Variability of the thermohaline circulation in an ocean general circulation model coupled to an atmospheric energy balance model

Abstract

The variability of the ocean's thermohaline circulation in an oceanic general circulation model (OGCM) coupled to a two-dimensional atmospheric energy balance model (EBM) is examined. The EBM calculates air temperatures by balancing heat fluxes, including that from the ocean surface; air temperature and ocean circulation evolve together without imposed temperature restrictions except specification of the solar constant. The heat coupling is scale dependent such that small-scale ocean temperature anomalies are damped quickly while large-scale ones lose heat slowly by longwave emission to space. These boundary conditions are more realistic than restoring conditions even when weak coupling is used, since they allow changes in air temperature and wholesale shifts in the planetary heat balance. It is found that coupling the EBM to the OGCM increases the stability of the ocean's thermohaline circulation. This increased stability arises from the ability of the coupled model to develop a four times greater sea surface temperature response to a given change in thermohaline overturning than when traditional restoring boundary conditions are used. The sense of this increased response works to stabilize the thermohaline overturning. The specific value of the small-scale thermal coupling coefficient also influences the stability even though the large-scale coefficient is always small (2 W m-2 C-1); this suggests that small-scale processes might determine the large-scale stability.