Mechanisms of Internal Atlantic Multidecadal Variability in HadGEM3-GC3.1 at Two Different Resolutions

Abstract

This study broadly characterizes and compares the key processes governing internal Atlantic multidecadal variability (AMV) in two resolutions of HadGEM3-GC3.1: N216ORCA025, corresponding to ∼60 km in the atmosphere and 0.25° in the ocean, and N96ORCA1 (∼135 km in the atmosphere and 1° in the ocean). Both models simulate AMV with a time scale of 60–80 years, which is related to low-frequency ocean and atmosphere circulation changes. In both models, ocean heat transport convergence dominates polar and subpolar AMV, whereas surface heat fluxes associated with cloud changes drive subtropical AMV. However, details of the ocean circulation changes differ between the models. In N216 subpolar subsurface density anomalies propagate into the subtropics along the western boundary, consistent with the more coherent circulation changes and widespread development of SST anomalies. In contrast, N96 subsurface density anomalies persist in the subpolar latitudes for longer, so circulation anomalies and the development of SST anomalies are more centered there. The drivers of subsurface density anomalies also differ between models. In N216, the NAO is the dominant driver, while upper-ocean salinity-controlled density anomalies that originate from the Arctic appear to be the dominant driver in N96. These results further highlight that internal AMV mechanisms are model dependent and motivate further work to better understand and constrain the differences.