Using merged sea level anomaly and absolute geostrophic velocity products from satellite altimetry and Argos drifter data,we analyzed the reversal process of the South China Sea(SCS) western boundary current(SCSwbc) from a summer to winter pattern in 2011 and important oceanic phenomena during this process.Results show that the outbreak time of the northeast monsoon over the southern SCS lagged that over the northern SCS by about 1 month.During the SCS monsoon reversal period,the SCSwbc reversed rapidly into the winter pattern at the Guangdong continental slope in late September.Subsequently,the southward Vietnam coastal boundary current strengthened.However,the northward Natuna Current maintained a summer state until mid-October.Thus,the balance between the southward and northward currents was lost when they met,their junction moved gradually southward.However,a loop current formed southeast of Vietnam because the main stream of the Vietnam Offshore Current(VOC) remained near its original latitude.Meanwhile,the VOC and associated dipole circulation system strengthened.After midOctober,the northward Natuna Current began to weaken,the loop current finally shed,becoming a cool ring.The VOC and its associated dipole sub-basin circulation system also weakened gradually until it disappeared. Using merged sea level anomaly and absolute geostrophic velocity products from satellite altimetry and Argos drifter data, we analyzed the reversal process of the South China Sea (SCS) western boundary current (SCSwbc) from a summer to winter pattern in 2011 and important oceanic images during this process.Results show that the outbreak time of the northeast monsoon over the southern SCS lagged that over the northern SCS by about 1 month. During the SCS monsoon reversal period, the SCSwbc reversed rapidly into the winter pattern at the Guangdong continental slope in late September.Subsequently, the southward Vietnam coastal boundary current strengthened.However, the northward Natuna Current maintained a summer state until mid-October.Thus, the balance between the southward and northward currents was lost when they met, their junction moved quite southward.However , a loop current formed southeast of Vietnam because the main stream of the Vietnam Offshore Current (VOC) remains near its orig inal latitude.Meanwhile, the VOC and associated dipole circulation-system strengthened. After midOctober, the northward Natuna Current began to weaken, the loop current finally shed, becomes a cool ring. VOC and its associated dipole sub-basin circulation system also weake gradually until it disappeared