DROUGHT 



The factors that determine the dur- 

 ation of droughts have not been well 

 explored and no predictive capability 

 exists. The droughts that have re- 

 ceived most attention are those that 

 have affected agricultural operations 

 — i.e., late spring and summer 

 droughts. Some of these have been 

 unable to survive the hardships of the 

 winter following, but others have 

 shown a tendency to recur the next 

 spring or summer, and these pro- 

 longed droughts are of great interest 

 economically as well as scientifically. 



There is evidence to indicate that 

 drought-producing systems tend to 

 develop in families (rather than as in- 

 dividuals), though each member may 

 not qualify as a drought according 

 to official definitions. For example, 

 Namias found that drought-producing 

 anticyclones over the agricultural 

 heartland of North America have 

 companion anticyclones on the Pacific 

 as well as on the Atlantic. Drought- 

 producing anticyclones in the lower 

 atmosphere appear to be associated 

 with distortions of the flow patterns 

 through deep layers. Our knowledge 

 of these conditions is meager; much 

 firmer information could be provided 

 through special analyses of existing 

 data. 



Although an official drought may 

 cover a relatively small region, the 

 associated atmospheric processes must 

 be studied in the context of the gen- 

 eral circulation of the atmosphere, in- 

 cluding the principal sources of heat 

 and moisture. 



The Causes of Drought 



The above-mentioned findings — 

 that drought-producing systems tend 

 to occur in families and that individ- 

 ual droughts may span one or more 

 annual cycles — are of considerable 

 scientific significance and hold out 

 hope of progress toward prediction. 

 These findings point toward the phy- 

 sical processes that create the large- 

 scale anomalies of which droughts are 



manifestations. Since extraterrestrial 

 influences can safely be ruled out, it 

 is clear that the forces, or energy 

 sources, that bring about these anom- 

 alies must develop within the earth- 

 atmosphere system itself. Further- 

 more, since an individual drought in 

 middle and high latitudes (where the 

 annual variation is large) may outlast 

 an annual cycle, it is plausible that the 

 underlying energy sources are rooted 

 in the equatorial belt (where the an- 

 nual change is small). 



Bjerknes has recently produced se- 

 lected analyses that indicate, with a 

 high degree of certainty, that the gen- 

 eral circulations of the atmosphere in 

 middle and high latitudes respond 

 readily and significantly to energy in- 

 puts resulting in variations in the 

 ocean-atmosphere interactions in low 

 latitudes. Of special importance is 

 the transfer of heat and moisture from 

 the oceans, and the freeing of latent 

 heat by condensation in the air. The 

 major site of interactions resulting in 

 varying inputs of energy is the equa- 

 torial belt from the west coast of 

 South America to beyond the date 

 line. Significant impulses can also be 

 traced to the Humboldt Current, the 

 Indian Ocean, and other areas. 



Bjerknes found that the upwellings 

 of cool water, resulting from the vary- 

 ing convergence of the trade winds, 

 undergo changes that may be large at 

 times, and these affect the rate at 

 which energy is supplied to the atmos- 

 phere in the equatorial belt. These in- 

 puts are, in turn, exported via upper 

 air currents as various forms of 

 energy to the mid-latitude belt, where 

 they bring about distortions of the 

 flow patterns, dislocations of the 

 storm tracks, and regional anomalies 

 of different kinds. Of particular in- 

 terest in connection with droughts is 

 the tendency for more or less sta- 

 tionary offshoots from the subtropical 

 belt of dryness to disrupt the mid- 

 latitude belt of wetness. Bjerknes' 

 findings are of great interest and raise 

 hopes for progress in long-range pre- 

 diction and other applied areas. 



Research Aspects — It is clear from 

 the foregoing discussion that our 

 knowledge of drought is fragmentary 

 and that much work remain 

 done before adequate descriptions of 

 individual or typical droughts can be 

 provided. An individual drought must 

 be recognized and described as a 

 member of a family of anomalies, and 

 its characteristics must be related to 

 the evolution of these anomalies. Un- 

 doubtedly, such descriptive studies 

 will lead to greater insights into the 

 underlying general mechanisms as 

 well as the many local or regional 

 factors that determine the severity of 

 droughts. In the past, research on 

 droughts has been conducted on an 

 ad hoc basis, with emphasis on local 

 or regional conditions. A concerted 

 effort, making full use of available 

 data and data-processing facilities, 

 seems justified in terms of national re- 

 quirements as well as available talent. 



Although the broad aspects of the 

 causes of droughts appear to be un- 

 derstandable on the basis of Bjerknes' 

 findings, much work remains to be 

 done to relate the evolution and the 

 characteristics of atmospheric anom- 

 alies to specific variations in the ap- 

 propriate ocean-atmosphere interac- 

 tions. Empirical studies should be 

 matched with construction of models 

 to simulate the behavior of the atmos- 

 phere in response to observed or in- 

 ferred ocean-atmosphere interactions. 



It is clear that the research oppor- 

 tunities in this general area are highly 

 promising. Data are available to sup- 

 port analyses of many cases, with ex- 

 tensions to longer time-spans. The 

 present recognition of a need for im- 

 proved understanding of our environ- 

 ment and better management of our 

 natural resources is likely to stimu- 

 late application. The research is likely 

 to appeal to young talent in several 

 disciplines. And the research is likely 

 to provide important inputs to the co- 

 operative schemes of the International 

 Decade of Ocean Exploration and the 

 Global Atmospheric Research Pro- 

 gram. 



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