Oceanography
My Blogs (olelog) are mainly based on my daily reading of earth science news. Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.

Ocean Waves

Notice that the amplitude is equal to one-half the wave height or the distance from either the crest or the trough to the still-water line. So amplitude is not the same as wave height. If the amplitude is 1 meter, the wave height is 2 meter!

When wind blows over the ocean, energy is transferred from the wind to the surface layers. Some of this energy is expended in generation of surface gravity waves Which lead to a small net movement of water in the direction of wave propagation - see below) - and some to drive currents. They are called gravity waves because gravity is the restoring force.

Ocean surface waves are often depicted as sine waves (see above), but experiments have shown that the waveshape is a "trochoid" (see above). The trochoid form of ocean waves changes shape as the amplitude increases for a given wavelength.

Wave propagation

As illustrated by the red dot in the image to the left water particles in a wave over deep water move in an almost closed circular path. At wave crests, the particles are moving in the same direction as wave propagation, whereas in the troughs they are moving in the opposite direction. The particles advance slightly further in the crest than they retreat in the trough, so a small net forward motion results. This forward motion is known as wave drift. In deep water the wave drift can be in the order of several millimetres to several centimetres per second.

At the surface, the orbital diameter corresponds to wave height, but the diameters decrease exponentially with increasing depth, until at a depth roughly equal to half the wavelength, the orbital diameter is negligible, and there is virtually no displacement of the water particles.

Internal Waves

The temperature of the ocean decreases towards the bottom of the ocean. Therefore the density of ocean water increases towards the bottom. The deep ocean is layered with the densest water on bottom and the lightest water on top.

Internal waves form at the interfaces between layers of different water density. Internal ocean waves were first seen from space by American astronauts on their way to the moon (as odd large scale ripples). Now their existence is well documented, but there is still much to be learned.

Oscillations are more easily set up at an internal interface than at the sea-surface, because the difference in density between two water layers is smaller than between water and air. Hence, less energy is required to generate internal waves than surface waves of similar amplitude. Internal waves travel more slowly than surface waves of similar amplitude.

Surface waves can be up to 20 m high, while internal waves can reach a height of 300 m or more, dependent of the thickness of the upper water layer.

Internal waves are of importance in the context of vertical mixing processes. Internal waves may transport plankton and fish larvae to other areas.

← Satellite image (from NASA) of internal waves.

The wave pattern results from the influence of the internal waves on the surface current, rather than the surface height. When the crests of the internal wave pass below, water in the surface layer essentially flows down the sides of the crest and pools into the area overlying the waves’s troughs. Because surface water is diverging over the crests and converging over the troughs, biological material and natural oils on the surface are alternately dispersed and concentrated in a wave pattern. These natural “slicks” calm the water surface and change how it reflects light. These surface slicks reveal the presence of the underlying internal waves. The pattern is especially obvious in areas of the image where the Sun’s reflection off the water, called sunglint, is brightest.

Kelvin Waves

A Kelvin wave is a type of low-frequency gravity wave. There are two types of Kelvin waves, coastal and equatorial. The amplitude of the Kelvin wave is several tens of meters along the thermocline, and the length of the wave is thousands of kilometres. The thermocline is a transition layer between (cold) deep water and (warm) surface water. The warm layer and the cold layer are relatively uniform in temperature, while the thermocline represents the transition zone between the two.

Kelvin waves occur where the deflection caused by the Coriolis force is either constrained (as at coasts) or is zero (as at the Equator). At the Equator Kelvin waves ("equatorial Kelvin wave") can only travel eastwards. Surface equatorial Kelvin waves travel very fast, at about 200 m per second. Kelvin waves in the thermocline are however much slower, typically between 0.5 and 3.0 m per second. They may be detectable at the surface, as sea-level is slightly raised above regions where the thermocline is depressed (see picture and my blog on Kelvin Wave) and slightly depressed above regions where the thermocline is raised.

Coastal Kelvin waves always propagate with the shoreline on the right in the northern hemisphere and on the left in the southern hemisphere.

El Niños often start with a Kelvin wave propagating from the western Pacific over towards South America.

Kelvin waves can be generated by an abrupt change in the winds. this may for example happen in the western Atlantic, so that Kelvin waves travel eastward.

The twice-daily rise and fall of sea-level corresponding to high and low water occurs in the form of coastal Kelvin waves, which progress anticlockwise round ocean basins (i.e. with the coast on the right in the Northern Hemisphere, and clockwise round basins in the Southern Hemisphere. The example shown to the right is from the North Sea in northwestern Europe.

Rossby waves

Rossby waves are also called planetary waves as they owe their origin to the shape and rotation of the earth. They are large-scale motions whose restoring force is the variation in Coriolis effect with latitude, and therefore they always travel from East to West, following the parallels They do not go fast - the speed varies with latitude and increases equatorward, but is of the order of just a few cm/s (or a few km/day, if you prefer). This means that at mid-latitudes (say, 30 degrees N or S) one such wave may take several months - or even years - to cross the Pacific Ocean. Their horizontal scale is of the order of hundreds of km, while the amplitude of the oscillation at the sea surface is just a few centimetres.

Tidal Waves

A tidal wave is the crest of a tide as it moves around the Earth. They are caused by the natural gravitational pull of the sun and moon as opposed to ocean surface waves which are caused by wind friction on the surface of the water.

The magnitude of the Sun's tide-producing force is only about 0.46 that of the Moon, because, although enormously greater in mass than the Moon, the sun is some 360 times further from the Earth

Tsunamis

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