692 



The Stratospheric Circulation 



core layer of this upper North Atlantic Deep Water (Fig. 330). In the western part of 

 the North Atlantic there is still a content of 25-30%, at the equator 20-18% in the 

 South Atlantic the Mediterranean content gradually falls to below 2%. The form of 

 the [r^l-relationship which is nearly a straight line indicates that the changes in the 

 core layer are due essentially to a simple mixing process. 



The great effect of the water flowing out from the Straits of Gibraltar on the compo- 

 sition of the water masses in the Atlantic is at first sight astonishing. A rough calcula- 

 tion shows, however, that it is of the right order. According to Schott (1939), about 



366 



Fig. 330. Standard curve of the [TiS] -relationship in the core layer of the upper North 

 Atlantic Deep Water (Mediterranean Water). 



52-000 km^ of water a year flows out from the Mediterranean into the Atlantic. For 

 a mean velocity of spread of about 2 cm/sec it would require about 6 years to spread 

 over the area between 45''N to 15° N. During this time the Straits of Gibraltar will 

 supply 312-000 km^ of Mediterranean Water, which, distributed evenly over a layer 

 of 500 m thickness from 45° N to 15° N., would mean a contribution of about 3-4%. 

 The layers inside the Spanish bay will, of course, show a considerably higher percen- 

 tage.* 



IsELiN (1936) has not quite agreed with the idea of an extension of Mediterranean Water to the 

 higher latitudes of the Southern Hemisphere. On the basis of "Atlantis" observations he investigated 

 the deviations of individual values from the standard value for the whole region using the Helland- 

 Hansen anomaly method (see Pt. I, p. 114). A positive anomaly is present at 1200 m depth only as far 

 as about 20° N. (until the North Equatorial Current), while farther south deficits appear due to the 

 effect of mixing with Antarctic intermediate water. According to Iselin the effect thus extends no 

 further than 20° N. This difference in viewpoint can be explained by differences in the definition of 

 the "Mediterranean Water"; the fact at least remains that traces of Mediterranean Water can be 

 followed far into the South Atlantic. 



The process of spread of Mediterranean Water through the Straits of Gibraltar 

 and out into the Atlantic is certainly of a twofold nature. During the first part of the 

 outflow and sinking of the heavier Mediten-anean Water, until it reaches the shelf and 

 the continental slope and until it finds the depth of equal density inside the Atlantic, 



* These percentages refer to the water present between 600 and 700 m depth west of the Straits 

 of Gibraltar which has a temperature of 11-9° C and a salinity of 36-5 %„ and was termed "Mediter- 

 ranean Water" by Wiist. If absolute values are required of the proportion of Mediterranean Water 

 from east of the Straits of Gibraltar then the given values must be reduced by half. 



