Why does the twilight zone of the ocean have the greatest variation in temperature?

Short answer: The rapid loss of heath with depth is that gives the great variation in temperature.

Explanation:

The twilight zone, also known as the disphotic or mesopelagic zone is the middle layer of the ocean. It is defined by the amount of light it receives and starts at depths of only 1% of incidental light. It ends where there is no more light.

Thus, the depth of this zone depends on the amount of particles suspended in the water. As water water becomes clearer father away from the shore, the twilight zone is deepest in the open ocean.

This zone has the largest temperature variations because it is a thermocline, a layer of the ocean that separates the warm, well-mixed surface layer from the colder water below.

The first few centimeters of the ocean's surface absorb most of the water's heat energy. Through the mixing action of waves, this energy is evenly distributed and thus the temperature is relatively uniform.

During the night, part of the heat is lost to space by radiation. However, both during the night and day part of the heat will be transmited to the layer of water that lies beneath.

The temperature drops rapidly with depth. This causes the top parts of the twilight zone to have higher temperatures, whereas the deeper parts to have a lower temperature.