In the first chapter we learned that according to one definition a tide is a distortion in the shape of one body induced by the gravitational pull of another nearby object. We also learned that on Earth the term is also used to refer to the rise and fall of the sea level. This change in sea level is of course a visible effect of the shape distortion affecting the solid Earth as well as the oceans. In this chapter we will have a closer look at the sea level change on a global scale.
The observable rise and fall of the sea level is influenced strongly by shoreline topography, ocean currents and the distribution of the continents on earth. As a result different tidal cycles can be experienced in different regions of the world. They are described as semi-diurnal, diurnal or mixed tidal cyles.
A semidiurnal tidal cycle is a cycle with two nearly equal high tides and low tides every lunar day. In the world map shown above regions experiencing a semidiurnal tidal cyle are marked in red. They have a period of 12 hours and 25 min, and a wavelength of more than half the circumference of Earth . It is also the type of tidal cycle one could expect from a planet covered entirely with water and without any continents obstructing the free motion of water. By looking at the oceans on Earth we can see that most places experience a semidiurnal tidal cycle. The following diagram shows the sea level change over time for a typical semidiurnal tidal cycle:
A diurnal tidal cycle is a cycle with only one high and low tide each lunar day. Diurnal tidal cycles can be found in the Gulf of Mexico and on the East coast of the Kamchatka Peninsula.
A mixed tidal cycle is a cycle with two high and low tides with different sizes each lunar day. The difference in height between successive high (or low) tides is called the diurnal inequality. Areas with a mixed tidal cycle can be found alongside the West cost of the USA, in parts of Australia and in South East asia.