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Why does the colour of Earths oceans vary? Coast lines around the world appear to have different shades of blue. The further one travels from the equator north or south the deep blue the ocean becomes. Is this effect a result of ocean temperature or salinity?

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  • $\begingroup$ Re "further from the equator", where are you getting your information? If from satellite photos, the color might be partly an artifact of viewing angle. From my (admittedly limited) experience of being up close & personal with northern waters, they tend to be gray or green rather than blue. $\endgroup$ – jamesqf Dec 3 '17 at 20:25
  • $\begingroup$ The question derives from physically being at various locations around the world and witnessing different colours. I chose to use blue as an example because this is how Earth Oceans are commonly referred by. I do agree that there is various different colour schemes. $\endgroup$ – Jeffrey Boettger Dec 5 '17 at 5:35
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The color of the ocean water does indeed vary depending on the chemical composition of the water. In addition, the color of the ocean in satellite images is affected by the position of the satellite, angel of the sun, and weather.

Color of water in general

Water appears blue because long wavelengths (red or yellow light) are absorbed more efficiently in water than short (blue or green) wavelengths. Therefore, when a water body — in a swimming pool, for example — is illuminated by a white light source (the sun) the light that scatters back from the water tends to be blue rather than red.

The color of the water can further be altered by its chemical composition. In nature, the apparent color of water also depends on the light source, that is, the weather. The same ocean can appear very different in bright sunlight and on a cloudy, stormy day.

Color depends on substances in the water

The color of the ocean water can be greatly altered by substances dissolved or suspended in the water. Typical substances that affect the color are chlorophyll (from waterborne algae) that make water appear green, or suspended particulate matter, such as silts and clay, that cause yellow or brown color. These are clearly seen in coastal areas, where local plankton blooms can turn water bright green, or river plumes carrying a high concentration of suspended particulate matter can be clearly identified from their brown waters. Other substances can alter the color of water as well. Colored dissolved organic matter (CDOM) related to decaying detritus, can turn water yellowish or green, for example. Water temperature and salinity themselves do not significantly affect the color of the water.

The color of the ocean, therefore, does depend on the location. For example, waters in areas with high primary production tend to be more green.

Color of oceans in satellite images

In satellite images, the color of the ocean can be influenced by the relative angle of the sun and the satellite versus the ocean surface. The ocean can appear darker if the satellite is at an oblique angle with respect to the ocean surface and/or if the sun is shining from an acute angle. In the case of geostationary satellites, the ocean near the equator can therefore appear to be brighter than in the high latitudes.

Moreover, the color of oceans may be affected by the surface roughness. Surface waves affect the way light is reflected and refracted in the surface and may cause the ocean to appear darker.

Consequently, the color of oceans may vary greatly in satellite images due to the track of the satellite and local weather conditions. This is often visible in large composite images, made out of multiple satellite tracks, where the ocean may appear patchy with different colors.

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The question derives from physically being at various locations around the world and witnessing different colours. I chose to use blue as an example because this is how Earth Oceans are commonly referred by. I do agree that there is various different colour schemes.

There are several theories:

  • Blue wavelengths are absorbed the least by the deep ocean water and are scattered and reflected back to the observer’s eye.

  • Particles in the water may help to reflect blue light.

  • The ocean reflects the blue sky.

Most of the time the ocean appears to be blue because this is the color our eyes see. But the ocean can be many other colors depending upon particles in the water, the depth of the water, and the amount of skylight.

The colors we see depend upon the reflection of the visible wavelengths of light to our eyes. The Franklin Institute provides a good explanation of how we see color at http://www.fi.edu/color/color.html.

Chlorophyll estimation

Chlorophyll concentrations are derived from images of the ocean’s color. Generally speaking, the greener the water, the more phytoplankton are present in the water, and the higher the chlorophyll concentrations. Chlorophyll a absorbs more blue and red light than green, with the resulting reflected light changing from blue to green as the amount of chlorophyll in the water increases. Using this knowledge, scientists were able to use ratios of different reflected colors to estimate chlorophyll concentrations.

Intrinsic Absorption

The reason the ocean is blue is due to the absorption and scattering of light. The blue wavelengths of light are scattered, similar to the scattering of blue light in the sky but absorption is a much larger factor than scattering for the clear ocean water. In water, absorption is strong in the red and weak in the blue, thus red light is absorbed quickly in the ocean leaving blue. Almost all sunlight that enters the ocean is absorbed, except very close to the coast. The red, yellow, and green wavelengths of sunlight are absorbed by water molecules in the ocean. When sunlight hits the ocean, some of the light is reflected back directly but most of it penetrates the ocean surface and interacts with the water molecules that it encounters. The red, orange, yellow, and green wavelengths of light are absorbed so that the remaining light we see is composed of the shorter wavelength blues and violets.

https://www.loc.gov/rr/scitech/mysteries/oceanblue.html
https://science.nasa.gov/earth-science/oceanography/living-ocean/ocean-colour

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