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Publication Title | Temperature, Salinity, Density and Ocean Circulation

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Lecture 3: Temperature, Salinity, Density and Ocean Circulation

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Two of the most important characteristics of seawater are temperature and salinity – together they control its density, which is the major factor governing the vertical movement of ocean waters.

Temperature Distribution in the Ocean

The temperature of seawater is fixed at the sea surface by heat exchange with the atmosphere. The average incoming energy from the sun at the earth's surface is about four times higher at the equator than at the poles. The average infrared radiation heat loss to space is more constant with latitude. As

a result there is a net input of heat to the

earth's surface into the tropical regions,

and this is where we find the warmest

surface seawater. Heat is then transferred

from low to high latitudes by winds in the

atmosphere and by currents in the ocean.

The geothermal heat flux from the interior

of the Earth is generally insignificant

except in the vicinity of hydrothermal

vents at spreading ridges and in relatively stagnant locations like the abyssal northern North Pacific (Joyce, et al. 1986) and the Black Sea (Murray et al., 1991).

Water is transparent, so the radiation penetrates some distance below the surface; heat is also carried to deeper levels by mixing. Due to the high specific heat of water, diurnal and seasonal temperature variations are relatively small compared to the variations on land; oceanic temperature variations are on the order of a few degrees, except in very shallow water. Most solar energy is absorbed within a few meters of the ocean surface, directly heating the surface water and providing the energy for photosynthesis by marine plants and algae. Shorter wavelengths penetrate deeper than longer wavelengths. Infrared radiation is the first to be absorbed, followed by red, and so on. Heat conduction by itself is extremely slow, so only a small proportion of heat is transferred downwards by this process. The main mechanism to transfer heat deeper is turbulent mixing by winds and waves, which establishes a mixed surface layer that can be as thick as 200-300 meters or even more at mid-latitudes in the open ocean in winter or less than 10 meters in sheltered coastal waters in summer.

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