ATLANT. DEEP-SEA EXPED. 1910. VOL. i] PHYSICAL OCEANOGRAPHY AND METEOROLOGY 



21 



general rule that the lifting of a water-layer will demon- 

 strate itself by sinking temperatures and salinities and 

 rising densities when the observations are made at a fixed 

 depth below the surface, while the sinking of a water- 

 layer will cause opposite variations in the observed values 

 of /, 6" and a,. 



Layers with great vertical variations are generally 

 called discontinuity layers. Although the conduction of 

 heat (conduction by turbulence disregarded) and the diffusion 

 of salt are exceedingly slow, it is probable that no real dis- 

 continuity ever exists in the open ocean. The variations do 

 not take place in jumps, but continuously, however at a 

 very variable rate. This makes an exact treatment of many 

 important problems difficult, or even impossible. For a 

 study of some of the temporal variations it is necessary 

 to study the rate of variation vertically. For the question 

 of vertical oscillations and boundary waves it is of special 

 interest to examine the variations in stability. 



We shall enter upon a closer discussion of the stability 

 in a later chapter, and now only refer to the numerical 

 values of this quantity given in Table III. A negative value 

 means a state of lability, indifferent equilibrium, and a 

 positive value a state of stability. 10^ E^ 1000 corresponds 

 nearly to a variation of 0-01 of o-, per metre. This is a 

 relatively large variation in the ocean, and when we obtain 

 values of 10*^ E amounting to some thousands we have to 

 deal with such water-layers as are generally called "discon- 

 tinuity-layers". From Table III we see that the stability is 

 especially large within the region of the Newfoundland Bank 

 at depths between 25 and 50 metres below the surface, but 

 it is also very considerable (lO*' E above 2000) at some small 

 depths below the surface in the sea west of the Azores 

 (Stat. 60, 50—75 m., Stat. 63, 10—25 m., Stat. 65, 50— 

 75 m., Stat. 66, 0—50 m.), in the eastern part of our 

 northern route between the Atlantic Longitudinal Ridge 

 and Europe where the observations were made in July 

 (Stats. 87 to 97 incl., especially between 25 and 50 metres 

 below the surface), and at some stations in the Rockall 

 Channel and the Faeroe-Shetland Channel. The conditions 

 at Stat. 115 in the Faeroe-Shetland Channel will be 

 specially discussed in the following chapter. 



When we combine all those consecutive points which 

 have the same temperature we get an isothermal surface. 

 We may construct a number of such surfaces, for instance 

 for every degree or every tenth degree centigrade. Similarly 

 we may construct isohaline surfaces combining points where 

 the same salinity exists, or isopycnal surfaces combining 

 points with the same density, in the latter case we shall 

 now disregard the compression and deal only with the 

 ordinary (j,-values. When the units are taken sufficiently 

 small, the vertical distance between two successive surfaces 

 of the same kind will vary. In "discontinuity-layers" 



they come close upon each other, while for instance in 

 the deep-water the distance may be comparatively very 

 large. 



These surfaces are generally not horizontal, but are 

 more or less sloping or may exhibit swellings or depres- 

 sions. The deviations from the horizontal may be due 

 to currents or to undulations caused by internal waves 

 or seiches. In any case such deviations mean more or 

 less considerable horizontal variations of temperature, 

 salinity and density in the sea. 



A convection current will cause a sloping of the iso- 

 pycnal and, consequently, of the isothermal and isohaline 

 surfaces. We shall go further into this subject later on, 

 and here only mention that, in the Northern Hemisphere, 

 these surfaces will be elevated on the left hand side of 

 a convection current so that here the water will appear 

 to be comparatively heavy, while the surfaces will be pressed 

 down and lighter water be found on the right hand side, 

 provided that the velocity of the current decreases down- 

 wards as is generally the case. The effect is directly pro- 

 portional to the vertical variations of the current velocities. 

 A convection current with a rapid decrease of velocity 

 downwards manifests itself by a comparatively dense series 

 of sloping isopycnal surfaces, and consequently also by 

 relatively large horizontal variations in a direction trans- 

 verse to the current. The slope of the isopycnal (and 

 isothermal) surfaces may amount to 1:100 or even more. 



It is obviously a common idea that the great currents 

 are rather broad, which would mean that the isopycnals 

 are sloping for a correspondingly great horizontal distance. 

 Recent investigations tend, however, to show that at any 

 rate some convection currents are much narrower than has 

 previously been assumed, corresponding to a sloping of 

 the isopycnals which is steeper, but at the same time 

 narrower in horizontal direction. We have of late made 

 numerous and very detailed investigations of the Atlantic 

 Current in the Faeroe-Shetland Channel and the southern 

 part of the Norwegian Sea. In some cases stations were 

 worked at much closer intervals than has previously 

 been done, namely every 3 to 10 miles. It proved 

 that in the Faeroe-Shetland Channel at places where the 

 velocity of the Atlantic Current was great, the inclination 

 of the isopycnals might be limited to the space between 

 two neighbouring stations, thus giving a much steeper 

 and narrower inclination than would have been found if 

 the distance between the stations had been for instance 

 20 miles, which would generally have been considered a 

 short distance. We have arrived at similar results with 

 regard to the Atlantic Current in the southern part of the 

 Norwegian Sea (north of Shetland) as also further to the 

 north off the coast of Norway. This indicates that a strong 

 current may be very narrow, and that very considerable 



