Rainfall in central Chile (30 ­ 35ES) occurs mostly during the austral winter (May­August), when occasional midlatitude fronts reach this subtropical region located between the Andes Cordillera and the Pacific coast. During the rest of the year, dry conditions prevail as the entire subtropical area is under the permanent influence of the South Pacific anticyclone. Several studies have confirmed the significant influence of the Southern Oscillation (SO) on the interannual rainfall variability in this region, with a tendency for abundant winter rainfall during El NiZo episodes. Mechanisms explaining this association have to do with the weakening of the subtropical anticyclone and an increase in blocking episodes to the southwest of the continent during the negative SO phase (Rutllant and Fuenzalida 1991). Diagnostic studies have also revealed a significant tendency for less than normal precipitation during La Nina episodes when the subtropical anticyclone is anomalously strong, thus blocking the entrance of fronts to central Chile.

Based on these relationships, a seasonal forecast of rainfall during the austral winter was developed for central Chile using a canonical correlation analysis (CCA) as described in Barnett and Preisendorfer (1987). Sea surface temperature (SST) in the region 150EE ­ 80EW, 20EN ­ 40ES, with a grid resolution of 4.5 x 7.5 (lat ­ lon) was used to predict rainfall at 15 Chilean stations between 30 and 40ES during the wet season (May­August) (Montecinos and Aceituno 1995a). Improved versions of CCA models developed for operational application (Montecinos and Aceituno 1995b) consider average SST fields during the trimester October­December (non-extended model) and during the three consecutive trimesters of Jun­Aug, Sep­Nov, and Dec­Feb (extended model) to forecast winter rainfall in 32 stations grouped in four regions, as indicated in Fig.1. It was determined that only the rainfall anomalies during the final months of the rainy season (July­August) have a reasonable predictability using these models.

Predictions for July­August 1995 proved to be partially successful (see Fig. 1). Overall, the correct category was forecast in 13 cases out of 32; this is 2.3 cases better than chance expectation. Best results were obtained in region 3 where forecasts indicated that the categories normal (N) and above normal (AN) were the most probable. Forecast were particularly bad in region 1, where the model mostly failed to anticipate the mainly below normal (BN) observed conditions. In fact, the rainfall deficit during the entire wet season worsened the already critical situation derived from a four-year drought in this region. The model also failed to anticipate the anomalously wet conditions observed in region 4, for which a precipitation in the range of below normal ­ normal was predicted. One reason for the failure in regions 1, 2 and 4 probably has to do with the lack of association between SST and rainfall when near normal conditions prevail in the equatorial Pacific, as explained next.

Figure 2 shows scatter diagrams of simultaneous average values of rainfall in each of the four regions described in Fig. 1 and SST anomalies in the NiZo 3 area during May­August of 1946­92. Most of the circulation anomalies associated with warm or cold episodes in the central Pacific are driven by teleconnection patterns concurrent with those SST anomaly fields. Hence, the results in Fig. 2 can be used to assess the upper limit of seasonal rainfall predictability in central Chile, when SST data are used as a predicting tool.

Figure 2 indicates that reliable forecasts of NiZo 3 SST anomalies could be useful in certain cases to anticipate rainfall anomalies in central Chile. Specifically, cold episodes characterized by SST anomalies lower than ­0.5EC are mostly associated with below average rainfall in all regions. Thus, a forecast calling for these cold conditions could be used to anticipate below normal or normal winter rainfall in central Chile. On the other hand, the existence of a reliable forecast indicating SST anomalies greater than +1.0EC could be associated with above normal rainfall in regions 1, 2 and 3, and with normal or above normal precipitation in region 4.

Figure 2 also indicates that rainfall predictability based on predicted SST anomalies in region NiZo 3 is considerably low when near normal SST is anticipated. This is particularly the case for the present year, as suggested by the currently predicted evolution of SST in the central Pacific by various dynamic and statistical models (Experimental Long­Lead Forecast Bulletin, March 1996 as well as the present issue).

Consistent with this lack of predictability, CCA rainfall forecasts for July­August indicate near normal or below normal precipitation in all 4 central Chilean regions shown in Fig. 1, although the associated probabilities of success are quite low and mostly statistically non-significant. Specifically, both extended and non­extended models call for below normal rainfall in regions 1 and 2. The probabilities of success of these forecasts, based on the functioning of the models during the period 1947­91, are around 20% for region 1 and 30% for region 2, which are well under the threshold of statistical significance. In regions 3 and 4, extended models anticipate below normal precipitation, with an associated probability of 46% in both cases. Non­extended models suggest instead that normal conditions will prevail in these two regions, with associated probabilities of 55% in region 2 and 44% in region 4. It should be noted that the only forecast with an associated probability reaching the 95% significance level is that for region 3, using the non­extended model.

Barnett, T. P. and R. Preisendorfer 1987: Origins and levels of monthly and seasonal forecast skill for United States surface air temperatures determined by canonical correlation analysis. Mon. Wea. Rev., 115, 1825­1850.

Montecinos, A., and P. Aceituno, 1995a: CCA forecast of rainfall in the subtropical west coast of South America (central Chile) for July­August 1995. Experimental Long­Lead Forecast Bulletin, 4(2), 17­19.

Montecinos, A., and P. Aceituno, 1995b: Pronostico estacional de la precipitacion en Chile central. Direccion General de Aguas. Ministerio de Obras Publicas, 39 pp + Anexos I, II.

Rutllant, J. and H. Fuenzalida, 1991: Synoptic aspects of the central Chile rainfall variability associated with the Southern Oscillation, Int. J. Climatol., 11, 64­76.

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