2025-10-14 九州大学
8月の月平均850hPa高度（黒実線）、外向き長波放射量（色実線）、水蒸気フラックス（矢印と陰影）の合成図。左から気候平均分布、偏差分布、気候平均＋偏差の分布。 偏差分布（中央図）において、日本の南方海上に巨大な低気圧の渦が見られ、モンスーンジャイア（モンスーン渦）に対応する。
＜関連情報＞
- https://www.kyushu-u.ac.jp/ja/researches/view/1346
- https://www.kyushu-u.ac.jp/f/63491/25_1014_01.pdf
- https://www.sciencedirect.com/science/article/pii/S016980952500609X
月ごとおよび総観スケールでの真夏の北太平洋西部におけるモンスーン渦 Monsoon gyres over the western North Pacific in mid-summer on monthly and synoptic scales
Keita Fujiwara, Kosuke Maruno, Naoki Yoshida, Ryuichi Kawamura
Atmospheric Research Available online: 27 September 2025
DOI:https://doi.org/10.1016/j.atmosres.2025.108517
Highlights
- The concept of a monsoon gyre (MG) is characterized by two different aspects.
- Synoptic-scale MGs are the collective manifestation of TCs’ outer circulations.
- The signature of the Rossby wave response is apparent on longer timescales.
- The moisture conveyor belt facilitates the replacement of a TC’s outer circulation.
Abstract
This study addresses the issues of why monsoon gyres (MGs) tend to appear over the western North Pacific in mid-summer and how tropical cyclone (TC) activity characterizes the MGs in terms of monthly and synoptic scales. On a monthly scale, a low-level anomalous cyclonic circulation to the south of Japan is well correlated with enhanced tropical convection to the southeast in active MG years, indicating a signature of the Rossby wave response to convective heating. The monthly mean features of MGs can be interpreted as a superposition of the climatological monsoon trough and the low-level cyclonic circulation anomaly due to the Rossby wave response. In addition, TCs tend to occur in the southern and eastern periphery of the MG in active MG years, suggesting a possible interaction between the MG and TCs. On a synoptic scale, the outer circulations of individual TCs accompany the moisture conveyor belt (MCB) and have the potential to induce MGs. Furthermore, the replacement of the outer circulation due to multiple TCs occurs repeatedly through the development of the MCB. Such replacement contributes to prolonging the life span of MGs, although the outer circulation of a single TC hardly maintains an MG for more than a week. It is found that synoptic-scale MGs are the collective manifestation of individual TCs and their outer circulations, while the characteristics of the Rossby wave response to TC-related heating become increasingly evident in MGs on longer timescales. These multiple-time-scale features are the essence of the MG.
