Some Effects of La Niña on Summer Rainfall,
Water Resources and Crops in Argentina
Guillermo J. Berri1Department of Atmospheric Sciences1
University of Buenos Aires
Buenos Aires, Argentina
mailto:berri@at1.fcen.uba.ar
Eduardo A. Flamenco2, Rafael Hurtado3, Liliana Spescha3, and Raúl A. Tanco4
Evarsa S.A., Argentina2, Faculty of Agronomy, University of Buenos Aires, Argentina3, Department of Geophysics, University of La Plata, Argentina4
Summer Rainfall
Different studies point out a significant relationship between ENSO (El Niño/Southern Oscillation), and seasonal to interannual climate variability in southeastern South America. ENSO has the ability to introduce a profound modification in the general circulation of the atmosphere. In the case of central and southern South America, the shifts of the subtropical jet is the mechanism that teleconnects the variability in the tropical Pacific Ocean with local climate variability. The flatlands of central and northeastern Argentina, known as the Humid Pampa, are the richest crop region of Argentina, with a total production of over 60 million metric tons. Almost one third of that is exported and represents a major source of income for the country. This results from the combination of good soils and favorable climatic conditions. However, most of this region is affected by sea surface temperature variability in the tropical Pacific Ocean. This effect is particularly notorious in the precipitation regime around the austral (Southern Hemisphere) summer, from October through April. In order to quantify the effect, we studied the monthly precipitation anomalies of 24 weather stations within the box defined by 57°W-63°W and 29°S-39°S (Berri and Tanco, 1998).
The original monthly values are converted in terciles, hereafter identified as below normal, normal and above normal, and averaged over the region. The final result is expressed as a three column matrix in which each column represents the percentage of the region that received precipitation within each tercile. Finally, two subsets are identified. One includes the years of cold ENSO events or La Niña (1946, 1954, 1964, 1970, 1973, 1975 and 1988), and the other one the years of warm ENSO events or El Niño (1951, 1953, 1957, 1963, 1965, 1969, 1972, 1976, 1982, 1986 and 1991). We consider only the year when the event started.
Figure 1, which corresponds to the La Niña composite, shows the percentage of the region that received above normal rainfall (upper tercile) and below normal rainfall (lower tercile). Between October and March there is a significant rainfall reduction that affects more than 60% of the region in December, while less than 10% of the region receives excess rainfall.
Figure 2 legend is the same as Figure 1, but it is for the average of 11 El Niño events during 1946-1993.
The rainfall shortage is more pronounced during November-December and March-April. During El Niño events (Figure 2), the region experiences excess rainfall in this period, to the extent that less than 20% of the region receives below normal rainfall. If ENSO had no effect on rainfall, all the bars should show values around 33%, which is the probability of occurrence of each tercile. A detailed study of 6 recent warm events, presented in Figure 3, reveals that there are always some significant dry regions, despite the generalized rainfall excess experienced in the region. In contrast to that, during cold events there are no significant wet spots. It can be concluded that ENSO modifies the climate regime of the region in such a manner that warm events enhance rainfall while cold events suppress it. The comparison of the two figures suggests that the ability of La Niña to suppress rainfall is stronger than the ability of El Niño to enhance it. It can also be concluded that the opposite phases of ENSO have a symmetric effect in the rainfall regime of the region.
Crop Yields
The humid Pampa region in Argentina, located approximately between 57°W-63°W and 29°S-39°S, is one of the richest regions of the world in terms of grain production. This result from the combination of good soils and favorable climatic conditions. On the other hand, the region is significantly influenced by ENSO-related seasonal to interannual climate variability. Wheat production in the region accounts for 80% of Argentina's production.
