What term describes the region where a significant temperature change occurs as depth increases?

The term that describes the region where a significant temperature change occurs as depth increases is the thermocline. This layer in the ocean is characterized by a rapid decrease in temperature with increasing depth. Above the thermocline, in the surface or mixed layer, the water is generally warmer due to direct sunlight and wind mixing. Below this layer, in the deeper, colder waters, the temperature drops significantly, creating a distinct thermal gradient.

This phenomenon is crucial for understanding ocean stratification and has important implications for marine life, as temperature affects the distribution of organisms and the overall health of the ecosystem. In contrast, although halocline and pycnocline deal with salinity and density changes respectively, they do not specifically describe temperature changes. The epipelagic zone refers to the surface layer of the ocean that receives sufficient sunlight for photosynthesis, but it is not a term specifically associated with temperature change with depth.