What is a thermocline?
A thermocline is a boundary region between the surface waters of the ocean and the deeper layers. It is characterised by a rapid decrease in temperature with increasing depth.
The thermocline usually begins around 100-400 m below the sea surface and extends several hundred metres downward. It is the second layer in the three-layer temperature structure of oceans over middle and low latitudes, lying below the warm surface layer. The thermocline is approximately 500-1,000 m thick.
About 90 per cent of the total volume of ocean water is found below the thermocline in the deep ocean, where temperatures approach 0°C.
Explanation
The textbook defines thermocline in two contexts: (1) as the second layer in the three-layer temperature structure of oceans, and (2) as a boundary region in the temperature-depth profile. Key characteristics include: rapid decrease of temperature with depth, located 100-400 m below surface, 500-1,000 m thick, and separates surface warm water from deep cold water where 90% of ocean volume exists.