Relationship between Significant Warming Event over Japan in Early Spring and Displaced Stratospheric Polar Vortex and its Predictability

抄録

This is a case study of March 2023 when Japan experienced significant warming, along with its prediction using the atmospheric reanalysis dataset and Japan Meteorological Agency's Global Ensemble Prediction System (GEPS). A major sudden stratospheric warming event occurred in mid-February 2023 with a shift of the center of the stratospheric polar vortex toward northern Eurasia. In conjunction with the stratospheric polar vortex shift, the tropospheric westerly jet over Japan shifted poleward, leading to significant warming in March 2023. Regression analyses were conducted using the GEPS ensemble forecast initialized on February 15, 2023, which predicted the significant warming phenomenon near Japan. The regressed anomalies of geopotential height from Japan to the mid-latitude North Pacific showed that the strengthening of the displaced stratospheric polar vortex over Siberia was related to the poleward shift of the westerly jet near Japan. This finding suggests that when the stratospheric polar vortex shifts toward Siberia, the subsequent significant warming near Japan can be predicted with reliability. The geographical position of the stratospheric polar vortex is likely to be a key predictor to improve the forecast skill of the weather conditions near Japan in early spring.