Significant Difference of the Summertime Eastward Jet Enhancement in the South China Sea During the El Nino Developing Summers of 1982 and 1997

The interannual variation of the South China Sea (SCS) summertime eastward jet (SEJ) contributes to the regional climate and ecology. Based on the Simple Ocean Data Assimilation data set, this study conducts a case analysis of SEJ in the SCS during two El Nino developing years, 1982 and 1997. In both years, the summer-mean circulation presented a stronger SEJ with an obvious dipole pattern; however, the enhancement of SEJ was significantly weaker in 1997 than that in 1982. The reasons for enhanced SEJ and the difference of the enhancement magnitude (e.g., 256% and 52%) between 1982 and 1997 are explored. The climatological summer-mean southwesterly wind stress is strongest in the middle SCS and decreases northward and southward (i.e., an arch-shaped structure), forming a wind stress curl field conducive to the generation of SEJ. There is a low-level anticyclonic anomaly during El Nino developing summer, with its northern flank enhancing the SCS monsoon. However, the structures of wind anomaly in the SCS are different in 1982 and 1997, which may relate to the earlier or later development of El Nino events modulated by the eastward migration of Madden-Julian oscillation. In 1982, the northern flank of the anomalous anticyclone strengthened the climatological arch-shaped structure, effectively enhancing the wind stress curls and SEJ. Whereas, in 1997, the northward deviation of the northern flank weakened the arch-shaped structure north of 11 degrees N, reduced the positive wind stress curl around 11 degrees N-14.2 degrees N, and caused the weaker enhancement of SEJ in 1997 than in 1982.