Answer: Most of South America is below Equator line, that is the reason why it is South America. The climate is strongly affected by the equator line winds (Coriolis effect) which blows everyday from east to west. Such wind push the ocean moisture in to the Amazon river low lands. Moisture escape is blocked by the western mountains (Andes Mountains) and turns counter clock wise to south and east. Such moisture is caught by the cold south pole winds and causes the tropical cyclone with a lot of rain.
The triangle shape of South America is very favorable for rains because it allows the polar winds (from west to east) to blow in to the continent, sometimes up to the equator line, or close to it, pushing more south ocean moisture and causing rains.
Another relevant factor for climate is the South Atlantic high pressure, but such is another subject matter.
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