Localized and global responses of the atmospheric circulation to midlatitude sea surface temperature anomalies

The sensitivity of atmospheric models to sea-surface temperature (SST) anomalies in the wintertime North Atlantic was studied. For this purpose, a series of numerical experiments with two atmospheric general circulation models (AGCMs), A547 and A5421, with different vertical resolutions (7 and 21 vertical levels, respectively) were performed. It was shown that the spatial pattern of the atmospheric responses depends significantly on the lifetime of the SST anomalies. On short-term time scales (up to two weeks), the response to a monopole-configured SST anomaly has a baroclinic structure and is localized in the North Atlantic area. The two atmospheric models lead to the responses that are close to each other in both the form and the time of transition to a steady state despite their different vertical resolutions. The spatial configuration of the localized response is such that it provides a negative feedback with its parent anomaly. For SST anomalies with considerably longer lifetimes, the models show global responses associated with the structure of the inherent low-frequency variability of the models. On the basis of a numerical experiment performed by the Monte Carlo method, it was found that the formation of the global response in the A547 model is a nonmonotone quasi-periodic process. The spatial structures of the global responses in models A547 and A5421 differ significantly from each other. In an experiment with model A5421, a procedure using the fluctuation-dissipation relation was applied to reconstruct the stationary response to a small time-independent external forcing.