A Statistical-Empirical Forecast of October- December 1998 Precipitation in Uruguay Based on the ENSO State

contributed by Gabriel Pisciottano, Gabriel Cazes, Alvaro Diaz and Jose L. Genta.

GDAO ¹IMFIA, ²Universidad de la Republica, Montevideo, Uruguay

¹Grupo de Dinamica de la Atmosfera y el Oceano

²Instituto de Mecanica de los Fluidos e Ingenieria Ambiental

Rainfall in Uruguay (30^o-35^oS, 54^o- 58^oW; see Fig. 1) occurs during all 12 months of the year. The complex interaction of several rainfall mechanisms produces a wet climate with a "flat" annual cycle of precipitation. This is the case especially in the southern part of Uruguay, which is influenced by the Rmo de la Plata and the Atlantic Ocean. In the northern part, fall and spring average rainfall exceeds that of winter and summer (Pisciottano et al., 1994; 1997).

Various studies have confirmed a significant influence of ENSO on interannual rainfall variability in Uruguay, with a tendency for below normal rainfall in association to cold events in the Pacific Ocean, especially between October and December. (Ropelewski and Halpert, 1987; Aceituno, 1988; Pisciottano et al., 1994, 1997; Diaz et al.;1998).

Pisciottano et al. (1998) presented a forecast calling for a wetter than normal April-July period in 1998 in northern Uruguay, with probabilities of above median precipitation of 75% in the NW region and 84% in the NE region. For the southwestern and southeastern regions no forecast was issued because no clear shifts in the conditional distribution were detected. Climatological values were suggested as a reference for "expected" values.

Table 1 shows that the predicted positive anomalies in the northern region compared favorably to the observations. In the southwestern region the observed value was very close to the climatological median. On the other hand, in the southeast the observed rainfall reached the 85th percentile of the climatological distribution.

Diagnostic studies have revealed, among other results, that there are highly significant negative precipitation anomalies between October and December of years with a high Southern Oscillation Index in all regions of Uruguay (Pisciottano et al. 1994). For diagnostic studies and for results presented here, we use the 1950-1997 time series of monthly values of the Niño 3.4 Index (N3.4=Niño 3.4 *100) of SST anomalies in the central tropical Pacific, available from NOAA. In previous studies (see Pisciottano et al. 1997, 1998) we had used the Index of Wright. This change is based on the current state of the SST field in the tropical Pacific Ocean, for which cold anomalies in the central equatorial sector co-exist with warm anomalies in the eastern tropical sector. The diagnostic studies of the relationships between the Index of Wright and precipitation in Uruguay, and between the Niño3.4 Index and precipitation in Uruguay appear to support the idea that a substantial part of these links is related to the SST field in the tropical central Pacific. Accordingly, it seems to be sufficient to select the index related to Niño 3.4 region to "forecast"precipitation anomalies in Uruguay, even when the available time series is considerably shorter than the Index of Wright used in previous studies. Using a shorter record has an impact on statistical significance. We also note that the rainfall climatology used in this study is defined by values from the last 48 years so that decadal variations cannot be accounted for.

Our precipitation data are monthly values from 13 rainfall stations in Uruguay (Fig. 1). The data come from the Direccisn Nacional de Meteorologma (DNM)-Uruguay. For diagnostic and prediction purposes we have grouped the stations into the same four regions defined in Pisciottano et al. (1997). For each region, we calculated a monthly regional precipitation value for each month in the common period spanned by the record. Time series for October-December are formed from the monthly values for each region.

We defined fixed equiprobable categories (quartiles) of the predicted variable. Changes in the probabilities given a specific climatic event (e.g. an ENSO event) are calculated for each category and used to form a probabilistic prediction. First the statistical distribution parameters (first quartile, median, and third quartile-called qcl, mcl and Qcl respectively) for the whole record are determined for October-December. These statistics characterize the rainfall climatology (cl) of the respective region. In the absence of any climate-determining information, a value "around mcl" is expected and the interval (qcl, Qcl) has a large (50%) chance of containing the real observed value. In this case the "forecast" would be mcl and an "error bar" would be (qcl, Qcl).

To incorporate the knowledge provided by the ENSO-rainfall relationships, we compare the statistical parameters of the "whole population" (all the values of the October-December precipitation ) with those of the "subpopulation" obtained by selecting only the October-December values for the years whose July-August mean N3.4 was lower than a "critical value" (N3.4cr). In agreement with previous studies, we find that in the so defined "subpopulation" there is a tendency for lower rainfall. We test this statistical tendency with a hypergeometric distribution using the frequencies of above and below median rainfall in the population versus the frequencies for the "subpopulation". We note a balance between the "intensity" of a required event (as prescribed by N3.4cr) and the statistical significance of the rainfall differences (in the sense of Ropelewski and Halpert 1987; Pisciottano et al. 1994; Cazes et al. 1994). A very low N3.4cr produces a small number of events, yielding a lower significance than a moderate N3.4cr. It is found that (Niño3.4=.45^oC) results in significance values higher than 99% for the 4 regions. Using this critical value, we estimate the statistical parameters q*, m*, and Q* for the "subpopulation" (Jul-Aug N3.4 < -45) for October-December for the 4 regions. The procedure is similar to that used by Ropelewski and Halpert (1996). The statistical parameters (qcl, mcl, Qcl) and (q*, m* and Q*) for the 4 regions are shown in Fig. 2. Table 2 illustrates the rainfall distribution shift associated to years having Jul-Aug N3.4 <45 through frequencies of occurrence of each of the four quartiles for each of the four regions.

