The northern South China Sea (NSCS) is a dynamically complex region whose shelf and slope currents are driven by different mechanisms. In this study, we used field measurements to identify clear interannual variations in the circulation related to the El Niño-Southern Oscillation cycle. To investigate the modulation mechanisms, we used a high-resolution numerical model that covers the shelf and slope regions of the NSCS. The results indicate that the stronger southwestward slope current during La Niña and stronger northeastward shelf current during El Niño in summer and winter are largely related to changes in wind forcing. The Kuroshio intrusion into the NSCS does not appear to significantly affect the circulation in the southwestern shelf region.