Thanks for the replays. I have flown many times to Argentina and Brazil usually in end of march, beginning of April, and I have to admit that I have never found any significant problem. Even sometimes no many clouds in the area. Some turbulences, but nothing drammatical.
Sorry for the offtopic, but not being a pilot, this kind of dark area, was something completely new for me. Weather is such an amazing issue in aviation, which I think we have lots to learn about it.