Spatiotemporal Evolution Of The Current-Eddy Structure Southwest Of Taiwan

The Kuroshio with high temperature and salinity intrudes into the South ChinaSea (SCS) mainly through the Luzon Strait, affecting the temperature, salinity,circulation, and eddy generation in the northern SCS. Due to lack of observationaldata and credible numerical model results, it is still a debated issue on how theKuroshio intrudes into the SCS. There is not an objective criterion to distinguish thethree types of Kuroshio intruding path off southwest of Taiwan: Kuroshio branch inthe SCS, Kuroshio Current Loop, and the Kuroshio leaps across the Luzon Strait. It isa region with high probability of eddy occurrence southwest of Taiwan. However, it isunclear in detail on the eddy formation, propagation, and dynamical mechanism. Doesits formation relate to the variation of different Kuroshio intruding path southwest ofTaiwan? This paper focuses on the spatiotemporal structure and evolution ofCurrent-Eddy southwest of Taiwan using historical hydrological data, Argo float data,satellite observational data, and numerical model data. An objective criterion wasconducted to distinguish the three types of Kuroshio intruding path off southwest ofTaiwan. We inspected that variation of different Kuroshio intruding path is the mainmechanism for eddy formation southwest of Taiwan. We also found that both theKuroshio intrusion into the SCS and eddy occurrence southwest of Taiwan have aweakening trend over the past two decades. The detail new ideas in this study arelisted as below:(1) Using satellite altimeter data, an objective criterion was created to distinguish thethree types of Kuroshio intruding path off southwest of Taiwan. The three types ofKuroshio intruding path were named as the leaking path, the looping path andleaping path, respectively. The leaking path is characterized with a branch of theKuroshio flowing into the northern SCS along the northern slope of the SCS. Thisbranch can reach farthest to112°E in longitude and18°N in latitude. Thelooping path is characterized with the Kuroshio flowing into the SCS in the middle of the Luzon Strait and flowing out in the northern part of the Luzon Strait,making an anticyclonic current loop southwest of Taiwan. A little water from theKuroshio may leak into the SCS. For the leaping path, the Kuroshio doesn’t cross120°E. The transport through the Luzon Strait is the least, while the SCS watermay flow into the western Pacific. The Kuroshio can transform from one path toanother in several weeks. This criterion was used to the output from HYCOMmodel, the characters of the vertical layer for the three Kuroshio paths weredepicted. Using the model data, the primary dynamical mechanism controlling thethree paths was also analyzed.(2) Formation and propagation of eddies southwest of Taiwan were investigated using17-year satellite altimeter data and W method. Cyclonic eddies (CEs) andanticyclonic eddies (ACEs) often coexisted, but the number of CEs is1.5timeslarger than the number of ACEs during the period from October1992to October2009. ACEs were stronger, and in general, lived longer than CEs. Occurrence ofACEs was more often in winter than other seasons, while CEs were more frequentin summer. ACEs likely propagate westward, bypassed the Dongsha Islands, andreach to the interior of the SCS, while CEs didn’t pass Dongsha Islands. Thepropagation speed of eddies varies from2cm/s to11cm/s during their lifetime.The radius, SLA, and relative vorticity of eddies also vary during their lifetime,suggesting complex interactions between eddies and the ambient currents in thenorthern SCS.(3) Correlation analysis of the local WSC and eddy activities suggested that the effectof local WSC on eddy occurrence was limited. Compared to the direct local windforcing, the Kuroshio path variability appears to be a dominant factor for eddyformation in this area. A conceptual model of eddy-Kuroshio interaction isproposed. In summer, there exists an outflow northwest of Luzon Island and theKuroshio likely leaps across the Luzon Strait. To the north of the outflow and leftof the Kuroshio axis, CEs are often formed, which in turn induce ACEs to thewest of CEs. In winter, under the influence of northeasterly monsoon the KuroshioCurrent Loop (KCL) appears southwest of Taiwan more frequently than other seasons, and ACEs are frequently shed from the KCL. Most of the ACEspropagate westward, and as a result, CEs are often spun up to the east of the ACEs.In this model, variability of the Kuroshio path is emphasized as the control factorfor eddies forming southwest of Taiwan, while the direct effect of local wind isless important. However, monsoons are important in driving the SCS outflow insummer and the KCL in winter, both of which have important effects on theseasonal patterns of the eddy occurrence. Therefore, the indirect effects ofmonsoon winds on eddy occurrence can’t be ignored.(4) The Kuroshio intrusion into the SCS has a weakening trend over the past twodecades derived from observed hydrological data in1990s and2000s. Satelliteand Regional Ocean Modeling System (ROMS) model results also show that theKuroshio intrusion into the SCS becomes weak from1993to2010. Associatedwith the weakening Kuroshio intrusion, the Kurosio Current Loop and eddyactivities southwest of Taiwan become weaker, whereas the salinity aboveintermediate minimum salinity becomes smaller in the northern SCS. Besides thedirect wind-induced Ekman transport, piling up of water induced by themonsoonal winds is an important mechanism changing the pressure gradientacross the Luzon Strait and eventually affecting the transport through the LuzonStrait. The pressure gradient between the western Pacific and the SCS becomessmaller over the past two decades which will contribute for the weakeningKuroshio intrusion.In summation, using available observational data, and numerical model data,spatiotemporal structure and evolution of Current-Eddy Southwest of Taiwan wasstudied, which will contribute to study on dynamical mechanism for Kuroshiointrusion, eddy formation dynamics in the SCS, eddy-flow interactions, eddy-eddyinteractions, theory of western boundary current, and so on.