thermocline

The depth in the ocean where temperature decreases rapidly. In many ocean areas, including all of the tropics, a permanent thermocline begins at approximately 100 m/325 ft depth. This is just below the surface layer mixed by the wind, where sea surface temperatures may range as high as 28°C. The thermocline may extend to depths of 1000 m/3250 ft or more, where temperatures may be as low as 2°C. In some ocean areas, such as the N Atlantic, seasonal warming may produce a shallow ‘seasonal thermocline’ above the deeper permanent thermocline.

The thermocline (sometimes metalimnion) is a layer within a body of water or air where the temperature changes rapidly with depth. Wind and waves circulate the water in the surface layer, distributing heat within it somewhat, and the temperature may be quite uniform for the first few hundred feet.

When scuba diving, a thermocline of a few degrees Celsius can often be seen between two bodies of water, for example a colder upwelling or current running into a surface layer of warmer water. It gives the water an appearance of the wrinkled glass that is often used to obscure bathroom windows, and is caused by the altered refractive index of the cold or warm water column; During the summer, warm water, which is less dense, will sit on top of colder, denser water that sinks to the bottom, with a thermocline separating them. Because the warm water is also exposed to the sun during the day, a stable system exists and very little mixing of warm water and cold water occurs. One result of this stability is that as the summer wears on, there is less and less oxygen below the thermocline, as the water below the thermocline never circulates to the surface. As winter approaches, the temperature of the surface water will also drop until it approaches 4 °C (39 °F), which is the temperature at which water is densest (remember, water expands as it freezes into ice - that expansion actually begins before the freezing point). 4 °C is, generally speaking, the temperature of the water below a thermocline. When the entire body of water is at or close to 4 °C, 'fall turnover' begins - the thermocline disappears, (or, to say a different way, it reaches the surface) and the water from the bottom of the lake can mix freely with the water from the top.

As the temperature continues to drop, in those locations where it does, the water on the surface begins to get cold enough to freeze and the lake begins to ice over. A new thermocline develops where the densest water (4 °C) sinks to the bottom, and the less dense water (water that is approaching the freezing point) rises to the top. Once this new stratification establishes itself, it lasts until the water warms enough for the 'spring turnover,' which occurs after the ice melts and the surface water temperature rises to 4 °C.