Abstract:

In this study, we use a layered model to investigate the vertical coupling of the SCS circulation. To reproduce the sandwich-like circulation in the SCS, the diapycnal mixing is parameterized as the exchange velocity (entrainment/detrainment) between the middle and deep layers, and its effects on the SCS circulation are examined. The model results indicate that 1) the cyclonic circulation in the deep SCS appears only when the mixing induces an entrainment of at least 0.72 Sv from the deep to the middle layer, which is equivalent to a diapycnal diffusivity of 0.65×10-3 m2s-1 or a net input rate of gravitational potential energy of 6.89 GW, respectively; 2) tidal mixing in the SCS is stronger than the threshold for forming the cyclonic circulation in the deep layer, but its distribution is essential in determining the circulation pattern such that the mixing needs to reach the whole basin rather than being concentrated near the Luzon Strait; 3) the pattern and evolution of the deep circulation and overturning circulation are closely associated with the variability of the mixing; and 4) the abyssal mixing is able to intensify the anticyclonic circulation in the middle layer but weaken the cyclonic circulation in the upper layer. Vorticity analysis suggests that the upward net flux induced by the mixing leads to vortex squeezing (stretching) and modulates the pressure gradient by redistributing the layer thickness, hence affects the pattern and strength of the circulation in the middle (deep) layer of the SCS, respectively. The depth-integrated effect of the thickness variation across all layers is able to transmit the perturbation from the abyss to the pressure gradient in the upper layer, hence influence the upper-layer circulation.

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(Host faculty: Prof. Jianping GAN)