Decadal variability in North Atlantic Deep Water transport measured by GRACE satellites and MOVE moorings

Decadal variability in ocean mass data from the GRACE (Gravity Recovery and Climate Experiment) satellite mission implies large-scale changes to the deep circulation in the North Atlantic, and is of interest for studying transport variability in the Atlantic Meridional Overturning Circulation (AMOC). However, trends in the GRACE data on these time scales are typically considered to be unreliable due to uncertainty in the corrections necessary to convert the raw measurements to the final ocean mass product. Here, we present two findings suggesting that decadal signals in GRACE reflect real oceanic variability rather than errors in the processing, and can therefore be used to study changes to the deep circulation: Near-decadal trends agree well with an independent measure of ocean mass from moorings and satellite altimetry, with a root mean square difference between the two methods at five sites of 5.5mm/decade, and spatial patterns in the GRACE trend are comparable to mean property distributions of North Atantic Deep Water (NADW).

Deep ocean circulation changes from 2002-2009 to 2010-2017 measured by GRACE show a large-scale anticyclonic anomaly between 20N and 40N. The change is linked to a strengthening of NADW transport in the Gulf Stream's recirculation gyres by about 7Sv, suggesting an increased role for the interior ocean relative to the Deep Western Boundary Current for its southward advection.

We further use GRACE data to obtain a reference level velocity for transports calculated from the MOVE (Meridional Overturning Variability Experiment) moorings at 16N, which are designed to observe the lower branch of the AMOC. Referencing to GRACE provides a measure of absolute transport variability of NADW through the section, and supports a strengthening of the transport from 2004-2014 inferred from the moorings alone. We also compare transports calculated in the same fashion at 26N to those measured as part of the RAPID-Meridional Overturning Circulation and Heatflux Array (RAPID/MOCHA) project, and discuss possible causes for differences between the two methods.