Interannual Variability of Global Overturning Circulation Dominated by Pacific Variability

The view prevails that the global meridional overturning circulation (GMOC) is primarily reflective of the Atlantic meridional overturning circulation (AMOC). While this is true in the time mean, this is likely not true when considering interannual variability. This paper shows that interannual variability of the GMOC in nine coupled climate models and one ocean reanalysis is heavily dominated by variability in the Pacific meridional overturning circulation (PMOC). This is apparent not just near the surface, but down to depths exceeding 4km. Furthermore, compared with AMOC variations, interannual PMOC variations are more strongly correlated with interannual variations of global oceanic meridional heat transport. PMOC variability has large scale organization. For example, in the Canadian Earth System Model version 2 (CanESM2), the first EOF of the PMOC streamfunction between 0-20N is an overturning cell that explains 78% of the total variability. Idealized experiments with CanESM2 show that PMOC variations are almost entirely driven by interannual variations in surface wind stress. The dominance of interannual PMOC variability is partially expected from linear theories which predict that off-equatorial baroclinic Rossby waves take more than a year to propagate across the Pacific basin, whereas such disturbances cross the Atlantic basin in less than a year. Analysis of meridional velocity anomalies suggests that these waves are excited by variations in the deep boundary currents, and so PMOC variations are not constrained to remain above the thermocline.