5

The equilibrium tide model is the simplest Newtonian model to predict principal tidal frequency. It considers that the Moon causes tides due to gravitational differentials over Earth, creating two tidal bulges pointing towards and away to the Moon. This model predicts two high-tides per day.

It is also known that the equilibrium tide model is far from accurate, there are no bulges and due to geography and continents there are many inconsistencies. Thus upgrading to a dynamical theory of tides where one accounts for geographical features one can predict better the tides for Earth. Other problems with the equilibrium model are the lack of Coriolis effect, and that the tidal bulges would have to be several times taller than the depth of the ocean to be synchronous with the rotation. See for example Does Earth really have two high-tide bulges on opposite sides?

As far as I know the Coriolis part is not that important as it only introduces a delay with respect to the Moon.

Now, what I would like to know is what happens in a non-rotating spherically perfect planet covered in water with a moon. Would there be tides? Would there be a bulge? How the prediction changes when the planet is rotating (with a tidally locked moon)? I have been trying to find out but most things I found about the dynamical theory discuss too many new factors at once.

Mauricio
  • 6,886

2 Answers2

2

Yes, even without rotation, you would have tides with bulges. The usual approach is to use the shallow water equations and linearize them to get the Laplace tidal equations. In spherical coordinates $(\theta,\phi)$ for a perfectly spherical Earth covered with a small layer of water: $$ \begin{align} \partial_t h &= - \frac H{R\sin\theta}\left(\partial_\theta(\sin\theta u_\theta) +\partial_\phi u_\phi\right) \\ \partial_tu_\theta &= -\frac gR\partial_\theta h \\ \partial_tu_\phi &= -\frac g{R\sin\theta}\partial_\phi h. \end{align} $$ $h$ is the height displacement (bulges will be positive $h$ values), $u$ is the 2D tangential velocity field. $H=3.7\cdot10^3m$ is the mean ocean depth, $R=6.4\cdot10^6m$ the radius of the Earth, and $g$ is the surface gravity, the usual $9.8m/s^2$. Naturally, $H\ll R$.

Under the influence of an external potential $U$ you need to add the additional terms: $$ \begin{align} \partial_t h &= - \frac H{R\sin\theta}\left(\partial_\theta(\sin\theta u_\theta) +\partial_\phi u_\phi\right) \\ \partial_tu_\theta &= -\frac1R\partial_\theta (gh+U) \\ \partial_tu_\phi &= -\frac1{R\sin\theta}\partial_\phi (gh+U). \end{align} $$

To solve the linear response, decompose the forcing into harmonic components by Fourier transform, then expand in spherical scalar/vector harmonics: $$ h = e^{i\omega t}\sum_{l=0}^\infty\sum_{m=-l}^lh_{lm}Y_{lm} \quad U = e^{i\omega t}\sum_{l=0}^\infty\sum_{m=-l}^lU_{lm}Y_{lm} \\ u = e^{i\omega t}\sum_{l=0}^\infty\sum_{m=-l}^lu_{lm}^{(1)}\Psi_{lm}+u_{lm}^{(2)}\Phi_{lm}. $$ The equations are now algebraic: $$ \begin{align} i\omega h_{lm} &= \frac HRl(l+1)u_{lm}^{(1)} \\ i\omega u_{lm}^{(1)} &= -\frac1R(gh_{lm}+U_{lm}) \\ i\omega u_{lm}^{(2)} &= 0 \end{align} $$ and easily solvable: $$ \begin{align} h_{lm} &= -\frac{\omega_l^2}{\omega_l^2-\omega^2}U_{lm} & u_{lm}^{(1)} &= -\frac{i\omega}{\omega_l^2-\omega^2}\frac{U_{lm}}R \\ u_{lm}^{(2)} &= 0 & \omega_l &= \frac{\sqrt{l(l+1)gH}}R \end{align} $$ with $\omega_l$ the resonant frequency of the mode. In particular, for $\omega=0$ you recover the Newton's static theory of tides: $$ h = -U \quad u = 0. $$ Physically, the water piles up at the minima of external potential and follow them quasi-statically as they rotate.

