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Main Thermocline The main thermocline is the central layer of the ocean. It is found at the base of the mixed layer and is marked by a rapid decrease of water temperature with depth.SEASONAL THERMOCLINE. At high latitudes there is no marked change in water temperature with the seasons, while in the mid-latitudes, a seasonal thermocline develops with the approach of summer. This seasonal thermocline comes about from the gradual warming of the surface waters. The warming takes place in the upper few hundred feet of the surface, and results in the seasonal thermocline becoming super-imposed on the main thermocline. Figure 1-2-3 illustrates the development of the seasonal thermocline in the mid-latitudes.The mid-latitude summer thermocline is more pronounced than the thermocline of spring or autumn. Bathythermograph traces of the summer thermocline show that it affects a much broader range of depth than at any other time of year. In our illustration, the seasonal thermocline is roughly 35 meters thick (90 to 125 meters deep). Note, also, that the winter temperature profile shows no seasonal thermocline. The mixed layer extends to a depth in excess of 160 meters. Come spring, the surface water is warmed and a seasonal thermocline develops between 35 and 60 meters. As summer takes hold, the water warms to 25C and the mixed layer extends to a depth of approximately 90 meters. The thermocline now exists between 90 and 125 meters. In summer, the seasonal thermocline is deeper and covers a broader range of depth than at any other season of the year. With the approach of autumn, the mixed layer continues to drive the thermocline deeper, but the water within the mixed layer is cooler than it was
Figure 1-2-3.-Upper level thermal structure of seasonal thermocline in middle latitudes. in summer. Just as in the spring, the cooler water in the mixed layer decreases the range of depths covered by the thermocline. In low latitudes, small seasonal temperature changes make it difficult to distinguish between the seasonal and the permanent thermoclines.
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