QUINN and NEAL: LONG-TERM VARIATIONS IN SOUTHERN OSCILLATION 



tion index and the greatly reduced amplitude in index 

 fluctuations (Fig. 1), has played a significant part in 

 bringing about this change. 



DISCUSSION 



Thermal and Pressure Changes and 

 Their Effects 



We were interested in determining the cause for the 

 persistent abnormally low sea level pressures in the 

 southeast Pacific subtropical anticyclone over the 

 past6yr. Petterssen (1940) explained that in the cen- 

 tral parts of anticyclones, or belts of high pressure, 

 the air is stagnant or slowly moving, and it therefore 

 has sufficient time to adjust its temperature and 

 moisture content to the underlying surface. The cir- 

 culation around anticyclones is divergent, and prop- 

 erties absorbed in the central parts are, therefore, 

 spread over large areas, while turbulence and con- 

 vective currents gradually distribute the absorbed 

 properties to higher levels (Petterssen 1940). Our 

 limited studies at other marine locations illustrate 

 this adjustment of properties in the overlying at- 

 mosphere to changes in the underlying ocean surface 

 (Quinn 1977, 1980a, b; Quinn et al. 1981). Rises in 

 SST relate to rises in surface air temperature, falls in 

 sea level pressure, and increased thicknesses be- 

 tween pressure levels aloft. Falls in SST relate to falls 

 in surface air temperature, rises in sea level pressure, 

 and decreased thicknesses between pressure levels 

 aloft. Talara data in figure 13 and table 4 of Quinn 

 (1980a) show close correlations between SST, sur- 

 face air temperature, and sea level pressure; 

 however, on the average there is about a 1 mo lead on 

 the part of SST changes over the associated changes 

 in air temperature and sea level pressure. (Changes 

 in sea level on the average lead changes in SST by 1 

 mo at Talara.) Figures 6 and 7 of Quinn et al. (1981) 

 show close associations between SST and thickness 

 changes between pressure levels aloft, and Table 2 of 

 this article indicates thickness changes between 

 pressure levels near the surface precede related 

 changes in thickness between pressure levels at 

 higher altitudes. 



In our opinion, it is primarily the relatively persis- 

 tent in-sync relationship over the past 6 yr, between 

 the generally above normal SST's and the generally 

 below normal sea level pressures over the subtropi- 

 cal southeast Pacific, which has caused the below 

 normal Southern Oscillation index and the reduced 

 amplitude of fluctuations in this index. As a represen- 

 tation of the Southern Oscillation, the index trend 

 would indicate a significant reduction in amplitude of 



the Southern Oscillation over this period. How much 

 longer this low index condition will prevail and what 

 will cause a return to the large fluctuations of the past 

 are not apparent at this time. However, a slow oc- 

 casionally interrupted rise in the index has been 

 noted over the past several years. 



The below normal pressures in the subtropical high 

 signify a generalized weakening of the associated 

 southeast trades and equatorial easterlies, and the 

 reduced amplitude of the index fluctuations signifies 

 a reduction in the El Niho/anti-El Nino extremes in 

 Southern Oscillation-related activity. 



It appears to us that the generally weaker southeast 

 trades and their reduced fluctuations in strength, 

 which have prevailed over the past 6 yr, would not be 

 capable of building up to the type of relaxation re- 

 sponse called for by Quinn (1974), Wyrtki (1975), 

 and Wyrtki et al. (1976), where Wyrtki's so-called 

 "back sloshing" of the built-up water accumulation in 

 the western Pacific produces a significant El Nino. 

 Under existing conditions we would expect weaker 

 responses to the smaller buildups, such as the 1979- 

 80 event reported by Donguy et al. (1 982). This event 

 would be considered a relaxation response to the 

 small 1978 anti-El Nino buildup. (Note the 1978 in- 

 dex peak in Figure 1.) 



Changes in strength of the southeast trades and 

 equatorial easterlies bring about changes in the Peru 

 (offshore and coastal) current and equatorial current 

 systems, and the Peru-Chile undercurrent, as dis- 

 cussed in the introductory section; and they, in turn, 

 cause environmental changes in the Peruvian and 

 north Chilean coastal fisheries. In addition to the 

 generally above normal subtropical surface water 

 temperatures, we also note that the Chimbote SST 

 anomalies remain generally high during this period of 

 low index anomalies (Fig. 1). One cannot fail to note 

 the sharp drop in the anchoveta contribution to the 

 overall catch after the 1976 El Nino in both the Peru- 

 vian and north Chilean fisheries; also, in both of these 

 fishery areas the catch of other types offish increased 

 over this same period (Ceres 1981; Caviedes 

 1981). 



Monitoring and Predicting 

 the Changes 



Considering the monitoring and prediction of El 

 Nino, it must be realized that when the use of the 

 Easter-Darwin index was initially proposed (Quinn 

 1974), it was intended for use on large interannual 

 fluctuations leading to the onset of relatively strong 

 events (e.g., 1957, 1972). This was particularly true 

 for purposes of prediction, since the time involved in 



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