Basic Principles of the General Oceanic Circulation 573 



where V is the velocity of the deep current produced, p is the average density of the 

 bottom current layer D and (P — E) is the difference between precipitation and 

 evaporation in the area under consideration. Estimation of the velocity in some 

 actual oceanic regions gave maximum velocities of the "evaporation currents" of 

 not more than 1-2 cm sec"^, but probably only fractions of this value are reached. 



This is valid for open sea surfaces. For partly enclosed basins the quantity (P — E) 

 may be of exceeding consequence ; the current processes occurring with water inter- 

 change in sea straits have been already discussed before (Chap. XVI, p. 513). Besides 

 the water transport through the sea straits also the salt transport stand in question. If 

 the inward water transport is A/,, the outward water transport Mq and the correspond- 

 ing salt transports are Si and Sq, then under stationary conditions the two equations 



MiSi = MoSo and M, = Mo-(P ~ E) (XVIII.2) 



are valid, and thus 



Mi =(P-E) ^^\ . (XVIII.3) 



This formula is identical with the simple Knudsen relations (p. 379). For example, 

 when the inflow through the Straits of Gibraltar is about 1-75 x 10^ tons sec-\ the 

 average salinity of the inflowing water about 36-25%o and of the outflowing water 

 37-75%o, then for the Mediterranean Sea according to the formula (XVIII.3) the quantity 

 E — P results to 0-07 x 10^ tons sec-\ which is in good agreement with other 

 estimates. 



More recently, Goldsbrough (1933) has dealt with ocean currents produced by 

 the given distribution of precipitation and evaporation. Already before that Hough 

 (1897) in his famous theoretical study of tides on a rotating globe has dealt with this 

 problem of currents produced by a zonal distribution of precipitation and evaporation. 

 Since he ignored frictional effects, he found a uniformly accelerated system of purely 

 east-west geostrophic currents as a consequence of these distributions. From the 

 impossibility of finding a steady state solution he concluded that precipitation and 

 evaporation cannot be a significant cause of ocean currents. Hough did not accept any 

 meridional boundaries in the ocean. Goldbrough took instead a model with precipi- 

 tation predominating in one hemisphere, evaporation in the other and assumed 

 meridional boundaries in the ocean. This model gave a steady current field, provided 

 that the integral of the precipitation-evaporation function taken along each parallel 

 of latitude between the two boundaries, vanishes. This is a very severe restriction which 

 no natural distribution of precipitation-evaporation necessarily fulfils. Figure 262 

 shows the current system produced in this case for one hemisphere; the other hemi- 

 sphere will be the mirror image of this. The field of pressure, the elevation of the free 

 surface and the flow will be steady. The horizontal velocity components will thus be 

 entirely geostrophic, and the current will flow along the isobars. The vertical component 

 will be zero at the bottom and will increase linearly from the bottom up to the sea 

 surface where it will equal the precipitation-evaporation rate. At the eastern edge of 

 the precipitation hemisphere there will be two low-pressure cells, and at the western 

 edge two high-pressure cells. At the poles the flow is directed from the region of 

 evaporation into the region of precipitation; however, in the opposite direction in 



