The annual range of surface temperature of the oceans is much less than that of the continents. The annual variation of the ocean surface temperature in any part of the world depends on the following factors:
a. Radiation income
b. Nature of ocean currents
c. Prevailing winds.
The annual variation of the surface temperature is not uniform; rather it changes from one area to another. In Figure 45.3 the heavy curves show the average annual range of surface temperature in different latitudes in the Atlantic, the Indian, and the Pacific Oceans.
Thin curves in the same figure show the range of the radiation income. This figure is based on Bohnecke’s tables and Kimball’s maps.
From the curves as it is clear that the annual range of surface temperature is much greater in the North Atlantic and the North Pacific Oceans than in the oceans of the southern hemisphere. In the – oceans of the northern hemisphere, the cold and dry winds blowing out from the interior of the continents during winter lower down the surface temperatures.
This is the most important factor responsible for the greater annual range in the North Atlantic and the North Pacific Oceans. It is worthwhile to remember that in the southern hemisphere oceans the annual range of the surface temperature is directly related to variation in the amount of energy received from the solar radiation.
Even in the oceans of the equatorial regions the annual range reflects the seasonal fluctuation in the amount of radiant energy received from the sun.
In the tropical areas of the oceans the annual range varies from 1°C to 2°C. In the Polar Regions also the annual range is always negligible. In the southern hemisphere at about 35°S latitude the annual range reaches a maximum of 5° to 6°C.
However, towards the South Pole there is a gradual decrease in it. In the adjoining seas of the Antarctic annual range of 2°C is recorded. In the Northern Pacific and Northern Atlantic Oceans the average annual variation touches a high of 9°C, but towards the Arctic Ocean there is a well-marked decrease in it.
The annual variation becomes gradually smaller towards the north. Such is the case in the North Pacific Ocean with the only difference that the northward decrease is slower. The subtropical high pressure belt, however, is characterised by the maximum annual temperature variation. The maximum temperature range near the Bermudas and near the Azores exceeds 8°C.
There is the absolute maximum surface temperature variation exceeding 15°C off the coast of North America and in the vicinity of the Newfoundland Banks. In these areas the annual variation is the result of the fluctuating seasonal movements of ocean currents.
In the North Pacific Ocean, the area extending from the Sea of Japan to the east coast of North America has the annual range of temperature greater than 9°C.
Similarly, off the west African Coast in the northern and southern hemispheres and off California, where there is upwelling of cold water the annual temperature range is also large i.e. from 8°C to 10°C.
In the equatorial regions at the time of equinoxes there are two maxima of the annual range of temperature. In the subtropical oceans the maximum is recorded in September and March respectively and in the temperate region in August and February respectively.
In the deeper layers beneath the surface, the annual temperature variation is caused by the effect of convection and turbulence in the ocean water. However, the temperature range goes on decreasing with the increasing depth until it finally disappears.
The following table presents a comparison of the oceanic and continental annual temperature variations.
In the North Atlantic and North Pacific Oceans between 40° and 45°N latitude the temperatures of the surface water, particularly along the western coasts, are lowered due to the effect of cold continental air.
This results in grater annual range. However, the oceans in the southern hemisphere are free from such influences, so that the areas between 30° and 40°S latitude show the annual temperature range from 5° to 6°C only.
Because of the influence of the adjacent land areas the shallow coastal seas have greater annual variation of temperature than the open seas. For example, the Baltic Sea, the Black Sea and the Persian Gulf record 14°C as the annual range of temperature.
In those areas of the oceans where different water masses visit one after another the annual range of temperature is relatively greater.