Current Clash
Over the past few decades anthropogenic climate change has been happening in the Southern Hemisphere.
There has been a strengthening of the circumpolar westerly winds and a weakening of the mid-latitude westerlies all the way from the stratosphere down to the Earth’s surface. This time however, greenhouse gasses are not considered to be to blame. The changes are thought to be due to effects of depletion of ozone over the Antarctic. In 2006 Wenju Cai of CSIRO, Australia, suggested that the surface wind changes resulted a southward movement and spin-up of the so-called super gyre, which links the South Pacific, Indian and Atlantic Oceans. The result is that more warm water is moving southward. This effect may be responsible for the unusually large warming seen in the southern mid-latitude oceans and also have the effect of allowing oceanic biota to migrate southward.
The movement of warm and salty water from the Indian Ocean to the Atlantic is called the Agulhas leakage. It is the only direct link between the Indian and Atlantic Oceans. South of Africa, large swirling water rings are produced from the retroflecting Agulhas Current, which traverse into the South Atlantic and then slowly decay releasing the water trapped inside.
The input of warm and relatively salty Indian Ocean water into the Atlantic can result in a considerable impact on the thermal and freshwater balance of the Atlantic Ocean. Model simulations suggest it may even influence the strength of the Atlantic overturning circulation, and hence the stability of the global climate system. It has also been suggested that the Agulhas Leakage has varied on glacial-interglacial time scales, with reduced leakage in glacial periods, as suggested by paleoceanographic records.
Knowledge of the Agulas leakage is required for forecasting future global climate trends. A study just published in Nature by Arne Bliastoch et al use a model to show that the Agulhas leakage has increased over the past few decades in concert with the other effects noted. They say it has contributed to the observed salinification of the South Atlantic thermocline waters. They add that there is also evidence from model and historical measurements that the Indian Ocean waters have begun to affect the North Atlantic with implications for the future of the thermohaline overturning circulation and its substantial climatic effects on northern Europe in particular.
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