Numerical study of the cross-shore range and the intensity of the Nearshore Kuroshio Branch Current (NKBC)

Temperature and salinity data, obtained by two snapshot surveys during 19-20 May 2019 and 12-25 September 2019 across the East China Sea (ECS) shelf, revealed that the Kuroshio intrusion to the north of 28 degrees N comprised the Nearshore Kuroshio Branch Current (NKBC) and the Offshore Kuroshio Branch Current (OKBC) at the bottom of the ECS during spring 2019, and that the NKBC was weak during autumn 2019. The Regional Ocean Model System was used to reproduce the distribution of water masses and the current structure over the continental shelf of the ECS during 2019. Analyses of the momentum balances indicated that the cross-shore range and the intensity of the NKBC were determined by the combination of the geostrophic flow and bottom Ekman current. In comparison with that in May 2019, a weakened shoreward bottom Ekman current and an increased offshoreward geostrophic flow caused the disappearance of cross-shore range of the NKBC in September 2019. Meanwhile, a diminished northeastward alongshore geostrophic flow in September 2019 also weakened the intensity of the NKBC. Sensitivity experiments indicated that a strong southwestward wind can push the western (eastern) boundary of the NKBC further offshoreward (shoreward) by increasing (decreasing) the offshore geostrophic flow (bottom Ekman current). A weak Taiwan Warm Current (TWC) can move the eastern boundary of the NKBC shoreward by decreasing the onshore bottom Ekman current. A weak Kuroshio Current (KC) can move the eastern boundary of the NKBC shoreward by increasing the offshoreward geostrophic flow. Furthermore, a strong (weak) southwestward wind, weak (strong) TWC, and strong (weak) KC can diminish (enhance) the intensity of the NKBC. Of the three factors, the wind plays the major role in influencing the NKBC.

Automated summary

Temperature and salinity data from two surveys in May and September 2019 over the East China Sea (ECS) shelf revealed spatial and temporal variations in the Kuroshio intrusion, with factors such as geostrophic flow, bottom Ekman current, and external winds playing significant roles. Among these factors, wind was found to have the most pronounced influence on the Nearshore Kuroshio Branch Current (NKBC).