Intrusion Phenomena Of The Kuroshio And Seasonal To Decadal Variations Of The Volume Transport In Main Strait Passages In China Seas

Based on long term grid data from HYCOM and OFES model outputs, combined with the WOA13 data, this paper investigated two shelfward intrusions of the Kuroshio and seasonal to decadal variations of the volume transport in the main passages in C hina seas.Shelfward intrusion of the Kuroshio not only affects the shelf temperature and salinity characteristics and the shelf circulation, offshore C hina, but also supply nutrients that enhance biological activity there. In this paper, based on previous researches, using buoy measurements and model outputs, we studied the two invasions in the Luzon Strait area and area off northeast Taiwan. We analyzed water mass properties of the Luzon Strait area and found that the T-S scatter diagram of each point in the study area shows inversed S-shaped structure, the isopycnal 25.8 is its interface; Above the interface, the temperature and salinity property of the study area is between the South China Sea water and the K uroshio water, and shows significant characteristic of subsurface high salinity water. Positions closer to the west correspond to water mass characteristics closer to the South C hina Sea water; while positions closer to the east correspond to characteristics closer to the K uroshio water. Latitude sections closer to the southern tip of Taiwan Island correspond to water mass properties closer to the K uroshio water, while latitude sections near Luzon Island, the seawater property distributes uniformly between the South C hina Sea water and the Kuroshio water. Below the isopycnal 25.8, the salinity decreases from the west to the east.In order to obtain low frequency variations of the upper- layer Luzon Strait volume transport(LST), we use the 62-year(1950~2011) hindcast outputs from an eddy-resolving ocean general circulation model revealed that LST has prominent interannual(roughly 3 years and 7 years) and decadal(roughly 14 years) variabilities. To seek local factors affecting the low frequency oscil lation of LST, deseasonalized anomaly fields of wind stress curl(WSC) over the Luzon cold eddy(LCE) active area were then analyzed using empirical orthogonal function(EOF), and dominant EOF modes of WSC anomaly were achieved. Results show that spatio-temporal variability of the first EOF mode is of prominent decadal oscillation with period of roughly 14.2 years and decadal southward shift of positive phase of the WSC anomaly, while the second EOF mode is of interannual oscillation of roughly 3.3 years and interannual northward shift of positive phase of the WSC anomaly. In addition, the near 7-year noticeable interannual oscillation can be regarded as an imprint of the variability of Kuroshio volume transport at its origin.Waters off northeast Taiwan is a major intrusion area after the K uroshio flows into the East C hina Sea and exchanges with shelf water. We obtained major temporal and spatial variation characteristics by doing vector EOF analysis to the current field anomalies off northeast Taiwan. The spatial field of the first EOF mode mainly reflects the Kuroshio’s mainstream oscillation accompanied with a cyclonic/anticyclonic circulation structure near 122.8°E across the 200 m isobaths, which represents the anti-phase relationship between the Kuroshio mainstream oscillation and its invasion onto the shelf off northeast Taiwan. This is consistent with the negative correlation between the K uroshio intensity change and its intrusion onto ECS shelf. Strong intrusion occurs near 122.2°E in winter, while weak or no intrusion in summer. Meanwhile, the first mode also shows the interannual characteristics, for example, invasions are extremely weak in summer of 1995, 1997 and 1999. Unlike the first mode, the second EOF mode represents oscillations on both sides of the Kuroshio mainstream and the accompanied cyclonic/anticyclonic circulation structure off north Taiwan. When time coefficients are positive, the K uroshio mainstream swings towards the continental shelf and can rush onto the 200 m isobaths, but it does not contribute to the shelfward intrusion off north Taiwan Island; when the time coefficient is negative, the Kuroshio mainstream swings towards ocean, and it is beneficial to the K uroshio intrusion to the north shelf off the Taiwan Island.There is a very good consistency between the seasonal variation of the surface- layer(0~60m) volume transport in the channels of the ECS and that of the surface wind stress. The first EOF mode of the ECS wind stress anomaly shows significant seasonal transition of East Asian monsoon between winter and summer, and there is almost no interannual change: since channel area along the Ryukyu Islands is controlled by northeasterly winds in winter, Ekman transport mechanism promotes flowing into the East C hina Sea through each channel; southwesterly winds in summer promotes flowing out of the ECS. At the same time, EOF analysis of channel transports of surface- layer integration indicates that the first mode time coefficient has not only seasonal but also interannual variations. In particular, EOF analysis of channel transports of entire-layer integration shows that its interannual variation is more significant than its seasonal one. This suggests that the wind stress field impacts the volume transport of each Ryukyu channel only in the surface layer, its impact on the whole layer is weak. In other words, there should be other factors affecting the entire-layer volume transport. For example, on interannual-scale variations, PDO may have certain modulation effect on the interannual variability of the volume transport of each channel in ECS, while the effect of ENSO is weak.