9:30 AM - 9:45 AM
[AOS15-09] Observations of upper ocean temperature and current variation under moderate wind conditions
Keywords: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.