142 DISCOVERY REPORTS 



(3) The bottom current, which is part of the slope current and which reaches from the 

 top of the "lower depth of frictional resistance" to the bottom, and turns contra solem 

 with increasing depth. 



This current system is illustrated by considering the currents which must arise in a 

 channel of uniform depth, running all around the earth between two parallels of latitude, 

 if a wind of uniform velocity blows in the direction of the channel. With applications to 

 conditions in the Antarctic in mind, it may be supposed that the channel is situated in 

 the southern hemisphere, that the wind blows from west to east, and that the depth is 

 considerably greater than the upper and lower depths of frictional resistance. Within 

 the pure drift current, which is limited to the upper depth of frictional resistance, the 

 transport is directed 90° to the left of the wind, which means in this case to the north. 

 This transport causes a piling up of the water along the northern boundary of the 

 channel and, therefore, the surface of the water must slope downwards from north to 

 south. This slope causes, on the other hand, a current which runs in a direction 90° to 

 the left of the slope, which means in the direction of the wind, from west to east. The 

 velocity and direction of the slope current are the same at all levels, except within the 

 bottom layer. When approaching the bottom the current turns to the right and de- 

 creases. Between the top of the lower depth of frictional resistance and the bottom, the 

 total transport has a component in the direction of the slope, which means from north 

 to south. Stationary conditions can exist when the total transport towards the north by 

 the pure drift current above the depth of frictional resistance equals the transport to- 

 wards the south below the lower depth of frictional resistance. On the assumption that 

 the upper and the lower depths of frictional resistance are the same, Ekman has found 

 that the velocity of the slope current must be equal to the velocity of the pure drift 

 current at the surface multiplied by -v/a. 



Thus, the principal effect of the wind is to uphold a slope current in the direction of 

 the wind and in addition to maintain a transversal circulation, which in the upper layer 

 transports water from south to north and near the bottom transports water from north 

 to south. At the northern boundary descending motion, and at the southern boundary 

 ascending motion must take place because of the continuity. The problem has not been 

 treated in three dimensions, but supposing the depth of the channel to be small in com- 

 parison with the width, the vertical components of the currents must always remain 

 small and, therefore, it is improbable that a complete analysis would give results which 

 would deviate considerably from the above. 



The simple system of three currents, the pure drift current, the slope current and 

 the bottom current is modified if the water is non-homogeneous. In this case the iso- 

 steres are generally not horizontal and, therefore, currents must be present which are 

 due to the distribution of density or in Ekman's terminology " convection currents." The 

 velocity and direction of these currents can be computed by means of the Bjerknes 

 theorem of circulation. Ekman considers the convection current as the fourth con- 

 stituent of the currents in the sea. He points out that the greatest changes of density in 

 a horizontal direction are found in the upper layers, from the surface to 1000 m. or less, 



