The Princeton Ocean Model is used to study the circulation in the South China Sea (SCS) and its seasonal transition. Kuroshio enters (leaves) the SCS through the southern (northern) portion of the Luzon Strait. The annually averaged net volume flux through the Luzon Strait is ∼2 Sv into the SCS with seasonal reversals. The inflow season is from May to January with the maximum intrusion of Kuroshio water reaching the western SCS during fall in compensation of summertime surface offshore transport associated with coastal upwelling. From February to April the net transport reverses from the SCS to the Pacific. The intruded Kuroshio often forms an anticyclonic current loop west of the Luzon Strait. The current loop separates near the Dongsha Islands with the northward branch continuously feeding the South China Sea Warm Current (SCSWC) near the shelf break and the westward branch becoming the South China Sea Branch of Kuroshio on the slope, which is most apparent in the fall. The SCSWC appears from December to February on the seaward side of the shelf break, flowing eastward against the prevailing wind. Diagnosis shows that the onshore Ekman transport due to northeasterly monsoon generates upwelling when moving upslope, and the particular distributions of the density and sea level associated with the cross shelf motion supports the SCSWC.

[1] Physical mechanisms for the summertime offshore detachment of the Changjiang Diluted Water (CDW) into the East China Sea are examined using the high-resolution, unstructured-grid, Finite-Volume Coastal Ocean Model (FVCOM). The model results suggest that isolated low salinity water lens detected west of Cheju Island can be formed by (1) a large-scale adjustment of the flow field to the Changjiang discharge and (2) the detachment of anticyclonic eddies as a result of baroclinic instability of the CDW front. Adding the Changjiang discharge intensifies the clockwise vorticity of the subsurface current (originating from the Taiwan Warm Current) flowing along the 50-m isobath and thus drives the low-salinity water in the northern coastal area of the Changjiang mouth offshore over a submerged plateau that extends toward Cheju Island. Given a model horizontal resolution of less than 1.0 km, the CDW front becomes baroclinically unstable and forms a chain of anticyclonic and cyclonic eddies. The offshore detachment of anticyclonic eddies can carry the CDW offshore. This process is enhanced under northward winds as a result of the spatially nonuniform interaction of wind-induced Ekman flow and eddy-generated frontal density currents. Characteristics of the model-predicted eddy field are consistent with previous theoretical studies of baroclinic instability of buoyancy-driven coastal density currents and existing satellite imagery. The plume stability is controlled by the horizontal Ekman number. In the Changjiang, this number is much smaller than the criterion suggested by a theoretical analysis.

An analysis of the water level and current data taken in Qiongzhou Strait in the South China Sea (SCS) over the last 37 years (1963 to 1999) was made to examine the characteristics of tidal waves and residual flow through the strait and their roles in the seasonal variation of the SCS circulation. The observations reveal that Qiongzhou Strait is an area where opposing tidal waves interact and a source of water transport to the Gulf of Beibu (Gulf of Tonkin), SCS. A year-round westward mean flow with a maximum speed of 10-40 cm s Ϫ1 is found in Qiongzhou Strait. This accounts for water transport of 0.2-0.4 Sv and 0.1-0.2 Sv into the Gulf of Beibu in winter-spring and summer-autumn, respectively. The outflow from Qiongzhou Strait may cause up to 44% of the gulf water to be refreshed each season, suggesting that it has a significant impact on the seasonal circulation in the Gulf of Beibu. This finding is in contrast to our current understanding that the seasonal circulation patterns in the South China Sea are primarily driven by seasonal winds. Several numerical experiments were conducted to examine the physical mechanisms responsible for the formation of the westward mean flow in Qiongzhou Strait. The model provides a reasonable simulation of semidiurnal and diurnal tidal waves in the strait and the predicted residual flow generally agrees with the observed mean flow. An analysis of the momentum equations indicates that the strong westward flow is driven mainly by tidal rectification over variable bottom topography. Both observations and modeling suggest that the coastal physical processes associated with tidal rectification and buoyancy input must be taken into account when the mass balance of the SCS circulation is investigated, especially for the regional circulation in the Gulf of Beibu.

Kuroshio intrusion into the South China Sea (SCS) has different forms. In this study, a Kuroshio SCS Index (KSI) is defined using the integral of geostrophic vorticity from 118°to 121°E and from 19°to 23°N. Three typical paths (the looping path, the leaking path, and the leaping path) were identified based on the KSI derived from the weekly satellite Absolute Dynamic Topography from 1993 to 2008. The KSI has a near normal distribution. Using ±1 standard deviation (σ) as the thresholds, the leaking path is the most frequent form with the probability of occurrence at 68.2%, while the probabilities of occurrence for the looping path and the leaping path are 16.4% and 15.4%, respectively. Similar analysis is also conducted on the daily Hybrid Coordinate Ocean Model (HYCOM) Global Analysis from 2004 to 2008. The results are generally consistent with the KSI analysis of the satellite data. The HYCOM data are further analyzed to illustrate patterns of inflows/outflows and the maximum/minimum salinity as representatives of the subsurface/intermediate waters. The Kuroshio bending and the net inflow through the Luzon Strait reduce from the looping path to the leaking path to the leaping path. However, the Kuroshio subsurface water intrudes farthest into the SCS for the leaking path. Vorticity budget associated with the different intrusion types is then analyzed. The tilting of the relative vorticity, the stretching of the absolute vorticity, and the advection of planetary vorticity are important for the change of vorticity, whereas the baroclinic and frictional contributions are three orders smaller.

[1] Oceanic eddies are active and energetic southwest of Taiwan. The formation and propagation of eddies in this area were investigated using 17 year satellite altimeter data. Cyclonic eddies (CEs) and anticyclonic eddies (ACEs) often coexisted, but there were more CEs than ACEs generated during the period from October 1992 to October 2009. ACEs were stronger and, in general, lived longer than CEs. ACEs occurred more often in winter than in other seasons, while CEs were more frequent in summer. Compared with the direct local wind forcing, the Kuroshio path variability appears to be a dominant factor for eddy formation in this area. A conceptual model of an eddy-Kuroshio interaction is proposed. In summer, there exists an outflow northwest of Luzon Island, and the Kuroshio likely leaps across the Luzon Strait. To the north of the outflow and left of the Kuroshio axis, CEs are often formed, which in turn induce ACEs to the west of CEs. In winter, under the influence of northeasterly monsoon, the Kuroshio Current Loop (KCL) appears southwest of Taiwan more frequently than in other seasons, and ACEs are frequently shed from the KCL. Most of the ACEs propagate westward, and, as a result, CEs are often spun up to the east of the ACEs. The surface South China Sea outflow in summer and the KCL in winter are, however, likely related to the monsoons. Therefore, the indirect effects of monsoon winds are also evident in the seasonal variations of eddy occurrence.