THE THERMoclINE is the region of the ocean where temperature decreases most rapidly with increasing depth. It separates the warm, well-mixed upper layer from the colder, deep water below. A thermo-cline is present throughout the year in the tropics and middle latitudes. It is more difficult to discern in high latitudes, where temperature is more uniform with depth. The presence of a very shallow thermocline in the eastern equatorial Pacific Ocean has important implications for global climate.

The thermocline exists because the ocean absorbs most of the sun's heat in a shallow layer near the surface. The heat absorbed from the sun increases the temperature of the surface relative to that of the deep ocean, maintaining the thermocline. This is in contrast to the atmosphere, where a much larger portion of incident solar radiation passes through to the Earth's surface.

Two important properties of the thermocline are its depth and its strength, or how rapidly temperature decreases with increasing depth. The thermocline's depth is influenced by the winds at the surface of the ocean. In the Atlantic and Pacific oceans, surface winds push warm surface water away from the equator toward the poles, bringing the thermocline close to the surface at the equator.

Water that diverges at the equator accumulates in the subtropics, increasing the depth of the thermocline there. The thermocline is generally (82 to 656 ft. (25 to 200 m.) deep in the equatorial regions and up to 3,281 ft. (1,000 m.) deep in the subtropics.

The thermocline is strongest in the tropics and weakest in high latitudes. This reflects the fact that the surface temperature of the ocean generally decreases from the tropics to the poles, whereas the temperature of the deep ocean is nearly the same at all latitudes. As a result, the temperature contrast between the upper ocean and the deep ocean is greatest in the tropics. The temperature can drop by as much as 18 degrees F (10 degrees C) in less than 164 ft. (50 m.) in the tropical thermocline.

In the extratropical oceans, the strength and depth of the thermocline vary from season to season. There is a main thermocline throughout the year between 6563,281 ft. (200-1,000 m.). During summer, the sun heats the ocean's surface more strongly than in winter. Most of the additional heat is absorbed in a very shallow surface layer, generating a sharper "seasonal" thermocline above the main thermocline. The seasonal thermocline is similar to the tropical thermocline in terms of its strength and depth. It erodes in the winter as the surface cools relative to the temperature in the main thermocline.

tropical oceans

The existence of a strong and shallow thermocline in the tropical oceans has important implications for climate. In the equatorial Pacific Ocean, westward surface winds lead to an accumulation of warm surface water in the west, depressing the thermocline there and raising it to near the surface in the east. The shallow thermocline in the east enables cold, nutrient-rich water to be mixed upward into the surface layer. Every few years the thermocline in the eastern equatorial Pacific deepens in association with an El Niño event. The mixing of cold, nutrient-rich thermocline water into the surface layer is reduced, the surface temperature of the eastern equatorial Pacific Ocean increases, and biological productivity decreases. The warmer surface temperatures associated with El Niño affect atmospheric circulation in the tropics and alter weather patterns throughout the world.

The depth of the eastern equatorial Pacific ther-mocline has varied significantly in association with changes in global climate. For example, during the early Pliocene period (between 4.5 and 3 million years ago; the most recent period with global temperatures significantly higher than today), the eastern Pacific thermocline was much deeper than it is today, much like it is during a modern El Niño event.

sEE ALso: El Niño and La Niña; Mixed Layer; Wind-Driven Circulation.

BIBLioGRAPHY. S. George Philander, James R. Holton, and Renata Dmowska, El Niño, La Niña, and the Southern Oscillation (Academic Press, 1989); George Pickard and

William Emery, Descriptive Physical Oceanography (Butterworth-Heinemann, 1990).

Gregory R. Foltz NOAA/Pacific Marine Environmental Laboratory

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