A slow, density-driven, vertical circulation occurs in the 

 ocean that leads to a stable density layering or stratification. Water 

 is heated in the tropics. This heated water decreases in density and 

 expands, spreads northward and southward along the surface, and gradu- 

 ally cools as it approaches higher latitudes. As warm surface water 

 is drained off from the tropics it is replaced by upwelling cold water 

 that originated in polar regions. The deepwater layer of all oceans 

 is derived from polar regions and has about the same characteristics 

 of salinity and temperature as the surface waters of those regions. 

 This happens because the water is uniformly cold in these regions and, 

 along with brine residue left behind in ice formation, produces the 

 heaviest waters in the ocean. This water sinks and spreads from one 

 pole to the other. The horizontal net drift in these deep thick layers 

 is very small, about 1 to 3 cm/s. This produces a low average tempera- 

 ture about 3.8°C for the whole ocean, and a conservative temperature- 

 salinity relationship. 



b. Three-Layered Vertical Temperature Structure 



Local convection is important in heat exchange near the surface. 

 Surface waters may be warmed down to 10 meters by solar radiation, and 

 may be cooled by evaporation, precipitation, and radiation. Both cooled 

 water and more saline water will be heavier than that beneath it and 

 will sink. These processes together with wind result in vertical mixing 

 and widespread formation of an isothermal surface layer. 



The basic vertical thermal structure is essentially a three- 

 layered system (fig. 5-5): (1) a relatively warm thin surface layer 

 mixed by wind and convection processes, (2) a very deep mass of much 

 colder water in which the temperature decreases very slightly and 

 uniformly with depth, and (3) a transition layer, known as the main 

 thermocline. This structure is an inherently stable one since the 

 density, which is largely controlled by the temperature, increases 

 with depth (pycnocline) . The warm surface layer is thickest in mid- 

 latitudes as is the thermocline or pycnocline. In latitudes higher than 

 about 50°N and S, no surface layer exists, and the entire water column is 

 the same temperature as the cold deen ocean water. Superimposed upon this 

 stable situation are marked diurnal and seasonal temperature variations 

 in the upper layers. Illustrating the three-layered ocean, figure 5-6 

 shows actual plots of temperature-depth curves for three different 

 latitudes for summer and winter. 



Figure 5-7 indicates seasonal effects in the near-surface layer 

 due to wind and convective mixing. If the spring condition is a 

 moderately deep mixed layer, then surface heating and calm conditions 

 of summer will create a surface thermocline leaving an isothermal 

 section in the profile (fig. 5-7A) . Winds then mix the near-surface 

 water creating a pair of mixed layers and thermoclines (fig- 5-7B) . 

 When strong winds blow, one deep mixed layer develops (fig. 5-7C) . In 

 autumn, surface cooling (fig. 5-7D) will induce convective mixing 

 (fig. 5-7E) , and by winter the mixed layer is very deep (fig. 5-7F) . 



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