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

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<sup>−1</sup> and 30 (15) cm s<sup>−1</sup>, 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

Eulerian and Lagrangian Statistics in the South China Sea as Deduced from Surface Drifters

“…The values of M(t) in regions 3-5 are negative at small time lags but become positive at large time lags, indicating that these regions are dominated by anticyclonic eddies at small scales but by cyclonic circulations (e.g., mean flow) at large scales. These results are consistent with previous eddy-related studies over the SCS (e.g., Li et al 2011;Wang et al 2003;Xiu et al 2010). Using satellite altimeter data, Wang et al (2003) showed that the southwest of Taiwan (roughly region 2) is dominated by anticyclonic eddies (see their Fig.…”

“…The angular momentum analysis for the asymmetric parts of the diffusivity tensor (k 12 and k 21 ) indicates that, in some regions (including the east and the northwest of the Luzon Strait and the exit region of Mekong River), the flows are dominated by anticyclonic eddies, while in other regions, such as the northern SCS, the flows are characterized by anticyclonic eddies at small scales but by cyclonic circulations at large scales. These results are consistent with previous eddy studies (e.g., Li et al 2011;Wang et al 2003;Xiu et al 2010), indicating that the computation of asymmetric parts of diffusivity tensor could be a valuable tool in accessing eddy features.…”

“…11). Li et al (2011) illustrated statistical features of eddies also using Lagrangian drifter data. Their results show (see their Fig.…”

“…Chow et al (2008) illustrated the typical features of Dongsha cyclonic eddy based on drifter observations and found that ;90% of the Dongsha cyclonic eddy formed in winter or spring except one that was influenced by a tropical cyclone. Li et al (2011) made a more systematic study of the eddy features in the northern SCS using drifter data up to 2010.…”

Oceanic eddy formation and propagation southwest of Taiwan

“…The analysis of eddy polarity by Chen et al [2011] also showed that there were more CEs than ACEs generated southwest of Taiwan. Li et al [2011] used drifter data from 1979 to the present to identify eddies in the northern SCS. Because of its sparse coverage compared with satellite altimeter data, drifter data might miss many eddies.…”