Three long-lived anticyclonic eddies in the northern South China Sea

Abstract

[1] In August 2007, three long-lived anticyclonic eddies (ACE1, ACE2, and ACE3) were detected by both satellite sea level anomaly (SLA) map and in situ measurements in the northern South China Sea (SCS). ACE3 had a two-core (ACE3(1) and ACE3(2)) structure. In situ stations along 18°N almost cut through the centers of ACE2 and ACE3 (2). Near the centers of ACE2 and ACE3(2), mean temperature and sound velocity are ∼0.65°C and ∼2 m s −1 larger than those in their surrounding areas, respectively, while mean salinity and density are ∼0.02 psu and ∼0.15 m 3 s −1 smaller than those in their surrounding areas due to downwelling near the eddy cores. The depths of maximum and minimum salinity near the eddy cores are ∼65 m and ∼35 m larger than those in their surrounding areas. The vertical depth with current speed larger than 0.05 m s −1 can reach ∼900 m. Their detailed evolutionary processes were depicted based on the variation of geostrophic currents and the trajectories of five drifting buoys. ACE1 lasted 147 days, while ACE2 and ACE3 lasted 168 days and 210 days, respectively. ACE1 had a smaller mean SLA (18.8 cm) in its lifetime than ACE2 (21.8 cm) and ACE3 (25.3 cm) but had a larger negative mean vorticity (−7.7 × 10 −6 s ). One short-lived anticyclonic eddy that split from ACE2 and another one that merged with ACE3 both had a smaller SLA, negative vorticity, and diameter than ACE2 and ACE3, respectively.

Progress of regional oceanography study associated with western boundary current in the South China Sea

“…Both eddies moved southwestward along the continental shelf in the northern SCS after their generations, and their moving speeds are similar to that of the first baroclinic Rossby wave. Nan et al [58] revealed the generation and dissipation processes and activities of three AEs around 18°N (Figure 4). Their results showed that the life spans of the three AEs were 147, 168 and 210 days, respectively (Figure 4(a)), and that the background circulation was anti-cyclonic (Figure 4(b)).…”

“…The nonlinear vorticity advection of the SCSWBC is of vital importance for the dipole mode (an anticyclonic eddy south of the eastward jet off central Vietnam and a cyclonic eddy north of it) in summer [33]. The [58]. monsoon influences the sea surface height (SSH) through baroclinic Rossby waves [14,60], and the characteristics of the meso-scale eddies could be indentified through the change of SSH anomaly [61,62].…”

“…monsoon influences the sea surface height (SSH) through baroclinic Rossby waves [14,60], and the characteristics of the meso-scale eddies could be indentified through the change of SSH anomaly [61,62]. Two regions of strong meso-scale eddies exist in the SCS: one along the SCSWBC, and the other across the central SCS from southwest to northeast [58]. Sometimes, the circulation in the northern SCS is mainly influenced by the Kuroshio intrusion [63,64], and most of the meso-scale eddies are formed by the Kuroshio intrusion [48][49][50][51][52].…”

A Gaussian-surface-based approach to identifying oceanic multi-eddy structures from satellite altimeter datasets

“…This three-cyclonic-eddy case demonstrated the ability of the proposed approach to split spurious multi-eddy structures and retain the real ones. Further, an eddy merger event which was previously reported with in situ observations [22] was also examined to validate the method's advantage over the splitting strategy [25]. Figure 5a-c are the detection results of applying the multi-eddy structure identification criterion.…”

“…Although various novel algorithms have been developed, few address the problem of identifying the multi-eddy structures (Fig. 1 b), which can physically form when multiple eddies are closely located [21][22][23][24]. Chelton, et al [8] proposed a SSH -based algorithm which may yield multi-eddy structures.…”