2025年2月上旬の帯広における記録的なドカ雪はなぜ起きた？～黒潮起源の海の渦がもたらした海洋熱波と気象条件の複合効果を解明～

2025-07-07

2025-07-07 立正大学,防災科学技術研究所,北海道大学

2025年2月3～4日、北海道帯広市で12時間降雪量120 cmの観測史上最多記録を樹立した「ドカ雪」は、十勝沖の海洋熱波と気象条件の複合効果によって引き起こされました。黒潮起源の渦によって海面が平年より＋4.6℃も高まり、これに伴う暖湿気が湿った不安定な空気を供給。さらに、温帯低気圧と前線の作用で上昇流が活性化し、雪雲を集中発達させました。領域気象モデルによる再現実験では、熱波がなければ約半分の降雪量にとどまったことが示され、海洋熱波が豪雪を増幅させる証拠が得られました。

【参考図】2025年2月上旬の帯広におけるドカ雪の領域気象モデル※4による数値シミュレーションの結果の可視化。白～灰色の領域は雪雲の3次元分布、陰影は海面水温、矢印は地上付近の水平風を示す。可視化にはMet.3D (https://met3d.wavestoweather.de/met-3d.html)を使用した。

＜関連情報＞

2025年2月に北海道帯広市で記録的な大雪が降ったメカニズム大気環境と海洋熱波の役割 Mechanisms behind the record-breaking snowfall in Obihiro, Hokkaido, Japan, in February 2025: Roles of atmospheric environment and a marine heatwave

Hidetaka Hirata, Kenta Tamura, Takehiro Morioka, Tomonori Sato
Scientific Online Letters on the Atmosphere（SOLA） Published:2025/07/03
DOI:https://doi.org/10.2151/sola.21C-001

抄録

In early February 2025, a 12-hour snowfall of 120 cm was observed in Obihiro, located on the Tokachi Plain of southeastern Hokkaido, Japan; it was the highest recorded snowfall in Japan. Concurrently, a marine heatwave (MHW) with pronounced warm sea surface temperature was observed offshore. While MHWs effect on rainfall are documented, their impact on snowfall remains poorly understood. Here, we demonstrated the mechanisms behind the record-breaking snowfall event, including the effects of the MHW. During the heavy snowfall, an extratropical cyclone drove strong easterly winds toward the coastal regions of the Tokachi Plain, and a surface front was located to the south of Obihiro. The easterly winds transported a convectively unstable layer from over the ocean into the front, and the frontal updrafts released the instability. Consequently, convective precipitation systems developed, yielding heavy snowfall at Obihiro on the cold side of the front. Notably, the MHW enhanced the frontal formation and convective instability, increasing precipitation around Obihiro by approximately 50%. This case study demonstrates that MHWs can significantly amplify snowfall under specific atmospheric conditions, advancing our understanding of compound extreme.