~^ IN MEIKS 



One reassuring result of this continuing study is that there have 

 been no signs of large changes in the distribution of temperature, 

 salinity, and other properties in the deep water over a period of a 

 few decades. Work on a similar scale is planned for the forth- 

 coming International Indian Ocean Expedition. 



The earliest observations on the deep water showed that, even 

 in equatorial latitudes, the temperature at great depths was very 

 near to freezing point. Recently, more detailed surveys have shown 

 that in the moderate latitudes of all oceans there is a more or less 

 steady fall in temperature with depth for about the first half mile or 

 more. Below this the temperature decreases more slowly. At a 

 depth of a mile the temperature nearly everywhere is down to 4°c. ; 

 and the bottom half of the water in all of the oceans has tempera- 

 tures lower than 4°c. Because salt water (unlike fresh water) be- 

 comes denser as the water is cooled below 4°c., there is a more or 

 less steady increase in density with depth in the oceans. 



This vast reservoir of surprisingly uniform cold water appears 

 to be formed from two sources — one in the extreme north of the 

 Atlantic, the other in the Weddell Sea on the borders of Antarctica. 

 In these two regions the surface waters may be cooled sufficiently to 

 make them heavy enough to sink and mix with the deep water, 

 setting up a density-driven deep circulation. The North Atlantic 

 Deep water can be traced as a distinct water mass throughout the 

 entire length of the Atlantic Ocean ; it is slightly warmer and saltier 

 than the Antarctic Bottom water coming from the Weddell Sea, 

 which is found underlying it in the South Atlantic. The deep layers 

 of the Pacific and Indian oceans appear to be made up of a mixture 

 of these two kinds of deep water. 



Overlying the deep water in much of the Southern Hemisphere 

 there is the Antarctic Intermediate water. This water mass — colder 

 than the North Atlantic Deep water but much less salty, so that it 

 has a lower density — is formed from the northward-drifting Ant- 

 arctic surface water which sinks at the Antarctic Convergence 

 (roughly about lat. 5o°s.), where it meets warmer water. 



In the northern Atlantic Ocean the Deep water is overlaid by a 

 much warmer and saltier water mass originating in the Mediter- 

 ranean Sea. Evaporation of the surface water of the Mediterranean 

 greatly exceeds rainfall and river runoff around the Mediterranean 

 seaboard. This means that the salt content of the surface water is 

 increased. In the winter some of the surface water is cooled suffi- 

 ciently so that it sinks and forms a dense deep layer of highly saUne 

 water which, as an undercurrent, overflows from the Mediterranean 

 into the Atlantic. Here the water quickly sinks to a depth of more 



This vertical section of tlie Atlantic from 

 the Antarctic to the Arctic shows the 

 Antarctic Bottom Current flowing beneath 

 the less dense water of the North Atlantic 

 Deep Current. (Salinity of the water is 

 given in parts per thousand by weight.) 



This chart illustrates StommeTs theory of 

 deep water circulation of the oceans. 

 Dots mark the two main sources of deep 

 water - in the extreme North Atlantic and in 

 the Weddell Sea. Thick lines along western 

 sides of the major oceans show the strong, 

 deep currents. Thinner lines show horizontal 

 movement of deep water that should be 

 everywhere directed toward the poles. 



