Decadal Strengthening of Interior Flow of North Atlantic Deep Water Observed by GRACE Satellites

The Gravity Recovery and Climate Experiment (GRACE) satellite mission provides information on changes to the Earth's gravity field, including ocean mass. Long-term trends in GRACE data are often considered unreliable due to uncertainties in the corrections made to calculate ocean mass from the raw measurements. Here, we use an independent estimate of ocean mass from satellite altimetry and in situ density data from five mooring sites and repeat hydrography to validate trends in GRACE over the North Atlantic, finding substantial agreement between the methods. The root-mean-square difference between ocean mass changes calculated with this method from the mooring data and those measured by GRACE is 3.5 mm per decade, much lower than the mean signal of 15.6 +/- 1.8 mm per decade for GRACE and 17.8 +/- 5.2 mm per decade for the altimetry-mooring estimate. The GRACE ocean mass data are then used to study the change in the deep circulation of the North Atlantic between the 1 April 2002 to 31 March 2009 and 1 April 2010 to 31 March 2017 periods, revealing a large-scale anticyclonic circulation anomaly off the North American coast. The change is associated with an increase of 13.9 +/- 3.3 Sv (1 Sv = 10(6) m(3) s(-1)) of southward North Atlantic Deep Water flow in the interior between 30 degrees N and 40 degrees N, largely balanced by a northward anomaly of 10.7 +/- 3.3 Sv for the boundary circulation. This implies an increased importance of interior pathways compared to the Deep Western Boundary Current for the spreading of North Atlantic Deep Water, which constitutes the lower limb of the Atlantic Meridional Overturning Circulation.