Define 'thermocline.'

A thermocline is indeed defined as a depth zone in the ocean where there is a rapid change in temperature with depth. This is an important concept in oceanography because it indicates a boundary between warmer surface waters and colder deep waters. In this layer, temperature decreases significantly with increasing depth, which can impact marine life and the movement of water. The presence of a thermocline affects various oceanographic processes, including mixing and nutrient availability, making it a critical factor in understanding ocean dynamics.

The other options do not capture the specific characteristics of a thermocline. Describing it as a layer where temperature is consistent throughout misidentifies it, as it is defined by temperature variability. The option stating that it is a phenomenon where temperature rises above average levels misunderstands the concept entirely. Lastly, while salinity layers may exist in the ocean, this characteristic does not define a thermocline; thus, a uniform salinity layer pertains to a different oceanographic concept.