Tidal asymmetry and tide-induced residual currents in the Yinggehai Coast, Hainan Island

Tidal asymmetry and tide-induced residual circulation play important roles in mass transport in estuarine and coastal seas. Previous studies have shown that the flood and ebb duration asymmetry corresponds well to the tidal velocity asymmetry in estuaries and embayments where standing tidal waves dominate. The tide-induced residual circulation is mainly from the nonlinear interaction between tides and bathymetry. This study takes the Yinggehai Coast as an example to study the tidal asymmetry and residual circulation in open coast with complex bathymetry, through combination of field observation and numerical simulation. Our results show that the flood duration is shorter than the ebb duration, while the velocity asymmetry manifests a complex spatial variability. The underlying mechanisms are that the velocity asymmetry is mainly induced by the interactions among K₁, O₁, and M₂ and the interactions among the residual flow and tidal constituents. The former contribution results in a flood dominance, while the latter contribution results in a spatial distribution similar to the residual current. Overall, the interactions among the residual current and tidal constituents determine the pattern of the velocity asymmetry. For the tide-induced residual current, the Euler residual is greatly stronger than the Stokes one. There exist many clockwise and anti-clockwise eddies in the study site, and the clockwise eddies are generally associated with the tidal sand ridges well, while the anti-clockwise eddies are located in the deep troughs. The conservation of potential vorticity largely determines the residual circulation pattern, along with the bottom friction playing a critical role as well.