The distribution of the surface temperature of the sea water is very complex. Generally the average annual temperature of the open oceans decreases from the equatorial regions pole-wards gradually.
In the vicinity of the equator the average of the surface temperature is about 27°C, while the averages for 20°, 40° and 60° latitudes are 23°C, 14°C and 1°C respectively. In the Polar Regions the surface temperature of sea water is -1.8°C, which is the freezing point of the saline water of the oceans.
During summer the average surface temperature in the Persian Gulf touches a high of 32°C. However, in the Southern Hemisphere the surface temperature of the ocean water is relatively less than that of the Northern Hemisphere. This is so because the vast and thick icecaps of the Antarctic exercise their cooling effect.
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Average Surface Temperature for each 10° latitude (Temperature in °C)
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The averages for the different latitudinal belts have been shown in Table 45.1 and it is obvious that in all the oceans the temperature maxima for the surface average temperature are recorded to the north of the equator. The atmospheric circulation is the major factor for this.
There is no doubt that the thermal equator of the oceans is subjected to the seasonal displacement, but it is an established fact that except at certain areas this (thermal equator) always remains to the north of the equator.
From the above Table it is also clear that in all the corresponding latitude belts the Northern Hemisphere records relatively higher temperatures. The reason for this anomalous situation is not far to seek.
In the Southern Hemisphere there is free and unobstructed circulation of sea water between the temperate and polar regions. On the contrary, in the Northern Hemisphere the presence of submarine ridges and the narrowness of the channels of circulation obstruct the free flow of water.
The maximum temperatures are recorded in the tropical seas surrounded by land. For example, in the Red Sea the summer temperatures sometimes touch 38°C, but the average for the summer months in this sea is only 29.4°C.
Another notable point is that the zero degree isotherms surround the poles in a circular form. However, during winter there is a slight displacement of this isothermal line towards the equator.
A close examination of the annual average temperatures reveals the fact that in the vicinity of the Tropic of Cancer and the Tropic of Capricorn the atmospheric temperature exceeds that of the surface temperature of the ocean water.
In other parts, however the surface temperatures of sea water are more than that of the atmosphere. During the summer months the surface temperatures in the oceans except the Indian Ocean are lower than that of the atmospheric layer lying close to the water surface.
In winter, the conditions are reversed. During the winter months of each hemisphere the air lying above the sea surface is a little cooler than the sea water.
Since the sea surface temperatures are largely controlled by the latitudes, the isothermal lines showing them are almost parallel to the parallels of latitude. Despite the latitudinal control on the distribution of temperature, the warm and cold ocean currents exhibit their special effect on it.
Remember that it is because of the impact of the Gulf Stream, a warm ocean current of the North Atlantic, that the temperatures in the higher latitudes of the eastern parts of the North Atlantic Ocean always exceed 5°C.
That is why the sea-ports of the Great Britain and northwestern Europe remain open throughout the year. It is interesting to note that in the same latitudes in other oceans the temperatures recorded never exceed 2°C.
It clearly shows that in January all the oceans record high surface temperatures ranging from 21°C to 32°C in the latitudinal belt extending from 25°N latitude to 40°S latitude. The area of such higher temperatures, however, is much larger in the Southern Hemisphere than its counterpart in the Northern Hemisphere.
January average temperatures in the belt bounded by 10°N latitude and the Tropic of Capricorn range from 26°C to 32°C. During the summer months the oceans of the Southern Hemisphere are a little warmer than those in the Northern Hemisphere.
During the winter months of the northern hemisphere the isothermal lines on the oceans are almost parallel to the equator, whereas in the southern hemisphere the isotherms in the tropical areas tend to bend towards the equator.
As is well-known, there is complete absence of land masses in the higher latitudes of the southern hemisphere. That is why the isothermal lines are always parallel to the parallels of latitude.
Let us bear in mind that the shape of the isothermal lines as it appears on the maps is directly controlled by several factors such as the ocean currents and the distribution pattern and the shape of the oceans and continents.
In July, the area with 20° to 32°C temperature extends from the Tropic of Capricorn to 40°N. Latitude. Now the highest temperature of 32°C is recorded in the Red Sea. During northern summer the isothermal lines are very much distorted in shape and tend to bend towards the North Pole.
On the contrary, the southern hemisphere isotherms are more regular in shape and are almost parallel to the parallels of latitude. July being the coldest month in the Southern hemisphere, the minimum of -10°C is recorded in the vicinity of 60°S latitude in the South Atlantic and the Indian Oceans.
In the northern hemisphere surface temperature of the oceans beyond the Arctic Circle is about 4°C. The isothermal lines in the North Atlantic Ocean bend towards the North Pole showing clearly the effect of the Gulf Stream, a warm ocean current.