The self‐organizing map (SOM) is used to study the linkage between the two western boundary currents, the Kuroshio Current and the Ryukyu Current, through the Kerama Gap. Four coherent ocean current patterns, extracted from a numerical model output for the Kerama Gap area, are used to describe the variability of the Kuroshio current axis and the mesoscale eddies in the Ryukyu Current system. The temporal evolution of these four patterns shows a robust cycle with an average period of 120 days. The shift of the Kuroshio current axis is found to be a dominant factor in determining the water exchange in the Kerama Gap, and the eddies associated with the Ryukyu Current are also important in affecting the strength of eddies in the Kerama Gap. The interaction between the Kuroshio Current and the Ryukyu Current through the Kerama Gap as revealed by the SOM provides new insights in understanding the water exchange between the East China Sea and the northwest Pacific.

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Abstract. Cyclonic and anticyclonic eddies from large scale to submesoscale in the northern South China Sea (NSCS) have been statistically characterized based on the satellitetracked Lagrangian drifters using our developed geometric eddy identification method. There are in total 2208 eddies identified, 70 % of which are anticyclonic eddies. If the submesoscale eddies are eliminated, the other eddies in the NSCS will show a 1.2:1 ratio of the number of anticyclones (210) to the number of cyclones (171). The spatial distribution of the eddies is regional: in southwest of Taiwan, the number of anticyclones dominates the number of cyclones, and most of them are the submesoscale anticyclones with small radii; in contrast, the large and medium cyclonic eddies are a little more than the same scale anticyclonic eddies in northwest of Luzon. The temporal distribution of eddy number in the NSCS has a close relation with the Asian monsoon. The number of the large and medium eddies peaks during the winter monsoon, while the submesoscale eddies are apt to generate in the summer monsoon. The spatial and temporal patterns have a good agreement with the results of the sea surface height anomaly (SSHA). The maximum and mean tangential velocities of anticyclones (cyclones) are 40 (30) cm s −1 and 25 (15) cm s −1 , respectively. The calculated normalized vorticities from drifters suggest that although the mesoscale eddies may be considered in geostrophic balance, ageostrophic dynamics and centrifugal effects may play an important role for the growth and decay of the mesoscale cores.

Cyclones and anticyclones from large scale to submesoscale on the Northern South China Sea (NSCS) have been statistically characterized based on the satellite-tracked Lagrangian drifter data using a geometric eddy identification method. There are totally 1972 eddies identified, 4/5 of which are anticyclonic eddies. If the submesoscale eddies are eliminated, the other eddies in the NSCS will show a 1.1–1 dominance with the number (133) of anticyclones over the number (122) of cyclones. The spatial distribution of all the eddies are: in Zone Z1, the number of anticyclones dominate the number of cyclones, most of which are the submesoscale anticyclonic eddies with small radii; whereas, in Zone Z2, cyclonic eddies are a little more than anticyclonic eddies. The temporal distribution of eddy number in the NSCS has a close relation with monsoon. The number of the large eddies peaks during the winter monsoon, while they tend to decrease quickly in the transition periods of monsoon. In contrast, submesoscale eddies are likely to generate in the summer monsoon, which may be related to the baroclinic instability in the NSCS. The spatial and temporal patterns have a good agreement with the results of SSHA. The maximum and mean tangential velocities of anticyclones (cyclones) are 45 (30) cm s⁻¹ and 30 (15) cm s⁻¹, respectively. Large scale eddies can be considered in geostrophic balance, but ageostrophic dynamics may be important for the submesoscale eddies where centrifugal effects cannot be ignored in the NSCS