Tides, which are typically observed along the coasts are the periodic, usually twice daily, rise and fall of the ocean resulting from gravitational interactions of the moon, sun and the earth. Although gravity provides the driving force behind tides, the rotation of the earth, size and geometry of ocean basins, and local climatic conditions play an important role in determining the local magnitude and peroidicity of tides.

Significance of tides

Terms and Definitions

Tidal flow:

Types of tides

Exercise: Describe the tides for the following sites: Captiva Island, Lubec, ME, Ostrov Atlasova, Kurile Islands, Amazon River, Brazil


Forces producing

Coriolis Force: An apparent force caused by the rotation of the earth, which is responsible for the deflection of surface currents toward the right in the northern hemisphere and to the left in the southern hemisphere (Diagram and movie from Topex/Poseidon online tutorial). The Coriolis force plays an important role in the formation of rotating standing waves (kelvin waves) in basins. (Discussed later)

 

Equilibrium Theory of Tide Formation

Newton described the effects of tide generating forces on a theoretical earth covered by an ocean of uniform depth and containing no continents.

The centrifugal force felt at each point on the earth is uniform whereas the lunar G force decreases as a function of distance squared. The total force felt at any point on the earth is the vector sum of these two forces. However, forces directed outward and perpendicular to the Earth's surface are counteracted by the Earth's gravitational field. The bulges are actually the result of the tractive (horizontal component) force that draws the water toward them. In determining tides one also needs to consider the influence of the Sun's gravitation force and the the fact that distances of the Moon and Sun vary relative to the Earth.

Exercise: Go to the moon phase calendar and make a plots for September and October 2003. On each plot mark the occurrence of the Spring and Neap tides.

Other variations:


Dynamic Theory of Tides
Harmonic Method of Tide Prediction
The observed tide is produced from a number of components (partial tides) having a given periodicity, amplitude and phase that can be represented by a sine wave. Each component is determined by the harmonic analysis of a record taken from a gaging station over a period of a year or more. Addition of all these components enables an accurate prediction of future tides.
O1= Principal lunar diurnal (25.82 hours)


Altimetric Determination of Tides

In the past decade sea surface elevations have been accurately measured from the TOPEX/Poseidon and Jason-1 satellites. These satellites contain altimeters that pulse microwaves toward the surface of the ocean and measure the time it takes the pulse to return. The distance obtained is then subtracted from either the distance from the center of the earth (TOPEX/Poseidon) or a reference elipsoid (Jason-1) to obtain the elevation of the ocean's surface. Satellite altimetry can be used to obtain tidal information in the deep ocean where tidal gages are lacking and to more accurately tune current tidal data along the coasts. Tides can now be predicted with an accuracy of 2 centimeters anywhere in the deep ocean.

For more on satellite altimetry go to AVISO How altimetry works.

Summary of factors controlling local tidal range and velocity

Many factors determine the magnitude and timing of tides. First, Considering that a tidal wave is a forced wave controlled by the the gravitational effects of the Moon and Sun on the Earth then variations in the positioning of the Moon and Sun relative to the Earth will cause consistant predictable tidal fluctuations. Second, the coriolis effect produced by the rotation of the earth and the constriction of the wave by ocean basins transforms the wave into a kelvin wave(amphidromic system) that rotates clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. Basin geometry influences the following:

Supperimposed on astronomical tides are the effects of local pressure and wind conditions that can depress or buldge the water's surface or pile up water onshore. Unlike astromonical variations, climatic variation add the less predictable element to tides that are experienced during storm surges.

1 Predicted ocean tides are only 50 cm. Higher tides are caused by amplification over broad continental shelves and funnel shaped embayments, and by resonance in a basin.



Sites to Explore

Online Articles and texts

Software for tide prediction

Online tide prediction

Sea Surface Altimetry

Lindley Hanson/email /Gls214
Department of Geological Sciences, Salem State College, Salem, MA
last updated 10/15/03