Grid points analysis for the identification of climatic extremes in the Uruguay river basin, South America

LARESE, MÓNICA; KROHLING, DANIELA

Laguna Mar Chiquita, Miramar, Córdoba

Otro; International Meeting “Holocene environmental catastrophes in South America: From the lowlands to the Andes.; 2005

ICSU Dark Nature – IGCP Proy. 490.

Monthly precipitation data available in gridded point format were processed to analyze the spatial and temporal variation of anomalies in the Uruguay River basin (365,000 km2) for the years: 1902-95. The interpolated grid points were extracted from a 0.50x 0.50 grid (New et al, 2000) for the region delimited by 25°- 35°8 and 48°-60° W. Anomalies were calculated as non dimensional coefficients which measure the precipitation that monthly deviates from the historical mean. The categories of extremes were constructed based on statistical analysis. The approach by quintiles allowed to discriminate five categories of extremes: dry, near dry, normal, near humid and humid. A dry category takes place when the anomaly is located inside the first quintile, whereas a humid category occurs when the anomalous value is in the last quintile. Regional maps were generated to identify geographic and stationary distribution patterns as well as persistence of the extremes at the same time. An animated set of 1,128 monthly maps was obtained. Extreme sequences were also numerically calculated to locate the extremes on the animation. lt allows to detect the initial month or a specific event by year of occurrence, by persistence and by grid points affected. Strongest dry periods were found in the years 1910 (Oct./Dec.; 63.85% grid points affected) and 1942/43 (Sep./Feb.; 69.74% grid points involved). Oppositely, extremely humid periods were detected in the years 1914/15 (Jan./May; 92.82% points affected) and 1990 (Set/Dec.; 70.77% points). Complementary gauging data from meteorological stations of the basin were also processed.  The results indicated that during dry extremes mean temperatures were lower than normal (several frosts); in contrast in humid periods mean temperatures were slightly higher (no frost). Both extreme categories experimented important shifts in minima with similar trend. Obtained results contribute to reconstruct the Holocene environmental scenarios in the Uruguay basin.