Tidal Effects on the Surface Water Cooling Northeast of Hainan Island, South China Sea

Abstract

Using a 3-D high-resolution ocean model, this paper explores the mechanisms that cause surface and subsurface water cooling northeast of Hainan Island during summer. Results show that two cooling centers in the regions east of Wenchang city and east of the Qiongzhou Strait occur when tides are included in the model. The cooling is primarily caused by large-scale changes in the circulation in the northwestern South China Sea (SCS) through anomalous horizontal pressure gradients caused by inhomogeneous tidal mixing between the near-shore and offshore regions. East of Wenchang city, the offshore tidal currents induce upwelling that brings the near-bottom cold water to the subsurface. For the region east of the Qiongzhou Strait, the tidally induced westward anomalous currents suppress the warm water transport from Qiongzhou Strait, resulting in a compensating intrusion of cold water from surrounding regions. In addition to the large-scale circulation changes, the local tidal mixing also contributes to the subsurface water cooling east of the Qiongzhou Strait by mixing near-bottom cold water with subsurface waters. Interannually, the cooling weakens or intensifies during different (cold or warm) phases of the El Niño-Southern Oscillation (ENSO) events. This is attributed to variability in surface forcings and large-scale circulation associated with different phases of ENSO leading to changes in stratification, which changes the tidally induced horizontal pressure gradient in the northwestern SCS. Plain Language Summary The surface water cooling around the eastern and northern coast of Hainan Island is one of the strongest in the South China Sea (SCS). During summer, strong surface water cooling occurs northeast of the island, which is associated with tidal effects. Variable tidal mixing between the coastal region and the open ocean changes the pressure gradient, which results in significant changes in the circulation in the northwestern SCS leading to cooling of the surface waters. On the interannual scale, the sea water cooling induced by tides northeast of Hainan Island weakens (enhances) during the cold (warm) phases of El Niño-Southern Oscillation (ENSO) events.

Using Stable Isotopes (δ18O and δD) to Study the Dynamics of Upwelling and Other Oceanic Processes in Northwestern South China Sea

“…These results indicate that the upwelling in summer is stronger than that in spring. This conclusion is consistent with previous studies (Guan & Yuan, 2006;Li et al, 2020).…”

“…(2015) found that pronounced surface cooling and upwelling related fronts are present in the NWSCS in summer season. Previous studies indicated that wind forcing, eddies, topography, and tides are the important factors affecting the upwelling in NWSCS (Guan & Yuan, 2006; Li et al., 2020; Xu et al., 2013). In general, most studies are based on physical methods to study water processes and upwelling in the South China Sea.…”

“…Coastal upwelling, as an important part of offshore ocean circulation and material transport, has significant impacts on the regional ecosystems, fisheries, and climate (Jing et al, 2011(Jing et al, , 2015Xiu et al, 2018). Traditionally, the upwelling characteristics are mainly studied by physical methods such as potential temperature and salinity, numerical model, and satellite remote sensing (Jing et al, 2011(Jing et al, , 2015Li et al, 2020;Lin et al, 2016;Xie et al, 2017). However, these methods are difficult to quantify the impact of upwelling.…”

Dynamical Ocean Responses to Typhoon Malakas (2016) in the Vicinity of Taiwan

“…The variations of momentum components of oceanic processes caused by Malakas will be further investigated to determine the contributing factors in Eddies 1 and 2, including advection, the pressure gradient force, the Coriolis force, and vertical viscosity (Li, et al. 2020; Pietrafesa, 1973, 1990; Sun & Pietrafesa, 1992). The momentum equations for the surface layer (without vertical integration) are expressed in Equation for the x and y directions: where is the water pressure.…”

Including Tides Improves Subtidal Prediction in a Region of Strong Surface and Internal Tides and Energetic Mesoscale Circulation

“…In this study, we investigate the influence of the tides on the predictability of the subtidal ocean circulation in a region that experiences both dynamic mesoscale circulation and energetic internal tides. The Philippine Sea region provides an ideal case-study for this work as it exhibits strong internal tides (Alford et al, 2011(Alford et al, , 2015Ramp et al, 2004), tidal mixing in the South China Sea (SCS) (Li et al, 2020), the origins of an important Western Boundary Current system (Rudnick et al, 2011;Yuan et al, 2006) and high eddy activity due to baroclinic instability between the Subtropical Counter Current and the North Equatorial Current (Qiu & Chen, 2010).…”