Phys.org published an article earlier this month on a new study of the Amazon rain forest which shows observationally that the end of the dry season there is triggered by water vapor emanating from the trees there. According to the article, until now scientists have not been able to determine why the rainy season begins when it does, since it starts before the onset of the monsoon winds and the movement of the Intertropical Convergence Zone, a belt of thunderstorms near the equator that bring copious rain to the tropics when they move into the area. You can read more about how the study at https://phys.org/news/2017-07-amazon-rainy-season.html.
