Abstract. 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.
Eddy characteristics in the northern South China Sea as inferred from Lagrangian drifter data