日本地球惑星科学連合2023年大会

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[J] 口頭発表

セッション記号 A (大気水圏科学) » A-OS 海洋科学・海洋環境

[A-OS15] 海洋物理学一般

2023年5月23日(火) 09:00 〜 10:15 105 (幕張メッセ国際会議場)

コンビーナ:土井 威志(JAMSTEC)、岡 顕(東京大学大気海洋研究所)、座長:土井 威志(JAMSTEC)、岡 顕(東京大学大気海洋研究所)

09:30 〜 09:45

[AOS15-09] Observations of upper ocean temperature and current variation under moderate wind conditions

*Chen-Chen Ho1、Ming-Huei Chang1,2、Je-Yuan Hsu1、Yu-Hsin Cheng3 (1.Institute of Oceanography, National Taiwan University、2.Ocean Center, National Taiwan University、3.Department of Marine Environmental Informatics, National Taiwan Ocean University)


キーワード:Diurnal Warm Layer, Shear instability

Under moderate wind conditions (less than or equal to 8 m s-1) and clear sky, the penetration of insolation into the upper ocean could form a diurnal warm layer (DWL), which can reach ~5 m in the open ocean. To better understand DWL, we analyzed the data collected by the Ship-based Air-sea Flux & Exchange System (SAFE) deployed in the Green Island wake in May 2021 and in the southwest of Taiwan in April 2022. SAFE is equipped with a 1200 kHz Acoustic Doppler Current Profiler (ADCP) and 50 temperature sensors to respectively measure current and temperature in the upper 20 m ocean at a resolution of 0.5-2 m. Significant thermal oscillations were observed in both experiments during the deepening (or forcing) stage of DWL. Under a weak wind speed of ~2 m s-1, the DWL in the island wake can reach ~20 m, suggesting strong interior mixing in the wake flow instead of wind-induced mixing, enhancing the deepening of DWL. Temperature oscillations observed are likely internal waves (IW) resulting from interior forcing. But some waves have a roll-up structure, a key feature of Kelvin-Helmholtz (KH) instability. In the southwest of Taiwan, the oscillation train is likely to be the mature KH instability resulting from wind forces, characterized by a combination of roll-up and breaking waveform. The KH billows cause turbulent kinetic energy (TKE) dissipation rate of O(10-6 W kg-1), two orders larger than the remnant layer. Therefore, the deepening during the development of DWL is likely due to the wind-induced turbulent mixing. Both observations of temperature oscillation satisfy the basic theorem of KH instability, i.e., (1) Miles-Howard criterion and (2) linear stability analysis, and empirical frequency of KH billow, near–N. Our observations unveil the detailed processes in response to the DWL deepening – both the IW and KH billow play pivotal roles.