Diagnosis of 3D Vertical Circulation in the Upwelling and Frontal Zones East of Hainan Island, China

Using the generalized omega equation and cruise observations in July 2012, this study analyzes the 3D vertical circulation in the upwelling region and frontal zone east of Hainan Island, China. The results show that there is a strong frontal zone in subsurface layer along the 100-m isobath, which is characterized by density gradient of O(10(-4)) kg m(-4) and vertical eddy diffusivity of O(10(-5) -10(-4)) m(2) s(-1). The kinematic deformation term S-DEF, ageostrophic advection term S-ADV, and vertical mixing forcing term S-MIX are calculated from the observations. Their distribution patterns are featured by banded structure, that is, alternating positive-negative alongshore bands distributed in the cross-shelf direction. Correspondingly, alternating upwelling and downwelling bands appear from the coast to the deep waters. The maximum downward velocity reaches -5 x 10(-5) m s(-1) within the frontal zone, accompanied by the maximum upward velocity of 7 x 10(-5) m s(-1) on two sides. The dynamic diagnosis indicates that S-ADV contributes most to the coastal upwelling, while term S-DEF, which is dominated by the ageostrophic component S-DEFa, plays a dominant role in the frontal zone. The vertical mixing forcing term S-MIX, which includes the momentum and buoyancy flux terms S-MOM and S-BUO, is comparable to S-DEF and S-ADV in the upper ocean, but negligible below the thermocline. The effect of the vertical mixing on the vertical velocity is mainly concentrated at depths with relatively large eddy diffusivity and eddy diffusivity gradient in the frontal zone.