Figure 5-8 shows frequency of occurrence by percent of mixed layer 

 depths in the North Atlantic by season. In summer and fall they are mostly 

 shallower than 250 feet (76 m) . In winter they are mostly deeper than 

 250 feet. 



c. Salinity and Water Masses 



Characteristics of ocean waters are controlled by surface 

 processes, for it is only at the sea surface that heat or freshwater 

 is added or removed in quantity. Thus, salinity and temperature of 

 midocean surface waters are determined by the distribution of evapora- 

 tion and precipitation processes as well as solar radiation. Surface 

 salinities reach maximum in belts centering at about latitudes 23 °N and 

 S. Evaporation is maximum along the outer edges of the trade wind belt, 

 where the air is descending and thus relatively dry. More precipitation 

 than evaporation occurs in the tropics and at latitudes above 40°N and 

 S. Salinity in the surface waters of the ocean is proportional to the 

 excess of evaporation over precipitation. This is demonstrated in 

 figure 5-9. 



Salinity effects on density are sizable, but the pycnocline is 

 controlled by development of the thermocline because of the relatively 

 large temperature range (-1.7°C to 30°C) of seawater compared with the 

 relatively small salinity range (33 to 37 °/oo) . 



In surface waters, the relationship between temperature and 

 salinity is highly variable because of surface heating and cooling, 

 evaporation, and precipitation. In the main thermocline and deepwater 

 layer, however, a direct correlation between temperature and salinity 

 exists. Knowing the temperature and water mass, one can predict the 

 salinity with almost as much accuracy as it can be measured by the 

 routine process (+0.02 °/oo). 



The permanency of the oceans' gross vertical structure, main- 

 tained by vertical circulation, allows identification of water masses 

 from their vertical salinity and temperature distributions. If the 

 vertical distributions of temperature and salinity are plotted on a 

 graph with temperature and salinity as coordinates, and density excess 

 ( fft ) as a parameter, then the resulting characteristic curve is 

 called a T-S curve. Figure 5-10 shows T-S curves for the Atlantic Ocean. 

 Figure 5-11 shows the water masses of the world oceans classified 

 from their T-S curves. 



d. Sound Speed vs. Depth Structure 



The density stratification of the ocean, with the three-layered 

 temperature model, leads to a typical sound speed vs. depth profile 

 such as seen in figure 5-12. 



In the thermocline, temperature changes rapidly and dominates 

 the sound speed profile. In the mixed layers, the temperature profile 



17 



