1 20 The Three-dimensional Temperature Distribution and its Variation in Time 



conditions at several stations in the central part of the Atlantic. In the subtropics 

 (30°-20°S. and 20°-25°N.) the isothermal layer extends down to about 100 m, 

 but is more shallow in the tropics and in regions close to the equator (in the west 

 about 75 m and in mid-latitudes 50 m or less). Off the African coast, especially in the 

 Gulf of Guinea, the thickness decreases to 25 m or less, and in the regions with cold 

 water upwelling it is entirely absent. Underneath the top layer there is a strong tem- 

 perature decrease that continues, gradually weakening, down to the lower limit of the 

 troposphere. The maximum of vertical temperature gradient (thermocline) is generally 

 found between 100 and 200 m, with a mean value of nearly 5°C per 100 m. The 

 meridional variation of the depth of thermocline is shown in Table 49 (Fig. 54). 



Table 49. Meridional variation of the depth of thermocline in the Atlantic Ocean 

 (Mean values for the entire ocean) 



Latitude 

 Depth (m) 



20° S. 

 141t 



15° 

 121 



10° 

 108 



5°S. 



77 



0° 

 69* 



2-5 °N. 

 83t 



5° 

 81 



10° 



53* 



15° 

 89 



20° 

 160 



25° N. 

 195* 



* Minimum; f Maximum 



20° S 10° 0° 10° 20° N 



Fig. 54. Meridional distribution of the depth of thermocline in the Atlantic. 



The thermocline rises steadily from a depth of 150 m in the subtropics to minimum 

 values in the equatorial regions. Approaching the equator from the Southern Hemi- 

 sphere a minimum of about 70 m is reached directly at the equator; however, coming 

 from the north the minimum (about 55 m) already shows in 10° N. Between these two 

 highest locations the thermocline drops about 1 5-20 m to a deeper level (approx. 80 m) 

 at 2-5° N. These changes in level are rather characteristic for the entire width of the 

 ocean and due to dynamical reasons are associated with the zonal oceanic circulation of 

 the equatorial water masses (see Chap. XVIII and XIX). The intensity of the thermo- 

 cline is greatest in the equatorial areas, where it has a mean value greater than 

 0-4 °C/m. An actual transition layer (temperature gradient >0-rC/m) properly 

 speaking only occurs between 15° N. and 15° S.; on either side of this belt the 

 gradient falls rapidly toO-05° C/m or lower and the transition layer shows only as 

 an intensification of the vertical temperature gradient. 



