The South China Sea (SCS) summer circulation often has a dipole structure associated with an eastward jet, appearing off central Vietnam. The dipole has an anticyclonic eddy (AE) south of the jet and a cyclonic eddy (CE) north of it. The life cycle of the dipole structure is analyzed using satellite altimetry data and a reduced gravity model. On average the dipole structure begins in June, peaks in strength in August or September, and disappears in October. The dipole evolution lags behind the basin scale wind by about 40 days, and 40 days are exactly what it takes for baroclinic planetary waves to cross the southern SCS. Our results show that the vorticity transports from the nonlinear effect of the western boundary currents are crucial for the generation of the dipole structure. In addition, the strength and direction of the offshore wind jet also play a significant role in determining the magnitudes and the core positions of the two concomitant eddies.
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