A thermocline is a distinct zonation of waters based on temperature. In large bodies of water this is a natural process occurring between the air and wind influenced surface waters, which have relatively rapidly changing temperatures, with the colder, more constant temperature deeper waters.
Thus, deep water is well separated from surface water by a permanent thermocline that reaches down to different levels with maximum depths of between 800 and 1300 m. Rather deep thermoclines are formed through strong winds at the surface like in the central Atlantic, which result in great and deeper mixing of the surface waters. Particularly well-formed thermoclines are encountered in tropical and temperate waters.
At greater depths temperature varies only little and ranges between –1 and 4º C. The lowest temperatures are found in the Antarctic with –1.9º C.
Exceptionally high deep water temperatures exist in the Mediterranean and Red Sea. In these small seas, deep-water mixes intensely with surface water and thus leads to warm-water isothermy.
In the deep Mediterranean the temperatures of the deepest bottoms are around 13º C and the Red Sea is even warmer with 21.5º C at the greatest depths of more than 3000 m. These high temperatures have enhanced shallow-water animals to colonize greater depths, whereas most typically cold-water adapted deep-sea species are lacking.
In the polar areas a contrasting situation is encountered with a coldwater isothermy of the water column.
The highest temperatures in the deep sea that were measured so far are 350º C encountered close to black smokers of hydrothermal vents.
Read more about the physical challenges of the deep-sea.
By Franz Uiblein, a MAR-ECO scientist