SECT. 2] LARGE-SCALE INTERACTIONS 97 



In middle latitudes, atmospheric circulations are entirely different, being 

 irregular and eddy-like near the ground, and showing an unsteady wave 

 pattern aloft in the three or four undulations of the subtropical jet stream. No 

 significant global-scale meridional cell circulations appear to exist. The flow is 

 characterized by transience and strong air-mass contrasts. Along these, frontal 

 cyclones develop at the surface, beneath shorter-lived jet-stream branches 

 which often move southward and merge with the more persistent subtropical 

 jet. The eddies or cyclones themselves play a major role in maintaining the 

 westerly jet streams, by release of the potential energy stored in their air-mass 

 contrasts or fronts. 



The important interaction between latitudes takes place in the energy and 

 momentum export, from the tropics poleward across the subtropical ridge lines 

 at about latitudes 35° North and South. While the seasonal magnitudes of the 

 various transports are now quantitatively estimated, the relative roles of the 

 several scales of motion in carrying them out is not yet settled beyond con- 

 troversy ; it is still not clear whether the tropical cell may be said to maintain 

 the subtropical jet by a rather direct kinetic energy and momentum supply, or 

 whether most of the energy goes around the much more involved circuit of 

 being stored in air-mass contrasts, released in synoptic (cyclonic storm) sized 

 eddies which in turn feed it back up the spectrum to drive the jet. The end fate 

 of the energy and momentum is, however, clear. The kinetic energy is dis- 

 sipated into heat, largely by ground friction and stresses of the rough oceans. 

 This tiny amount of heat suffers the same fate as the huge remainder received 

 from the tropics, most of it being used to balance the radiation losses of the 

 long polar nights. The westerly momentum shipped poleward across the sub- 

 tropical ridge has also been gained from the tropical oceans, where the trade 

 winds have given off their easterly momentum to drag the equatorial currents 

 and pile up the waters for creating Gulf Stream and Kuroshio. This westerly 

 momentum is given back irregularly to the earth's surface in middle latitudes, 

 completing the northern portions of the ocean gyres and the remaining part of 

 the stress curl that maintains the wind-driven ocean-current systems. 



The average annual heat-energy transports by both ocean and atmosphere 

 together are computable as a function of latitude from the distribution of net 

 radiative sources and sinks for the air-sea system as a whole. The magnitude 

 of the energy transactions required of the earth's fluids in order to maintain 

 our planet's heat balance is suggested in Table I. The transport unit is lO^^ cal/ 

 sec, which is two orders of magnitude greater than the rate of kinetic energy 

 dissipation by all winds and nearly five orders of magnitude greater than the 

 rate of power consumption by human civilization. Total fluxes deduced from 

 the radiation results reported by three recent authors (Houghton, 1954; 

 London, 1957; Budyko, 1956) are shown in the first three columns. On the 

 right, Budyko's figures are used to show separately the contributions from sea 

 and atmosphere, derived in a manner to be described in Section 4 of this 

 chapter. Particularly noteworthy is the significant role attributed to the ocean ; 

 its fluxes are a non-negligible fraction of the total poleward transiDort. While it 



