Why desert and forest flip at 30°S in the Andes?

Between about 30 degrees south of the equator, in a region called the horse latitudes, the Earth's rotation causes air to slant toward the equator in a northwesterly direction in the southern hemisphere. (The opposite in the northern hemisphere). This is called the Coriolis Effect. The trade winds to switch directions south of the Horse Latitudes. This means (in the southern hemisphere) from 30 degrees to the equator, the prevailing trade winds move from East to West creating a rain shadow on the western side of the Andes. South of the horse latitudes, it switches and the winds move west to east - just like across North America. The rain shadow south of these latitudes is on the eastern side of the Andes - like the rocky mountains in the USA.