Interannual variability of circulation in the northern South China Sea

The South China Sea (SCS) current system is characterized by strong seasonality in wind forcing.Superimposed on seasonality is the significant interannual variability, especially with weakened circulation gyres and upwelling under the condition of weakened monsoon winds during El Niño. In the northern SCS(NSCS),there are three adjacent band-like currents with alternating directions in winter, namely the Guangdong coastal current, the SCS Warm Current (SCSWC), and the southwestward slope current. A high-resolution numerical model is set-up to investigate the interannual variability of these currents. The model is based on the Regional Ocean Modeling System (ROMS), covering from the western Pacific to the eastern Indian Ocean, with a spatial resolution of 1/24 degree, and it is forced with daily winds and air-sea fluxes of heat and freshwater.Nine-year hydrographic observations spanning from 2004 to 2013 in the NSCS are used to validate the model. Modeled outputs during the observation period are statistically compared with measured temperature and salinity profiles, as well as the Acoustic Doppler Current Profilers (ADCP) measured current profiles for different years. We focus our study on the interannual variability of the currents associated with El Niño and La Niña events. Two driving mechanisms, the Kuroshio intrusion through the Luzon Strait and atmospheric effects via changes in wind and heat flux are diagnosed and compared to infer the dominant interannual control. With the help of nine-year in-situ data, this study provides an opportunity to examine the mechanisms regulating circulation changes in the NSCS.