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Rainfall Prediction for the Austral Winter in Central Chile
based on a CCA Forecast and a Niño 3 Analog Evolution Approach
contributed by Aldo Montecinos and Patricio Aceituno
Department of Geophysics, Universidad de Chile, Casilla 2777, Santiago, Chile
The rainfall regime in central Chile (30 - 35S) is characterized by a strong annual cycle with 80%
of the annual rainfall concentrated during the austral winter months (May to August). According
to several authors, the interannual rainfall variability in this region is significantly modulated by El
Niño/Southern Oscilla-tion with warm (cold) events in the central equatorial Pacific associated
with wet (dry) condition in central Chile. 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 Niña episodes when the subtropical anticyclone is anomalously strong, thus blocking
the entrance of extratropical fronts to central Chile. The strength of the association between Niño
3 SST anomalies in the May-August period and simultaneous rainfall averaged over each of four
central Chilean region is shown in Aceituno and Montecinos (1996; hereafter AM).
CCA FORECASTS FOR 1997
The CCA method (Barnett and Preisendorfer, 1987) is used to predict rainfall during the austral
winter in central Chile, as described in AM. Sea surface temperatures (SST) in the region
150E-80W, 20N- 40S, with grid resolution of 4.5 x 7.5 (lat-lon), were used to predict rainfall at
32 Chilean stations between 30S and 36S grouped in four homogeneous regions (see Fig.1 of
AM). For operational applications, 2 predictor timing schemes are considered: mean SST fields
during the trimester of Oct-Dec (non-extended model) and during the three consecutive trimesters
of Jun-Aug, Sep-Nov and Dec-Feb (extended model). It was determined that only the rainfall
anomalies during the final months of the rainy season (July-August) have a reasonable
predictability using these models (Montecinos and Aceituno 1995, and AM).
CCA rainfall forecasts for July-August 1997 indicate, in general, near normal (N) or above
normal (AN) conditions in all 4 central Chilean regions, although the associated probabilities
are non-significant in some cases. Specifically, non- extended models (2 season-lead) predict N
condition for regions 1 and 3 with associated probabilities of 46% and 59%, respectively; and N
or AN conditions for region 4 with 93% probability. Extended models (4-months lead) predict
AN condition for regions 1 (47% probability) and region 3 (55% probability); and N or AN
conditions for regions 3 and 4 with 95% and 84% probabilities, respectively.
ANALOG FORECASTS USING NINO 3 SST
A simple approach is used to predict rainfall conditions for the whole wet season in central Chile
(May-August). Monthly SST anomalies for the Niño 3 region between January 1996 and April
1997 (1996-97) are used to search for years with analogous time evolution (correlation higher
than 0.5) in the period of 1946 - 1992. Using this procedure, five analog years were chosen:
1950-51, 1956-57, 1962-63, 1971-72 and 1985-86. Then the observed rainfall conditions
(expressed as terciles) during the austral winters of 1951, 1957, 1963, 1972 and 1986 are
composited.
Fig. 1 shows the time evolution of the Niño 3 SST anomalies during analog years (top and middle
panels), as compared with the current evolution of the Niño 3 SST anomalies from January 1996
to April 1997. In all cases, SST anomalies during the previous years were negative with a rapid
transition to warm conditions during the austral summer months. The bottom panel in Fig. 1
shows the observed May-to-August rainfall categories in each of the four regions for each of the
five analog cases. In general, above normal and near normal conditions were observed
during years with evolution of Niño 3 SST anomalies similar to that of the present year.
Moreover, Fig. 2 in AM indicates that rainfall predictability based on predicted SST anomalies in
region 3 is considerably higher when SST anomalies greater than +0.5C are expected during the
austral winter. This is the case for the present year, as suggest-ed by the current predicted
evolution of the SST in the central equatorial Pacific by several dynamic, hybrid and statistical
models (Experimental Long-Lead Forecast Bulletin, March 1997; and even more strongly in this
June 1997 issue). According to the simultaneous relationship between mean rainfall in each of the
four regions and SST anomalies in the Niño 3 area during May-August of 1946-92 (Fig. 2 of
AM), normal to above normal rainfall conditions are expected in central Chile this 1997
winter.
References
Aceituno, P. and A. Montecinos, 1996: Assessing upper limits of seasonal predictability of rainfall
in central Chile based on SST in the equatorial Pacific. Experimental Long-Lead Forecast
Bulletin, 5(2), 37-40.
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, 1825-1850.
Montecinos, A. and P. Aceituno, 1995: CCA forecast of rainfall in the subtropical west coast of
South America (central Chile) for July-August 1995. Experimental Long-Lead Forecast Bulletin,
4(2), 17-19.
Rutllant, J. and H. Fuenzalida, 1991: Synoptic aspects of the central Chile rainfall variability
associated with the Southern Oscillation, Int. J. Climatol., 11, 64-76.
Fig. 1. Time evolution of the Niño 3 SST anomalies during analog years: 1950-51, 1956-57, and
1962-63 (top panel), and 1971-72 and 1985-86 (middle panel). Current evolution of the Niño 3
SST anomalies for January 1996 - April 1997) is shown in both panels (thick line with open
circle). The bottom panel shows the observed rainfall categories for May-August for each of the 5
analog cases in each of the 4 central Chilean regions.
