TemperatureLearning Objective | 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. |
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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