Comparison of AMOC variability in ocean reanalysis products back to 1960

AMOC variability has been invoked as a significant driver for ocean heat content change in the subpolar North Atlantic. One of the important outcomes of the various analyses of the CMIP5 suite of decadal prediction experiments was the development of a mechanistic paradigm for the role of low-frequency AMOC variability in preconditioning the ocean state for the skillful prediction of upper ocean heat content (e.g. Yeager et al. 2012; Robson et al. 2012; Msadek et al. 2014). However, the low-frequency variability of AMOC in the ocean state estimates used for initialization of these models is dramatically different.

We show the mean and variability of the Atlantic Meridional Overturning Circulation (AMOC), as represented in six ocean reanalysis products from 1960-2007. Particular focus is on multi-decadal trends and interannual variability at 26.5degreeN and 45degreeN. Reference simulations obtained from the same models and forcing datasets but without the imposition of subsurface data constraints are also included for comparison. The AMOC as simulated in these two sets are presented in the context of results from the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II), wherein a common interannually varying atmospheric forcing data set was used to force a large and diverse set of global ocean-ice models.

Relative to the reference simulations and CORE-II forced model simulations we show that:

** The reanalysis products tend to have greater AMOC mean strength [in better agreement with RAPID estimates at 26.5degreeN];

** The reanalysis products have enhanced multi-decadal trends and inter-annual variance

** The reanalysis products are inconsistent in their representation of the year-to-year changes in AMOC