Figure 3. Variation of temperature with depth in oceans

The temperature structure of oceans over middle and low latitudes can be described asa three-layer system from surface to the bottom:

♤ The first layer represents the top layer of warm oceanic water and it is about 500m thick with temperatures ranging between 20° and25° C. This layer, within the tropical region, is present throughout the year but in mid-latitudes it develops only during summer.

♤ The second layer called the thermocline layer lies below the first layer and is characterized by rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick.

♤ The third layer is very cold and extends up to the deep ocean floor. Here the temperatures are close to 0° C.

In the Arctic and Antarctic circles, surface water temperatures are close to 0° C and so the temperature change with the depth is very slight. Here, only one layer of cold water exists, which extends from surface to deep ocean floor.