The Sensitivity of Modelled Atlantic Meridional Overturning Circulation Variability to Parameterized Nordic Sea Overflows in CCSM4

The inclusion of parameterized Nordic Sea overflows in the ocean component of the Community Climate System Model version 4 (CCSM4) results in dramatically reduced variance of the Atlantic meridional overturning circulation (AMOC) maximum transport on decadal and longer timescales compared to a control simulation without this parameterization. In particular, this commonly used measure of the AMOC variability shows a variance of 1.7 Sv^2 in the control case in contrast with a variance of only 0.6 Sv^2 with the overflows. The reduced variability is due to the stabilizing effects of dense overflow waters which fill the abyss of the Labrador Sea and inhibit the decadal variability of deep convection. However, the overflow impacts on the AMOC variability are not uniform in latitude, depth, or frequency-space. For example, at 26.5N the enhanced North Atlantic Deep Water transport associated with the Nordic Sea overflow parameterization generally results in somewhat greater AMOC variability compared to the control case with variances of 0.6 and 0.5 Sv^2, respectively. We study the differences in the AMOC behavior with and without the parameterized Nordic Sea overflows to deduce spatial and spectral characteristics of the key physical processes associated with the AMOC variability at particular timescales and latitudes. Finally, the insights gained are used to shed light on the differences in AMOC variability in control simulations of the CCSM4 and its previous version CCSM3.