The mixing of surface waters to depths greater than 1,000 metres in the northern North Atlantic Ocean has returned during the winter of 2007/2008, scientists report online this week in Nature Geoscience. This deep mixing, an essential part of the Atlantic Ocean circulation, has been absent for almost a decade, and is an important regulator of both carbon dioxide uptake by the oceans and heat transfer between the ocean and atmosphere. The lack of recent deep mixing in the Labrador Sea had been linked with climate warming, leading to concerns about possible future changes in ocean circulation.
Kjetil V?ge, Robert Pickart and colleagues use data from a network of measuring floats to detect the deep mixing of surface waters. They also evaluate local observations, computer reconstructions of past climate and satellite data to understand the mechanisms that lead to deep convection. They find that a combination of air temperatures in the northern hemisphere, storm pathways, flow of freshwater to the Labrador Sea and the distribution of pack ice cooled the ocean surface, allowing convection to mix surface waters to greater depths.
The authors conclude that the convective system in the North Atlantic Ocean is too complex to allow straightforward predictions of future deep mixing events.