Intensified drought over tropical South America, their causes and link to decadal climate variability and global climate change

We have observed an intensified drought severity over the tropical South America during the past several decades. Such a change is mainly contributed by a delay of wet season onset and rainfall reduction during late wet season. These changes are consistent with an increase of atmospheric stability in all seasons and with weakened cold air incursions which are important to jump start the wet season. These changes are correlated with warming of sea surface temperatures (SSTs) in the eastern tropical Pacific and tropical Atlantic, a pole-ward shift of southern hemispheric jets, and land surface feedbacks to these externally forced changes. Comparison of climate model simulations with and without anthropogenic forcing suggests that this delay of wet season is explained at least in part by anthropogenic forcing, although the increase of drought severity itself appear to be dominated by decadal changes of SSTs in the late 20th century. Although non consensus can be reached by all the climate model that participated in the IPCC AR4 as to whether drought severity will further increase in late 21st century, global climate models that agree well with observations in the 20th century indicate a likelihood over the next century of further delays of the wet season, and of a greater intensity of droughts.