AriANT. DEEi'SEA EXPED. 1910. VOL. i] PHYSICAL OCEANOGRAPHY AND METEOROLOGY 



37 



rents have not the same direction nor the same velocity 

 in the different layers at a given moment. 



The tidal currents appear, whether a convection current 

 exists or not, as a transport of water masses with veloci- 

 ties that vary periodically. In an area with no appreciable 

 convection currents, /. c. where the isopycnal and isother- 

 mal surfaces are horizontal, the tidal currents will not 

 materially affect the horizontal position, provided that the 

 basin is so extensive that possible effects of pressing against 

 a coast may be disregarded. The relative thickness of the 

 water layers and their depth below the surface will then 

 be maintained, and no appreciable vertical variations in 

 the physical properties be observed as a primary effect 

 of the water transport. But in the case of a strong 

 convection current and, consequently, a marked sloping 

 of the isopycnal surfaces, an appreciable effect may appear, 

 // the velocity of the tidal curreiU is great in a direction 

 transverse to the main direction of the convection current, 

 the transport of zcater zenith the tides will make the sloping 

 isopycnal, Isothermal and isohaline surfaces move later- 

 ally for quite a long distance. Repeated series of obser- 

 vations at a fi.xed station within the domains of the 

 convection current will then reveal considerable variations 

 in the depth of, for instance, the isothermal surfaces. 

 This effect may easily be estimated when the tidal currents 

 and the convection currents are known. 



In order to illustrate the relations now discussed we 

 shall assume that the convection current at Stat. 115 runs 

 towards NE with a velocity of 30 cm. sec. at 200 metres, 

 20 cm. /sec. at 300 m. and 10 cm., 'sec. at 400 m. The 

 sloping of the isothermal, isohaline and isopycnal surfaces 

 is then so steep, that a semi-diurnal tidal current at 300 

 metres needs only to have a mean component of velocity 

 of less than 7 cm.'sec. along the direction SE-NW in 

 order to cause a variation in the observed depths of the 

 said surfaces corresponding to the value of B found above 

 (24 metres), the total distance measured along the hori- 

 zontal between the extreme positions of the surfaces 

 being about 3 kilometres. 



This example shows that the transport of water by 

 tidal currents, in an area with strong convection currents, 

 may account for appreciable variations observed as "ver- 

 tical" oscillations. The above-mentioned variations in the 

 orientation of the ellipses of the tidal currents, in the 

 ma.ximuni velocities and the phases, will make such 

 quasi-vertical oscillations alter in phase and magnitude 

 from layer to layer downwards. If the excentricity of the 

 tidal ellipse is considerable, an orientation of the major 

 axis in a direction forming a right angle to that of the 

 convection current causes oscillations of temperature which 

 are considerably greater than those observed if the major 

 axis has the same direction as the convection current. 



The vertical variations in the tidal currents make the 

 isothermal, isohaline and isopycnal surfaces alter their 

 position not only by parallel movements, but also by some 

 degree of twisting and by variations in the sloping of the 

 surfaces. It is probable that the variations in phase and 

 amplitude of the oscillations of temperature described in 

 some of the preceding sections may to a great extent be 

 accounted for in this way. 



It follows from this discussion that the vertical varia- 

 tions in the tidal currents create periodic variations in 

 the intensity of a solenoid field, as a result of the diffe- 

 rences in the water transport. The deformation of the 

 solenoid field must in its turn obviously affect the tidal 

 currents, these being sometimes retarded and sometimes 

 accelerated by the variations of the solenoid forces. But 

 then the tidal wave itself will be influenced by a solenoid 

 field, and the more so the stronger this field is. The tidal 

 waves and other waves will be somewhat deformed in 

 a sea ivith appreciable variations of density, in comparison 

 zvith waves in homogeneous water. These questions have 

 not yet been taken up for discussion, and we shall not 

 enter more closely into them here. 



From what has been said above it seems reasonable 

 to assume that the oscillations of temperature observed 

 at various fixed stations in the sea may to a great extent 

 be accounted for by a horizontal transport of water with 

 tidal currents. This corresponds to the result already 

 deduced, viz. that the oscillations seem to have periods 

 corresponding to tidal periods and to be most marked in 

 those areas where the tidal currents as well as the con- 

 vection currents may be assumed to be comparatively 

 strong. All vertical oscillations with tidal periods are 

 probably not accounted for in this way. They are evidently 

 to some extent caused by sub-surface waves too.') 



/*. Boundary waves. 



It is a well known fact that waves are created at the 

 boundary between two strata of fluids with different densi- 

 ties when the strata move with different current-velocities. 

 The theory of the boundary waves is based not only upon 

 the assumption that there is a discontinuity at the boun- 

 dary with regard to density as well as velocity, although 

 the differences in both elements may be very small, but 

 also that the two layers, above and below the discontinuity, 

 are in themselves homogeneous. The speed of propagation 

 of such waves is proportional to the square root of the 



') We liave recently made numerous current-measurements in tlic 

 Faeroe-Shclland Channel, the Norwegian Sea and the North Sea, in 

 connection with observations for calculating the solenoid field and its 

 variations. This vast material is now being arranged and will be 

 discussed in a publication by I'rofessor V. W.M.iKio Ek.man and the 

 present author. 



