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Abstract:
Many theoretical and numerical studies have been carried out to understand the mechanisms for the meridional overturning circulation (MOC), especially how its strength varies with for example, the magnitude of Ekman pumping, the density contrast between the two polar regions, and the vertical diffusivity. However, these studies were mostly done under idealized configurations, it is still unclear how the strength of global MOC changes in the realistic climate as simulated by climate models and why it changes in the way it does. Here we first present climate simulations for the whole Phanerozoic (541 million years ago to now) performed using a fully coupled atmosphere-ocean general circulation model (GCM), CESM1.2. A snapshot of the climate was obtained for every 10 million years, from which a time series of the MOC strength was constructed. The strength here is defined as the maximum zonally integrated stream function at the equator and varies from 2 Sv to 32 Sv. Then we carried out many sensitivity experiments to test whether the MOC strength was due to change of climate or continental configuration. It is found that both the climate state and the continental figuration have significant influence on the MOC strength. We further identified the roles of the mid-latitude westerly winds and the polar density contrast in affecting the MOC strength, with the aid of idealized experiments done with MITgcm. It is found that the wind-induced Ekman pumping plays a more important role than the density contrast.
