Summertime eastward jet and its relationship with western boundary current in the South China Sea on the interannual scale

The western boundary current (WBC) in the South China Sea (SCS) flows northward in summer, closely following the Vietnam coast, and splits near 11.2 degrees N into one eastward offshore branch named summertime eastward jet (SEJ). SEJ is significant between 110-112 degrees E, and gradually becomes wider, shallower, weaker and warmer toward eastward until it disappears after 113.5 degrees E. Based on the monthly mean Simple Ocean Data Assimilation (SODA) data, SEJ and its relationship with WBC on the interannual scale are studied. At the cross section 111.2 degrees E, the current axis of SEJ (Y-SEJ) swings in the north-south direction with climatological value of 11.7 degrees N and standard deviation of 0.9 degrees, and the standard deviation of surface intensity and volume transport of SEJ (SISEJ and VTSEJ) is about 50% and 88% of the climatological value 28 cm s(-1) and 4.6 Sv, respectively. In El Nino developing summers, SEJ increases its volume by 37% with 0.2 degrees southward deviation, while in the decaying summers, SEJ decreases its volume by 43% with 0.5 degrees northward deviation. More interestingly, SEJ has prominent correlation with the southern WBC (0.54 for VT and 0.41 for SI, > 99%), and both of them strengthen (weaken) in El Nino developing (decaying) summers. The response of SEJ and the southern WBC to El Nino can be attributed to the setting up of the anticyclonic circulation in Sverdrup balance driven by the negative wind stress curl in the southern SCS. The negative wind stress curl controls the southern WBC and further determines the transport of SEJ based on Sverdrup balance.