Meeting Documents

Reconstructing the Atlantic Meridional Overturning Circulation at Mid-Latitudes Using Indirect Observations

Abstract

Since 2004 the Atlantic meridional overturning circulation (AMOC) has been observed at 26°N by the RAPID project, and in 2014 it was joined by the Overturning in the Sub-Polar North Atlantic Programme (OSNAP) at around 58°N. These two array-based programmes have provided many insights into the AMOC and its variability over a range of timescales, however there is little coherence between the two time series, not helped by OSNAP's relatively short duration.

The MEZCAL (Methods for Extending the Horizontal Coverage of the AMOC Latitudinally and retrospectively) project seeks to leverage indirect observations and existing reconstruction methods to create an AMOC framework covering the North Atlantic between these two existing observing systems. The primary reconstruction method was developed at 41°N, a latitude between the subpolar and subtropical gyres, and combines data from Argo floats and satellite altimetry to estimate the AMOC from 2002 onwards.

Here we examine how the assumptions of this method hold as we move northwards and southwards from 41°N into the more active gyre regions. We evaluate the resulting AMOC estimates against the ECCO v4r4 state estimate, higher resolution ocean reanalysis products, and other observation-based AMOC time series; and how well the method captures different ocean processes over varying timescales. We also show at which latitude it becomes important to define the AMOC in density-space rather than depth-space, and how the reconstructions are affected by features like the Gulf Stream extension.

The increased spatial coverage these reconstructions provide show AMOC changes and variability at a greater range of latitudes, give insight into the connectivity and coherence between latitudes, and provide a comparison basis for numerical models. The increased temporal coverage increases the likelihood of identifying climate-driven low frequency changes to the Atlantic meridional overturning circulation.