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

Abstract

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) m2 s−1. The kinematic deformation term SDEF, ageostrophic advection term SADV, and vertical mixing forcing term SMIX 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 × 10−5 m s−1 within the frontal zone, accompanied by the maximum upward velocity of 7 × 10−5 m s−1 on two sides. The dynamic diagnosis indicates that SADV contributes most to the coastal upwelling, while term SDEF, which is dominated by the ageostrophic component SDEFa, plays a dominant role in the frontal zone. The vertical mixing forcing term SMIX, which includes the momentum and buoyancy flux terms SMOM and SBUO, is comparable to SDEF and SADV 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.

Response of Coastal Upwelling East of Hainan Island in the South China Sea to Sudden Impact and Long-Term Variability of Atmospheric Forcing

“…The wind-forced offshore Ekman transport is the general mechanism for the coastal upwelling [16,[19][20][21]. In recent decades, with global climate change, the long-term and interannual variability of upwelling, particularly its response to extreme events, such as the El Niño-Southern Oscillation (ENSO) events, has been paid more and more attentions [22][23][24][25][26][27].…”

“…As shown in Figure 1, upwelling east of Hainan Island (UEH) is seasonal coastal upwelling in the northwestern South China Sea (SCS). It occurs mostly within a range of 40 km off the coastline in summer months as the southwesterly monsoon prevails [16,21,32]. The wind-driven offshore Ekman transport and Ekman pumping, as well as the topography effect, are main mechanisms of the UEH [33][34][35].…”

“…Since pioneer studies in the 1960s, researchers have investigated the hydrographic features and the variability of the UEH [16,31,33,[36][37][38][39]. In recent years, the 3D vertical circulation and diapycnal turbulent mixing in the UEH zone have been investigated from cruise observations [21,40].…”

Effect of a Mesoscale Eddy on Surface Turbulence at the Kuroshio Front in the East China Sea

“…Its energy dissipation mechanism has attracted attention for a long time (Ferrari & Wunsch, ). The ocean front has an important effect on turbulence mixing in the upper ocean (Xie et al, ). Recent field observations have shown that the turbulent kinetic energy (TKE) dissipation rates within the ocean surface boundary layer can be elevated by about 2 orders of magnitude by the Kuroshio front (D'Asaro et al, ).…”

Satellite Observations of Wind Wake and Associated Oceanic Thermal Responses: A Case Study of Hainan Island Wind Wake

“…The major circulation of the SCS is driven by the monsoon wind [8], with a southward coastal current along the western boundary in the winter and a northward coastal current in the summer [9,10]. Concerning regions around Hainan Island, many previous studies have focused on the summertime upwelling systems off the east coast (e.g., [11][12][13]). For example, the work of Su and Pohlmann [13] revealed that the southwesterly and southerly winds in the summer are responsible for the formation of the cold water centers off the eastern Hainan coast, while the combined effects of wind and topography lead to the uneven distribution of the upwelling centers.…”