Why is there a tide?

This is the title of Chapter 4 of an Elsevier volume called “Journey through Tides”

Sophie Ward, David Bowers, Mattias Green, Sophie-Berenice Wilmes,
Chapter 4 – Why is there a tide?,
Editor(s): Mattias Green, João C. Duarte,
A Journey Through Tides,
Pages 81-113,
ISBN 9780323908511,
Abstract: Tides are created by the gravitational pull of the Moon and Sun on the ocean. More exactly, it is the variation in these forces that creates tides. The Earth and Moon are held in orbit by their mutual gravitational attraction. The Moon’s gravity is exactly right at the center of the Earth, but it is a little too strong in the Earth hemisphere facing the Moon and a little too weak in the opposite hemisphere. These discrepancies make the tide generating force. As the Earth spins, the ocean experiences an oscillating force which creates long tide waves – the crest of the wave is the high tide and the trough low tide. In the deep ocean, the amplitude of the tide wave is small, but on the continental shelf, the wave is amplified by resonance, making the large tidal range we see at some coasts.
Keywords: Tides; Tide generating force; Cotidal charts; Tidal dynamics; Tidal dissipation

The domain experts selected to answer this question assert this:

“While this is not an exhaustive list of why the tide is important, it is important to note here that perhaps the most physically far-reaching influence of the tide, long-term, is on the change in day length.”

The day length impact is straight-forward to understand for a rotating solid body as the total angular momentum is conserved between the Earth, smaller Moon, and much larger sun. This is essentially a linear perturbation causing the Earth’s rotational period to slightly change leading the length-of-day (LOD) to cycle. But what is it for the Earth’s oceans, which isn’t pinned to its base?

“Internal waves are another form of gravity wave which occur within the water body on internal interfaces, for example, when the interface between water masses of different densities is disturbed.”

National Central University, Institute of Geophysics, Thesis
Analysis of the wavelet spectrum and elasticities of day length variation over 46 years
Earth’s Response to Long-Term Ocean Tides

Graduate student: Zou Zhiwei
Supervisor: Dr. Feng Zhao
(June 1999, Republic of China)

This thesis does the smart move of taking the LOD derivative to isolate tidal constituents.

Related: Spatio-Temporal Variations of Sea-Level for ENSO: Intercomparison Study of Geodetic Satellite Data

One thought on “Why is there a tide?

  1. Analytical computation of total topographic torque at
    the Core-Mantle Boundary and its impact on tidally
    driven Length-of-Day variations

    Click to access articleTopo_LOD_March2023.pdf

    “Possible amplifications in tidal LOD variations”
    “We look for resonance frequencies that can be close to tidal frequencies corresponding to longperiod tides. In practice, we search for the solutions of the equation. These solutions refer to all periods larger than 2 days. The spectrum is divided in frequency bands with central periods near the tidal periods at 9.1, 9.18, 9.54, 9.61, 13.61, 13.66, 13.81, 14.19, 14.77, 26.88, 27.55, or 182.62 days. These resonances would amplify the total third component of the torque leading to enhanced LOD variations.”

    Effects of non-hydrostatic core-mantle boundary topography and core dynamics on Earth rotation
    Xiaoping Wu, John M. Wahr
    Geophysical Journal International, Volume 128, Issue 1, January 1997, Pages 18–42, https://doi.org/10.1111/j.1365-246X.1997.tb04069.x
    Published: 01 January 1997


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s