State Dependence of Atmospheric Response to Extratropical North Pacific SST Anomalies

Abstract By performing two sets of high-resolution atmospheric general circulation model (AGCM) experiments, the authors find that the atmospheric response to a sea surface temperature (SST) anomaly in the extratropical North Pacific is sensitive to decadal variations of the background SST on which the SST anomaly is superimposed. The response in the first set of experiments, in which the SST anomaly is superimposed on the observed daily SST of 1981–90, strongly differs from the response in the second experiment, in which the same SST anomaly is superimposed on the observed daily SST of 1991–2000. The atmospheric response over the North Pacific during 1981–90 is eddy mediated, equivalent barotropic, and concentrated in the east. In contrast, the atmospheric response during 1991–2000 is weaker and strongest in the west. The results are discussed in terms of Rossby wave dynamics, with the proposed primary wave source switching from baroclinic eddy vorticity forcing over the eastern North Pacific in 1981–90 to mean-flow divergence over the western North Pacific in 1991–2000. The wave source changes are linked to the decadal reduction of daily SST variability over the eastern North Pacific and strengthening of the Oyashio Extension front over the western North Pacific. Thus, both daily and frontal aspects of the background SST variability in determining the atmospheric response to extratropical North Pacific SST anomalies are emphasized by these AGCM experiments.