TEMPEEATtTKE OF THE SURFACE WATERS OF THE ATLANTIC OCEAN 



17 



c. Eqnatorial Ocean. 



The zonal boundaries of the bodies of water at the 

 surface of the ocean have been located by means of areas 

 with steeper temperature gradients, which appear pre- 

 dominantly in the middle and high latitudes. In regions 

 where the climatic contrasts are smaller, such a pro- 

 cedure is, however, not feasible. 



The subtropical convergence, which in most cases 

 cannot be distinguished by the distribution of temper- 

 ature, is shown by current displacement. In the case 

 of the equatorial part of the ocean with its large areas 

 of evenly heated masses of water, however, another 



northerly position. It then runs from Cape Verde 

 (15° N.) on the African Coast to 10° N. at 40° W. and 

 to 25° N. in the Gulf of Mexico, where the warm water 

 of the equatorial current is warmed stiU further by local 

 heating. In October the returning movement south- 

 ward sets in, so that as early as November the thermal 

 equator reaches the Yucatan Sea and the Gulf of 

 Guinea. While the thermal equator on the east side of 

 the ocean remains at 2° N. as its most southerly position 

 for the series on the northern hemisphere, it crosses the 

 earth's equator in the west as early as January and 

 comes on the average up to 7° S. in March and April. 



■ ^Position of the thermal equator at the surface for the months January, April, July, and October, 



J =Position of the thermal equator at the surface for the months February, March, August, and September. 



^„^^P =Average southern extreme position. 

 _ _ _ ^D =A verage northern extreme position. 



. E =A ver^e position according to the annual average of the temperature. 



Figure 18. — Position of the thermal equator at the surface of the hydrosphere. 



criterion must be sought to establish the boundary of 

 the tropic zone in the sea. The thermal equator of the 

 hydrosphere, which indicates the momentarily warmest 

 zone in the surface water and follows, with a certain 

 amount of lag, the solstitial point, presents such a 

 criterion. 



If the one-degree fields are combined with the highest 

 temperature values on the monthly charts of the temper- 

 ature (plates VI-XVII), the course of the thermal 

 equator in the surface water is derived, as shown in 

 figure 18. In August and September it reaches its most 



The northward movement first begins in the west, 

 where in June it again crosses 21° N., whereas it does not 

 leave the Gulf of Guinea until July. For the sake of 

 clarity, only the months near the extreme positions, 

 i. e., July to October, are recorded on figure 18 for the 

 northern positions and January to April for the south- 

 ern ones. In November and December and in May 

 and June, the thermal equator assumes intermediate 

 positions. 



The way in which the seasonal displacements are 

 expressed on individual meridians is shown in figures 19 