These results enable us to "predict" the expected precipitation as the median of the "subpopulation" (m*) rather than the climatological median (mcl). The interval (q*, Q*) from the subpopulation is the "error bar" of the prediction. This technique circumvents problems associated with the nonlinearities of the SST-rainfall relationships and skewness of the rainfall distributions, and has been used during the past several years to experimentally predict rainfall in Uruguay. For example, in 1996 and 1997, a version of this technique predicted rainfall in three regions of Uruguay for November 1996-February 1997 and for September-December 1997 reasonably well. These forecasts were used by rice growers for water resource management (Pisciottano and Diaz, 1997; Cabral and Pisciottano 1997).

Based on the current values of the Pacific SST anomalies and the distributional shifts discussed above, we issue the following rainfall predictions for the October-December 1998 period for the 4 regions covering Uruguay.

Region I-NW Uruguay: A value approximately 255 mm of rainfall is expected, with 50% chance of between 213 and 291 mm. The probabilities of rainfall in the 1st and the 2nd quartile of the climatological distribution are 56% and 33%, respectively; and of rainfall below the median (322 mm) is 89%.

Region II-NE Uruguay: ~216 mm is expected, with 50% chance of between 174 and 270 mm. The climatological 1st and 2nd quartiles have probabilities of 78% and 11%, respectively; and of rainfall below the median (334 mm) is 89%.

Region III-SW Uruguay: ~173 mm is expected, with 50% chance of between 148 and 235 mm. The climatological 1st and 2nd quartiles have probabilities of 67% and 22%, respectively; and of rainfall below the median (276 mm) is 89%.

Region IV-SE Uruguay: ~196 mm is expected, with 50% chance of between 163 and 240 mm. The climatological 1st and 2nd quartiles have probabilities of around 56% and 44%, respectively; and of rainfall below the median (247 mm) is almost 100%.

In summary, a drier than normal October-December period is expected this year in all regions of Uruguay with probabilities of below median precipitation ranging from 89% to 100 % respectively.

References:

Cabral, A. and G. Pisciottano, 1997: Elementos para entender el fensmeno 'El Niño' y sus aplicaciones en la agricultura. Revista de la Asociacisn de Cultivadores de Arroz del Uruguay. Junio 1997, 32-42.

Cazes, G., J. L. Genta and G. Pisciottano, 1994: Generacisn de informacisn hidrolsgicamente relevante a partir de informacisn y diagnsstico climatico. Aplicacisn en Uruguay. Memorias XVI Congreso Latino-americano de Hidraulica, Santiago, Chile, 3, 121-127.

Diaz, A., C. Studzinski and C. R. Mechoso, 1998: Relationships between precipitation anomalies in Uruguay and southern Brazil and sea surface temperature in the Pacific and Atlantic Oceans. Journal of Climate, 11, 251-271.

Pisciottano, G., G. Cazes and A. Diaz, March 1998: A statistical-empirical forecast of April-July 1998 precipitation in Uruguay Based on the ENSO state. COLA - Experimental Long-Lead Forecast Bulletin.

Pisciottano, G., A. Diaz, G. Cazes and C.R. Mechoso, 1994: El Nino-Southern Oscillation impact on rainfall in Uruguay. J. Climate, 7, 1286-1302.

Pisciottano, G. and A. Diaz, 1997: Diagnsstico y prediccisn climatica en Uruguay. Memorias del Taller "Variabilidad climatica interanual: mitodos de pronsstico e impactos asociados". Santiago, Chile.

Ropelewski, C.F., and M.S. Halpert, 1987: Global and regional scale precipitation patterns associated with the El Nino/Southern Oscillation. Mon. Wea. Rev., 115, 1606-1626.

Ropelewski, C.F., and M.S. Halpert, 1996: Quantifying Southern Oscillation-precipitation relationships. J. Climate, 9, 1043-1959.

Wright, P. 1989: Homogenized long-period Southern Oscillation Indices. Int. J. of Climatol., 9, 34-54.

Table 1. Climatology median, predicted (when appplicable) and observed precipitation for April-July 1998 for each of the 4 regions in Uruguay. An interval of precipitation with 50% probability is indicated together with central value.

Table 2. Frequencies of occurrence of October-December rainfall in each of the 4 quartiles of the climatological distribution, given a year having Jul-Aug N3.4 <-.45, for the four regions in Uruguay. The quatriles are sorted according to increasing precipitation, i.e., q.1 is the lowest quartile and q.4 is the highest quartile.

Fig. 1. The 13 rainfall stations used in this study and the 4 regions in Uruguay. Stations: 1 Artigas; 2 Salto; 3 Paysandu; 4 Rivera; 5 Tacuarembo; 6 Melo; 7 Colonia; 8 Mercedes; 9 Trinidad; 10 San Jose; 11 Treinta y Tres; 12 Rocha; 13 Montevideo. Regions: I-NW, Northwest; II-NE, Northeast; III-SW, Southwest; IV-SE, Southeast.

Fig. 2. a) Medians and quartiles of the distribution of precipitation for the period October-December, in the region I-Northwest, for all the cases (Climatology) and for the subpopulation of cases when the Nino 3.4 index averaged in the previous July-August period was below -45 (N3.4 < -45). The data record spans from 1950 to 1997.b) Same as a), for the region II-Northeast. c) Same as a), for the region III-Southwest. d) Same as a), for the region IV, Southeast.