North Atlantic Modeling of Low-Frequency Variability in Mode Water Formation

Abstract

Abstract The generation of interannual and near-decadal variability in the formation of mode waters in the western North Atlantic is investigated in the realistic framework of an isopycnic coordinate ocean model forced with atmospheric data from 1946 to 1988. At Bermuda, the model reproduces quite well the observed potential vorticity and isopycnal depth anomalies associated with the subtropical mode water (STMW). Heat storage and preconditioning of the convective activity are found to be the important factors for the generation of STMW variability, with persistence of cold (warm) conditions, associated with anomalous heat loss (gain) over the western subtropics, being more significant for the generation of the simulated variability than are strong anomalous events in isolated years. In the Labrador Sea, the model captures the phase and order of magnitude of the observed near-decadal variability in the convective activity, if not its maximum amplitude. The simulated potential vorticity anomalies are, as observed, out-of-phase with those in the western subtropics and correlate well with the North Atlantic Oscillation (NAO) at near-decadal timescales, with the oceanic response lagging the NAO by ∼2–3 years. These results support the idea that the variability in water mass formation in the western North Atlantic can be attributed, to a large extent, to changes in the pattern of the large-scale atmospheric circulation, which generate sensible and latent heat flux variability by modifying the strength and position of the westerly winds and the advection of heat and moisture over the ocean. To the authors' knowledge, this is the first time that the interannual and near-decadal subsurface variability associated with STMW and Labrador Sea Water, and its relationship to the NAO, has been simulated in an ocean general circulation model.