The Kuroshio Loop Current west of Luzon Strait observed by the long-term South China Sea Mooring Array.

Based on concurrent mooring array and satellite observations between June/2014 and June/2017, spatiotemporal variabilities and generation mechanisms of the Kuroshio loop current (KLC) in the northeastern South China Sea (SCS) were investigated. The 3-year moored observations revealed that the KLC mainly occurred in winter and it exhibited significant interannual variability with moderate, weak, and strong strengths in the winters of 2014/2015, 2015/2016 and 2016/2017, respectively. Through analyzing the historical altimeter data, we found that the KLC event in the 2016/2017 winter was the strongest since 1993 in respect to sea level anomaly, relative vorticity and lifespan. This super KLC was observed to carry huge amount of warm, salty and nutrient oligotrophic Kuroshio water into the SCS. Although the KLC structure was initially confined to the upper 500 m near the Luzon Strait (LS), it became more barotropic with increased deep-layer velocity when extending into the SCS interior. Further analysis suggested that the super KLC was primarily caused by the combined effects of strengthened local wind forcing and impingement of a strong Pacific anticyclonic eddy, and the role of nonlinear hysteresis of the upstream Kuroshio was insignificant. For the Pacific eddy impingement, it perturbed the Kuroshio path and generated the initial loop structure in the LS, while for the strengthened wind stress curl, it input more vorticity and energy into the Kuroshio loop. The strengthened northeast wind in 2016/2017 winter was further suggested to be associated with the La Niña event occurred in that year.