Seasonal and mesoscale-eddy-caused variability in the Kuroshio surface current northeast of Taiwan differentiated using nine years of ocean radar observations

Seasonality and mesoscale eddies are the major sources of variability in the Kuroshio current northeast of Taiwan. Although previous studies have acknowledged these sources, none has differentiated their respective contributions. The surface current northeast of Taiwan was observed during 2001–2010 by a high-resolution spatiotemporal ocean radar system deployed on Ishigaki and Yonaguni Islands. We used the data from this system to differentiate between the contributions of the two sources of variability by comparing Kuroshio speed and path during different combinations of the phases of the two sources of variability: seasonality (summer or winter) and eddy type (anticyclonic or cyclonic). We found that the two sources of variability are independent. The period-averaged surface current structures for each combination of phases revealed two typical states for the Kuroshio. One is the “straight, fast” Kuroshio Current, which occurs when anticyclonic eddies arrive in summer (anticyclonic-summer phase) and the other is the appearance of the Kuroshio Branch Current, which happens when cyclonic eddies arrive in winter (cyclonic-winter phase). These typical Kuroshio surface current structures, revealed by classifying the phases of the sources of variability and conditions for their occurrence, should advance our understanding of the Kuroshio and its influences on the surrounding regions.