Multi-centennial modulation of Atlantic multi-decadal variability in a 2000-year climate integration

Abstract

We explore the amplitude and frequency of Atlantic Multi-decadal Variability (AMV) in a 2,000-year pre-industrial control simulation with the FOCI-OpenIFS coupled climate model. We find a statistically significant AMV-like mode on the 20-year and 80-year time scales. We also find a mode of multi-centennial variability where the North Atlantic Ocean shifts a regime of a warm period to/from a cold period of ~400 years. The warm period is characterised by mean states of a stronger and deeper Atlantic Meridional Overturning Circulation (AMOC), less Arctic sea ice, and more deep convection in the Labrador Sea than the cold period.  We find that the AMV has a much higher amplitude in the cold period compared to the warm period, and also that the lead-lag relationship between the AMOC and the AMV is different between the two periods. In the warm period, AMOC leads the AMV; a strong AMOC enhances the oceanic poleward heat transport which warms the North Atlantic Ocean both at the surface and deeper down, producing a positive AMV. In the cold period, however, AMV leads AMOC; a warm surface anomaly reduces the sea ice in the Labrador Sea which enhances local air-sea interactions and deep convection, and later a stronger AMOC. In the cold period, the warm anomaly associated with the AMV does not extend below the mixed layer, suggesting that it is driven by the atmosphere and not ocean dynamics.