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

Abstract. Cyclonic and anticyclonic eddies from large scale to submesoscale in the northern South China Sea (NSCS) have been statistically characterized based on the satellite-tracked 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.