Adjustment to Radiative Forcing in a Simple Coupled Ocean-Atmosphere Model

This study calculates the adjustment to radiative forcing in a simple model of a mixed layer ocean coupled to the overlying atmosphere. One application of the model is to calculate how dust aerosols perturb the temperature of the atmosphere and ocean, which in turn influence tropical cyclone development. Forcing at the top of the atmosphere (TOA) is the primary control upon both the atmospheric and ocean temperature anomalies, both at equilibrium and during most of the adjustment to the forcing. Ocean temperature is directly influenced by forcing only at the surface, but is indirectly related to forcing at TOA due to heat exchange with the atmosphere. Within a few days of the forcing onset, the atmospheric temperature adjusts to heating within the aerosol layer, reducing the net transfer of heat from the ocean to the atmosphere. For realistic levels of aerosol radiative forcing, the perturbed net surface heating strongly opposes forcing at the surface. This means that surface forcing dominates the ocean response only within the first few days following a dust outbreak, before the atmosphere has responded. This suggests that, to calculate the effect of dust upon the ocean temperature, the atmospheric adjustment must be taken into account explicitly and forcing at TOA must be considered in addition to the surface forcing. The importance of TOA forcing should be investigated in a model where vertical and lateral mixing of heat are calculated with fewer assumptions than in the simple model presented here. Nonetheless, the fundamental influence of TOA forcing appears to be only weakly sensitive to the model assumptions.