Mesoscale Dongsha Cyclonic Eddy in the northern South China Sea by drifter and satellite observations

Near the center of the northern South China Sea (NSCS), the Dongsha atoll is situated on the continental slope that runs from Taiwan to Hainan. A mesoscale cyclonic eddy, south or southwest of Dongsha, seems to appear annually and has been identified as the Dongsha Cyclonic Eddy (DCE). Historical drifter (1988–2005) and satellite altimeter (2000–2005) data were adopted to study the variations to, and characteristics of, the DCE. Some typical features were derived such as mean tangential speed of ∼20 cm/s, moving speed of ∼8 km/d (i.e., 9 cm/s), eddy size of ∼22 × 103 km2, center sea surface lowering (height difference, DIF) of ∼6 cm, and angular velocity of ∼9°/d. Mean tangential speeds of 12–16 cm/s in late spring and summer are slower than those of 16–31 cm/s in winter and early spring. Moving speeds are of the order of the Rossby wave at NSCS latitudes. Variations in the time series of eddy size and DIF are generally similar and show a positive linear relation between them. The DCE starts near Dongsha and moves southwestward along the continental slope at a depth of 1000–2000 m. The eddy peaks with maximum eddy size and DIF when reaching 114°E, and then diminishes south of Hainan because of trapping of the underwater bay formed by the continental slope. The DCE typically exists 1–2 times per year and lasts 1–3 months. Roughly 90% of observed DCEs formed in winter or spring. A unique DCE was identified in the summer of 2000 and lasted for over 4 months. Its birth was associated with typhoon Kai‐Tak. The positive linear relationship between eddy size and DIF in this case was unique before reaching 114°E and then returned to the typical relationship of other DCEs. The DIF of this unique DCE attained the highest of 24 cm right at the climax longitude. According to AVHHR and SeaWIFs images, such a DCE is associated with a core of which sea surface temperature is ∼2°C lower and chlorophyll‐a concentration is ∼0.1 mg/m3 higher than those of surrounding seas. Migration of the DCE may be linked to tuna fish farms in the NSCS.