'Neglected' current could keep Europe warm

By Richard Black
Environment correspondent, BBC News

  • Published

Changes to a "neglected" ocean current near the southern tip of Africa could keep Europe warm even if the Gulf Stream switches off, scientists say.

Warm water in the Agulhas Current flows from the Indian Ocean into the Atlantic where it brings changes further north.

Researchers say this could compensate if the main northwards flow of heat, carried by the Gulf Stream, drops.

Writing in the journal Nature, they say this effect has largely been overlooked as a factor affecting climate change.

The Agulhas Current flows southwards down the eastern coast of Africa.

When it reaches the continent's southern tip - Cape Agulhas - most of the water swings eastward and back into the Indian Ocean.

But some of it forms giant eddies and rings, up to 300km (200 miles) across and extending from the top of the ocean to the bottom, that go in the other direction - rounding the cape and flowing into the Atlantic.

This bit is known as the Agulhas Leakage.

Exactly how much water travels in this direction is not known, and is thought to vary markedly from year to year.

But this team of scientists - drawn from the US and Europe - say wind shifts further south make it likely that leakage is increasing.

"This prediction comes from a [computer] model - the leakage itself is very difficult to measure because it happens over a wide corridor of ocean and because of its eddying nature," said Lisa Beal from the Rosenstiel School of Marine and Atmospheric Science in Miami, US.

She told BBC News that research on the current had been sparse mainly because it is remote from Western research centres, making studies expensive and difficult.

Salty whirls

Once in the Atlantic, the warm and salty Agulhas water acts to strengthen the main current system, the Atlantic Meridional Overturning Circulation (AMOC).

Image caption,
A weakening Gulf Stream would bring colder weather to the shores of Western Europe

Part of this circulation is the Gulf Stream, which brings hot water northwards, keeping parts of Western Europe and eastern North America several degrees Celsius warmer than they would otherwise be.

Thanks largely to the 2004 movie The Day After Tomorrow, the possibility that this would "switch off" in a warmer world is one of the best-known potential climate change impacts, even though there is a lot of uncertainty about whether it will happen.

The Intergovernmental Panel on Climate Change (IPCC) said in its landmark 2007 report: "Most [computer] models suggest increased greenhouse gas concentrations will lead to a weakening of the [A]MOC, which will act to reduce the warming in Europe".

But the scientists behind the Nature article say an increase in Agulhas Leakage could compensate.

"This could mean that current IPCC model predictions for the next century are wrong and there will be no cooling in the North Atlantic to partially offset the effects of global climate change over North America and Europe," said Dr Beal.

"Instead, increasing Agulhas Leakage could stabilise the oceanic heat transport carried by the Atlantic overturning circulation."

Unsteady eddies

Analysis of sediments shows the Agulhas Leakage has varied hugely in the past, notably at transitions between Ice Ages and the warm periods in between.

Its modern behaviour is being studied by satellites and by instruments in the sea; but still, the record is short and much clearly remains to be discovered.

Further north in the Atlantic, UK scientists said last year that the amount of water being carried in the AMOC varies, naturally, by almost a factor of 10; so discerning a long-term trend becomes very difficult.

Image caption,
Autonomous underwater gliders are among the tools gathering data on Atlantic currents

The same appears to be true with the Agulhas circulation.

"The main thing this shows to me is how complicated the region is; it's a very complex situation," said Harry Bryden from the UK's National Oceanography Centre in Southampton.

"How to find out how much of this water goes into the South Atlantic and stays there is the critical question - and as a research scientist, I'm not sure how you would go about it."

Professor Bryden is part of a UK-US project deploying measuring equipment in the northern Atlantic in a bid to understand AMOC behaviour and variability.

Last year, Lisa Beal's group put an array into the Agulhas Current that may provide some answers.

If better measurements are one aim of scientists in the field, better computer models are another, with existing global models not able to replicate the circular eddies typical of the Agulhas system.

In the long term, putting all of this together should lead to much better understanding of how the AMOC behaves - in particular, whether it can shut down stably for long periods, and what that would mean for Europe.

"If you think about evaporation over the Atlantic, the ocean is clearly losing water, so the circulation system brings new water in to balance that," said Richard Wood, a climate scientist from the UK Met Office who studies ocean currents.

"There are two pathways - warm, salty water from the Indian Ocean and colder, fresher water from the Drake Passage [between the southern tip of South America and Antarctica].

"And there's pretty strong evidence that the balance between those pathways indicates whether or not the Gulf Stream is safe".

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