Seasonal And Inter-Annual Variation Of Kuroshio Intrusion Into The Taiwan Strait And Its Physical Mechanisms

As a warm current which had distinct characterist of high temperature and salinity, the Kuroshio had a significant effect on the hydrological environment and ecosystem in the Taiwan Strait(TWS). But so far, researches about the Kuroshio intrusion into the TWS and its effect on biogeochemistry remained qualitative. The quantitative analysis was rarely seen. Based on HYCOM data, as well as using CTD and NCEP data, the aim of this paper was to study the seasonal and inter-annual variation of Kuroshio intrusion into the TWS and its physical mechanism through quantitative analysis.The result of analyzing historical CTD data showd that apart from summer, there were Zhemin coastal current and Kuroshio intrusion water in the TWS. And there was no Kuroshio intrusion water in the TWS in summer. The water mass in the TWS in summer mainly came from South China Sea Current which had relatively lower salinity. According to HYCOM data, this paper first analyzed the seasonal variation of the sea surface temperature and sea surface salinity in the TWS. After that, we showed the Kuroshio intrusion path into the TWS and its seasonal variation throuth studying the salinity distribution in the 22°N section and in the bottom sea. Finaly, the Kuroshio intrusion index(KII) was designed to quantify the degree of Kuroshio intrusion into the TWS through finding the 34.2 psu in the section along the Penghu Channel. The result manifested a distinct seasonal variation of the Kuroshio intrusion into the TWS. The degree of Kuroshio intrusion into the TWS in descending order was Spring, Winter, Summer and Autumn, respectively. The Kuroshio intrusion was blocked in winter under the peak northeast monsoon; With the northeast monsoon in spring weakened, some of the Kuroshio intrusion water could enter the TWS. The intrusion degree reached its maximum in May. The Kuroshio intrusion water was replaced by South China Sea water in Summer, leading the low salinity hydrological environment in the TWS. The pattern in Autumn was the same as that in Summer.As for the study of inter-annual variation of Kuroshio intrusion into the TWS, the EOF analysis was first used to analyze the inter-annual variation of KII. It showed that KII was maximum in 2003 while minimum in 2008. A time-lag correlation analysis was applied to KII and Ni?o3.4 Index. The result showed that the correlation coefficient reach its maximum value 0.34 when Ni?o3.4 Index lead 5 months. Which indicated that the ENSO had effect on the Kuroshio intrusion into the TWS 5 months later. The composite analysis under El Ni?o and La Ni?a event according to Ni?o3.4 Index manifested that the anomalous area of the salinity in the section along the Penghu Channel was mainly above 100 m. When El Ni?o occured, the salinity in Penghu Channel was anomalously high, indicating more Kuroshio intrusion water into the TWS. However, when La Ni?a occured, the salinity in Penghu Channel was anomalously low, indicating less Kuroshio intrusion water into the TWS. Wavelet analysis of KII and Ni?o3.4 Index showed that KII and Ni?o3.4 Index had the same period of 46.8 months, indicating the close relationship between these two time series.Since KII laged 5 months behind Ni?o3.4 Index, it was considered that there was no direct relation between ENSO and the inter-annual variation of Kuroshio intrusion into the TWS. ENSO may affect the degree of Kuroshio intrusion into the TWS through other factors such as wind. The paper futher studied the dipole wind stress curl southwest of Taiwan and its effect on the inter-annual variation of Kuroshio intrusion into the TWS. EOF analysis of wind field and wind stress curl field manifested that the variation of dipole wind stress curl pattern southwest of Taiwan is notably seasonal, which mainly occurs from October to April. For the convenience, the paper defined DWS through caculating the difference value of regional mean wind stress curl field between the near sea area of Penghu and Gaoxiong. The time-lag correlation analysis of WSCI, KII and Ni?o3.4 Index showed that the the correlation coefficient between WSCI and KII reach its minimum value-0.22 when KII laged 3 months behind WSCI. And the correlation coefficient between Ni?o3.4 Index and WSCI reach its minimum value-0.42 when WSCI laged 1 months behind Ni?o3.4 Index. To sum up, ENSO did effect the inter-annual variation of WSCI. The occurance of El Ni?o event would lead to the weakening of the dipole wind stress curl southwest of Taiwan 1 month later, which would futher result in the intensification of the Kuroshio intrusion into the TWS; The occurance of La Ni?a event would lead to the intensification of the dipole wind stress curl southwest of Taiwan 1 month later, which would futher result in the weakening of the Kuroshio intrusion into the TWS. Wavelet analysis of KII and WSCI showed that KII and WSCI had the same period, indicating the inter-annual variation effect of WSCI on KII.