This chapter provides a descriptive introduction to tides, tidal processes, and tidal forces.
Tides are gravity waves characterized by the rhythmic fall and rise of the sea level. Tidal flow and ebb at the coast is a manifestation of the general rise and fall in sea level caused by a long-wavelength wave motion that affects the open ocean as well as shallow coastal waters. Because of their long periods and long wavelengths, tidal waves behave like shallow-water waves. The horizontal tidal currents are the horizontal movements associated with the rise and fall of these shallow-water waves (see 2.2).
Sea level is varying around a reference value. In an introduction section, we will discuss the shape of the earth and the shape of the mean sea level called the geoid. We will identify different tidal frequencies in observed sea-level measurements. We will then consider the forces that cause the tides. These are gravitational forces exerted by the moon and the sun upon the earth and the oceans. To derive the Tide-Producing Force, we will express the effects of the gravitational attraction of a body orbiting around Earth. In the next sections, we then thus review the relative motions of the earth, the moon, and the sun that give rise to tractive forces that displace the oceans and yield complex patterns of tidal events. In 5.5, we will give a more realistic description of how the tides vary around the world and what are the effects of the Coriolis force.
Note that the Atmosphere is also subjected to tides, and to a lesser extent the solid Earth too. But because Earth is denser and more viscous than water or air, when it reacts to the tidal forces, the forces have already changed. The crustal tides are very small compared to the movements in the ocean. Seawater is free to move and reacts quickly to tidal forces with fluctuations of the sea level.