FISHERY BULLETIN: VOL. 81, NO. 2 



ing and low Southern Oscillation indices) and its tim- 

 ing in relation to the regular regional seasonal 

 relaxation determine the strength of the resulting El 

 Nino (Quinn 1979). During the period of prolonged 

 strong southeast trades and equatorial easterlies, the 

 south equatorial current is intensified, coinciding 

 with an east-to-west buildup in sea level and an ac- 

 cumulation of warm water in the western Pacific; and, 

 as soon as the wind stress relaxes, the accumulated 

 water flows eastward, probably in the form of an in- 

 ternal equatorial Kelvin wave (Wyrtki 1975). This 

 wave leads to the accumulation of warm equatorial 

 undercurrent water off Ecuador and Peru and to a 

 depression of the usually shallow thermocline there. 

 In addition to the generation of internal Kelvin waves 

 and Rossby waves, as discussed by Hurlburt et al. 

 (1976) and McCreary (1976), it is assumed that the 

 eastward-flowing currents (i.e., the North Equatorial 

 Countercurrent, South Equatorial Countercurrent, 

 and Equatorial Undercurrent) are intensified, and 

 the westward-flowing South Equatorial Current is 

 weakened when the relaxation occurs (Wyrtki et al. 

 1976). Hydrographic data off the coasts of Ecuador 

 and Peru confirm the thermal structure depression 

 and poleward spreading during El Nino (Enfield 

 1981); although upwelling may continue, it is from 

 the accumulated warm water above the base of the 

 thermocline, and this too causes coastal surface 

 waters to be much warmer and less productive than 

 water from the usual source, the Peru Current. 



At times we use the broader connotation "El Nino 

 type" when describing events, in order to avoid 

 arguments as to what is and what is not an El Nino; in 

 this way, we can account for events that evolve in a 

 similar manner (associated with falling and low 

 Southern Oscillation indices), but which vary in tim- 

 ing, intensity, and extent (Quinn et al. 1978). We will 

 also at times refer to the contrasting anti-El Nino 

 phase where a strengthening and strong southeast 

 trade and equatorial easterly system prevails (asso- 

 ciated with rising and high Southern Oscillation in- 

 dices) (Quinn et al. 1978). It appears that most of the 

 large short-term climatic changes, and their charac- 

 teristic current and weather patterns over the lower 

 latitudes of the Pacific, are associated with either El 

 Nino or anti-El Nino phases of the Southern 

 Oscillation. 



METHODS 

 Data Processing 



Rainfall data for Santiago and Valparaiso prior to 



1931 were obtained from Taulis (1934). Rainfall data 

 for Valparaiso 1931-80 were obtained from the Ser- 

 vicio Meteorologico, Armada de Chile. Rainfall data 

 for Santiago 1931-80, La Serena 1869-1980, and 

 Tarawa 1966-81 were obtained for applicable years 

 from the World Weather Records (Clayton 1927, 

 1934; Clayton and Clayton 1947; U.S. Department of 

 Commerce 1959, 1968) and the Monthly Climatic 

 Data for the World (U.S. Department of Commerce 

 1961-81). The sea level atmospheric pressure data 

 and upper level pressure versus geopotential height 

 data (used to obtain atmospheric thickness data be- 

 tween the 850 and 200 mbar levels in Figure 1) were 

 obtained for applicable years from previously listed 

 U.S. Government sources. Sea surface temperature 

 (SST) data for Chimbote were obtained from the In- 

 stituto del Mar del Peru. SST analyses for the 

 southeast Pacific were obtained from Fishing Infor- 

 mation (National Marine Fisheries Service 1976-80) 

 and a continuation of those analyses for 1981-82 by 

 Forrest Miller of the Inter-American Tropical Tuna 

 Commission at the Southwest Fishery Center, 

 National Marine Fisheries Service, NOAA, in La 

 Jolla, Calif. 



The use of the triple 6-mo running mean filter on 

 monthly anomalies of various atmospheric and 

 oceanic variables (i.e., Fig. 1) has been discussed in 

 Quinn etal. (1978). An 11-yr running mean filter was 

 used on the longest annual rainfall records available 

 for Santiago and Valparaiso (Fig. 2). Our selection of 

 the 11-yr running mean for smoothing the data was 

 based in part on the desire for a decadal filter and in 

 part on the sunspot cycle having an average length of 

 11.1 yr. Although 11-yr cycles have been suggested 

 for various tropospheric phenomena, none of these 

 has been substantiated (Hushke 1959). 



Classifications of Activity 



The classification of El Nino events by intensity was 

 accepted from Quinn et al. (1978). Since we had a 

 Southern Oscillation index and more reliable infor- 

 mation concerning El Nino intensities available after 

 1860, our study concerning El Nino intensities, event 

 frequencies, and the corroborative subtropical 

 Chilean rainfall data was limited to the period from 

 1861 on (see Tables 2-6). 



In Tables 3, 4, and 5 the rainfall classifications of 

 <200 mm for Santiago and<300 mm for Valparaiso 

 were selected to represent unusually dry years; and 

 the classifications of >500 mm for Santiago and > 

 600 mm for Valparaiso were selected to represent 

 unusually wet years. 



366 



