QUINN and NEAL: LONG-TERM VARIATIONS IN SOUTHERN OSCILLATION 



1830 1840 1850 I860 1870 1880 1890 1900 1910 1920 1930 1940 1950 I960 1970 



900 



800 



1 



E 

 — 700- 



Z. 



o 



< 600 



LJ 500 



a: 

 a. 



< 400 



< 300 



o 



Or 



>- 



200 



100 



T 



T 



r 



T 



T 



T 



"SANTIAGO 



900 



800 



700 



600 



500 



400 



300 



200 



100 



± 



_L 



_L 



1825 1835 1845 



1855 1865 1875 1885 1895 1905 1915 



YEARS 



1925 



1935 1945 1955 1965 1975 



FIGURE 2.— Eleven-year running mean (1 1-YRM) plots of annual precipitation amounts (millimeters) for Santiago (lat. 

 33°26'S, long. 70°50'W) and Valparaiso Oat. 33°0TS, long. 71 39'W), Chile, for 1853-1980 and 1824-1980, 

 respectively. 



LONG-TERM VARIATIONS 

 Southern Oscillation-Related Activity 



Large demands on the Peruvian anchoveta fishery 

 over the past couple of decades to support fishmeal 

 production have caused increasing concern over El 

 Nino occurrences, since they adversely affect this 

 fishery. As a result, many articles on El Nino, both 

 scientific and popular, have appeared in periodicals 

 and other news media over recent years. So much em- 

 phasis has been placed on recent El Nino-type events, 

 regardless of intensity, that those without the histori- 

 cal background on this subject might think this type 

 of activity has increased in frequency and intensity 

 over recent years. Our findings indicate this is 

 definitely not the case. 



It was Berlage's (1957) opinion that the Southern 

 Oscillation (the large-scale atmospheric circulation 

 fluctuation with which El Nino is associated) was less 

 regular and less intensively developed after 1925 

 than it was before. Accepting Berlage's breakpoint 

 after the 1925 El Nino and using tables 3 and 5 in 

 Quinn et al. (1978) to obtain times between onsets of 

 moderate/strong events and between onsets of all 

 events (very weak, weak, moderate, and strong), re- 

 spectively, we derived the data in Table 2. The 

 average time between event onsets, regardless of in- 

 tensity, was the same for 1925-76 as it was for 1864- 



TABLE 2.— Statistical information pertaining to the frequency of 

 occurrence of the highly significant moderate/strong El Nino (Case 

 I) and all events regardless of intensity (Case II) (from Quinn et 

 al. 1978). 



1 Here we refer to separate events, the onset times for separate events, and the inter- 

 val between onset times (e.g., the 1 925-26 El Nino was a single event with onset in 

 1925, the 1957-58 El Nino was also a single event with onset time in 1957). 



1925. However, the average time between onsets of 

 the significant (moderate/strong) EINinos was much 

 less for 1864-1925 (4.1 yr) than it was for 1925-76 

 (6.4 yr). Therefore, it appears that the average fre- 

 quency of occurrence of the irregular atmospheric 

 circulation fluctuations, known as the Southern Os- 

 cillation, has not changed significantly over the past 

 1 20 yr; but the number of those fluctuations resulting 

 in moderate/strong El Nihos has decreased con- 

 siderably over the past half century. 



In our investigation of this intensity shift, we 

 followed up on findings in Quinn et al. (1981) which 

 showed the Chilean subtropical rainfall fluctuations 

 (for Santiago and Valparaiso) to be closely related to 

 the El Nino/anti-El Nino conditions for 1875-1930. 



367 



