72 



BJ0RN HELLAND-HANSEN 



(REP. OF THE "MICHAEL SARS" NORTH 



names tropo-hydrosphere and straio-hydrosphere in con- 

 tradistinction to the corresponding terms for the atmos- 

 phere, but when no misunderstanding can arise the 

 shorter names are sufficient. 



Where to fix the limit between the troposphere and 

 the stratosphere in the sea is more or less a matter of 

 opinion. The vertical extension of the two spheres is 

 very variable. In some small areas the troposphere may 

 at times be absent, while in other regions it may reach 

 1000, or 2000 metres, or more, but in most cases the 

 transition to the more uniform stratosphere is very gradual. 

 For a special ocean we may agree in fixing a certain 

 isotherm as the limit between the two spheres. In the 

 Arctic and Antarctic regions as well as in the Norwegian 

 Sea we may, for instance, take the isotherm of 0' C, 

 in the Mediterranean that of 13° C. etc. With regard to 

 the North Atlantic it seems reasonable to select the 

 isotherm of 3° C as a limit. In the greater part of this 

 ocean 3° C. is found at depths between 2000 and 3000 

 metres below tlie surface. The water of the stratosphere, 

 with temperatures below 3° C, is here called the deep 

 water, although in some regions it may arise to high 

 levels. But in the upper strata of this water there is a 

 vertical gradient of temperature too, even if it generally 

 is considerably less than in the lower part of the tropos- 

 phere. What is here called the deep water is often called 

 the bottom-water, but we shall use this therm only with 

 reference to the lowermost water-layers, or to deep water 

 where the temperature in situ is either uniform or in- 

 creasing downwards. The bottom-water is thus synonymous 

 with the deeper part of the deep water. 



Quite a number of obseivations have been taken 

 from the bottoin-water (in combination with soundings), 

 but until recently only very few vertical series have been 

 secured from depths below 3000 metres. In 1921—22 the 

 Danish expedition in the "Dana", and in 1925—27 the 

 German expedition in the "Meteor" have worked a 

 number of stations in the Atlantic with serial observations 

 from great depths, but the observations are as yet only 

 partly published. At present we possess, therefore, scanty 

 material for studying the conditions in the real deep- 

 water of the Atlantic, to say nothing of other oceans. 



The deep water of the North Atlantic has, in the 

 main, its origin in the northern regions of this ocean. 

 After a discussion of the bottom-water and the cooling 

 of the Ocean Nansen [1912] summarizes his results in this 

 way: "the bottom-water of the North Atlantic has a 

 salinity of about 34-90 "yW ^nd temperatures about 

 24° C. It is chiefly formed at the sea surface in a 

 limited area southeast of Greenland. It is also to some 

 extent formed by water from the Norwegian Sea, flowing 

 across the Faeroe-Shetland-Greenland submarine ridge". 



The very salt and relatively warm water coming from 

 the Mediterranean as an under-current through the Strait 

 of Gibraltar is heavier than any water in the Atlantic and 

 sinks here. In sinking it is mixed with Atlantic water. 

 The mixed water attains such a density that the bulk of 

 it is found at about 1000—1200 metres where the anomaly 

 of salinity has a maximum, though at 2000 metres also 

 the influence of the Mediterranean is very marked over 

 wide areas in the eastern part of the North Atlantic. Even 

 in the deep water at much greater depths such an in- 

 fluence may be traced by slightly increased salinities and 

 temperatures. 



On the other hand, deep water from the South At- 

 lantic passes the equator and makes itself felt some way 

 to the north, especially in the western part of the North 

 Atlantic [Bohnecke, 1927; WOst, 1928]. The temperature 

 and the salinity of the deep water are lower in the South 

 Atlantic than in the North Atlantic. 



In discussing the deep water of the North Atlantic 

 we have, then, to pay attention not only to its chief 

 source in the northern regions, but also to the arrival of 

 water from southern latitudes and the effect of water from 

 the Mediterranean. 



During the "Michael Sars" Expedition only a few 

 observations were taken from the deep water. Our ob- 

 servations at and over a depth of 3000 metres are grouped 

 in the following table where ©means the potential tempe- 

 rature (referred to the surface of the sea) and rrn the 

 corresponding density: 



The stations 10 A, 49 C and 91 were situated in the 

 eastern part of the North Atlantic (/. e. east of the cen- 

 tral longitudinal ridge) and Stat. 63 in the western. 



The temperature at Stat. 91, 4750 metres was observed 

 by means of a Nansen thermometer, fixed in the lid of a 

 Pettersson-Nansen insulating water-bottle (section 12). 

 When the water-bottle arrived on deck, the temperature 

 was 2^00° C., but is was rising and would certainly have 



