Implications of Both Statistical Equilibrium and Global Warming Simulations with CCSM3. Part II: On the Multidecadal Variability in the North Atlantic Basin

The nature of the multidecadal variability in the North Atlantic basin is investigated through detailed analysis of multicentury integrations performed using the low-resolution version of the Community Climate System Model, version 3 (CCSM3), a modern atmosphere-ocean coupled general circulation model. Specifically, the results of control simulations under both preindustrial and present-day perpetual seasonal cycle conditions are compared to each other and also to the results of five simulations with increasing CO2 concentration scenarios. In the absence of greenhouse gas-induced warming, the meridional overturning circulation (MOC) variability is shown to be dependent on the details of the simulation. In the present-day control simulation, the MOC is characterized by a broad spectrum of low frequencies, whereas, in preindustrial control simulations, MOC variability is characterized either by a well-defined periodicity of 60 yr or by a broad spectrum of low frequencies. In all the control simulations, the MOC appears to respond with a delay of 10 yr to synchronous temperature and salinity anomalies in the deep water formation sites located in the subpolar gyre, but salinity dominates the density anomalies. The explanation of the modeled MOC periodicity is therefore sought in the creation of these density anomalies. The influence of increased sea ice coverage under cold/preindustrial conditions is shown to modify the salinity variability, but it is not a sufficient condition for the support of the MOC periodicity. Instead, its source appears to be a modified subpolar gyre circulation resulting from interaction with the bottom bathymetry, which is able to sustain strong coupling between the horizontal and overturning circulations. Based on the global warming analyses, for the simulations initialized from the cold/preindustrial statistical equilibrium run, the North Atlantic variability continues to be dominated by strong coupling between the horizontal and overturning circulations if the imposed forcing is weak. More generally, the delayed response of the MOC to surface density anomalies in the deep water formation regions is preserved under weak forcing.