Asymmetry and Seasonality of the Atmospheric Circulation Response to a Meridional Shift of the Oyashio Extension Sea Surface Temperature Front

There is growing evidence that SST anomalies associated with Kuroshi-Oyashio Extension frontal shifts have a significant impact on both the local and large-scale atmospheric circulation. Furthermore, some studies suggest the atmospheric response may be sensitive to the sign of the SST anomalies and also impacted by the seasonal cycle of the background circulation. We examine the asymmetry and seasonality of the atmospheric response to SST anomalies due to meridional displacement of the Oyashio Extension using ensemble atmospheric GCM simulations. Experiments were conducted using the Community Atmospheric Model version 5 with 0.25° horizontal resolution, with the SST front either in a northward position (“warm”), its climatological position (“control”) or a southward position (“cold”). Asymmetry in the atmospheric response is examined by comparing the anomalies from the warm-minus-control and cold-minus-control cases. The basin-scale equivalent barotropic circulation responses in winter are found to be highly asymmetric during the entire winter, and different in early (December-January) and late (February-March) winter. In early winter, a significant weakening of Aleutian Low is found in response to the warm SST anomalies, while the responses are weak and insignificant in the cold SST anomaly case. On the other hand, in late winter, both the warm and cold cases show strengthening of the Aleutian Low, but the response in the cold case is almost twice stronger. Daily evolution of the responses reveals that the ensemble mean responses consist of low-frequency transients occurring in at a few week intervals. In addition, the synoptic transient eddy heat flux response in the upstream forcing region over the Oyashio Extension also exhibits a strong asymmetry as well as the difference between the early and late winter. Furthermore, the upstream transient eddy variability slightly precedes the downstream height variability, which suggest forcing of the downstream low-frequency height variability by the upstream synoptic transient eddies.