The observed North Atlantic Meridional Overturning Circulation, its Meridional Coherence and Ocean Bottom Pressure

Analyses of meridional transport time series from the RAPID MOC array at 26°N and from Argo float and altimetry data at 41°N reveal that, at semi-annual and longer time scales, the contribution from the western boundary dominates the variability of the North Atlantic Meridional Overturning Circulation (MOC), defined as the transport in the upper 1000 m of the ocean. Since the variability of the western boundary contribution is associated with a geostrophic overturning, it is reected in independent estimates of transports from gradient of ocean bottom pressure (OBP) relative to and below 1000 m on the continental slope of the western boundary at three nominal latitudes (26°N, 39°N and 42.5°N).

Time series of western meridional transports relative to and below 1000 m derived from OBP gradient, or equivalently derived from transport shear profile, exhibit approximately the same phase relationship between 26°N and 39–42.5°N as the western contribution to the geostrophic MOC time series do: the western geostrophic MOC at 41°N precedes the MOC at 26°N by approximately a quarter of an annual cycle, resulting in a zero correlation at this time scale. This study therefore demonstrates how OBP gradients on basin boundaries can be used to monitor the MOC and its meridional coherence.