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



available, to eliminate aperiodic changes. This elimination is done by the "tempera- 

 ture anomaly" method given by Helland-Hansen (1930). According to the mean 

 [rS'l-diagram (see Chap. VI, 3) there is normally, for every value of the salinity in the 

 water mass under consideration, a definite mean temperature {>. If an observation 

 (^1, S) is obtained in this area, then the difference ?^i — i^ is termed the "temperature 

 anomaly" of this observation. Experience shows that the temperature anomalies in a 

 given set of data is of a considerably smaller scatter than the original values and that 

 the aperiodic change of it has very largely been eliminated. The annual temperature 

 variation in particular is shown much better than by the original values. 



By these methods Helland-Hansen has worked out the annual temperature variation 

 for the water layer down to 200 m depth for three ocean areas in the North Atlantic. 

 Figure 51 gives the results for the Bay of Biscay (area B) for the surface, as well as for 



Fig. 51. Annual temperature variation in the water layer down to 100 m depth in the Bay 

 of Biscay (area B) (according to Helland-Hansen). 



the depths of 25, 50 and 100 m. Table 46 presents the time of occurrence of the 

 maxima and minima in this area and in the area between Portugal, Morocco and 

 Madeira and also gives the amplitude at different depths. The amplitude at 25 m 

 is still quite considerable and not very much smaller than the surface amplitude. 

 However, lower down it decreases more rapidly and at 200 m the annual variation is 

 more or less insignificant. The shape of the curve is almost the same in both areas and 

 quite characteristic. In late autumn and in winter the surface water cools rapidly and 

 the resulting convection also involves the deeper layers in this coohng. Thus, the ver- 

 tical temperature gradient decreases continuously and becomes almost zero in spring. 

 Heating now raises rather rapidly the temperature of the uppermost 25 m layer. 



Table 46. Annual temperature variation in the Bay of Biscay (B) 

 and in the area between Portugal, Morocco and Madeira (C) 



