Numerical study on the currents around Taiwan

[[abstract]]A multiple grid-size nesting ocean model system is developed in this work to perform studies on the variations of the flow in the Taiwan Strait and the Kuroshio east of Taiwan. The transport in the Taiwan Strait is studied using the East Asian Marginal Seas (EAMS) model. Three model experiments using different wind data sets (ERA40, NCEP Reanalysis version 2, and QuikSCAT/NCEP blend wind) were performed. Model experiments suggested that the best simulation is achieved when the model is driven by the QuikSCAT/NCEP blend wind forcing. Involving the strong wintertime southward flow events in the Taiwan Strait, the annual averaged modeled transports through the Taiwan Strait is 1.09 Sv (1 Sv=106 m3/s). The result suggests that shipboard Acoustic Doppler Current Profiler (sb-ADCP) observations are biased toward estimates in summer and fair weather since bad weather during the winter northeast monsoon often prevents seagoing observations. Linear regression lines are also proposed to give simple relations between transport and wind stress for roughly evaluating the transport through a known wind stress value. The spatial and temporal variations of the Kuroshio east of Taiwan are investigated using model outputs, surface drifter trajectories, satellite-based altimetric data, and wind data. From the simulation of the EAMS model over a span of 24 years from 1982 to 2005, the variability of the Kuroshio east of Taiwan is studied in detail. Between 22 and 25?N, the mean state and variability of the Kuroshio, such as the two paths observed in the trajectories of surface drifters southeast of Taiwan and the branching of the Kuroshio northeast of Taiwan, are well reproduced by the model. Southeast of Taiwan, the Kuroshio is mostly in the top 300 m in the inshore path but extends to 600 m in the offshore path. Northeast of Taiwan, the Kuroshio follows the shelf edge in the East China Sea, but sometimes branches along a path south of the Ryukyu Islands. The latter path often meanders southward, and a significant portion of the Kuroshio transport may be diverted to this path. The Kuroshio extends from the coast to 123?E ~ 123.5?E between 22?N ~ 25?N with currents reaching a depth of 1000 m at some latitudes. The Kuroshio transports averaged over five sections east of Taiwan are 28.4 ? 5.0 Sv and 32.7 ? 4.4 Sv with and without the contribution from the countercurrent, respectively. Using satellite data and the Seas Around Taiwan (SAT) model simulation, the intra-seasonal variation of the Kuroshio southeast of Taiwan is further studied. Superimposed with the main stream of the Kuroshio, two intra-seasonal signals longer than 2 weeks are revealed in the study region, 20 ~ 30 days and 40 ~ 90 days. The variation of 20 ~ 30 days is only significant between Taiwan and the Lan-Yu Island. A mechanism is proposed to describe how the wind stress curl in the northeastern South China Sea modulates the circulation southeast of Taiwan on this timescale. The fluctuation with a longer period of 40 ~ 90 days is resulted from the westward propagating eddies.