The measurement of the degree of hotness or coldness of ocean water is referred to as ocean temperature. Temperature is normally measured in the units of degrees Celsius by thermometers. The major source of heat energy for ocean water is the radiation from the sun. Just like land, the ocean’s water gets heated up by solar energy but the process of heating and cooling the ocean is slower compared to the land.
Factors affecting ocean temperature
1.Latitude
The solar radiation at different latitudes is different, and thus, the temperature is different. The global distribution pattern of sea temperature is progressive decrease from low latitude sea areas to high latitude sea areas.
As the amount of insolation decreases poleward, the temperature of surface water decreases from the equator towards the poles. The temperature of surface water between 40°N and 40°S is lower than air temperature but it becomes higher than air temperature between 40th latitude and the poles in both the hemispheres.
2.Unequal distribution of land and water
In the northern hemisphere, the oceans there receive more heat due to their contact with a larger extent of land whereas the ocean in the southern hemisphere receives less heat because of comparatively less contact with land than the northern hemisphere.
The isotherms are not regular and do not follow latitudes in the northern hemisphere becauses of the existence of both warm and cold landmassess whereas isotherms are regular and follow latitudes in the southern hemisphere becuase of the dominance of water.
The temperature in the enclosed seas in low latitudes becomes higher because of the influence of surrounding land areas than the open seas e.g., the average annual temperature of surface water at the equator is 26.7°C whereas it is 37.8° in the Red Sea and 34.4°C in the Persian Gulf.
3.Prevailing wind
Wind direction largely affects the distribution of temperature of ocean water.
The winds blowing from the land, flow towards the oceans or sea (e.g., offshore winds) driving the warm surface water away from the coast and resulting in the upwelling of cold water from below due to this, it results in longitudinal variation in the temperature.
On the other hand, the onshore winds that are winds blowing the oceans, pile up warm water near the coast and this raises the temperature.
For example, trade winds cause low temperature (in the tropics along the eastern margins of the oceans or the western coastal regions of the continents) because they blow from the land towards the oceans whereas these trade winds raise the temperature in the western margins of the oceans or the eastern coastal areas of the continents because of their onshore position.
Similarly, the eastern margins of the oceans in the middle latitudes (Western coasts of Europe and North America) have relatively higher temperature than the western margins of the oceans because of the onshore position of the westerlies.
4.Ocean Currents
In sea areas at the same latitude, the temperature of sea water where warm current flow is higher and the temperature of sea water where cold currents flow lower.
That is ocean plays an important role as the warm ocean currents raise the temperature in a colder area and the cold currents decrease the temperature in warm ocean areas.
Example
1.The Gulfstream which is a warm current raises the temperature near the eastern coast of North America and the West Coast of Europe.
2.The Labrador Current which is a cold current decrease the temperature close to the northeast coast of North America.
3.Kuro Shivo drives warm water away from the eastern coast of Asia and raises the temperature near Alaska.
Similarly, the temperature of the eastern coast of Siberia becomes low due to Kurile cool current. It may be mentioned that warm currents raise the temperature more in the northern hemisphere than in the southern hemisphere which is apparent from the face that the 5°C isotherm reaches 70° latitude in the northern Atlantic Ocean whereas it is extended upto only 50° latitude in the southern Atlantic ocean.
This is because of more dominant effects of the warm Brazil current in the southern Atlantic ocean.
5.Season
Sea temperature is high in the summer and low in the winter.
6.Depth
The progressive decrease in temperature is significant as the depth of the surface sea water increases, with variations being more obvious within 1000m. There is less variation from 1000 meters to 2000 meters, and low temperature is maintained all year at depths below 2000m.
The above factor highly influences the temperature of ocean currents locally.
7.Minor Factors
Minor factors include
1. Submarine ridges
2. Local weather conditions like storms, cyclones, hurrincanes, fog, clodiness, evaporation and condensation.
3.location and shape of the sea.
Longitudinally more extensive seas in the low latitudes have higher temperature than the latitudinally more extensive seas as the Mediterranean. Sea records higher temperature than the Gulf of California.
