434 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



the surface there is, as might be expected, a flow of water deep below 

 the surface, to the south. A number of compensating flows of deep 

 water are found in all of the major oceans and they, in turn, are linked 

 to vertical movements, the transfer of water back and forth between 

 the surface and the deeper layers. 



SINKING WATER 



The net gain of surface water to the North Atlantic is balanced by 

 water which leaves the surface and sinks below in areas between 

 Greenland and Iceland, in the Labrador Sea, and to the west of where 

 the Mediterranean communicates with the Atlantic. Each of these 

 three downward movements removes from the surface about one-third 

 of what the Southern Equatorial Current brings to the North Atlantic. 

 There are reasons for these vertical movements, based upon unequal 

 heating and cooling of sea water and upon evaporation and rainfall. 

 The hotter sea water becomes, the lighter it is, so that it tends to rise 

 to the surface. The cooling of sea water has the reverse effect and 

 gives it a tendency to sink. Evaporation of sea water at the surface, 

 due to winds and the heat of the sun, makes it saltier and heavier. 

 The addition of fresh water by heavy rains has the effect of reducing 

 the salinity and so causing surface water to become lighter. 



SIX MILLION TONS PER SECOND 



The surface of the Mediterranean Sea loses more fresh water by 

 evaporation than it gains from river discharge and rainfall and there- 

 fore becomes heavier than the water in the adjacent Atlantic. The 

 result is that in the Straits of Gibraltar there is a 2-knot inflow of 

 lighter surface water from the Atlantic to the Mediterranean, and this 

 is compensated for by a deep subsurface flow of the heavier Mediter- 

 ranean water into the Atlantic. This continues to sink and joins the 

 deep south-flowing stream on its way to cross the Equator. A similar 

 amount of surface water joins the deep southward flow by sinking 

 off Greenland and in the Labrador Sea at places where the cold Arctic 

 water, meeting warmer, but saltier Atlantic waters, especially in 

 winter, cools the latter by mixing until the surface waters become 

 heavier than those below. The North Atlantic loses in this way a total 

 of 6 million tons of surface water a second, but this amount returns 

 to the surface in the South Atlantic, owing to other forces, where 

 it exactly replaces the six million tons of water which originally 

 crossed the Equator to enter the North Atlantic circulation. 



SOURCES OF FERTILIZER 



There is another type of vertical movement in the sea. When winds 

 or other causes bring about a divergence of currents, water will well 



