Understanding Atlantic Multidecadal Variability and associated decadal predictability

Abstract:

Atlantic Multidecadal Variability (AMV) has many important regional and global-scale climate impacts. Different mechanisms of AMV would have very different implications for future predictions of AMV-related climate impacts. The correlation/regression between net surface heat flux and SST anomalies at low frequency have been identified as key indicators for the relative roles of oceanic vs. atmospheric forcing in SST anomalies, and they suggest that the oceanic forcing has a dominant role in the low frequency subpolar North Atlantic SST anomalies associated with AMV. The observed decadal persistence of the subpolar North Atlantic SST anomalies associated with AMV will lead to much higher Atlantic decadal predictability than that obtained from the slab ocean models or the red noise process. Multidecadal Atlantic Meridional Overturning Circulation (AMOC) variability is a major source of enhanced low-frequency variability and thus decadal persistence in the subpolar North Atlantic SST anomalies associated with AMV. Observational analyses reveal high coherence among subpolar North Atlantic SST, sea surface salinity (SSS), upper ocean heat and salt content, and the AMOC fingerprint at low frequency. This key AMV feature cannot be explained as responses forced by atmospheric white noise, but are consistent with the ocean dynamics mechanism (e.g. low frequency AMOC variability). One key mechanism for enhanced decadal predictability of the subpolar North Atlantic upper ocean heat content is the slow southward propagation of AMOC anomalies from northern high latitudes to the subtropics and associated anomalous Atlantic meridional heat transport convergence. Atlantic decadal predictability is much higher in fully coupled models with relatively stronger multidecadal AMOC variability than in fully coupled models with relatively weaker multidecadal AMOC variability, and almost disappears in models coupled with a slab ocean (i.e. slab ocean models) with no AMOC variability. Multidecadal AMOC variability also has predictive impacts on Atlantic major hurricane frequency associated with AMV. The simulated AMOC-AMV linkages, and associated decadal predictability and climate impacts in CMIP5 models could be substantially hampered due to the underestimation of multidecadal AMOC variability.