Observed decrease in Deep Western Boundary Current transport in subpolar North Atlantic

Abstract

The lower limb of the Atlantic Meridional Overturning Circulation is an important feature of Earth’s climate system as it returns recently ventilated water to the deep ocean and is a major sink for anthropogenic carbon. The Deep Western Boundary Current—the primary component of the lower limb—flows southwards along the eastern flank of Greenland transporting dense water formed in the Nordic seas. Since 2014, the Deep Western Boundary Current has been continuously monitored at this location from a mooring array to observe the current’s velocity and hydrographic structure close to its source. Here we find that the Deep Western Boundary Current transport has decreased by 26% over the first six years of observations, due to (1) a thinning of the traditionally defined Deep Western Boundary Current layer (σθ > 27.8 kg m−3) from a known freshening signal propagating through the subpolar region (56%), and (2) weakening velocities (44%). Despite this decrease, the Atlantic Meridional Overturning Circulation has remained relatively steady over the same period. Ultimately, this difference is due to the methods used to define these two circulations. Finding such notably different trends for two seemingly dependent circulations raises the question of how to best define these transports. The amount of water transported southwards by the Deep Western Boundary Current shows a 26% decline since 2014 in mooring array monitoring data, despite the strength of the Atlantic Meridional Overturning Circulation remaining stable.