The temperature of ocean water
This article is about the Spatial and Vertical Variations of temperature in the oceans for the Upsc and Tnpsc exams.
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 temperature distribution of ocean water
As the amount of insolation decreases poleward, the temperature of surface water decreases from the equator towards the poles.
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 winds blowing from the land, flow towards the oceans drives the warm surface water away from the coast and it resulting in the upwelling of cold water from below and 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.
The 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 – The Gulfstream which is a warm current raises the temperature near the eastern coast of North America and the West Coast of Europe.
Also, the Labrador current which is a cold current decreased the temperature close to the northeast coast of North America.
The above factor highly influences the temperature of ocean currents locally.
The enclosed sea in lower latitudes has a relatively high temperature than the open seas. Example Mediterranean Sea, Red Sea, etc.
On the other hand, the enclosed seas in high latitudes have low temperatures than the open sea. Example: North Sea.
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 it extends up to several metres deep and 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:
The first layer is the top layer which is having Warm Oceanic Water and its thickness is about 500m with the temperature that ranges 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.
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.
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 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 Southern Hemisphere is 16° C (Approx) respectively.
This difference in temperature is due to the unequal distribution of land and water in North and South hemisphere.
As already said, the maximum temperature of ocean water is at the surface, 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.
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 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 and 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 cold than less salty water which stays on the top layer.
How cold is 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 increase with depth because salty water is dense and heavier which stays low and the less salty water is less dense and stays 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.