Phosphate Transport Into The East China Sea Via Kuroshio On The Interannual Time Scale And Its Influence On Biological Processes

The East China Sea(ECS)is one of the most important marginal seas in the Northwest Pacific and it is also one of the most productive regions of the global ocean because of huge amount of externally sourced nutrients.Previous studies show that the nutrient transport into the ECS via Kuroshio intrusion,especially phosphate transport,plays a crucial role in the variation of the ECS ecosystem.However,there is lack of the study for the interannual variation of phosphate transport into the ECS via Kuroshio intrusion and its influence on the East China Sea ecosystem,as well as the specific mechanism.Using a three-dimensional Pacific physical-biological model with a 12.5-km spatial resolution,we investigate the interannual variation of phosphate transport into the ECS via Kuroshio intrusion(KIPT)and its influence on the ECS biology process during 1997-2016.The KIPT into the ECS mainly occurs in the northeast of Taiwan and southeast of Kyushu,with the maximum phosphate flux in the subsurface water(below the depth of 100 m).On the interannual time scale,the variation of phosphate flux into the ECS via Kuroshio intrusion is more significant in the subsurface water of the northeast Taiwan than that in the southwest of Kyushu.The KIPT shows significant variation with the period of about 2 and 4 year,which is similarly to that of the volume transport via Kuroshio intrusion.After further analysis,we confirm that the interannual variation of KIPT is dominated by Kuroshio velocity variation,while it is also affected by the variation of Kuroshio phosphate concentration.Based on the numerical simulation,we demonstrate that the interannual variation of KIPT has a significant impact on the surface chlorophyll in the coastal region of the ECS,especially in the Hangzhou Bay estuary-Zhoushan Islands and the movement of the Kuroshio axis in the subsurface water plays an important role on the interannual variation of KIPT.The specific mechanism of the effect of KIPT on the ECS ecosystem is as follows.When the Kuroshio axis in the subsurface water is closer to(farther away from)the ECS shelf,the strength of the bottom upwelling increases(decreases)in the northeast of Taiwan.The process induces more(less)significant topographic beta spiral,which causes an anticyclonic(cyclonic)eddy-like bottom velocity anomaly feature in the north of Taiwan.Eventually,more(less)phosphate is transported to the ECS inner shelf,which increases(decreases)chlorophyll concentration around the Zhoushan Islands.This study highlights the important of Kuroshio intrusion in connecting the inner shelf of the ECS on the interannual time scale.