Absorption values are approximately 0.065 dB/km at 1 kHz and 1 dB/km 

 at 10 kHz (T = 4.0°C). The absorption coefficient is that component 

 of the attenuation coefficient that passes directly into heat in the 

 ocean. 



Absorption of sound in pure water, computed from Rayleigh's 

 formula based on shear viscosity, is about one- third of the measured 

 absorption. The water molecule has a bent-triangle structure, with an 

 angle of 109° between the oxygen and two hydrogen atoms. Two modes 

 of packing exist for water molecules, ice and liquid, both of which 

 are always present in varying relative amounts in liquid water. The 

 density maximum at 4°C and atmospheric pressure for the liquid is due 

 to a relative maximum amount of the closely packed structure. Molecular 

 packing is pressure dependent, and in the presence of a compressional 

 sound wave, there is a relaxation effect causing the packing of some 

 molecular aggregates to shift back and forth between liquid and ice 

 structure. The extraction of acoustic energy in this process is the 

 pure water absorption. 



In seawater, the additional absorption over that of pure water 

 is due to a pressure sensitive relaxation process at about 70 kHz, 

 arising from a shift between hydrated and nonhydrated magnesium sulphate 

 ions (fig. 4-6). Magnesium sulphate accounts for 5 percent of the salts 

 present in seawater. The effect of sodium chloride, accounting for 

 78 percent of seawater salinity, is to inhibit absorption. An 

 expression for the absorption of sound in seawater for frequencies 

 above 2.5 kHz as a function of temperature, salinity, and pressure is 

 given in figure 4-7. 



A relaxation occurring at about 1 kHz increases seawater 

 absorption further, by a factor of 10 over the magnesium sulphate 

 absorption, at very low frequencies below 1 kHz. This relaxation 

 process is due to the boric acid-borate system in seawater. Thorp's 

 empirical expression for absorption of sound in seawater over the 

 entire frequency range is given in figure 4-8. 



5. The Steady State of the Ocean 



a. Heat Transport by Winds and Currents 



Since the average temperature of Earth as a whole is not 

 changing appreciably, then the total amount of heat received from the 

 Sun must equal the amount lost by radiation back into space. However, 

 at any one time and place. Earth is either warming or cooling. The 

 intensity of solar radiation is largely a function of latitude and 

 season. 



Figure 5-1 shows the radiation balance between insolation and 

 outgoing radiation as a function of latitude. The surplus in the 

 tropical regions is balanced by the deficit in polar regions. The 



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