Observed and projected changes in rainfall extremes in the Metropolitan Area of Sao Paulo

Changes in rainfall extremes and flooding are becoming more frequent in many countries, particularly in large cities where people and assets are concentrated. In the Metropolitan Area of the city of Sao Paulo (MASP) region, heavy or extreme precipitation events have important effects on society. Flash floods and landslides, associated with intense, but often brief, rainfall events, may be the most destructive of extreme events. Observations since the mid-1930s in the MASP region have shown significant increases in total and heavy rainfall and decreases in light rain. This was probably due to natural climate variability, but with some signals of the urbanization effect, especially during the last 40 yr. Here projections of future changes in rainfall extremes in the MASP region were derived from the Eta-CPTEC 40 km regional model nested in the HadCM3 global model, with 4 available realizations of the global model for the A1B emissions scenario to the end of the 21st century. Trends were assessed for significance using the nonparametric Mann-Kendall test. Projections, based on percentiles and on the number of days with rainfall above a certain limit, suggested: (a) an increase in total precipitation, (b) an increase in heavy precipitation and in the contribution to total precipitation from more intense rainfall events, and (c) the possibility of longer dry periods separating days with intense rain in the MASP region. The trends were stronger and more significant in the second half of the 21st century. We are aware that dynamical downscaling may not provide information at the weather station level and that climate modeling does not resolve all uncertainties. However, we believe that this exercise enables climate assessments that, in time, can be used for general public information.