Precipitation trends in Southeastern South America: relationship with ENSO phases

MOIRA DOYLE; VICENTE BARROS

Foz do Iguazú, Brasil

Congreso; 8th International Conference on Southern Hemisphere Meteorology and Oceanography; 2006

AMS- American Meteorological Society

During the second half of the twentieth century, in subtropical South America to the east of the Andes, rainfall trends were observed in precipitation series from local networks. The relationship of these trends with the different phases of El Niño-Southern Oscillation (ENSO) is explored in this paper. As precipitation changes are characterized by linear trends, under certain conditions, the total linear trend is the result of adding the partial linear trends corresponding to each of the ENSO phases, i.e. El Niño, La Niña and Neutral events. When splitting a series in various sub series, the linear trends of these sub series add up to the same value as the linear trend of the entire series if each of the sub series has its elements ordered in time around the same mean value and variance as the complete series. A linear trend is a special case of a linear regression slope when the independent variable is time. These conditions were approximately fulfilled during the period 1960-1999. This property permits to isolate the linear trends of any variable corresponding to each phase, and therefore, helps to highlight or discard any possible relationship between the linear trends of the variables and the ENSO phases. Annual rainfall linear trends were calculated applying a linear regression model, to each grid point of a 3ºx3º mesh between 70º– 40ºW and 16º– 40ºS; regression coefficients were tested for significance through a t-test. There are positive linear trends south of 22ºS and negative to the north. In the southern area, positive significant trends of 4mm/year or more can be observed, i.e. an increase in precipitation exceeding 160mm in the last 4 decades of the twenty century, which is about 30% of the initial value. These annual rainfall trends took place predominantly during the neutral phase of the ENSO. Significant values extend northward from its southern nucleus along the east side of Andes Mountains where the climate is considerably dry. In northeastern Argentina, southern Brazil and Paraguay, half of the annual rainfall trend was also positive and came from El Niño phase with a minor contribution from La Niña and the neutral phases. Indeed, in this region more than a trend there was a change in mean value after 1980, well in phase with similar changes in the ENSO indexes. On the other hand, the region of the South Atlantic Convective Zone (SACZ) has a negative trend of up to 4mm/year, which in a total of 40 years adds to almost 160mm decrease in rainfall. Here the contribution of the neutral phase predominates, but only in the core of the negative trends Almost everywhere in the region, no trends or very small ones took place during La Niña periods. Hence, in most of subtropical South America, east of the Andes, precipitation trends during the extreme phases of the ENSO constitute only a small part of the trends of the last 40 years of the twentieth century