Mechanisms of AMOC variability simulated by the NEMO model

We have investigated dominant mechanisms of the Atlantic Meridional Overturning Circulation (AMOC) variability at 26.5° N (without the Ekman component) on monthly timescales using 1° and 1/4° NEMO model data. All data were detrended and the seasonal cycle removed. The spatial lead-lag correlations of different hydrodynamic fields with the AMOC time series were calculated. The analysis shows that the AMOC depends on the strength of wind over the North Atlantic on different time scales. At ～ 1 yr the January–June difference of mean sea level pressure between high and mid-latitudes in the North Atlantic defines (according to different model runs) 35–50% of the annual AMOC variability. At interannual time scales ～ 4 yr after strong (weak) winds over the North Atlantic the AMOC transport becomes higher (lower) by means of an increase (a decrease) in deep water formation in the North Atlantic subpolar gyre. The analysis of the 1/4° NEMO model shows that about 30% of the AMOC variability is due to density changes in the top 1000 m in the Labrador and Irminger seas occurring about 4 yr early.