Mechanisms of Atlantic Meridional Overturning Circulation variability simulated by the NEMO model

We have investigated mechanisms for the Atlantic Meridional Overturning Circulation (AMOC) variability at 26.5 N (other than the Ekman component) that can be related to external forcings, in particular wind variability. Resolution dependence is studied using identical experiments with 1 and 1/4 NEMO model runs over 1960–2010. The analysis shows that much of the variability in the AMOC at 26 N can be related to the wind strength over the North Atlantic, through mechanisms lagged on different timescales. At ∼ 1-year lag the January–June difference of mean sea level pressure between high and mid-latitudes in the North Atlantic explains 35–50 % of the interannual AMOC variability (with negative correlation between wind strength and AMOC). At longer lead timescales ∼ 4 years, strong (weak) winds over the northern North Atlantic (specifically linked to the NAO index) are followed by higher (lower) AMOC transport, but this mechanism only works in the 1/4 model. Analysis of the density correlations suggests an increase (decrease) in deep water formation in the North Atlantic subpolar gyre to be the cause. Therefore another 30 % of the AMOC variability at 26 N can be related to density changes in the top 1000 m in the Labrador and Irminger seas occurring ∼ 4 years earlier.