The overturning circulation in the Atlantic ocean does not consist of a single vertical circulation cell, as previously thought, but varies in strength between low and high latitudes, shows a study online this week in Nature Geoscience. The Atlantic overturning circulation transports heat from tropical waters northwards and has been projected to slow down as the climate warms.

Susan Lozier and colleagues investigated changes in the properties of the Atlantic ocean over the second half of the twentieth century using ship-based observations along with a numerical model. They found that the Atlantic overturning circulation has slightly weakened in the subtropics over this period, but has become slightly stronger further north, in the subpolar region.

The authors conclude that the Atlantic overturning circulation cannot be viewed as a single loop current, and suggest that the observed changes probably reflect climate variability and not a consistent climate trend.

“Lozier et al. provide clear evidence for latitude-dependent changes in the Atlantic meridional overturning circulation”, says Agatha M. de Boer in an accompanying News & Views article.