A Response Climatology of Idealized Midlatitude Thermal Forcing Experiments with and without a Storm Track

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Several GCM studies indicate that eddy activity may have a considerable influence on the atmospheric response to midlatitude sea surface temperature anomalies. The effect of eddy activity on the atmospheric equilibrium response to idealized midlatitude thermal forcing is analyzed for an atmosphere with or without an idealized storm track. Experiments using a simplified global circulation model forced by thermal anomalies of different sign and location are discussed. Consistent with the linear theory the geopotential height field displays a baroclinic response with a shallow low (high) somewhat downstream of the warm (cold) anomaly; farther downstream an equivalent barotropic response occurs with positive (negative) amplitude increasing with height. Eddy feedbacks have weak impact on the baroclinic part, but the equivalent barotropic response is strongly enhanced if the bandpass-filtered streamfunction tendency is in-phase with the linear geopotential height response. This is the case in an experiment with a warm anomaly near 40°N, located southwesterly of the idealized storm track. In the corresponding experiment with a cold anomaly the two patterns are out-of-phase and the equivalent barotropic response is slightly reduced. It is weakened (strengthened) if a warm (cold) anomaly is shifted about 10° poleward or equatorward relative to the idealized storm track. Midlatitude heat sources generate wave trains that extend equatorward and poleward developing large-scale correlations between the flow at remote locations (teleconnections). The space–time variability can be changed considerably by eddy feedbacks developing stronger variance for large-scale retrogressive traveling and standing waves. Partially, blocking-like events develop.