Blocking activity and the warm SST anomaly over the midlatitude North Pacific in 2022 winter

In the 2021-22 winter, the midlatitude North Pacific was characterized by a prominent warm sea surface temperature (SST) anomaly associated with a negative phase of the Pacific Decadal Oscillation (PDO), whereas the eastern tropical Pacific was charactered by a cool SST anomaly associated with La Niña. This study investigates the potential impact of these SST anomalies separately on the atmospheric circulation based on a set of atmospheric general circulation model (AGCM) ensemble experiments. It is also assessed how sensitive the atmospheric responses can be to the horizontal resolution of the AGCM. The horizontal resolutions used in this study were 50 km and 100 km. The 50-km AGCM experiments suggest that the enhanced blocking activity observed over the western North Pacific in February 2022 was likely due to a response to the midlatitude North Pacific SST anomaly but unlikely due to remote influence from the tropical SST anomaly. Significant responses in the North Pacific blocking cannot be obtained in the 100-km AGCM experiments. Our experiments suggest potential importance of the midlatitude North Pacific SST anomaly in causing anomalous blocking activity in winter, whose reproduction may require high-resolution AGCMs.