Study On The Feature And Origin Of The Nearshore Kuroshio Branch Current

The Nearshore Kuroshio Branch Current(NKBC),as an important branch current of the Kuroshio invading our country’s nearshore area through northeastern Taiwan,continuously transports the low-temperature,high-salt,and nutrient-rich Kuroshio subsurface water to the nearshore area of the East China Sea(ECS).It has an important influence on hydrology,ecology and fishery production in the nearshore area of the ECS.Therefore,it is of great practical significance to strengthen the research on the change and characteristics of NKBC for the people’s livelihood and economy in China.At present,the study of the NKBC focuses more on its downstream path characteristics,three-dimensional structure and dynamic mechanism at the origin of northeast Taiwan.There are few kinds of research on the changes and mechanisms of the path of the NKBC,especially in the middle reaches.And the research on the flow amplitude and intensity of the NKBC is needed too.In addition,the specific origin depth of the NKBC in northeast Taiwan needs to be quantified.Based on the four large-scale observations of the ECS in 2017 and 2019,based on the high-resolution three-dimensional model of the ECS and the adjacent sea area,combined with the satellite altimeter data and wind field reanalysis data,this paper makes a deep study on the changes and dynamic mechanism of the path,flow amplitude,intensity and origin of the NKBC,and obtains the following conclusions:(1)Based on CTD observation data of the East China Sea in September 2017 and September 2019,this study found that the middle paths of the NKBC in September2019 is more eastward than that in September 2017.By comparing the differences of sea surface height,geostrophic current,and wind during the observation period of the two years,we explained the dynamic mechanism of local sea surface wind field influence NKBC path.The southwestward wind which prevails in the ECS from August to September in 2019 is stronger,compared with that in 2017.The stronger southwestward wind in 2019 pushed the water to accumulate towards more shoreward and southwestward in the nearshore area.Therefore,from August to September 2019,affected by the distribution of pressure gradients,alongshore geostrophic current and offshore geostrophic currents occurred in the coastal waters of the ECS.The stronger offshore geostrophic current and the offshore bottom Ekman current which is induced by the southwestward alongshore geostrophic current jointly restrained the inshore intrusion of the NKBC.(2)Temperature and salinity data,obtained by two snapshot surveys during 19–20 May 2019 and 12–25 September 2019 across the ECS shelf,revealed that the Kuroshio intrusion to the north of 28°N comprised the Nearshore Kuroshio Branch Current and the Offshore Kuroshio Branch Current at the bottom of the ECS during spring 2019 and that the NKBC was weak during autumn 2019.Based on the results of the ROMS model,we have obtained the spatial distribution of the flow field at the bottom of the ECS in 2019,specifically given the changing trend of the amplitude and intensity of the NKBC,and explained the dynamic mechanism.Analyses of the momentum balances indicated that the amplitude and the intensity of the NKBC are determined by the combination of the geostrophic flow and bottom Ekman current.In comparison with May 2019,a weakened shoreward bottom Ekman current and increased offshoreward geostrophic flow caused the amplitude of the NKBC to disappear 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 Strait 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 a major role in influencing the NKBC.(3)We superimpose the Lagrangian particle tracking method on the basis of the ROMS model to quantitatively study the specific origin depth of NKBC east of Taiwan Island.In spring 2019,the NKBC flows east of Taiwan at a depth ranging from 100 m to 450 m,with an average depth of about 260 meters.Through sensitive experiments on the intensity of the Taiwan Strait current and the intensity of the Kuroshio east of Taiwan,this article further concludes that the strong(weak)Kuroshio speed in the east of Taiwan will reduce(increase)the intensity of the NKBC.The strong(weak)Taiwan Strait current will increase(decrease)the intensity of the NKBC.At the same time,the strong(weak)Kuroshio speed in the east of Taiwan will decrease(increase)the average depth of the origin of the NKBC.The strong(weak)Taiwan Strait current will also reduce(increase)the average depth of the origin of the NKBC.