An advanced artificial neural network classification algorithm is applied to 20 years of multisatellite geostrophic velocity data to study the Kuroshio intrusion onto the southern shelf of the East China Sea. The results suggest that the on-shelf intrusion may occur year round, but winter intrusion events are more frequent than summer events. Both stronger intrusion (which is closely correlated to surface heat flux gradient) and weaker intrusion (which is correlated to wind forcing) occur during wintertime, but the former dominates in late winter while the latter prevails in early winter. There is a significant year-to-year variation of the winter on-shelf intrusion. Although on-shelf intrusion is the major characteristic of the region during wintertime, seldom intrusion events have been identified in the winters of 1997-1998 and 2002-2003. Two conditions are responsible for the cause of the nonintrusion events. During the two nonintrusion winters, the upstream Kuroshio transport anomaly is insignificant (the Kuroshio is not weakened) and no significant winter cooling off northeast Taiwan. Thus, the Kuroshio tends to flow along the shelf break following the 200 m isobath and on-shelf intrusion ceases. Those two nonintrusion events take place during the winters when both the Ni~no 3.4 index and the PDO index are large and positive.
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