Tidal Cycles

Due to the mobility of water, the tidal effect on the oceans is differnt than the effect on the land mass. If water could move unobstructed by continents it would move slowly with the tidal bulges whilst the planet is rotating underneath. On an Earth without continents tidal bulges would move westwards and every place on the planet would experience two high and low tides of equal size on every lunar day.

In reality continents are blocking the free motion of water. The observable rise and fall of the sea level is influenced strongly by shoreline topography, ocean currents and the distribution of the continents. As a result different tidal cycles can be experienced in different regions of the world. They are called semi-diurnal, diurnal or mixed semidiurnal tidal cycles.

Distribution of tidal cycles on earth. Based on this NOAA image

In this chapter we will have a closer look at the sea level change on a global scale and on the three different types of tides found on Earth.

Semidiurnal Tides

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 cycle 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 [5]. 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:

Left: Tidal height vs. time for a semidiurnal tidal cycle.
Right: Global distribution of semidiurnal tides.

Diurnal Tides

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.

Left: Tidal Height vs. time for a diurnal tidal cycle.
Right: Global distribution of diurnal tides.

Mixed Tidal Cycle

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.

Left: Tidal Height vs. Time for a mixed tidal cycle.
Right: Global distribution of mixed tides.