

Figure VI-12 — FREQUENCY OF TROPICAL CYCLONES 



North Atlantic Ocean 73 



North Pacific — off west coast of Mexico _. 57 



North Pacific Ocean, west of 170°E ..... . 211 



North Indian Ocean, Bay of Bengal 60 



North Indian Ocean, Arabian Sea 15 



South Indian Ocean, west of 90°E 61 



South Indian Ocean, northwestern Australia 9 





The table shows the frequency of tropical storms per 10 years. The numbers are 

 only estimates of the number of tropical cyclones to be expected, since, until 

 recently, there have been no reliable statistics except for the Atlantic, where ship 

 traffic has been heavy and island stations numerous for many years. Surveillance 

 by satellites will provide worldwide coverage of tropical cyclones. 



dilations favorable or unfavorable to 

 tropical storms, is definitely needed. 

 Clearly, such storms are not mere 

 nuisances. A single hurricane can re- 

 place the function of the equatorial 

 trough zone in the Atlantic for verti- 

 cal transport of heat and moisture 

 and their transmission to higher lati- 

 tudes. 



Altogether, the true value of such 

 storms — when, where, and under 



what circumstances needs to be 



stressed and measured. Coastal dam- 

 age and associated flooding from hur- 

 ricanes in areas such as southeastern 

 United States usually receive the 

 widest publicity. It is forgotten that, 

 as these storms move slowly inland 

 and turn into unspectacular inland 

 rains, they have on occasion saved 

 the cotton crop and even relieved 

 water shortages of cities such as At- 

 lanta. Lowered water tables over 

 southern Florida and other areas, with 

 their danger of salt-water intrusion 

 into the water supplies of cities like 

 Miami, can also be counteracted by 

 hurricane precipitation. In terms of 

 dollars, then, hurricanes can often 

 bring benefits that are comparable to 

 the damage they cause. 



Impact of the Tropics 

 on World Weather 



Long-Period Trends — As the en- 

 ergy and momentum source for the 



general circulation, the tropics are 

 most likely to have an important im- 

 pact over long time-scales (from 

 months to years). The excess of 

 energy acquired and held by the 

 tropical oceans may undergo slow 

 variations of possibly great impor- 

 tance for long-period circulation 

 anomalies. Bjerknes, for example, 

 has speculated on the equatorial Pa- 

 cific and its influence over large areas 

 beyond the tropics. 



Expanded observational networks 

 at sea and, again, satellite data now 

 appear sufficient for empirical re- 

 searches to begin on such aspects 

 of general circulation. Theoretical 

 modeling would also be useful to in- 

 dicate how much variation in the 

 tropics is needed to produce an even- 

 tual circulation upheaval elsewhere. 

 From models that have been run so 

 far, it appears that the heat accumu- 

 lations or deficits need not be very 

 large. 



The intensity of the mean merid- 

 ional circulation is also a matter for 

 serious study. Data are marginally 

 sufficient to calculate this circulation 

 on a monthly, if not weekly, basis. 

 Variations in the cell have hardly 

 been considered at all; yet they would 

 profoundly affect, among other 

 things, the energy and momentum bal- 

 ance picture, subtropical jet streams, 

 stress in higher latitudes on the 



ground, and relations to the intensity 

 of the Siberian winter high. 



Short-Period Fluctuations — Vari- 

 able exchanges with the tropics may 

 be responsible for the "index cycle" 

 of the general circulation in the west- 

 erlies on a two- or three-week scale. 

 Prediction experiments now planned 

 in connection with the Global At- 

 mospheric Research Program (GARP) 

 may or may not lead to an under- 

 standing of such influences. Sepa- 

 rate studies — using diagnostic data 

 from the National Maritime Commis- 

 sion and other hemisphere analyses 

 and data storages — would also be of 

 considerable value. Such studies 

 could also investigate whether the 

 exchanges are forced from higher 

 latitudes, and in this way learn more 

 about the mechanisms for the vari- 

 ability of the atmospheric machine. 



For prediction equations, much em- 

 phasis has been given to parameter- 

 ization of cumulonimbus convection, 

 since a few thousand cumulonimbus 

 cover roughly 0.1 percent of the 

 tropics at any one time. Much re- 

 search on this subject is under way, 

 although some dispute remains as to 

 the form the research should take. 

 GARP takes the view that a master 

 tropical experiment must be con- 

 ducted for final clarification. While 

 a series of smaller projects might be 

 inadequate for the problems to be 

 solved, the master experiment may 

 not succeed either, since experimental 

 difficulty increases nonlinearly with 

 the size of an experiment. Further- 

 more, there is a deplorable tendency 

 to ignore the results of past expedi- 

 tions in writing the prospectus for 

 new ones; in present planning, for 

 example, such large undertakings as 

 the German Atlantic Expedition and 

 its results have been generally over- 

 looked. 



Emphasis should not be placed ex- 

 clusively on oceanic observations. 

 Obviously, the oceans hold much of 

 the key to world weather; but pre- 

 dictive models should eventually be 

 geared mostly for continental areas, 



189 



