoch. More research is needed to assess the impact these variations may have on the strength of the global “conveyor belt”.

Biastoch added, “This is a computational problem that – once again – we can only solve with the help of supercomputers.

Link to the study: www.nature.com/../ncomms10082.html