Effects of the Pearl River plume on the vertical structure of coastal currents in the Northern South China Sea during summer 2008

Analyses of moored current meter data from the South China Sea (SCS) Coastal Oceanographic Process Experiment Pilot Project (SCOPE-PP) reveal that strong spatial variation in the vertical structure of coastal currents in the northern SCS (NSCS) was caused by the interaction between the Pearl River plume (PRP) and the alongshore coastal jet. Observations indicate that the NSCS coastal current was generally eastward, spreading the PRP water eastward. The lighter brackish plume residing on the top of the saline, coastal water strengthened the stratification near the surface and blocked the downward momentum transport, resulting in a strong vertical shear of the velocity at the mooring site S205. Meanwhile, the plume water raised the sea level. Because of the geostrophic balance in the across-shore direction, the eastward coastal current in the upper layer strengthened on the onshore side (near S205) and weakened on the offshore side (near S206 and S305) due to the plume water intrusion. Therefore, the vertical shear of the velocity was enhanced at S205 and reduced at S206 and S305. Moreover, because the plume varied substantially with the wind, the vertical shear of the velocity responding to the wind was different between S205 and S206. The numerical simulation during summer 2008 confirms that the PRP enhances (weakens) the vertical shear of the velocity on the onshore (offshore) side by increasing (decreasing) the upper-layer current by about 0.2m s(-1) and decreasing (increasing) the middle-layer current slightly.