Numerical study on the pattern and origins of Kuroshio branches in the bottom water of southern East China Sea in summer

Abstract

Pattern and origins of Kuroshio branches in the bottom water of southern East China Sea (ECS), were carefully examined by numerical simulations based on the Regional Ocean Modeling System (ROMS) together with observations. Model results show that in the bottom water of ECS, the intrusion pattern of Kuroshio is mainly composed of an Offshore Kuroshio Branch Current (OKBC) which, bifurcated from the Kuroshio northeast of Taiwan, flows nearly along the isobath of ∼100 m, and a Nearshore Kuroshio Branch Current (NKBC) which, originated from the Kuroshio northeast of Taiwan, upwells northwestward gradually from ∼250 m to ∼60 m, then turns to northeast around 27.5°N, 122°E, thereafter flows northeastward along the isobath of ∼60 m, and finally reaches at 30.5°N where it turns to east. Furthermore, we found that the NKBC mostly originated in the Kuroshio subsurface water (120–250 m) east of Taiwan, whereas the OKBC mainly stemmed from the Kuroshio water (60–120 m) east of Taiwan. This pattern and origins of OKBC and NKBC well addressed the observational phenomena that off the coast of Zhejiang province, China, there were colder, less saline, and more phosphate‐rich bottom water near the isobath of ∼60 m rather than near the isobath of ∼100 m in August 2009. Finally, it is proposed that on southern ECS continental shelf, Kuroshio exhibits its intrusion branches by an anticyclonical stair structure: bottom stair NKBC, middle stair OKBC, and top stair Kuroshio surface branch (KBC).