海南岛莺歌海近岸的潮汐不对称与潮致余流研究

潮汐不对称与潮致余流在河口海岸区的物质输运中扮演着重要角色。已往的研究表明，在驻波占主导的河口海湾中，涨落潮的历时不对称与流速不对称有较为良好的对应关系。而潮致余流主要由地形与潮波的非线性作用所致。本研究以海南岛莺歌海附近为代表，结合实际观测与数值模型，研究复杂地形的开阔近岸区的潮汐不对称与潮致余流。结果表明，在莺歌海近岸区，涨落潮历时不对称皆表现为涨潮历时短于落潮历时，而流速不对称则出现复杂的空间变化。对流速不对称的机制分解表明，研究区的流速不对称主要由K₁、O₁与M₂的相互作用，以及潮余流与各潮汐分潮的相互作用所控制。其中前者产生涨潮流速大于落潮流速的涨潮优势，而后者则与余流的方向相对应，出现多个涨潮优势与落潮优势的区域。总体而言，研究区的流速不对称由余流与各潮汐分潮的相互作用所决定。这表明，采用涨落潮历时的不对称来确定潮汐不对称的方法在开阔近海区可能并不适用。对潮致余流的研究表明，研究区的欧拉余流远大于斯托克斯余流。欧拉余流表现为多个顺时针与逆时针的涡流。涡流分布与地形具有较好的对应关系，潮流沙脊区多发育顺时针涡流，而深槽区则以发育逆时针涡流为主。摩擦力在涡流的发育中起着重要作用。

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.