Chapter 16 — FUNDAMENTALS OF OCEANOGRAPHY 



2-. The main thermocline — the central layer 

 of the oceans in which there is a rapid decrease 

 of water temperature with depth. 



3. The deep layer — the bottom layer of water 

 which in mid latitudes exists below 4,000 feet. 

 The deep layer is characterized by fairly con- 

 stant cold temperatures, generally less than 

 4°C. 



We will now consider the physical properties 

 of these layers, how they are determined, and 

 their ranges and variations. 



Two variables — temperature and pressure — 

 determine the physical properties of pure water. 

 When studying sea water, a third variable — 

 salinity — must be considered. 



It can readily be seen that some physical 

 properties of sea water are dependent upon 

 pressure, temperature, and salinity, while 

 other properties are affected by the suspension 

 of minute matter and motion characteristics. 

 The latter variable cannot be accurately 

 measured; whereas pressure, temperature and 

 salinity can be determined with a greater degree 

 of accuracy. 



TEMPERATURE 



About three-fourths of the earth's atmos- 

 phere is underlain by the water surfaces of the 

 oceans and other bodies of water. It is the 

 temperature of the sea water, primarily in the 

 upper portions of the mixed layer, which has 

 such a marked effect upon the atmospheric and 

 climatic conditions over the surface of the 

 earth. 



The transport of heat from lower to higher 

 latitudes takes place partly by air currents 

 (winds) and partly by ocean currents. 



Temperatures of the oceans will range from 

 about -2° C to 30° C near the surface. There 

 are of course variations dependent upon such 

 factors as ocean currents, the character of the 

 atmospheric circulations, latitude, variation of 

 the amount of heat absorbed at different depths, 

 and the effect of vertical motion. 



Figure 16-2, depicting the three-layered 

 ocean concept, illustrates the average vertical 

 temperature distribution in middle latitudes, 

 showing little temperature change through the 



TEMPERATURE (°C) 



5,000- 



i 

 \- 

 o. 



LlI 

 O 



< 



UJ 



o 



10,000 - 



10 

 i 



MIXED LAYER 



20 



TEMP UNIFORM OR 

 CHANGING SLIGHTLY 

 WITH 0EPTH 



MAIN 



THERMOCLINE 

 LAYER 



TEMPERATURE 

 DECREASING RAPIDLY 



15,000 - 



209.319 

 Figure 16-2. — Typical thermal structure 

 of the oceans (winter conditions in 

 mid-latitudes). 



mixed layer, sharp decrease through the ther- 

 mocline layer, and a return to a gradual 

 decrease in temperature through the deep water 

 layer. In the lower latitudes, the mixed layer 

 extends to approximately 300 feet, increasing 

 to approximately 1,500 feet in middle latitudes 

 and decreasing to less than 50 feet in higher 

 latitudes. (See fig. 16-3.) The lack of mixed 

 layer thickness at higher latitudes is a result 

 of surface cooling and a sinking of water to 

 deep or bottom layers. 



SALINITY 



The term "salinity" as defined earlier 

 refers to the quantity of dissolved solid material 

 in sea water. 



Salinity in the open ocean varies between 

 the limits of 33 to 37 %o . Variation in surface 

 salinity depends upon the ratio of evaporation 

 to precipitation. In areas of excess evaporation, 

 salinity is high; and in areas where precipita- 

 tion is excessive, salinity values are low. 



375 



