Spatio-temporal variability of daily and weekly precipitation extremes in South America

Spatial and temporal variability of precipitation extremes are investigated by utilizing daily observations available at 2.5 degrees gridded fields in South America for the period 1940-2004. All 65 a of data from 1940-2004 are analyzed for spatial variability. The temporal variability is investigated at each spatial grid by utilizing 25-a moving windows from 1965-2004 and visualized through plots of the slope of the regression line in addition to its quality measure (R-2). The Poisson-generalized Pareto (Poisson-GP) model, which is a peaks over threshold (POT) approach, is applied to weekly precipitation maxima residuals based on the 95%-quantile threshold, while daily data are utilized to analyze the number of consecutive daily extremes and daily extremes in a month based on the 99%-quantile threshold. Using the Poisson-GP model, we compute parameters of the GP distribution, return levels (RL) and a new measure called the precipitation extremes volatility index (PEVI). The PEVI measures the variability of extremes and is expressed as a ratio of return levels. From 1965-2004, the PEVI shows increasing trends in the Amazon basin except eastern parts, few parts of the Brazilian highlands, north-west Venezuela including Caracas, north Argentina, Uruguay, Rio De Janeiro, Sao Paulo, Asuncion, and Cayenne. Catingas, few parts of the Brazilian highlands, Sao Paulo and Cayenne experience increasing number of consecutive 2- and 3-days extremes from 1965-2004. The number of daily extremes, computed for each month, suggest that local extremes occur mostly from December to April with July to October being relatively quiet periods.