150 



ANALYSIS OF THE ENVIRONMENT 



pressure) than the cooling." This theorem 

 has been successfully tested in experimental 

 models. Its appUcation to oceanic circula- 

 tion presents some difficulties, since on first 

 inspection it would appear that heating and 

 coofing of ocean water both occur at the 

 same level, that is, at the surface. Closer 

 study reveals that the heating actually takes 

 place, in part, at some distance below the 

 surface. 



The heated, fighter water spreads from 

 the tropical regions over the surface of the 

 ocean toward high latitudes, where it gives 

 off heat and becomes denser. The water 

 then sinks and flows back toward the equa- 

 torial region at some depth below the sur- 

 face. So far, the conditions reafize the 

 generafized scheme presented earfier (p, 

 141). The returning water is heated by 

 conduction before it actually reaches the 

 surface in the tropics; hence heating does 

 occur at a greater depth than coofing and 

 so accords with the theorem of Bjerknes. 

 The depth scale is not great when con- 

 trasted with the long north and south ex- 

 panse of the oceans. When the surface 

 water near Spitzbergen at 80 degrees north 

 latitude has a temperature of 3.3° C, the 

 Vertical lowering of the warming point of 

 water for the North Atlantic cannot be more 

 than about one and a quarter miles in con- 

 trast to something over 5000 miles of hor- 

 izontal distance. The warming of deeper 

 water by conduction is not efficient, since 

 water is a poor conductor of heat, hence the 

 direct thermal component underlying oce- 

 anic circulation of water is not an effective 

 driving force. 



Density differences related to safinity also 

 play a role in these large-scale, oceanic cur- 

 rents. Surface water evaporates, especially 

 in the warm, dry regions, and leaves an in- 

 creased concentration of salt. Such water 

 sinks. The density-safinity component of the 

 global circulation of sea water has been fit- 

 ted into an extension of the theorem of 

 Bjerknes by Sverdrup, Johnson, and Flem- 

 ing (1942) as follows: "If a thermohafine 

 circulation shall produce energy, the expan- 

 sion must take place at a greater depth 

 than the contraction." 



Again, let us examine the situation in 

 the Atlantic Ocean. Thanks to tropical 

 rains, the equatorial region tends to have 

 diluted surface water which, moreover, is 

 warm. Both factors make for lowered den- 

 sity. Heating is less intense on both sides of 



the equatorial belt, and the dry, subtropi- 

 cal trade winds cause surface evaporation. 

 The expansion does take place at a greater 

 depth than the contraction, and the density- 

 dilution gradients in this part of the ocean 

 tend to reenforce the thermal circulation. 

 Poleward, the conditions are reversed, the 

 density relations tend to run counter to the 

 thermal component, and circulation is re- 

 tarded. The final result is a compromise. 



Winds exert a strong force on water un- 

 derlying them. The driving power is exerted 

 by the frictional contacts between air and 

 water and is greatest when winds blow 

 steadily over the water from the same 

 direction. The correlation of orientation of 

 flow of winds and of ocean currents is high 

 in the open sea and may entirely ovfirride 

 the primary density relations within the sea 

 water itself. 



Coastlines are a disturbing influence. In 

 the northern hemisphere, along coasts that 

 fie to the right of the direction of wind 

 flow, the warmer surface water tends 

 to be piled up on the coast, and replacing, 

 colder water wells up at some distance out 

 from shore. In seas that approach being 

 landlocked, wind-driven water piles up on 

 the lee shore under conditions that closely 

 resemble seiches in lakes, and small seiches 

 are also known, even under usual wind con- 

 ditions, along coasts that are practically 

 open to the sea. Larger masses of wind- 

 driven water are all too well known because 

 of their destruction of human life and prop- 

 erty. 



If the coast fies to the left of the wind 

 direction in the northern hemisphere, 

 fighter water is carried out to sea, and the 

 colder, denser water wells up near the 

 shore. The upwelling is usually from mod- 

 erate depths. Similar phenomena occur in 

 the southern hemisphere, only there, in 

 keeping with the effect of the rotational 

 force of the earth (Coriofi's force), the 

 directional relations are reversed, and the 

 fighter water is carried to the left of a per- 

 son who is facing down wind. 



The same winds that produce an upwell- 

 ing of deeper water near shores also ad in 

 setting up currents that flow parallel with 

 the shore line and in the same general di- 

 rection as the prevaifing wind. 



Other forces act to bring deeper water 

 up to the surface; the steady flow of off- 

 shore winds has this effect. Thus, off the 

 African coast in the South Atlantic, the 



