Satellite observations have shown the abundance of generally westward-propagating eddies in the subtropical regions in the North Pacific Ocean, especially north of 10°N. Eddies transport mass, and can significantly impact the circulation as well as the heat, salt and nutrient balances of the western Pacific marginal seas. This study uses a numerical model to examine the conditions when eddies can or cannot freely propagate westward through the Luzon Strait into the South China Sea (SCS). Composite analyses on the 10-year model data show that the fates of eddies depend on the strength and path of the Kuroshio. In one path which exists mostly during fall and winter, the Kuroshio loops westward into the SCS, the potential vorticity (PV) across the current is weak, and eddies are likely to propagate freely through the Luzon Strait. In another path which exists mostly during spring and summer, the Kuroshio tends to leap directly northward bypassing the SCS, the PV across it strengthens, and eddies are then blocked and are constrained to also follow the northward path. Nonlinear eddy-current interaction and the existence of a cyclone north of the Luzon Island during the looping phase explain why eddies of both signs can pass through the strait. It is shown also that the upstream state of the Kuroshio in the western tropical Pacific plays an important role in dictating the different paths of the Kuroshio. The looping (leaping) path is caused by a weakened (stronger) Kuroshio transport related to the northward (southward) shift of the North Equatorial Current in wintertime (summertime).
During El Niño/Southern Oscillation (ENSO) events, the Kuroshio weakens and a large portion of the Philippine Sea water passes through the Luzon Strait. The intensity of the Kuroshio is capable of influencing the seasonal upwelling in the SCS. Seasonal upwelling events along the east coast of Vietnam and west coast of Luzon have been demonstrated by satellite data. In a normal year, a strong eastward jet is associated with the cooling and upwelling off Vietnam. Strong ENSO events have been recorded in the years 1997 and 1998: the satellite data clearly show a vigorous upwelling off Vietnam in August 1997, but a wan one in August 1998. Abnormal warming interrupts the normal upwelling generated off Vietnam and Luzon.
Two defined indicators of sea surface temperature anomaly (SSTA), the upwelling off the coast of East Vietnam in summer and off the coast of west Luzon Island, reveal seasonal upwelling activities. Both these upwellings appear in normal years, but they diminish during the active warming period. The seasonal upwelling dynamics is dependent on ENSO and is also coherent with the South Eastern Tropical Indian Ocean (SETIO). The intensity of prevailing monsoon varies the proportion of the response to oceanic circulation and heat content, that is, weak monsoons do not promote water in vertical motion and less heat is released. The ocean heat content anomaly (OHCA) indicates whether the budgets for both the ascending and descending heat content are initiated at the western boundary. Although the OHCA in conjunction with the vortex has not been directly reported, the results of model dynamics studies are favorably comparable with the satellite data.
Large-scale Indian-Pacific Ocean meteorological variations have certain strong impacts on the SCS. The teleconnection between ocean and atmosphere shows that the seasonal upwelling is controlled by the complex interplay between the internal and external sea-air interacting processes. The regional monsoon system changes rapidly in response to oceanic variations. The weak wind stress curl during the evolution of the La Niña cycle of 1998 affects the atmosphere-ocean coupling and hampers the generation of the upwelling. Variations in both the large-scale air-sea interaction and the strength of the Kuroshio transport impact (1) the generation of vortices over the SCS and (2) the path of movement vortices in the Luzon Strait.

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