Using Density Changes over Continental Slopes to Estimate Atlantic Meridional Overturning Circulation Variability on Inter-annual to Decadal Timescales

The Atlantic meridional overturning circulation (AMOC) is an important element of the global climate system. A number of studies have sought a link between sea level and AMOC variability as a complement to temporally and spatially limited in-situ AMOC monitoring. Coastal sea level is of particular interest as it offers the promise of a relatively inexpensive method of monitoring the AMOC, and a tide gauge based reconstruction of past variability extending back over more than a century. Yet, there is contradictory evidence as to the degree to which AMOC variability influences coastal sea level, calling into question the veracity of coastal sea level-based AMOC proxies. Through geostrophic balance, a more direct link must hold between ocean bottom pressure on the Atlantic’s continental slopes and the zonally integrated meridional transport, with pressure changes on the western boundary appearing from modelling studies to be dominant. However, the tendency of bottom pressure recorders to drift over time renders them unsuitable for measuring changes in bottom pressure associated with inter-annual to decadal changes in AMOC strength. As a potential alternative, here we show, in the context of a data-assimilating ocean model, how density observations alone can be used to determine across-slope pressure gradients associated with inter-annual to decadal fluctuations of the AMOC. Thus, according to the model, it may be possible to estimate long-term AMOC variability using only density over the continental slopes. In the North Atlantic, AMOC fluctuations can be determined from density changes over only the western boundary slope, confirming the dominance of this boundary found in previous studies, while in the South Atlantic density changes over both the eastern and the western boundaries is required. Finally the approach is used to estimate of past AMOC variability from observed density fields and this is compared with a tide gauge-based reconstruction. This comparison is used to shed light on the contradictory evidence regarding the relationship between coastal sea level and the AMOC.