The Three-dimensional Temperature Distribution and its Variation in Time 109 



everywhere with increasing depth, most rapidly at the equator and least at the poles. 

 This case will be considered later in connection with the actual temperature distribu- 

 tion in the deep ocean (see p. 123). 



Another solution for the third special case is 



^• = ^11 



m e' 



cos 



z with jS = 



V 



11 ^ 2/ 



Obviously the solution of this distribution of 5 is identical with that of the first case on 

 p. 106 if j8 is put equal to a, that is if 



This means that in a vertical cross-section a tongue-shaped distribution of s can be 

 equally well regarded as the effect of a horizontal advection with velocity u in the 

 direction of the tongue and as a vertical turbulence with an exchange coefficient A'^, 

 or as the sole effect of pure mixing in horizontal and vertical directions without any 

 advection. See Vol. I, Part II, Chap. XIII, 3, for a theoretical discussion of turbu- 

 lent mixing in ocean currents. 



3. Diurnal and Annual Variation of the Temperature in the Ocean 



The daily variations in temperature at the surface of large bodies of water (lakes 

 and seas) are confined within narrow limits as was mentioned previously. In lakes, 

 away from the shore, there may be diurnal variations exceeding 2 °C. They decrease 

 rapidly with depth so that at 4-6 m they may be not more than 0-1 °C (see particu- 

 larly the investigations by Homen (1913) in Lake Logo (Finland). Some idea of 

 the diurnal temperature variation (of the air and the water) is afforded by the in- 

 vestigation of Merz (1911) in the Gulf of Trieste (an enlcosed basin, relatively close 

 to the land). The amplitude of the water temperature was 0-87 °C, for the air it was 

 3-l°C, which is considerably more. For a discussion of the diurnal and annual varia- 

 tions of the surface temperature in a shallow water especially in the North Sea and 

 in the Bahic, see Dietrich (1953). 



(a) The Diurnal Temperature Variation in the Open Sea 



The diurnal variations of temperature in the open sea are even smaller than in 

 lakes; usually smaller than 0-4 °C and can rise at the most to about 1 °C in calm and 

 fair weather. The most accurate measurements of the daily temperature variation in 

 the open sea are obtained at anchor stations (fixed location). Four equatorial stations 

 between 12-5° N. and 4° S. of the "Meteor" Expedition in the Atlantic Ocean (De- 

 FANT, 1932) gave the following values (Table 43). 



Table 43. Mean daily temperature variation from four "'Meteor'' anchor stations 



Minimum; j Maximum 



