Interactions between global SST anomalies and the midlatitude atmospheric circulation

A review is given of the processes contributing to variability of the atmosphere-ocean system on interannual timescales. Particular emphasis is placed on the relationships between midlatitude atmospheric fluctuations and sea surface temperature (SST) anomalies in various geographical sites. Various hypotheses are tested using output from a coordinated set of general circulation model experiments, which are subjected to time-varying SST forcing observed during 1946-88 in different parts of the world's oceans. It is demonstrated that tropical Pacific SST fluctuations associated with El Nino-Southern Oscillation (ENSO) episodes produce a strong extratropical response in the model atmosphere, whereas the atmospheric signal associated with midlatitude SST anomalies is less robust. Analysis of a 100-yr control experiment, which is conducted in the absence of any interannual SST forcing, indicates that a substantial fraction of the simulated atmospheric variability may be attributed to internal dynamical processes alone. The observed coexistence of tropical ENSO events with SST anomalies in the extratropical North Pacific is successfully reproduced by forcing the model atmosphere with tropical Pacific SST variations and allowing the atmospheric perturbations thus generated to drive a simple ocean mixed layer model inserted at ocean grid points outside the tropical Pacific. This simulation affirms the role of the atmospheric circulation as a ''bridge'' linking SST changes in different parts of the world's oceans. The midlatitude model responses in the presence of local air-sea interactions are noticeably stronger than the corresponding responses without such interactions. This finding is indicative of the positive feedback processes inherent in extratropical air-sea coupling.