WATER MASSES AND CURRENTS OF THE OCEANS 217 



sea level in both hemispheres. That is the case in the Atlantic Ocean, 

 where deep water is formed in the Northern Hemisphere. The water 

 that sinks in high latitudes or flows out of the Mediterranean Sea spreads 

 at great depths, continues across the Equator, and is replaced by hori- 

 zontal flow from the South Atlantic Ocean. In the Indian Ocean a 

 similar mechanism operates, but on a very small scale, because sinking 

 takes place only in the Red Sea, and the amounts of deep water that are 

 formed there are small and exercise a significant influence only upon 

 conditions to the north of the Equator. In the Pacific Ocean no mecha- 

 nism exists that will give rise to a considerable exchange of water across 

 the Equator, because conditions are such that no deep water is formed 

 in the North Pacific. 



On the other hand, since continuity must exist between the South and 

 the North Pacific, the depths of the North Pacific Ocean are filled by 

 water of the same character as that which is found in the northern 

 portion of the South Pacific. The most accurate data that are available 

 indicate that a very small exchange of deep water takes place between the 

 two hemispheres and that a sluggish motion to the north may occur on 

 the western side of the Pacific Ocean, whereas a sluggish motion to 

 the south may occur on the eastern side. The uniform character of the 

 Pacific Deep Water is illustrated in fig. 61. 



In the preceding discussion the term "flow of water" has been used 

 freely, but one has to deal actually with such slow and sluggish motion 

 that the term ''flow" is not appropriate, since the average velocities 

 must often be measured in fractions of a centimeter per second. 



In conclusion it can be stated that appreciable exchange of deep and 

 bottom water across the Equator takes place in the Atlantic Ocean only. 

 It is rudimentarily present in the Indian Ocean and practically absent 

 in the Pacific. Superimposed upon such an exchange between the 

 hemispheres there exist independent circulations in the three southern 

 oceans, because Antarctic Intermediate and Bottom Waters return to the 

 Antarctic as deep water. This circulation is well established in the 

 Atlantic Ocean, and in the Indian and Pacific Oceans the existence of 

 such a circulation is derived partly by analogy with the Atlantic Ocean 

 and partly by an examination of the few available precise salinity 

 observations. 



Ice In the Sea 



Ice and Icebergs in the Antarctic. Two forms of ice are encoun- 

 tered in the Antarctic Ocean : sea ice, which is formed by freezing of sea 

 water, and icebergs, which represent broken-off pieces of glaciers. The 

 appearance of both sea ice and icebergs varies widely. Several classi- 

 fications have been proposed by Transehe, Smith, Maurstad, and 

 Zukriegel, and several codes for reporting ice have been introduced, one 



