The modeled atmospheric response to midlatitude SST anomalies and its dependence on background circulation states

The atmospheric response to a midlatitude SST anomaly in the North Pacific and its dependence on background flow are examined in a GCM. Experiments are conducted using the same warm SST anomalies but two different model states: perpetual January and perpetual February. The atmospheric responses to the SST anomalies are statistically significant in both January and February but are completely different. The anomalous circulation in January is characterized by a trough decaying with height immediately downstream of the SST anomalies. In February, the anomalous circulation is dominated by a downstream ridge growing with height. The patterns of the anomalous heights in the two months are nearly orthogonal. Vorticity and thermodynamic budgets are diagnosed to illustrate how the anomalous circulations are maintained. Over the SST anomalies, low-level convergence and ascent are observed in both months. In January the anomalous convergence is balanced by a residual due primarily to the forcing by submonthly transients. In February the convergence is balanced by the advection of planetary vorticity. Analysis of the thermodynamic budget indicates that the intensity of the mean meridional wind downstream of the SST anomalies plays a critical role in determining the nature of the responses in the two months. The ''warm SST-ridge'' type of response is favored when the background meridional flow is relatively weak. These results demonstrate that the atmospheric response to a midlatitude SST anomaly is strongly dependent on the background flow.