Numerical Study on Tidally Induced Cross-Frontal Mean Circulation

Abstract

ABSTRACTAn analytical solution for a circulation across a tidal front obtained by Dong, C., Ou, H-W., Chen, D., & Visbeck, M. (2004). Tidally induced cross-frontal mean circulation: Analytical study. Journal of Physical Oceanograph, 34, 293–305) suggests that the cross-frontal circulation can be decomposed into four parts: frontal cell, Ekman cell, Bernoulli cell, and Stokes drift. This study examines the analytical solution thoroughly using a two-dimensional numerical model solving primitive Navier-Stokes equations. The direct comparison between the numerical and the analytical models of three cases (winter tidal front, summer tidal front, and no front) with the same configurations demonstrates that the analytical solution captures the major features of the cross-frontal circulation. A series of numerical experiments are applied to study the sensitivity of the cross-frontal circulation to physical variables: tidal intensity, horizontal topography scale, frontal strength, and vertical eddy viscosity. The ratio of the tidal excursion distance to the topographical scale is crucial to the Ekman cell in the homogenous ocean. With a greater density gradient, both the Bernoulli and Ekman cells are enhanced. Assumptions made in the analytical model are also examined. The uniform eddy viscosity and linearization in the analytical model could overestimate the bottom flow in the Ekman cell and underestimate the Bernoulli cell in the shallower region, respectively. The influence of the internal tide on the cross-frontal circulation is discussed.