Trends in the distributions of aggregated monthly precipitation over the La Plata Basin

Abstract

Positive trends in annual rainfall in the La Plata Basin (LPB), south of 20°S observed in the last four decades of the twentieth century were not reversed and became more statistically significant when calculated until 2005. These trends were part of a more general change in the monthly precipitation distribution including extreme precipitation. Precipitation in dry and extremely dry months (below the 35th percentile) has been decreasing in the whole LPB region south of 22°S. On the contrary, precipitation in the above normal (between the 65th and 90th percentile) and the extremely high rainfall (above the 90th percentile) ranges has been increasing accounting for most of the annual precipitation trends. More than a steady trend, there has been an abrupt change in extreme monthly precipitation concentrated between 1977 and 1983. The analysis of intensity and frequency of extreme events was done fitting Generalized Extreme Values (GEV) and Poisson distributions. Each distribution was fitted with and without trends in the location parameter and tested to determine the best fit in each case. The regions where GEV with a positive trend was the best fit coincide with areas affected by extensive floods during the last decades. Spatially aggregated results highlight the signal of change towards higher maximum monthly precipitations for a wide span of return periods. The atmospheric circulation associated with cases where extreme monthly precipitation was observed in most of the stations was studied through the integrated water vapour transport in the lower troposphere and its associated divergence. During warm months, an intense northern low-level water vapour flow with two convergence nuclei, one over eastern Argentina, southern Brazil and Uruguay, and the other over western Argentina, along with a weakened south Atlantic Convergence Zone was associated with the more extreme precipitation months favouring the occurrence of Mesoscale Convective Systems.