Tidally Induced Cross-Frontal Mean Circulation: Analytical Study*

Abstract

An analytical model is developed to study the tidally induced mean circulation in the frontal zone. Four distinct forcing mechanisms are identified, which result in the generation of the counterclockwise Bernoulli cell, the clockwise Ekman cell, the clockwise frontal cell, and the Stokes drift (facing in the direction with the shallow water to the left). The decomposition of the cross-frontal circulation provides a dynamical framework for interpreting and understanding its complex structure. To illustrate the underlying physics, three model configurations are considered pertaining to a homogenous ocean and winter and summer fronts. For a homogeneous ocean, the circulation is dominated by three cells; for the winter front, the offshore Bernoulli cell is strengthened; and for the summer front, two counterrotating cells are found in the vertical direction, associated with the two branches of the front. The dependence of the cell structure on the Ekman, Burger, and other dimensionless numbers is examined.

Numerical study of the diapycnal flow through a tidal front with passive tracers

“…The Eulerian along-frontal current (Figure 1b) flows in the direction with the shallow water on the right side. The Eulerian cross-frontal circulation (Figure 1c) is in a multiple-cell structure [Dong et al, 2004;Dong et al, submitted manuscript, 2003]. The vertical viscosity decreases dramatically in the frontal zone due to the presence of stratification.…”

Tidally induced upwelling off Yangtze River estuary and in Zhejiang coastal waters in summer

“…In the coastal seas with strong tidal signals, tidal mixing often results in easily discernable fronts. For the difficulty of field observation [20] , two-dimensional models are commonly employed in the studies of cross-frontal circulation [20][21][22][23] . Given ideal density fields, the models often predict cell structure on the transect perpendicular to shoreline.…”

“…Given ideal density fields, the models often predict cell structure on the transect perpendicular to shoreline. Depending on the density distribution, such circulation can be composed of one single cell [21] , or two cells including a dominant and a subsidiary component [22,23] .…”

“…The bottom flow of the cell is onshore, while the upper flow is offshore. Studies also show that the frontal circulation can be driven by tidal rectification, which is irrelevant to density field [20,23] .…”

“…Simpson et al [18] pointed out the possible vertical motions near the front on the basis of observed sea surface convergences and SST minima. The previous two-dimensional model studies indicate that typical frontal density structure, which is prescribed in the model, results in cross-frontal cells in the vertical plane [20][21][22][23] . The bottom flow of the cell is onshore, while the upper flow is offshore.…”