Characteristics And Impact Factors Of The Seasonal And Interannual Variations Of The Surface Kuroshio Onshore Intrusion Northeast Of Taiwan

Kuroshio onshore intrusion(KOI) northeast of Taiwan has an important impact on the circulation structure in East China Sea and coastal ecosystems of China. In this article, we aim at investigate characteristics of KOI northeast of Taiwan and impact factors of its seasonal and interannual variations. Besides, we design and implement a series of sensitivity experiments using ROMS model to explore the role of different impact factors. The finding and conclusions are as follows.Firstly, seasonal characteristics of surface flow are obtained by using WOCE-SVP project drifting buoy data, and features of the Kuroshio onshore volume transport across the 200 m isobaths are obtained by using HYCOM reanalysis data. The main route of KOI is very close to the northern coast of Taiwan island in winter, while the KOI is not happened at surface in summer. The significant spatial patterns and their temporal variations of interannual characteristics are obtained from empirical orthogonal function(EOF) analysis on surface geostrophic velocity anomaly from 1993 to 2014 provided by AVISO. The first EOF mode(EOF1) illustrates a east-west shift of Kuroshio axis and a KOI anomaly pattern in the form of anticyclonic/cyclonic circulation anomaly structure. The second mode(EOF2) shows a throughflow anomaly pattern across the 200 m isobaths and the north-south shift of Kuroshio axis.The impact factors of the multi-temporal-scale variations of KOI are analyzed. The flow direction of upstream Kuroshio truns anticlockwise(clockwise) relative to the climate mode in winter(summer), and the Kuroshio’s axis makes an onshore(offshore) shift, which corresponding to the enhanced(weakened) intrusion. Wind stress forcing maybe make an effect on the intrusion in the late autumn and early winter. Surface heat flux may influence the KOI northeast off Taiwan by cooling in winter. Seasonal changes of the flow through the Taiwan Strait forcing by large-scale wind may have an impact on the KOI.On the interannual scale, the time coefficient of EOF1 shows negative correlations with the upstream flow, zonal wind and sea surface heat flux, while is positively correlated with the direction of upstream flow. A weakened upstream Kuroshio and a relatively anticlockwise shift of flow direction promote the surface onshore intrusion. Strengthened westerly wind and cooling northeast of Taiwan is corresponding to the strengthened intrusion. For the first mode, ENSO, PDO and other oceanic factors may influence the intrusion through a medium such as eddies arrived at east off Taiwan(122.5oE~124oE, 22.5oN~24oN). The time coefficient of EOF2 shows a correlation with the anomaly flow in the range of 122.3oE ~ 122.8oE east of Taiwan, and a negative correlation with the interannual variability of the meridional wind and the volume transport through Taiwan Strait.Finally, a series of sensitivity experiments are implemented to compare the contribution of different factors on the KOI northeast of Taiwan. For the volumn transport across the 200 m isobaths west of 122.8oE, the flow through Taiwan Strait is the most important factors, which strengthen the intrusion in autumn and winter. The upstream Kuroshio can cause obvious onshore intrusion in winter and spring. Wind stress strengthens the intrusion in late autumn and early winter. In winter, the cooling enhances intrusion in February and March. For the interannual variability of the surface and vertical-integrated onshore intrusion, upstream Kuroshio and the flow through Taiwan Strait are the main factors. Wind stress effects the intrusion in the period of the decadal conversion from 1996 to 2001, and has no obvious effect in the rest time. Surface heat flux forcing is not an important impact factor.