A revisit of the interannual variation of the South China Sea upper layer circulation in summer: correlation between the eastward jet and northward branch

The interannual variation of the South China Sea upper layer circulation in summer is revisited based on analysis of current derived from altimetry data, Acoustic Doppler Current Profilers moorings, and numerical simulations. Results show not only the interannual variation of the eastward jet (eastward branch), but also its anti-correlation with the northward branch. On interannual time scale, when the eastward branch is enhanced, the northward branch is weakened, and vice versa. Their variations are largely related to the change of the South China Sea summer monsoon (SCSSM), and are strongly influenced by the Luzon strait Transport (LST). Composite analysis reveals a stronger SCSSM and LST into the SCS in the developing phase of El Niño would lead to an eastward branch dominant circulation pattern, whereas a weaker SCSSM and reduced LST into the SCS in the decaying phase of El Niño favors a northward branch dominant circulation pattern. The distinct composite patterns appear in El Niño and Southern Oscillation cycles, rather than episodic event or multiyear El Niño or La Niña. Contribution of the transport of major straits in the SCS to the interannual variation of the SCS summer circulation is quantitatively evaluated for the first time, and the results show that the change of the planetary vorticity flux through three major straits (Luzon strait contributes most) is as equally important as the vorticity input change from local wind stress curl.