to the sea. 
Exploration of temperature and salinity patterns together 
with theoretical work in fluid dynamics has already provided a quali- 
tative understanding of large-scale processes in the ocean. We know, 
for instance, that the sea resembles a great convective cell in which 
the upper layers are stirred horizontally by wind-driven currents. 
In mast areas, these upper layers have the highest salt content be- 
cause of evaporation from the surface. Heat and salt are carried down 
to deep water by turbulent diffusion. While we know there is a return 
flow of heat and salt to the surface and that it must take place by 
vertically-rising advective currents, we do not know precisely how this 
takes place. The stirring mechanism which drives the vertical flow is 
provided by the sinking of cold water in the polar regions where sur- 
face waters radiate their heat and increase in salinity by the forma- 
tion of sea ice. Bottom water thus formed flows to the deep basins 
of the oceans as an abyssal current which supplies the upward return 
to the surface. Oceanographic and geochemical studies have indicated 
that the abyssal current originates in two areas: the North Atlantic 
and the Weddell Sea in the Antarctic region of the South Atlantic. 
Wnere this current flows, how fast it travels, the nature of the flow 
in the different oceans and whether or not other sources exist are 
questions that remain to be studied. 
In addition, we know that vertical and horizontal gradients 
of chemical constituents in the sea are smoothed by turbulent diffu- 
sion which acts at a rate some 100 times faster than molecular diffu- 
