Coriolis and tidal motion in shelf seas

Introduction

Tidal waves in wide basins are strongly influenced by earth's rotation. Tidal waves turn around so-called amphidromic points, where the vertical motion is almost nil. At some distance from the basin boundaries the tidal current vector describes an ellipse during the tidal period.

Assumptions

We will restrict the discussion to situations where the following assumptions approximately hold:

• a homogeneous shelf sea of uniform depth;
• a single (semi-diurnal) tidal component with radial frequency [math]\omega[/math] ;
• tidal motion driven by the ocean tide at the shelf boundary;
• tide-generating forces of moon and sun are negligible for the selected domain;
• latitudinal variation of the Coriolis parameter [math]f[/math] can be neglected;
• frictionless tidal flow;
• tidal amplitude much smaller than depth;
• current velocities much smaller than tidal wave celerity [math]c[/math].

Tidal equations

Inclusion of Coriolis acceleration, [math]f=2\Omega \sin \phi[/math], in the tidal equations yields:

[math] \partial u / \partial t - f v + g \partial \eta / \partial x =0 ,[/math]

[math] \partial v / \partial t + f u + g \partial \eta / \partial y =0 ,[/math]

[math] \partial \eta /\partial t + h ( \partial u / \partial x + \partial v /\partial y ) =0 .[/math]

The following conventions are used:

• [math]u,v[/math] are the depth-averaged tidal current velocities in [math]x,y[/math]-directions, respectively;
• [math]\eta[/math] is the tide level;
• [math]h[/math] is the average depth;
• [math]\Omega[/math] is the radial frequency of earth's rotation;
• [math]\phi[/math] is the latitude expressed in radians (positive for the northern hemisphere, negative for the southern hemisphere).

The current velocities [math]u,v[/math] can be eliminated from the tidal equations; this gives

[math] \partial^2 \eta / \partial t^2 + f^2 \eta = gh ( \partial^2 \eta / \partial x^2 + \partial^2 \eta / \partial y^2 ) . [/math]

Dispersion relation

Figure 1: At some distance from the coastal boundaries the tidal current vector describes an ellipse during the tidal cycle. The current vector runs the ellipse in clockwise direction on the northern hemisphere.

The wave solution of the tidal equation has the form

[math] \eta(x,y,t)= \Re [A \exp(ik_x x+ik_y y - \omega t) ] ,[/math]

where the symbol [math]\Re[/math] indicates the real part. Substitution of this solution yields the dispersion relation

[math] gh(k_x^2+k_y^2)=\omega^2-f^2 .[/math]

The dispersion relation shows that non-damped propagating waves ([math]k_x, k_y[/math] real) only exist at low and medium latitudes ([math]\phi \lt 70[/math] degrees), where [math]\omega\gt f .[/math]

For the tidal wave solution the corresponding velocity components are given by

[math] u = \Re[ g \eta (k_x \omega + i k_y f )/ (\omega^2-f^2)] , \; v = \Re[g \eta (k_x \omega - i k_y f )/ (\omega^2-f^2) ] . [/math]

The propagation direction is given by the vector [math]\Re \vec k = (k=\Re k_x, l=\Re k_y[/math]) and the phase speed by [math]c=\omega / \sqrt{k^2+l^2} .[/math] We now choose at [math]x=0[/math] the [math]x[/math]-axis along the propagation direction (i.e. [math]k=\omega / c, l=0[/math]). Then at this particular location we have

[math]\eta=A \cos \omega t, \; u=(A \omega /kh) \cos \omega t, \; v = - (A f/kh) \sin \omega t .[/math]

The current vector ([math]u,v[/math]) thus rotates anti-cyclonically around an ellipse with demi-axes ([math]A \omega / kh, A|f|/kh[/math]), see figure 1.

Kelvin wave

Figure 2: Co-tidal lines (indicated by dashed lines, with phase hour) and lines of equal amplitude (solid lines) according to the solution of G.I. Taylor, for tidal propagation in a rectangular semi-enclosed basin with uniform depth.

A particular solution of the tidal equations corresponds to

[math]k_x= k = \omega /c , \; k_y = if /c , \; c=\sqrt{gh} .[/math]

For this solution, which is called Kelvin wave, we have [math] v=0[/math] and

[math] \eta(x,y,t)= A \exp(-fy/c) \cos(kx-\omega t) ,\; u= (ac/h) \exp(-fy/c) \cos(kx-\omega t). [/math]

The Kelvin wave is a reasonable approximation for tidal waves in the vicinity of a straight coastline [math]y=y_0[/math], where [math]v \approx 0[/math].

The Kelvin wave is largest near the coastline and exponentially damped further away; it propagates along the coast with the coastline at the right hand in the northern hemisphere ([math]f,y\gt y_0[/math]) and with the coastline at the left hand in the southern hemisphere ([math]f,y\lt y_0[/math]).

Amphidromic points

A tidal wave propagating in a rectangular basin of length [math]L[/math] can be described by the sum of two Kelvin waves, an incoming wave propagating in negative [math]x[/math]-direction along the right boundary [math]y=0[/math] and a reflected wave propagating in positive [math]x[/math]-direction along the left boundary [math]y=b[/math],

[math]\eta = -(A/2) \left[ \exp(-fy /c) \cos(kx+\omega t) + \exp(fy/c) \cos(kx-\omega t) \right] .[/math]

The closed landward boundary is at [math]x=0[/math] and the open seaward boundary at [math]x=L[/math]. At [math]x=\pi/2k, y=0[/math] the vertical wave motion is zero; this is the so-called amphidromic point. In long basins another amphidromic point exists at [math]x=3\pi/2k, y=0[/math]. In long wide basins the tidal wave composed of incoming and reflected Kelvin waves turns around amphidromic points, see figure 2.

