Climatic Controls on the Interannual Variability of Shelf Circulation in the Northern South China Sea

We comprehensively investigate the interannual variability of multi-scale motions in the northern South China Sea (NSCS) and associated influences from the terrestrial, atmospheric, and oceanic processes over the period 1994-2018 based on observations and a numerical simulation. We find that the interannual variabilities of the NSCS circulation and hydrographic properties, in the form of a cross-scale interactive dynamic system, are sensitive to the tropical climate variability represented by the El Nino-Southern Oscillation (ENSO). In coastal areas with less than 30 m depth, the anomalous warm (cold) and less (more) saline waters are regulated by the rate of Pearl River discharge due to the predominant precipitation (evaporation) and positive (negative) heat-flux anomaly in the Pearl River catchment in El Nino (La Nina) years. Controlled by the interannual variabilities of slope current and atmospheric forcings, the shelf current at 30-200 m depth responds asymmetrically to the ENSO, which establishes a stronger northeastward flow anomaly in El Nino years than the southwestward flow anomaly in La Nina years. We further show that the interannual variability of the slope current is jointly governed by the atmospheric and oceanic processes that tend to offset each other in interannual timescales. In particular, the weakened (strengthened) cyclonic wind stress curl over the ocean basin tends to moderate (intensify) the slope current in the NSCS, while this process is mitigated by the stronger (weaker) Kuroshio intrusion in El Nino (La Nina) years.