Surface estimates of the Atlantic overturning in density space in an eddy-permitting ocean model

Grist, Jeremy P., Josey, Simon A. and Marsh, Robert (2012) Surface estimates of the Atlantic overturning in density space in an eddy-permitting ocean model. Journal of Geophysical Research, 117 (C6), C06012. (doi:10.1029/2011JC007752).

Abstract

A method to estimate the variability of the Atlantic meridional overturning circulation (AMOC) from surface observations is investigated using an eddy-permitting ocean-only model (ORCA-025). The approach is based on the estimate of dense water formation from surface density fluxes. Analysis using 78 years of two repeat forcing model runs reveals that the surface forcing–based estimate accounts for over 60% of the interannual AMOC variability in ?0 coordinates between 37°N and 51°N. The analysis provides correlations between surface-forced and actual overturning that exceed those obtained in an earlier analysis of a coarser resolution-coupled model. Our results indicate that, in accordance with theoretical considerations behind the method, it provides a better estimate of the overturning in density coordinates than in z coordinates in subpolar latitudes. By considering shorter segments of the model run, it is shown that correlations are particularly enhanced by the method's ability to capture large decadal scale AMOC fluctuations. The inclusion of the anomalous Ekman transport increases the amount of variance explained by an average 16% throughout the North Atlantic and provides the greatest potential for estimating the variability of the AMOC in density space between 33°N and 54°N. In that latitude range, 70–84% of the variance is explained and the root-mean-square difference is less than 1 Sv when the full run is considered.

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