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



95 



wind. Owing to the friction the water-masses lieiow the 

 surface are also moved but with decreasing velocities and 

 more and more cum sole as one goes downwards from 

 the surface. At a depth called the depth of frictional in- 

 fluence the direction of the pure wind-current is exactly 

 contrary to the direction of the current at the surface, 

 but at this depth the velocity is reduced to a small frac- 

 tion only — say 4 "/'o — of the surface velocity. The 

 average flow of the water in a pure wind-current goes 

 at a right angle cum sole from the direction of the wind. 

 The surface layers will thus be driven to the right of 

 the wind in the northern and to the left in the southern 

 hemisphere. A sloping of the surface is thus established. 

 In homogenous water the pressure will increase at all 

 levels from the surface to the bottom in the regions where 

 an accumulation takes place. The e.xcess pressure creates 

 a gradient current extending from the surface nearly to 

 the bottom. After a short while the gradient current will 

 move more or less in the direction of the wind. In the stra- 

 tified water of the ocean, the surface layers will be pressed 

 down on the right hand side of the wind in the northern 

 hemisphere, so the isopycnal, isothermal and isohaline 

 surfaces will assume a slanting direction, deepest to the 

 right and highest to the left (and vice versa in the south- 

 ern hemisphere). In this case an interior field of force 

 is established in the sea. Such a field of force in stratified 

 water will be called a solenoidal field, in accordance 

 with the terminology of Professor V. Bjerknes. 



Even if there were no winds a system of ocean cur- 

 rents would be created on account of regional differences 

 of density. As an example it will suffice to mention the 

 differences of density between low and high latitudes. In 

 low latitudes the water is heated so much that it becomes 

 lighter than the water in higher latitudes even if the salinity 

 is increased by evaporation. The light water has a tendency 

 to spread over the heavier water. When the water-masses 

 move, the rotation of the earth acts in such a way that the 

 motion does not go in the direction of the force but at 

 right angles to it, to the right in the northern and to 

 the left in the southern hemisphere (cum sole)}) 



The actual solenoidal field may be the composite 

 result of solenoidal fields created indirectly by the wind 

 and more directly by thermal Influences, evaporation etc. 



The 'Coriolean force' with which the rotation of the 

 earth acts upon unit mass ('the accelerating force') is 

 expressed by the well-known equation : 



R = 2 m v sin (f , (a) 



') The conditions in tlic sea are analogous to tliose in the at- 

 mosphere, where the wind does not blow in the direction of the 

 pressure gradient but nearly along the isobars. 



where w is the angular velocity of the earth (0-0000729), 

 V the velocity of the mass particle and <f the geographic 

 latitude. Under stationary conditions this force is di- 

 rected contrary to the resultant of the real physical 

 (moving) forces and has the same value. The movement 

 takes place at a right angle cum sole from the physical 

 forces and contra solem from the Coriolean force. 



Apart from the wind stress at the surface, the phy- 

 sical forces in the ocean are those of gravity, pressure 

 and friction. The acceleration oi gravity, g, varies slightly 

 with the latitude and with the depth below the surface, 

 g increasing with latitude as well as with depth, apart 

 from some insignificant local irregularities. The free sur- 

 face of a liquid which is motionless relatively to the earth, 

 is perpendicular to the direction of gravity (the plumb-line) 

 and forms a level surface. No work will be required to 

 move a weight along such a surface if gravity is the only 

 acting force. In other words, a level surface is a surface 

 of constant gravity potential (an equipotential surface). 

 If, as an effect of atmospheric conditions or for other 

 reasons, the sea surface is inclined at an angle y from 

 the level surface, the gravitational force will have a com- 

 ponent along the sea surface, this component being^. 5//;;- 

 per mass unit. The angle will always be so small that 

 we can simply write gy- Any number of equipotential 

 surfaces may be constructed, each of them characterized 

 by being perpendicular to the plumb-line. When a mass 

 m is lowered from such a surface to the next one, the 

 gravitational force performs a work, w, which is equal to 

 m.g.h, h being the vertical distance between the two sur- 

 faces. One may represent the gravitational field by con- 

 structing a series of surfaces at a distance from one another 

 which corresponds to unit increase of gravitational work 

 per mass unit {g.h --^ 1). The distance between two 



such level surfaces will then be h -- - . As g is nearly 



S 

 10, the distance between two succeeding level surfaces 

 will be about 01 metre in the metre-ton-second system 

 of units. This distance has been called by V. Bjerknes 

 a dynamic decimetre, and a distance ten times as great 

 a dynamic metre. Since g varies with latitude and depth, 

 the dynamic metre is not a constant length like the or- 

 dinary metre. At sea-level the dynamic metre is 1-02246 

 ordinary metres at the equator and 101716 inetres at the 

 pole. The length of the dynamic metre decreases slightly 

 with the depth below sea-level because jg' increases down- 

 wards in the ocean. A dynamic metre will always, how- 

 ever, be nearly 102 cm. Inversely we have 1 metre = 



p- 



or about 0-98 dyn. m. The dynamic metres are 

 10 ^ ^ 



always used to measure vertical distances only. If D 



