Fickle ocean current foils climate modellers

New Scientist: The North Atlantic is stirring fitfully. A new monitoring system has shown that the ocean's currents change rapidly, surging or slowing from one week to the next. That makes it difficult to judge whether they really are slowing down over the long term, as one study has suggested.

An overall slowdown might be bad news for Europe, which is warmed by a current called the North Atlantic Drift, and it might be even worse for the rest of the world because the North Atlantic forms a vital piece of planetary plumbing. When the North Atlantic Drift reaches the Arctic, it cools, sinks and flows back to the south, helping to drive global ocean circulation.

The risk is that climate change will melt ice and increase rainfall around the Arctic, flushing fresh, less dense water into the Arctic Ocean. That could interfere with the crucial sinking process, slowing the current down or even cutting it off. Some have even claimed this could plunge Europe into a new ice age, though most climatologists now dismiss this extreme scenario as just another climate change myth.

Most ocean models predict that a substantial slowdown is unlikely until late this century, but in 2005 a team of oceanographers led by Harry Bryden of Southampton University, UK, announced evidence that the vital "overturning circulation" of the North Atlantic had already slowed by 30%.

However, they conceded that natural variability might explain their data, because the only measurements available were intermittent ones made over the past half century by oceanographers on ships.

RAPID measures

Now for the first time scientists can monitor the ocean continuously, thanks largely to RAPID, an array of instruments strung on cables moored to the seabed. RAPID measures the ocean's pressure profile, which scientists can use to calculate how water is flowing.

An international team, including Bryden, has now taken the first year of data from RAPID and combined it with two other sources: space-based measurements of wind-blown surface currents, and the flow of the gulf stream between Florida and the Bahamas, which is revealed by its electrical effect on submarine telephone cables.

What they see is that the overturning circulation fluctuates wildly, between 35 million tonnes a second and just 4 million tonnes a second. All the earlier measurements lie within that range. "It is now going to be difficult to make robust estimates of overturning based on earlier results," says Stuart Cunningham, also of Southampton University and lead author of the new study. "This defines an unambiguous baseline against which future change may be gauged."

Cunningham still suspects that overturning circulation is slowing down, pointing to an ocean model published last year by Carl Wunsch of the Massachusetts Institute of Technology in which it fell by 14% between 1993 and 2004.

However, finding out for sure will take some time. At present RAPID is funded until 2014, but it may be decades before scientists understand the natural variability of the ocean and identify any effects of human-driven climate change. "We hope to be able to explain all variability eventually," Cunningham told New Scientist.