Slowing of the Atlantic meridional overturning circulation at 25° N

Atlantic Ocean trends The circulation across the 25° N latitude line in the Atlantic Ocean between Africa and the Bahamas has become the benchmark for estimating the Atlantic meridional overturning circulation, an important component of global ocean transport that carries warm upper waters into far northern latitudes via the Gulf Stream and returns cold deep waters south across the Equator. Its heat transport contributes to the moderate climate of maritime and continental Europe. A new hydrographic section across 25° N was taken in 2004, and comparison with measurements from 1957, 1981, 1992 and 1998 reveals a slowing of almost a third between 1957 and 2004. This means that more Gulf Stream waters are now recirculating southwards at mid-ocean depths, and that southward transport of cold lower North Atlantic Deep Water has halved. Some climate models suggest that the anthropogenic increase in atmospheric CO 2 will result in a slowdown of the Atlantic overturning circulation, so this latest finding will add fuel to the debate on climate change. The Atlantic meridional overturning circulation carries warm upper waters into far-northern latitudes and returns cold deep waters southward across the Equator 1 . Its heat transport makes a substantial contribution to the moderate climate of maritime and continental Europe, and any slowdown in the overturning circulation would have profound implications for climate change. A transatlantic section along latitude 25° N has been used as a baseline for estimating the overturning circulation and associated heat transport 2 , 3 , 4 . Here we analyse a new 25° N transatlantic section and compare it with four previous sections taken over the past five decades. The comparison suggests that the Atlantic meridional overturning circulation has slowed by about 30 per cent between 1957 and 2004. Whereas the northward transport in the Gulf Stream across 25° N has remained nearly constant, the slowing is evident both in a 50 per cent larger southward-moving mid-ocean recirculation of thermocline waters, and also in a 50 per cent decrease in the southward transport of lower North Atlantic Deep Water between 3,000 and 5,000 m in depth. In 2004, more of the northward Gulf Stream flow was recirculating back southward in the thermocline within the subtropical gyre, and less was returning southward at depth.