Large scale air-sea interactions with a simple general circulation model

We have coupled Lorenz's (1960) two-layer atmospheric model to a “copper plate” ocean to obtain a simple model that can be used to study the effects of large-scale sea surface temperature (SST) anomalies on the dynamics of the atmosphere. With 164 degrees of freedom, the atmospheric model mimics the observed northern hemisphere energy cycle in fair detail. In three experiments, each lasting one year, we consider an ocean (a) with fixed temperature that depends only on latitude; (b) with fixed large-amplitude SST anomalies; and (c) with temperature determined by heat exchange with the air. Only in (b) are significant dynamical effects observed. These consist of a weak monsoon response in the subtropics and a tendency for storms to intensify over warm water at higher latitude. Experiment (c) developed SST anomalies that resemble the observed anomalies; however, the atmosphere and ocean in (c) differ insignificantly from the atmosphere and re-computed “slave ocean” in (a), even when the flows are averaged in reference frames moving with the anomalies. Our results suggest that the atmosphere may be too noisy to be much affected by SST anomalies on time scales over which the anomalies are themselves predictable.