The Three-dimensional Temperature Distribution and its Variation in Time 143 



(1) The isotherms tend to be arranged zonally, especially in higher southern latitudes 

 in all three oceans, where they almost parallel the latitude circules. This is due to the 

 homogeneous climatic conditions over this almost exclusively oceanic area. 



(2) The major equatorial ocean currents to a large extent run from east to west. 

 At east coasts of the continents they diverge and the isotherms do the same. The 

 western sides of the oceans are thus appreciably warmer than the eastern sides. These 

 differences are particularly pronounced in the Atlantic; here in temperate and higher 

 latitudes this difference between east and west is actually reversed, and from about 

 35° N. the east is appreciably warmer than the west. However, this phenomenon 

 does not occur in the Southern Hemisphere. Again, the major current system at the 

 sea surface can be considered to be the cause of different behaviour of both hemi- 

 spheres. The horizontal advection of water with a different temperature produces 

 almost stationary contrasts in temperature between the eastern and western side of 

 the ocean. In addition the distribution of land and sea and in some regions local 

 oceanographic-meteorological phenomena, such as upwelling water, and piling up 

 ("Anstau"), influence the temperature distribution. 



(3) There is another phenomenon apparent on the chart which is not clearly shown 

 in the Southern Hemisphere because of the sparsity of the observations, although it 

 has long been recognized in the Northern Hemisphere. This is the uneven, stepwise 

 change in temperature towards higher latitudes. Already Fig. 50 (see p. 1 13)shows clearly 

 this phenomenon, as it appears in the Atlantic. In both the Northern and the Southern 

 Hemisphere there is an increase in the meridional temperature gradient in the zone 

 between 40° and 50° which, during the year, is displaced towards and away from the 

 poles following the movements of the sun. The concentration of the isotherms into a 

 narrow belt between the Gulf Stream and the Labrador Current and between the 

 Atlantic water and the Greenland Current is quite obvious. This boundary is called, in 

 analogy with the atmospheric polar front, the "oceanic polar front" which indicates 

 the position of the Arctic convergence where the two different types of water are 

 brought into close contact. Its southern continuation along the east coast of North 

 America has long been known as the "cold wall". This discontinuity appears in the 

 chart of mean values because the aperiodic displacements of the ocean currents are 

 confirmed within narrow limits. Accurate information about this sharp discon- 

 tinuity has only been obtained from numerous thermographic recordings made by ship- 

 ping across the whole system of currents off the east coast of North America (Church, 

 1937; Spillhaus, 1940). Figure 60 shows the most important of the results obtained by 

 analysis of these recordings. The coastal water with a slowly increasing temperature 

 eastwards borders the warm belt of water in the Gulf Stream which is barely 50 km 

 wide. Towards the east the Gulf Stream is separated almost as sharply by a rapid 

 fall of temperature from the water of the Sargasso Sea, where the temperature rises 

 again slowly towards the east and south-east. 



The "band" character of the Gulf Stream does not show very clearly in the hori- 

 zontal temperature charts, since the temperature is recorded at one or two degree 

 squares which completely blurs this phenomenon, and the strong aperiodic dis- 

 placements of the discontinuity along the right-hand side of the band (looking down- 

 stream) contribute to this blurring when mean values are taken. The observations are 

 also not strictly synoptic but are only obtained with differing time. 



