Quasi-geostrophic (QG) dynamics play a central role in the oceanic mesoscale processes. Previous observational and numerical studies have already suggested that the vertical mode of the QG flow is significantly affected by topography, which leads to a surface-intensified baroclinic mode against the classic first baroclinic mode over a flat bottom. However, this issue has not been attracted any attention in the marginal seas (as far as we are aware), where the slope is two orders of magnitude larger than that in the open oceans. In this study, we use several moorings and remote sensing data to reveal the substantial surface mode in the northern South China Sea (NSCS). Results indicate that the amplitude of the slope current is dominated by a surface-intensified mode, which decays rapidly in a scale of about 1/5 of the local water depth. The flow appears to be dynamically decoupled in the vertical, oscillating as the planetary (topographic) Rossby waves in the upper (lower) layer. The dispersive characteristics of the observed surface energy spectrum is closer to the theoretical curve of the surface mode Rossby wave, which propagates much faster than that in the first baroclinic mode. Moreover, the surface mode is significantly modified by the seasonal circulation of the NSCS and tends to be energetic in a scale larger than 100 km because of a larger deformation radius in the triad interactions.