The relationship between the sea surface temperature (SST) anomalies in the Niño3 region of the Pacific Ocean and wheat yields from 127 districts, during the period 1970-1997 has been investigated (Hurtado and Berri, 1998). The time series of wheat yields is detrended by means of a linear adjustment. A positive correlation coefficient between every 3-month average SST during the crop cycle June to December and wheat yields is obtained in the southern part of Buenos Aires province. In particular, August-October produced the largest correlation coefficient. The positive sign of the correlation coefficient means that higher wheat yields are associated with the warm phase of ENSO, while lower yields are associated with the cold phase or La Niña. Soybean yields have also been studied within the core region (Spescha and Berri, 1998). Soybean is one of the most important crops in the country, totaling 16 million metric tons during
the harvest of 1997/1998. Soybean yields of all districts in the region are correlated with the SSTs in the Niño3 region, averaged over every 3-month period during the crop cycle (November through March). The results show two clearly distinguished regions with opposite responses. Most of the provinces of Santa Fe and the northern part of Buenos Aires show a negative correlation with Niño3, while the southeastern part of the province of Cordoba shows a positive correlation with Niño3. Again, a positive correlation with Niño3 means higher yields (lower yields) during warm (cold) ENSO events. The opposite situation holds in the case of a negative correlation coefficient. This opposite response to ENSO events in neighboring regions may have important implications for regional agricultural planning.
Water Resources
Figure 3 Regions with rainfall equal or less than decile 3 during November-January of the warm events initiated in 1957, 1972, 1976, 1982, 1986 and 1991. We present an example of the practical use of climate information in water resources management (Berri and Flamenco, 1998). The Diamante River, located in western Argentina, approximately between 34°S-35°S and 69°W-70°W, has its source in the high ranges of the central Andes mountains. The hydrological regime presents a well defined spring and summer maximum, when the melting of the snow accumulated during wintertime takes place. The October-March period accounts for 70% of the annual water volume. The total drainage area of the basin is 2,750 km2. The two hydropower plants in the system, Agua del Toro and Los Reyunos, have a combined power of 500 Mw. Important irrigation areas totaling 800 km2 are located downstream these water reservoirs, which are dedicated to vegetables crops, grapes and other fruits, which represent an important economic activity in the region. For the purpose of hydroelectricity management, the accumulated water volume flowing during the October-March period is used as a measure of the water available in the system. At the end of the winter, in September, an estimate of the volume of the snow deposited in the catchment is made and the first seasonal volume prediction is issued. The model in use converts the snow volume accumulated in the catchment into an equivalent water volume that will flow during the period October-March. This prediction is used by the power utility company to produce future electricity generation estimates.A statistically significant positive correlation is found between the seasonal volume October-March and the sea surface temperatures (SSTs) in the Niño3 region of the Pacific Ocean (5°S-5°N, 90°W-150°W), during March-April and November-December. A multiple linear regression model for the October-March volume predictions is developed, making use of Niño3 anomalies observed during March-April and November-December Niño3 SST. A validation analysis is performed for the 1981-1994 period, developing the model coefficients for the training period 1949-1980. Since the November-December Niño3 SST anomalies are input to the model, they are replaced with 6-month Cane-Zebiak model predictions (Chen et al, 1995; Zebiak and Cane, 1987), performed in May.
| Below | Normal | Above | |
| Below | 2 | 2 | 0 |
| Observed Normal | 2 | 3 | 0 |
| Above | 0 | 0 | 5 |
References
Berri, G.J. and E. Flamenco, 1998, Seasonal volume forecast in the Diamante River, Argentina, based on El Niño observations and predictions, submitted to Water Resources Research.Berri, G.J. and R. Tanco, 1998, Some effects of El Niño in the summer rainfall in the humid Pampa of Argentina, X° Brazilian Congress of Meteorology, Brasilia, Brazil, October 1998.
Chen, D., S.E. Zebiak, A.J. Busalacchi and M.A. Cane, 1995, An improved procedure for El Niño forecasting: Implications for predictability, Science, 269, 1699-1702.
Hurtado, R. and G.J. Berri, 1998, Relationship between wheat yields in the humid Pampa of Argentina and ENSO, during the period 1970-1997, X° Brazilian Congress of Meteorology, Brasilia, Brazil, October 1998.
Spescha, L. and G.J. Berri, 1998, On the effect of ENSO on the soybean yields in the humid Pampa of Argentina, X° Brazilian Congress of Meteorology, Brasilia, Brazil, October 1998.
Zebiak, S.E. and M.A. Cane, 1987, A model of El Niño/Southern Oscillation, Mon. Wea. Rev., 115, 2262-2278.