Temporal variability of the current in the northeastern South China Sea revealed by 2.5-year-long moored observations

The current in the northeastern South China Sea (NSCS) has been extensively studied by both observations and numerical models, but its vertical structure and temporal variability are still not well understood because of the lack of long-term and full-depth direct observations. From August 2010 to March 2013, the current velocity in the NSCS was directly observed by Acoustic Doppler Current Profilers from a subsurface mooring at 117°52.07′E, 21°06.68′N. The observed current velocity was coherent in the vertical direction and nearly showed the same sign in the nearly full water column (960 m). The observed current presented strong intraseasonal variabilities (ISVs) within periods between 30 and 90 days. Analysis of the altimeter observation suggested that the ISVs were closely related to the propagating mesoscale eddies. Corresponding to the seasonally varying circulation in the NSCS, the ISV-removed current roughly showed a semiannual cycle. The seasonal U (V) averaged over 80–200 m depths reached the peak in March and October (January and July) and the trough in July and February (April and August), respectively, with a peak-to-trough amplitude of 0.14 m/s (0.19 m/s). Dynamical modes analysis suggested that the observed current velocity showed dominated energy in the barotropic mode during most of the observation period but enhanced energy in the first baroclinic mode during eddy events because of the enhanced vertical shear of velocity.