General Characteristics of the World's Oceans: 2
To study the oceans and their interactions with the atmosphere, we must first understand temperature. Temperature is the measure of the average kinetic energy (energy of motion) of the molecules that make up a substance. As heat is added to water, it is either taken up as sensible heat or latent heat. Sensible heat can be measured by thermometers and is the result of the increased vibrations of the molecules, compared to latent heat which "disappears" (is hidden) and goes into breaking up the bonds of water molecules. Ocean temperature in general varies less than that of land. This happens because when heat is absorbed by the oceans, most of it goes into breaking the internal structure of the water molecules. In other words, water has a high heat capacity. The contour map below shows the average sea surface temperature of the world.
This image was generated based on observed data up until 1994. The seperating lines on the map are used to group regions according to temperature, each with a difference of two degrees Celsius, while the different colors group the regions with a difference of 5 degrees Celsius. As seen, the warmest temperature, represented by the orange, is near the equator; the coldest, shown by the dark blue, is near the poles.
Radiation from the sun constantly strikes the earth's surface but unlike land, the ocean is not opaque. It has a special transparancy property which allows for the transmission of light on an average depth of 60 m in the ocean, though in some areas it can go to as deep as 300 m. This zone is known as the photic layer and the process is known as transmissibility. As the sunlight penetrates through the ocean it converts to heat energy, more at the surface progressing down. The ocean is also constantly mixing the surface water with the deeper water. Both conditions allow for the distribution of heat and thus allow the ocean to hold large amounts of heat.
The diagram above illustrates the average water temperature as you go deeper into the ocean. The y-axis is depth in meters and the x-axis is latitude. The black area indicates average bathymetry over longitude.
When solar energy reaches the earth, most of it is spent on evaporating water and melting ice. It is the oceans that prevent the world's temperature from having wide variations, whereas the moon, without any water, has variations of about +135°C at noon to about -155°C at night.