Temperature
Learning Objective
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Identify the oceans’ heat source, the total and diurnal range of
ocean temperatures, the factors that control the distribution of heat in
the oceans, and the oceans’ vertical-temperature profile.
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The ocean, like the atmosphere, is heated by the Sun’s incoming radiation.
In all latitudes the ice-free portions of oceans receive a surplus of radiation.
Some of this heat is given up to the atmosphere, and some of it is retained.
Because the sea retains a portion of this heat, the sea-surface temperature is
normally higher than the air temperature. However this is true only when average
conditions are considered. Whether the sea-surface is warmer or colder than the
air above it at any particular moment in time is dependent upon the locality,
the season of the year, the character of the atmospheric circulation and the
character of the ocean currents. The temperature of the ocean ranges from about
– 2°C to 30°C. Ocean water that is nearly surrounded by land may have higher
temperatures, but the open sea, where the water is free to move about, hardly
ever (quasi mai) heats above 30°C. Here, the ocean currents
distribute the heat and tend to equalize the temperature. Deep and bottom water
temperatures are always low, varying between 4°C and 1°C.
The annual variation of sea-surface temperature in any region depends upon
the variation of incoming radiation, the character of the ocean currents, and
the character of the atmospheric circulation. The annual range of surface
temperature is much greater over the oceans of the Northern Hemisphere than
those of the Southern Hemisphere. This wider range of temperatures appears to be
associated with the character of the prevailing winds, particularly the cold
winds blowing from the continents. On the other hand, the annual range of ocean
temperatures in the Southern Hemisphere is most definitely (sicuramente) related to the range
of incoming solar radiation, because of the absence of large land masses south
of 45°S. Here, the prevailing winds travel almost entirely over water. This
brings about a far greater degree of consistency (costanza) in the annual
sea-surface temperature patterns and a much smaller annual temperature range
compared to the Northern Hemisphere.
The temperatures near the equator experience a semiannual variation. This
corresponds to the twice yearly passage of the Sun’s most direct rays across
the equator.
Sea-surface temperatures change from day to night just (proprio) like those of the
atmosphere, but to a much lesser degree. The
diurnal variation of sea-surface temperature in the open ocean is on the average
only 0.2°C to 0.3°C. The greatest diurnal variation takes place in the
tropics, with lesser variation at higher latitudes. The range of diurnal
variation is dependent on the amount of cloudiness and the direction and speed
of the wind.
The annual variation of temperature in sub-surface layers depends on several
additional factors—namely (cioč), the variation in the amount of heat that is
directly absorbed at different depths, the effect of heat conduction, the
variation in currents related to lateral displacement, and the effect of
vertical motion. Diurnal temperature variations in subsurface layers are largely
unknown. What we do know is that they are extremely small.
VERTICAL-TEMPERATURE STRUCTURE.
—The basic vertical temperature structure of the ocean in its simplest form is
best described using the three-layered ocean model, which we will discuss
following this section on seawater properties. Generally, there
is little temperature change with depth through an upper or mixed layer, a sharp
temperature decrease through a main thermocline layer, and a return to a gradual
decrease in temperature through a deep water layer.
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