LONG-TERM VARIATIONS IN THE SOUTHERN OSCILLATION, 

 EL NINO, AND CHILEAN SUBTROPICAL RAINFALL 



William H. Quinn and Victor T. Neal 1 

 ABSTRACT 



A 1 20-year record of Southern Oscillation-related activity along the west coast of South America was studied 

 in order to better understand the causes and variations in this activity with time. Significant decreases in the 

 frequency of occurrence of moderate/strong El Ninos and the related abnormally heavy amounts of Chilean 

 subtropical rainfall were noted over the past half century. Work done by Berlage on the Southern Oscillation 

 and findings here concerning the above decreases in activity indicate a significant change took place between 

 1927 and 1931. A significant decrease in average annual rainfall was noted over subtropical Chile after 

 1944. 



Below normal pressures in the southeast Pacific subtropical high over the past 6 years coincide timewise 

 with generally above normal sea surface temperatures over this region. An associated weakening of the 

 southeast trade system is hypothesized to be responsible for environmental changes that contribute to the 

 large alteration in composition of the pelagic biomass in both the Peruvian and northern Chilean fishery 

 regions over recent years. 



Recent findings on large-scale climatic fluctuations 

 over a large part of the tropical and subtropical 

 Pacific (Quinn et al. 1978, 1981), the availability of 

 additional sources of corroborative evidence of these 

 changes (Chilean subtropical rainfall data), and the 

 establishment of an unusual trend in the Southern 

 Oscillation indices over the past 6 yr led to our 

 further investigation of the long-term climatic 

 changes and their causes. 



In Quinn et al. (1978) we compiled evidence on El 

 Nino developments, their intensities, and their fre- 

 quency of occurrence. However, at that time we did 

 not consider the possibility of long-term variations in 

 El Nino intensity with time. In Quinn et al. (1981) we 

 found the Chilean subtropical rainfall amounts to be 

 closely associated with the El Nino (low Southern Os- 

 cillation index anomaly)/anti-El Nino (high Southern 

 Oscillation index anomaly) type conditions for 1875- 

 1930. Here we have extended the rainfall records 

 from Taulis (1934) up through 1980 (Table 1) for a 

 study of the changes in activity with time. 



In the past we noted a relationship between the 

 Southern Oscillation indices and the productivity of 

 the Peruvian anchoveta fishery (Quinn 1976; Quinn 

 et al. 1978). Ordinarily when a moderate/strong El 

 Nino occurred, the fishery suffered a significant set- 

 back, but following such events there was usually a 

 prolonged anti-El Nino period during which the 

 fishery recuperated. However, since the 1976 El 



Table 1. — Annual rainfall amounts (in mm) at Santiago (lat. 

 33°26'S, long. 70°50'W) and Valparaiso (lat. 33°01'S, long. 

 71°39'W), Chile, for 1931-80. 



'School of Oceanography, Oregon State University, Corvallis, 

 OR 97331. 



Manuscript accepted October 1982. 

 FISHERY BULLETIN: VOL. 81, NO. 2, 1983. 



Nino the index has remained unusually low (Fig. 1), 

 and coincidentally there has been a startling change 

 in the makeup of the pelagic biomass in the fishery. 

 This change was reported in a Cerescope item (Ceres 

 1981) from the FAO/Norway Regional Acoustic Cen- 

 tre (the Centre) for Latin America in Lima, Peru. 

 (The Centre became operational in May 1975 under 



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