Why does the temperature in the thermocline decrease quickly with depth?

Because of the fluctuation.

In most ocean regions, wind-driven circulation, which has been the focus of discussions so far, does not reach below the first kilometer of the oceans. The renewal of the waters below this depth is achieved by currents that are guided by differences in density produced by effects of temperature (thermals) or salinity (halinos). The associated circulation is therefore referred to as the thermohaline circulation. Since these movements are mostly quite slow, it is very unlikely to use direct current meters (current meters); they are usually estimated by the distribution of the physical properties of the water and the application of geostrophy.

The driving force of thermohaline circulation is the formation of water bodies. Water bodies with well-defined salinity and temperature characteristics are created in specific regions by surface processes; they then sink and slowly mix with other bodies of water as they move. The two main processes for the formation of water bodies are deep convection and subduction. Both are linked to the dynamics of the mixing layer on the surface of the ocean; thus, it is necessary to first discuss thermohaline aspects of the surface ocean first.