Assessing the Importance of Prominent Warm SST Anomalies and Northward Shift of Oceanic Frontal Zone over the Midlatitude North Pacific in Forcing Atmospheric Anomalies in October
In contrast, the model reproduction of atmospheric anomalies observed in summer was overall unsuccessful. This appears to arise from the fact that, unlike in October, the midlatitude SST anomalies tended to accompany negative anomalies in upward heat fluxes from the surface, indicative of atmospheric thermodynamic forcing on the SST anomalies in 2011 summer.
The distinct July-October difference in the AGCM responses to the warm SST anomalies may also be contributed to by the seasonality of background westerlies and stormtrack. In October, they are collocated with the midlatitude oceanic frontal zone, which is favorable for the stormtrack activity to be modulated in responding to SST anomalies. In addition, in October 2011, no strong Rossby wave was injected into North Pacific and response to the warm SST anomaly can be observed in the observation, not being masked.
In October, anticyclonic anomaly tends to be located downstream of the poleward-shifted frontal zone in the AGCM experiment prescribed with observed SST field in 1982-2010. Furthermore, analysis of atmospheric reanalysis data for the same period revealed that upper-tropospheric anticyclonic anomaly tends to appear in the Central North Pacific followed by poleward-shift of the frontal zone. These are consistent with the observed and simulated anticyclonic anomaly in October 2011 and suggest an importance of poleward shift of the North Pacific frontal zone in October to overlying atmosphere.