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Abstract

Sea surface temperatures (SST) in the North Pacific were extremely warm in the 2021-22 winter associated with the negative phase of the Pacific Decadal Oscillation (PDO). We investigate the large-scale atmospheric responses to the warm SST anomalies through ensemble time-slice experiments of an atmospheric general circulation model (AGCM) with two different horizontal resolution settings: 100 km and 50 km. In both the 100-km and 50-km experiments, anticyclonic responses in the winter-mean 500-hPa geopotential height fields are found over the North Pacific. The anticyclonic response is stronger in the 50-km AGCM, and its center is located slightly to the west of that obtained in the 100-km AGCM. Blocking frequency over the North Pacific in the 50-km AGCM is significantly increased under the influence of the warm SST anomalies, which may cool the lower stratosphere in the Arctic through suppressing upward planetary-wave propagation from the troposphere. Such increased blocking and cooling in the polar stratosphere are not observed in the response in the 100-km AGCM, suggesting the need for a high-resolution model to obtain a realistic blocking response to midlatitude SST anomalies.