1 1 2 The Three-dimensional Temperature Distribution and its Variation in Time 



(larger than 15 °C) off the coast of North America and in the region of the Newfound- 

 land Banks. There, as already mentioned, the annual variation in temperature is 

 caused by the fluctuating seasonal movements of ocean currents. Similar conditions 

 occur in the North Pacific; the absolute maxima of more than 20 °C in the Yellow 

 Sea, and in the Sea of Japan, are associated with a zone of maximum annual ampli- 

 tude (greater than 9°C) extending from Japan eastward towards the east coast of 

 North America. In the Southern Hemisphere the subtropical maxima of temperature 

 variations are of a smaller extension. The annual temperature range is also large 

 (8-10°C) in the areas of cold water upwelling (off" western Africa in the Northern and 

 Southern Hemispheres and off California) in accordance with the seasonal variations 

 in these phenomena. 



The geographical distribution of the annual temperature variation at the sea 

 surface is not difficult to explain. In the tropics the small amplitude is due in the first 

 place to the constant high altitude of the sun throughout the whole year and also to 

 the relatively high cloudiness, so that there are only small annual variations in the in- 

 coming radiation. In the subtropics the absorption of solar radiation has a much 

 greater influence on the development of a marked annual temperature variation be- 

 cause of the already larger seasonal changes in the zenith distance of the sun, and also 

 because of the stronger effect of back-radiation due to the low cloudiness prevalent in 

 these areas. With increasing latitude the incoming radiation becomes less effective 

 and the autumn and winter convection, which is able to penetrate down to greater 

 depths here, still further reduces the annual amplitude of the temperature variation 

 until it reaches a minimum in the polar regions. Table 44 shows mean annual variations 

 in temperature for equatorial, temperate and high latitudes, by the use of the mean 

 temperatures for zones of 10° latitude given by Bohnecke (1938). In the equatorial 

 zone there are two maxima at the time of the equinoxes. In the subtropics the maximum 

 occurs in September and March, respectively, and in the extra-tropical regions in 

 August and February, respectively. The minimum values in the first area occur in 

 March and August respectively, and in the latter, in February and September, re- 

 spectively. 



Table 44. Annual variation in the water temperature at the sea surface in the 



Atlantic 

 (Deviation from annual mean 0-1 °C) 



* Minimum; f Maximum 



In general the surface temperature minimum is retarded about two to three months 

 after the sun reaches its lowest height; the maximum is retarded also by about the 



