96 The Three-climensionol Temperature Distribution and its Variation in Time 



Only in the absence of any more rapid processes could the lower temperature of the 

 deep sea be taken, as was previously assumed, as evidence of a much lower former 

 temperature at the surface of the ocean (ice ages). Periodic changes of temperature 

 at the surface of the sea will be transmitted to the deeper layers and cause a periodic 

 change there also. The theory of conductivity shows that when the surface change 

 has the simple form 



&0 = Oq cos y / 



it will have at a depth r the form 



^2 = Qq e""" cos 



(y'-4 



where a = ■\/{TrjaT) = ■\/{Cj,pttIXT) and Tis the period of the oscillation. The amplitude 

 of the change in temperature decreases according to the e-function and at the same time 

 there is a phase shift. For the diurnal variation in temperature the amplitude at the 

 surface is reduced to 1% at a depth of 28 cm and the extremes at this depth have 

 already been shifted by three-quarters of the period (18 h). The corresponding values 

 for the annual variation are 5 m with here also a phase change of three-quarters of the 

 period (268 days). The general effect of molecular thermal conductivity confines both 

 these periodic changes to the very uppermost layers of the sea. 



(c) Thermo-haline Convection 



A much more rapid process than molecular thermal conductivity is the vertical 

 displacement of small quanta of water which occurs when a small part of a water mass 

 is heavier than the water underneath it. To restore the disturbed equilibrium the heavier 

 water tends to sink and the lighter to rise. Associated with these forced vertical move- 

 ments of small water quanta there is also a transport of the characteristic properties 

 of sea-water in vertical direction which leads to an equalization of any vertical 

 differences in these properties which may be present (see p. 195). 



This has a rather important effect on the state of the deep water layers. An 

 increase in the weight of small water particles at the surface may be caused either by 

 an increase in salinity due to evaporation or by the formation of ice or it may be due 

 to cooHng. If the temperature of a small water particle falls, its specific volume also 

 decreases as long as the salinity is greater than 24-7%o (see p. 46). In a volume of 

 water with a horizontal cross-section of 1 cm^ and a height of // cm the temperature 

 change AS- due to a removal of an amount of heat AQ\^ given by 



Cj>h 



If a layer of water of e mm thickness evaporates from the top of such a column of 

 water with 5'%o salinity then the increase in salinity when evenly distributed over the 

 column of water is given with sufficient accuracy by 



AS, = ^ €. 



If at the top of a similar column an ice layer of e cm thickness is formed with a salt 



