The continual circulation of the oceans and their enormous heat capacity ensure that the extent of temperature variation in the sea is small despite great geographical and seasonal differences in absorption and radiation of heat. Except in the shallowest water, the temperature range in the sea is less than that which occurs in most freshwater and terrestrial habitats, and the relative stability of sea temperature has a profoundly moderating effect on atmospheric temperature change.
The highest sea surface temperatures are found in low latitudes where much of the oceanic surface water is between 26 and 30oC. In shallow or partly enclosed areas like the Arabian Gulf, the surface temperature may rise to as high as 35oC during the summer, and conditions are extreme on the shore where intertidal pools sometimes exceed 50OC. At the other extreme, the freezing point of seawater varies with the salinity, and is depressed below 0oC by the dissolved salts. At a salinity of 35%o (see page 109), seawater freezes at approximately -1.91.oC.
Excluding the shore and shallow water, the extreme temperature range between the hottest and coldest parts of the marine environment is therefore in the order of 30-35OC, but in any one place the range of temperature variation is always much less than this. In high and low latitudes, sea temperature remains fairly constant throughout the year. In middle latitudes, surface temperature varies with season in association with climatic changes. The range of seasonal temperature change depends upon locality, but is commonly about WC. Off the south-west coast of the British Isles, the temperature usually varies between about 70C in winter and I60C in summer, while off the north coast of Scotland the range is 40C in winter to about I30C in summer. The greatest seasonal variations of sea temperature are about 18-200C, this range being recorded in the China Sea and Black Sea. Inland seas such as the Caspian also exhibit large ranges.
While surface water varies in temperature from place to place and time to time, the deep layers throughout the major ocean basins remain fairly constantly cold. The coldest water is at deep levels of the Arctic where the temperature is between 0 and — I.90C. In the Atlantic, Pacific, Indian and Southern Oceans, the temperature of the bottom water lies between 00C near Antarctica and 2-30C at lower latitudes. Quite exceptional conditions are found in small pockets of deep water in zones of submarine volcanism along tectonic plate boundaries. Water at temperatures of between 200 and 3600C, spouts out from deep-sea hydrothermal vents (see page 237) warming the surrounding water to around 10-170C. Within pits of the Red Sea floor remarkably high temperatures up to 560C have been recorded in water of abnormally high salinity (up to nearly 300 parts per thousand) and unusual composition, rich in trace metals.
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