The Three-dimensional Temperature Distribution and its Variation in Time 97 



content 5^, smaller than S, then, as a first approximation the corresponding increase 

 in salinity is given by 



0-9g {S - S,) 



If the ice contains no salt (Sg = 0) then 



In these quantities AQ,AS, and ASe (heat loss, salinity increase by evaporation and by 

 ice formation) lies the primary cause of every thermo-haline convection. In lower lati- 

 tudes where there are only small variations in the temperature the heat loss is out- 

 weighed by the effect of evaporation ; in temperate latitudes the heat loss by radiation 

 is the decisive factor, while in polar regions, in addition to these processes, the increase 

 in salinity due to the formation of ice is also effective. 



Only very small changes in the specific volume are needed to initiate convection in 

 the uppermost surface layer since the resistance to be overcome is not large, a hun- 

 dredth %o salinity or a hundredth degree centigrade is sufficient. 



The range of effectiveness of convection depends entirely on the vertical density 

 distribution in the water mass in which it occurs. For a given surface disturbance it 

 can only extend down to that depth at which the displaced quantum of surface water 

 reaches, water having the same specific volume. If there is a rapid decrease in the spe- 

 cific volume, then the convection will cease in the upper layers ; this is liable to occur 

 particularly at the density transition layer (thermocline) which acts as a barrier layer 

 and confines the thermo-haline convection to the top layer of the sea (thin homo- 

 geneous layer of uniform density). On the other hand, a randomly initiated disturbance 

 of any size at the surface leads to convection which extends in a homogeneous water 

 mass down to the bottom. The range of effectiveness of convection is a maximum only 

 when the density disturbance of the sinking water quantum is retained while it sinks. 

 If, as is to be expected, it mixes with the surrounding water the disturbance will be 

 rapidly decreased and the depth of influence of convection will be correspondingly 

 less. The larger the density difference between the sinking water and its surroundings 

 the more rapidly the difference between them will be diminished and the greater the 

 reduction in the depth of the convection layer. 



When the sea has a normal stable structure (tropics, subtropics and temperate 

 latitudes) the nocturnal convection before sunrise will extend to a depth of 10 or 20 m. 

 The seasonal convection processes, caused by prolonged cooling during the autumn 

 and the winter, will extend to greater depths, normally to about 300 m. The con- 

 vection is developed to its greatest extent in polar and subpolar latitudes, where it is 

 assisted by a very uniform temperature and salinity distribution. The question for the 

 primary cause initiating these major convection processes, which are of decisive 

 importance for the deep-sea circulation of the ocean, has been the subject of a con- 

 troversy that is still not without interest. The initiation and maintenance of the vertical 

 convection in higher latitudes could be due to the cooling of the upper layers by radia- 

 tion, or it could be due principally to contact with melting ice. Pettersson (1904) 

 supported the strong cooling effect of the ice that is so plentiful in these latitudes, 

 while Nansen (1912) favoured the direct cooling of the surface layer by outgoing 



