Coherent changes of the circulation in the deep North Atlantic from moored transport arrays

TL;DR

This study uses boundary array measurements at 16°N and 26°N in the North Atlantic to analyze low-frequency deep water density changes and their impact on ocean circulation and meridional overturning circulation (MOC) variability.

Contribution

It provides new insights into the coherent deep circulation changes and their influence on the MOC using in situ boundary measurements at two latitudes.

Findings

Deep waters became fresher and less dense between 2004-2014.

Density changes caused increased shear and altered transport estimates.

Opposing MOC trends depend on the transport constraint method.

Abstract

In situ boundary arrays have been installed in the North Atlantic to measure the large-scale ocean circulation. Here, we use measurements at the western edge of the North Atlantic at $16^\circ$N and $26^\circ$N to investigate low-frequency variations in deep densities and their associated influence on ocean transports. At both latitudes, deep waters (below 1100 dbar) at the western boundary are becoming fresher and less dense. The associated change in geopotential thickness is about $0.15$ $\mbox{m}^2\mbox{s}^{-2}$ between 2004-2009 and 2010-2014, with the shift occurring between 2009-2010 and earlier at $26^\circ$N than $16^\circ$N. Without a similar density change on the east of the Atlantic, a mid-depth reduction in water density at the west drives an increase in the shear between the upper and lower layers of North Atlantic Deep Water of about 2.6 Sv at $26^\circ$N and 3.9 Sv at $16^\circ$N. While these transport anomalies result in an intensifying tendency in the meridional overturning circulation (MOC) estimate at $16^\circ$N, the method of applying a zero net mass transport constraint at $26^\circ$N results in an opposing (reducing) tendency of the MOC.