Observed decadal variability in precipitation and circulation over South America during austral summer

Abstract:

The Southeast coast of Brazil (SE) is among the most densely populated areas in South America. Previous studies using 70yr records have shown that both intensity and frequency of rainy days have increased in the southern portion of SE whereas precipitation has become more concentrated in fewer extreme events in the northern portion of the region during the wet season. The present study investigates changes in regional circulation associated with the observed trends in precipitation using daily National Center for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR), at 0.5° spatial resolution. We compare the difference in the climatology between 1979-1988 and 2003-2012 decades of the following variables: sea level pressure, geopotential height (850hPa and 200hPa), wind (850hPa), temperature (850hPa), precipitation, and precipitable water. For each decade, we estimate the difference in the composites of the variables during days when precipitation was above the median during austral summer (December to February) and we perform these analyses separately for the two portions of SE. The difference in the climatology indicates an increase in precipitable water accompanied by the weakening of circulation along the east coast of Brazil in the last decade. These results seem independent on the El Nino/Southern Oscillation variability and are consistent with some robust climate change scenarios resulting from the increase in anthropogenic greenhouse gases. The weakening in circulation is more intense for events over northern SE, which could explain the decrease in the number of rainy days observed over the area. Nevertheless, when the necessary dynamic conditions are met, the increased precipitable water along the coast could result in more intense events. Events occurring over southern SE are associated with an increase in precipitable water and stronger northwesterly flow, which could reinforce moisture flux convergence and intensify rainfall.