Atlantic overturning circulation in decline

Major currents in the North Atlantic Ocean seem to be slowing down. The findings show that a recently measured slowdown of 10-15% may be part of larger decline that began in the 1990s and shows no sign of stopping yet. This slowdown could have big impacts on weather in Britain and elsewhere.

A new analysis led by Dr Jon Robson and published in the journal Nature Geoscience suggests that we might expect a further decline in the strength of these Atlantic currents over the coming decade, which could impact the climate of Europe and beyond.

The strength of the Atlantic Meridional Overturning Circulation (AMOC) is thought to be an important regulator of North Atlantic sea surface temperatures which, in turn, have been linked to Northern Europe’s summer rainfall patterns. The recent relatively warm state of the North Atlantic has been implicated in the tendency for wash-out British summers.

But, the evidence that the AMOC is involved with these changes in surface temperature is essentially from indirect sources. This is because we have lacked the direct observations from which to measure the strength of the AMOC. However, since 2004 the UK-US RAPID array has measured a significant decline in the strength of the overturning. But, is this a short-term temporary change, or is this part of a larger, more persistent event?

To answer this question requires putting the RAPID observations in a wider context. One relationship that holds true across most climate and ocean models is that density anomalies in the North Atlantic tend to precede changes in the strength of the AMOC, and the historical observations of density show some remarkable changes, with a dramatic decrease in the density since the mid-1990s.

Using this wider context suggests that the current observed decrease in the AMOC as observed by the RAPID array is not just a temporary blip, but is instead part of a longer term adjustment. Simple calculations suggest that the AMOC could have already reduced by 20% or more. The observations also show no sign that the fall in North Atlantic density has stopped, so a further decrease in the strength of the AMOC should be expected. This could lead to a corresponding relative cooling of the North Atlantic over the next few decades, and a increased tendency for drier summers in Northern Europe.

However, there is still a great deal we do not know about the AMOC, so these large ongoing events in the North Atlantic are an enormously important opportunity to understand and test the theories of AMOC dynamics. Importantly, we may soon have a much better idea of the AMOC’s role in climate.

This entry was posted in Climate, Weather. Bookmark the permalink.