In your setup, $U$ is only due to the Moon. The simplest model is to assume that the Moon executes uniform circular motion, centered on the Earth. Since it is far away, the tidal forces is approximated by its quadrupole component. Furthermore, aligning the spherical coordinates so that $\theta=\pi/2$ is the plane of the Moon's orbit, you get: $$ U = e^{-i2\omega t}U_{2,-2}Y_{2,-2}+U_{2,0}Y_{2,0}+e^{i2\omega t}U_{2,2}Y_{2,2} $$ with $\omega$ the angular velocity of the Moon's revolution and the amplitudes ($D=3.8\cdot10^8m$ the Earth-Moon distance, $g_m = \frac{GM_m}{D^2} = g\frac{M_m R^2}{M_eD^2} = 3.4\cdot10^{-6}g$ the gravity due to the Moon on Earth): $$ U_{2,0} = -\frac{g_mR^2}{D}\sqrt{\frac\pi5} \quad U_{2,2} = U_{2,2} = \frac{g_mR^2}{D}\sqrt{\frac{6\pi}5} \\ U = \frac{g_mR^2}{D}\left(-\frac34\sin^2\theta\cos[2(\phi-\omega t)]+\frac{3\cos^2\theta-1}4\right) $$ The amplitude of the potential's variations is due to the $|m|=2$ terms: $$ \Delta U = \frac{3g_mR^2}{4D}. $$ You therefore only have a quadrupole response in the height field, so two bulges and tide amplitude: $$ \Delta h = \underbrace{\frac{\omega_2^2}{\omega_2^2-4\omega^2}}_{dynamic}\underbrace{\frac{3g_mR^2}{4gD}}_{static} $$

In this simple model, you therefore just need to compare $\omega$ and $\omega_2$. If $\omega<\omega_2$, then you get you get a qualitatively similar result as in the static case. Only as $\omega$ increases to $\omega_2$, the tides are dynamically amplified by dynamics, diverging at the resonant frequency $\omega_2$. For $\omega>\omega_2$, past the resonance, you have a reversal in the sign (dips rather than bulges) and at very high frequency you have essentially no tides.

Plugging the numbers, the static theory predicts tides of the order of the centimetre: $\Delta U/g \sim 10^{-2}m$. For the dynamic theory, use $\omega = 7.27\cdot 10^{-5} rad/s$ for a period of $1$ day and $\omega_2 = 7.30\cdot 10^{-5} rad/s\sim\omega$ of also roughly a day. Therefore, the $m=2$ harmonic is past the resonance with an attenuation of $-1/3$. You therefore still have tides of the order of the centimetre, but in opposite phase. Geometrically, it predicts the bulges to be a right angle to the Earth-Moon line.

Naturally, this model is too simplistic for modelling actual tides. Rotation needs to be taken into account, which couples the harmonics. It is not negligible, it has the same order of magnitude as the driving frequency and the resonant frequency. Furthermore, to really get large tides as observed on Earth, you need to look at how the dynamics interacts with the topography.

Hope this helps.

LPZ
  • 17,715
  • 1
  • 10
  • 36
0

From a Libretexts book:
Authors: Judith Bosboom, Marcel J.F. Stive
Oceanography, coastal dynamimics

Section 3.8.1

Dynamic theory of tides

Quote from halfway down the page:

Only in the Southern Hemisphere at the latitude of about 65°S, an equilibrium tide can more or less exist. To the south of Africa, South America and Australia the earth is circled by an uninterrupted band of water such that the tidal wave can travel around the earth. Besides, at this latitude the water depth is not so much of a limitation for the propagation velocity so that the original ellipsoid can develop.

In the opening paragraph the authors give for the amplitude of the tidal wave 'order 1 meter'.


The way I understand this:
The band of ocean water at around 65°S is where there is opportunity to transfer energy to the ocean water, sustaining a tidal effect that goes all the way round.

From that region where the tidal wave goes all the way round: waves propagate out and enter the various ocean basins, such as the pacific ocean and the atlantic ocean.

My understanding is that wherever the wave encounters coastline the energy of the wave dissipates.


To the question in the title:

The case of a planet that is perfectly spherical, and tidally locked to its Moon.

I don't know what in that case the height of the water tide would be. Remark: think it's worth emphasizing: that scenerio is not physically realistic.

The solid Earth is in hydrostatic equilibrium. Over geologic time scale the Solid Earth deforms to the shape of lowest energy.

So in the end a planet, tidally locked to its moon, will not have the effect of the water on the surface displaying two liquid tidal bulges above a spherical solid planet. The solid planet will deform down the potential gradient, and the equilibrium state will be one with tidal bulding of the solid planet, so that there won't be a separate liquid tidal bulge; both the solid planet surface, and the water surface will be (globally averaged) parallel to a global equipotential surface.

Cleonis
  • 24,617