OCEAN CURRENTS 147 



MELTING ICE. 



According to Pettersson, melting ice plays an important 

 part in causing ocean currents, not only in the displacement 

 of large water masses in the Southern Hemisphere, but also 

 in the formation of important ocean currents, of which the 

 East Greenland Current may be taken as a type.* 



ATMOSPHERIC PRESSURE AND OCEAN CURRENTS. 



Atmospheric pressure is an important factor in the causa- 

 tion of ocean currents, and it works in two ways partly by 

 local variations in pressure and partly through wind. The 

 ocean is really a gigantic water barometer. When the 

 barometer rises over a given water area, it is equivalent to an 

 extra pressure on the water surface, which consequently sinks. 

 Pressure gradients are consequently formed as the result of a 

 variation of atmospheric pressure. 



During the monsoon period gradients are caused in the 

 transitional period from high to low atmospheric pressure. 

 The extra pressure on the water when the barometer is high 

 causes a flow of water to areas of lower atmospheric pressure. 

 With a low barometer off the Shetlands and a high one off the 

 Continental coast, the North Sea water is forced in two or 

 three days to the north. When the barometer falls over the 

 Baltic and rises over the northern part of the North Sea, water 

 is forced in through the Belt into the Baltic. Knudsen says 

 that the strength of the current at the entrance to the Baltic is 

 directly proportional to the variations in atmospheric pressure 

 in the North and Baltic Seas. If v be the velocity of the 

 stream, then 



v = P ~ e + c (B-B'), 



when p is the rainfall, e the evaporation, a the section of the 

 exit (0-8 square kilometre), c a constant (estimated by Knudsen 



* Geographical Journal, vol. xxiv., p. 285, and vol. xxv., p. 279. 