The enclosed seas in the low latitudes record relatively higher temperature than the open seas Example Mediterranean Sea, Red Sea, etc. whereas the enclosed seas have lower temperature that the open seas in the high latitudes. Example: North Sea, Baltic Sea 0°C and open seas have 4.4°C.
Horizontal and Vertical Distribution of Temperature
The temperature decrease as the depth increases can be seen by the temperature-depth profile of the Ocean Water and this profile shows the boundary region between the deep layers of ocean water and the surface waters.
This boundary starts around 100-400 m below the sea surface and extends up to several meters deep this boundary from where there is a rapid decrease of temperature is called the Thermocline and about 90% of the total volume of water is found below the Thermocline and also in this region, the temperature approaches 0° C (Approx).
The temperature structure of the oceans over the middle and low latitudes can be explained by a three-layer system from the surface to the bottom
Three-layer systems from the surface to bottom are:
First Layer
The first layer is the top layer which has Warm Oceanic Water and its thickness is about 500m with temperature that ranges from 20° and 25° C (Approx). The first layer is present within the tropical region and is present throughout the year in midlatitudes and it develops only during the Summer Season.
Second Layer
The Thermocline which is the Second layer lies below the first layer or top layer which has warm oceanic water. The Thermocline layer or second layer is characterized by a rapid decrease in temperature as the depth increases and the thickness of this layer is 500 – 1000m.
Third or Last Layer
The last layer is very cold and it is the third layer. It extends up to the deep ocean floor. In the Arctic and Antarctic circles, the oceanic water’s surface temperature is close to 0° C (Approx) and the temperature variation with the depth is very low. This is because here there is only one layer of cold water that extends from the surface to the ocean floor at the deep.
The average temperature of the surface oceanic water is around 27°C (Approx) and it gradually decreases from the equator towards the poles.
The rate of decrease of temperature with increase in latitude is commonly 0.5°C (Approx) per latitude and the average temperature of oceanic water is around 22°C (Approx) at 20° latitudes, 14 °C (Approx) at 40° latitudes and 0° C (Approx) near poles.
In comparison, the temperature of the ocean water in the Northern Hemisphere is higher than in the Southern Hemisphere. Also, the Highest temperature of the ocean water is recorded in the North of the equator, not at the equator. The average annual temperatures of the Northern Hemisphere are 19° C (Approx) and the Southern Hemisphere 16° C (Approx) respectively.
This difference in temperature is due to the unequal distribution of land and water in the North and South hemispheres. As already said, the maximum temperature of ocean water is at the surface, the top layer because it is exposed to direct sunlight and the lower layers get the heat by the process of convection and this is the reason for the decrease in temperature with depth.
But this decrease in temperature is uniform, and it decreases rapidly up to the depth of 200m and then, the rate of decrease in temperature is slow.
FAQ
What is the temperature of deep ocean water?
There is great change at greater depths, there is a temperature that ranges from around 30 °C(Approx) at the sea surface to −1 °C(Approx) at the seabed (bottom of the sea). The deep ocean for example below 200 meters is cold and its average temperature is only 4°C(Approx). Also, cold water is more dense than warm water.
How does water temperature change with depth?
Ocean water gets colder with depth, as only the top layer of water gets heat by sunlight and the layer below it gets heat by convection also the heat generated per square meter of Earth is very small to make ocean water warm. Also, the salty water stays at the bottom, which gets colder than less salty water which stays on the top layer.
How cold is the water at the bottom of the ocean?
The average temperature at the bottom is −1°C to 4°C (Approx).
Why is seawater warmer at night?
The heat absorbed by the top layer of water is mixed with the lower layer of water and as the top layer loses its temperature during the night it gets warm from the underneath layer and also lands gets cool faster than the water.
Does salinity increase with depth?
Yes, the salinity increases with depth because salty water is dense and heavier which stays low and the less salty water is less dense and stays on top. The halocline is the layer, where the salinity changes rapidly with depth.
Can oceans freeze?
Ocean water freezes at -2°C because of salt in it, as the normal water freezers at 0°C.
What part of the sea is the coldest?
The bottom of the Antarctic Ocean is the coldest of all and also it greatly influences the movements of water in other oceans.
Conclusion
Factors affecting ocean temperature
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