Read here. There is a major climate pattern, often referred to as the South American Monsoon System (SAMS), that brings the annual rainy season to portions of S. America, including the Amazon area. Scientists (Bombardi, R.J. and Carvalho) analyzed the output of IPCC global climate models and discovered that not only did they fail to accurately forecast/predict the actual precipitation levels, they also failed at predicting the start and end points of the monsoon season.
"Bombardi and Carvalho report that over northern South America the annual precipitation cycle "is poorly represented by most models," and, more specifically, that "most models tend to underestimate precipitation during the peak of the rainy season." In addition, they say that "the misrepresentation of the Inter-Tropical Convergence Zone and its seasonal cycle seems to be one of the main reasons for the unrealistic out-of-phase annual cycles simulated near the equator by many GCMs," and that "poor representation of the total monsoonal precipitation over the Amazon and northeast Brazil is observed in a large majority of the models. As a consequence, therefore, they note that "simulations of the total seasonal precipitation, onset and end of the rainy season diverge among models and are notoriously unrealistic over [the] north and northwest Amazon for most models."
Additional climate model failures.