In the ocean the semidiurnal lunar tidal wave turns around amphidromic points, see figure 3. This illustrates that ocean tides can be approximately characterised by a system of Kelvin waves.

Figure 3: Co-tidal lines (solid lines) and lines of equal amplitude (dashed lines, amplitude in meters) of the semidiurnal lunar M2-tide in the oceans. The tidal range along the coast is indicated in dark for tidal ranges larger than 4 m, in grey for tidal ranges between 2 and 4 m and in white for tidal ranges below 2 m.

Poincaré waves

Because Kelvin waves are not a full solution of the tidal equations, the condition [math]u=0[/math] at [math]x=L[/math] is satisfied only for [math]y=0[/math]. Other wave solutions have to be taken into account near the head of the basin. These are the so-called Poincaré waves, which are periodic in the lateral coordinate [math]y[/math] in order to satisfy the condition [math]v=0[/math] at the lateral basin boundaries [math]y=(0,b) [/math],

[math]v = \sum_{n=1}^{n=\infty} (C_n \exp (\nu_n x) + D_n \exp (-\nu_n x)) \sin (n \pi y /b) \cos(\omega t) .[/math]

From the dispersion relation we have

[math]\nu_n^2=-k_x^2=(n\pi /b)^2-(\omega^2-f^2)/gh .[/math]

In basins which are much narrower than the tidal wavelength, [math]\nu_n[/math] is a real positive number, [math]\nu_n \approx n \pi /b[/math]. In this case Poincaré waves are significant only near the head of the basin and strongly damped elsewhere. The contribution of the Poincaré waves to the tidal elevation can be derived by eliminating [math]u[/math] from the tidal equations. The result is

[math] \eta = \frac{h}{\omega^2+gh \nu_n^2} (f \frac{\partial v}{\partial x} + \frac{\partial^2 v}{\partial y t}).[/math]

The coefficients [math]C_n,D_n[/math] are determined from the boundary conditions

[math]u_K(0,y,t)+u_P(0,y,t)=0[/math] and [math]\eta_K(L,y,t)+\eta_P(L,y,t)=\eta(L,y,t)[/math]

at the landward and seaward boundaries. The subscripts [math]K,P[/math] indicate the Kelvin and Poincaré wave contributions, respectively. The solution was first given by G.I. Taylor in 1921 , see figure 2.

Figure 4: Bottom friction rotates the crestlines of a Kelvin wave in cyclonic direction with respect to the propagation direction (to the left in the northern hemisphere).

Influence of friction

Inclusion of friction influences the tidal wave in several important ways. Analytical expressions can be derived if the tidal equations are linearised. Because calculations are quite lengthy, the reader is referred to the literature , . Here we only mention some major consequences of the inclusion of friction terms in the tidal equations.

(1) The crests of Kelvin waves are not perpendicular to the coastline, but are rotated in cyclonic direction, see figure 4.

(2) The amphidromic points are not situated on the center line of semi-enclosed basins, but they are shifted in cyclonic direction relative to the propagation direction of the incoming tidal wave, see figure 5.

Figure 5:Left panel: Co-tidal lines of the M2-tidal wave in the North Sea. The tidal wave enters mainly from the north, follows the UK coast at the right hand and returns along the Dutch, German and Danish coast. The amphidromic point in the central North Sea is shifted cyclonically with respect to the propagation direction of the incoming tidal wave. In the southern North Sea, the tide is influenced by the tidal wave entering through the Channel; this explains why the amphidromic point is not shifted toward the Dutch coast. Right panel: Tide curves along the Dutch coast at locations I-V. Tidal asymmetry (faster tidal rise and slower tidal fall) increases along the propagation direction (from I-IV). At location V the asymmetry has vanished, because the tide is mainly determined by the amphidromic system of the central North Sea.

Figure 6: Transformation of a rectilinear tidal current near the surface to an elliptic tidal current near the bottom, by the combined influence of Coriolis acceleration and bottom friction.

(3) The current velocity vector rotates in cyclonic direction with increasing distance from the water surface ,, see figure 6. This can be shown by decomposing the tidal current in two circular rotating components, one component rotating in cyclonic direction and the other component in anticyclonic direction. If the magnitude of the two components is the same, the tidal current is rectilinear. Frictional damping depends on the rotation sense. The anticyclonic component is more strongly damped with increasing depth than the cyclonic component; the latter therefore dominates near the bottom.

(4) In shallow coastal zones, with significant tide compared to the depth, the Kelvin wave propagates faster at high tide than at low tide. The tidal wave therefore becomes increasingly distorted in the propagation direction; tidal rise becomes faster than fall. See figure 5. Fast tidal rise causes stronger flood currents than ebb currents in adjacent tidal lagoons and estuaries. This tidal asymmetry stimulates sediment import. It can result in silting up of lagoons and in the formation of turbidity maxima in estuaries.

References

The main author of this article is Job Dronkers Please note that others may also have edited the contents of this article. Citation: Job Dronkers (2016): Coriolis and tidal motion in shelf seas. Available from http://www.coastalwiki.org/wiki/Coriolis_and_tidal_motion_in_shelf_seas [accessed on 23-11-2024] For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Dronkers J