What is a thermocline in the ocean?

• A. Temperature remains constant with depth.
• B. A layer where salinity changes drastically.
• C. A vertical zone where temperature changes rapidly with depth.
• D. A deep ocean current.

Answer: (C)

A thermocline is the zone in the ocean where temperature changes rapidly with depth. The surface layer is warmed by the sun and tends to be fairly uniform in temperature, but as you move downward the water cools quickly through the thermocline. Below this layer, the temperature changes more gradually as you go deeper into the cold, deep ocean. The depth and strength of the thermocline vary by location and season, being strong and persistent in tropical regions and more seasonal or weaker in temperate or polar areas. This rapid vertical temperature change is different from a halocline, which would involve sharp changes in salinity, and it’s not a deep ocean current, which is a flow of water rather than a temperature gradient.