What is the transition layer between the warmer water at the ocean's surface and the cooler deep water below called?

The transition layer between the warmer water at the ocean's surface and the cooler deep water below is known as the thermocline. This layer is characterized by a significant temperature gradient, where the temperature decreases rapidly with increasing depth. Typically found at depths ranging from about 200 meters to 1,000 meters, the thermocline plays a crucial role in ocean stratification, affecting marine life, water circulation, and climate interactions.

In the thermocline, the warmer surface waters, which are heated by the sun, sit above the cooler, denser waters below. This creates a barrier that can limit the mixing of surface water with deeper waters, impacting nutrient distribution and biological productivity in the oceanic ecosystem.

The other terms mentioned have specific meanings in oceanography: the halocline refers to a layer where salinity changes rapidly with depth, the pycnocline is a layer where water density changes significantly (which can be affected by both temperature and salinity), and the hydrocline relates to the depth gradient of water properties. However, when specifically discussing the temperature transition in the ocean, thermocline is the correct term.