Geostrophic Adjutment And The On-shelf Intrusion Of The Kuroshio Northeast Of Taiwan

Intrusion of the Kuroshio onto the East China Sea northeast of Taiwan exerts strong influence on the circulation system and marine ecosystems of the China Seas. The study on the on-shelf intrusion of the Kuroshio northeast of Taiwan and its accompanying "Cold-Dome" feature in summer has been a continual focal point of research about the circulation dynamics of the East China Sea. It is also a representative dynamic process of the interaction between a western boundary current and its adjacent marginal seas. However, so far there is no consensus on the dynamic mechanism of on-shelf intrusion of the Kuroshio. In addition, existing hydrological observations were not able to reveal a good three dimensional structure of the intrusion, nor its long-term scale variation.In this study, the dynamics of on-shelf intrusion of the Kuroshio northeast of Taiwan was studied with a high-resolution numerical model. Analysis of the momentum balance from the results of the numerical model suggests that spatial geostrophic adjustment of the Kuroshio, due to loss of the Taiwan Island in support of Kuroshio’s cross-shelf pressure gradient of the Kuroshio, is the principal mechanism for its on-shelf intrusion northeast of Taiwan. The advection term in the momentum equation plays a major role in the process of the adjustment. The bottom friction term influences mainly the adjustment of the subsurface layer of the Kuroshio. The horizontal viscosity plays a secondary role in the momentum balance. The eastward turning of the shelfbreak orientation further north affects the spatial extent, as well as the pattern, of the geostrophic adjustment. The sub-marine canyons across the shelfbreak northeast of Taiwan do influence the pattern of the current and the position of the "Cold Dome" in summer.Consistent features of the Kuroshio intrusion were also obtained by analyzing the along-track satellite altimeter data, the sea-surface drifter data and the numerical model results. (1) In summer, the on-shelf intrusion of the Kuroshio mainly manifests as the upwelling of the subsurface cold water. The "Cold Dome" formed by the upwelled water often can be seen in surface layer. The intensity of the cold water, i.e., the area and the temperature anomaly of the cold water, increases with depth, and the cold center at the near bottom level is also closer to Taiwan. The patterns of the current and voricity induced by the geostrophic adjustment are the cause of the upwelling. The Kuroshio surface water also intrudes directly onshore in summer at the location of the sharp eastward turning continental slope northeast of Taiwan. (2) In winter, on-shelf intrusion of the surface Kuroshio water follows two routes. Near the northern shore of Taiwan on-shelf intrusion of the Kuroshio from the geosptrophic adjustment manifests as a large anti-cyclonic loop current, accompanied by a strong thermal front between the Kuroshio water and the shelf water. Further north on-shelf intrusion of the Kuroshio takes on a straight northward path onto the shelf when it turns sharply eastward following the sharp eastward turning continental slope of the ECS. The inertia of the Kuroshio is the main mechanism of the intrusion here. Subsurface upwelling induced by geostrophic adjustment also exists in winter, with the main upwelling region near the shelf break just north of Taiwan. (3) Interannual variation of the on-shelf intrusion of the Kuroshio northeast of Taiwan is obvious. The intrusion is controlled by the local wind field, the air-sea heat flux and the variability of the Kuroshio. The pattern and variability of surface intrusion in winter depend mostly on the local air-sea heat flux and the inertia of the Kuroshio.