[A-CG36] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[ACG36-12] Multiple-scale variability in the Kuroshio driven by Asian Monsoon: the recurrent tendency of the seasonal response

*Hirohiko Nakamura¹, Zhenlong Zhan¹, Ayako Nishina¹, YUXIANG QIAO¹, Hong-Sik Min², Hanna Na³, Jae-Hun Park⁴ (1.Kagoshima University, Faculy of Fisheries, 2.Korea Institute of Ocean Science and Technology, 3.Seoul National University, 4.Inha University)

Keywords:Moored ADCP array, Satellite along-track surface geostrophic velocity, East China Sea, Nonlinear Ekman pumping

Our previous studies revealed that the mechanism underlying seasonal variability of the upper-layer velocity field in the Kuroshio is primarily regarded as the response to local wind stress over the Kuroshio; the seasonal velocity variation maximum associated with the offshore shift appears primarily in summer (July) under the wind blowing along the Kuroshio, while the minimum along with the inshore shift occurs in autumn (October to November) and winter (January to February), depending on the region, under the wind blowing against the Kuroshio. It was hypothesized that a driving mechanism is due to the nonlinear Ekman pumping over the jet caused by the Northeast Asian Monsoon. However, the observational evidence previously provided for this mechanism was derived from a climatological mean state, and so the recurrent seasonal tendency to each year has been unclear. This study, therefore, describes such a tendency based on daily time series data from ADCPs moored in the East China Sea (ECS), which have been maintained since 2015 under the collaboration between Japan (Kagoshima Univ.) and Korea (KIOST, SNU, and Inha Univ.), and satellite along-track surface geostrophic velocity (SGV) data in the ECS.
The main conclusions are as follows: Velocity time series from a moored ADCP and time series of SGVs during 2015/06–2020/06 showed that 1) the surface current speed of the Kuroshio responded recurrently every year to the seasonal wind stress variation over the Kuroshio, 2) such a response occurred over the entire thermocline water. Velocity time series from four moored ADCPs and SGVs during 2019/06–2020/06 showed that 3) the entire current field in the cross-section shifted inshore, following that the wind over the Kuroshio had changed from the northeastward direction in summer to the southeastward one in winter. In addition, the year-to-year amplitude modulation of seasonal current speed variability followed the year-to-year amplitude modulation of seasonal wind stress variability over the Kuroshio. These results support our previous hypothesis that the mechanism underlying seasonal variability of the upper-layer velocity field in the Kuroshio is primarily regarded as the response to local wind stress over the Kuroshio.