CHAPTER Mi 

 THE MIXING OF OCEAN WATERS 



Section 27. The Concept of Mixing 



Processes continually occur in the ocean which change the vertical and horizontal distribution 

 of temperature and salinity (and other physical-chemical characteristics). 



Some of these processes which are caused by the biological activity of organisms occur 

 throughout the entire depth of the ocean. These processes, which change the relationships between 

 the chemical compounds contained in sea water in small amounts, find practically no expression in 

 the general physical-chemical state of ocean water. There are other processes which sharply 

 change this state; i.e. , the absorption and radiation of solar energy, evaporation, precipitation, 

 etc. ; these develop at the surface of the ocean. In addition, water with a specific temperature and 

 salinity is formed on the surface in one region of the ocean, and is transported by ocean currents to 

 different regions and depths. Here, they mix with waters created under different conditions, and 

 in this manner vertical and horizontal hydrologlcal gradients are continually created and maintained 

 on the ocean's surface and in its depths. 



As a result of all these processes, water masses of different composition and dimensions are 

 created in the ocean; these differ from one another in temperature, salinity, oxygen content, etc. , 

 and are separated from each other by surfaces of separation, i.e. , by frontal surfaces. In vertical 

 cross sections, these water masses appear as superposed layers with the upper layers less dense 

 than the lower ones, as a rule. Only very seldom is a density decreasing with depth observed. 



It is evident from the TS diagram in Section 4 that identical densities can be obtained with 

 different combinations of temperature and salinity. Normally, temperature decreases with depth, 

 while salinity increases, but often anomalies of vertical temperature and salinity distributions can 

 be observed. Borrowing some meteorological terms, temperature increase with depth is called a 

 temperature inversion, and the decrease in salinity downward, a salinity inversion. We have al- 

 ready seen that the downward increase of temperature may be due to the pressure of the upperlying 

 layers. Such an inversion is called an adiabatic temperature inversion. Naturally, in contrast to 

 an ordinary temperature inversion, an adiabatic temperature inversion can be observed in the same 

 water mass. Continuing the analogy, we call a decrease in density downward a density inversion. 



Simultaneously with the creation of water masses in the ocean, processes occur which tend 

 toward equalization of the states and which can be classified under the general heading "mixing." 



Thus, the continual and chaotically uniform thermal movement of molecules comprises 

 molecular mixing.* But the most important and decisive role in the ocean's regime is played by 

 turbulent mixing, subdivided into frictional and convective. 



*The coefficients of molecular diffusion, thermal conductivity, and friction are so small that 

 molecular processes exert practically no influence on the ocean's regime, and they may be neg- 

 lected when solving general problems. 



6.5 



