西部赤道太平洋の季節内変動のメカニズムを解明（Mechanism of Deep Intraseasonal Variability in Western Equatorial Pacific）

2025-10-22 中国科学院 (CAS)

中国科学院海洋研究所の王嘉寧教授らは、西部赤道太平洋の深層（1500～3000m）で顕著な季節内変動を発見した。5年間の係留観測データ分析により、40～90日周期の運動エネルギーが確認され、その約81％はヤナイ波（ロスビー・重力混合波）によるものであった。波源は西部境界近傍の中層（1000～1750m）にあり、境界流の水平シアにより励起されたエネルギーが東方・深層へ伝播していると判明。さらに、約2500m深での変動周波数は太平洋南北循環上層深枝の東西流速と逆相関を示し、ドップラー効果による位相の西進が確認された。これにより、従来上層起源とされた季節内変動エネルギーが中層から深層へ直接伝達される新たな経路が提示された。本成果は、海洋モデルにおける鉛直エネルギー収支の改善に寄与するとしている。

Time-depth variation of intraseasonal kinetic energy observed by moorings, and vertical profiles of the time-averaged intraseasonal kinetic energy for the zonal and meridional velocity components. (Image by IOCAS)

＜関連情報＞

西赤道太平洋における深層季節内変動の観測：西境界から発生したヤナイ波 Observed Deep Intraseasonal Variability in the Western Equatorial Pacific Ocean: Yanai Waves originated from the Western Boundary

Hang Zhang,Jianing Wang,Zhixiang Zhang,Weidong Ma,Fan Wang,and Yan Du
Journal of Physical Oceanography Published:01 Oct 2025
DOI:https://doi.org/10.1175/JPO-D-25-0031.1

Abstract

Substantial deep intraseasonal kinetic energy (IKE) over 1000-3000 m depth with a 20–90-day period was detected from five-year mooring data in the western equatorial Pacific Ocean. Deep intraseasonal variability (ISV) is mainly concentrated in the meridional velocity (V) with a peak period of 50 days. The frequency-vertical mode number spectrum of V suggests that 81% of IKE is related to Yanai waves. The IKE is mainly concentrated on vertical modes of 2–6. Furthermore, the core frequency of deep ISV at 2464 m has a seasonal variation and negative correlation with the zonal velocity, which belongs to the upper deep branch of Pacific meridional overturning circulation (U-PMOC) and has a strong seasonal variability. Such a relationship suggests the Doppler shift effect and westward phase propagation of Yanai waves. The outputs from a reanalysis product identify observed characteristics of deep ISV and are used to study its energy source. The IKE over 1500–3000 m at the mooring site of 142°E originates from the intermediate layer over 1000-1750 m between 134-137°E, which is generated by barotropic instability associated with the horizontal shear of the intermediate currents. Yanai waves are responsible for energy transmission from the intermediate layer of the western side to the deep layer of the eastern side. This research highlights the critical role of Yanai waves in the energy transfer between the intermediate and deep layers of the equatorial ocean.