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

Sets of AGCM experiments are conducted to assess the importance of prominent positive anomalies in SST observed over the midlatitude North Pacific in locally forcing a persistent large-scale anticyclonic anomaly in 2011 summer and autumn. The North Pacific anticyclonic anomaly observed in October is well reproduced by AGCM forced only with the warm SST anomaly in the midlatitude North Pacific, which brings poleward shift and expansion of the North Pacific oceanic frontal zone. The anticyclonic anomaly is maintained under strong transient eddy feedback forcing associated with the poleward-deflected stormtrack. In addition, both barotropic and baroclinic energy conversions from the climatological-mean state also substantially contribute to the maintenance, indicative of its characteristic as a dynamical mode inherent to the extratropical climatological-mean flow over the North Pacific. The corresponding AGCM response over the North Pacific to the tropical SST anomalies is in the same sense but substantially weaker and less robust, suggesting the primary importance of the prominent midlatitude SST anomaly in forcing the large-scale atmospheric anomalies observed in October 2011.

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.