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The Amazon forests and climatological precipitation patterns in South America are interrelated. A fundamental question is how these patterns depend on the presence of forests. Here we investigate this relationship by studying how precipitation varies with distance from the ocean along wind streamlines linking the Atlantic Ocean to northwestern and southern South America through the Amazon forests. Through a robust observation-based analysis, we found that precipitation exponentially increases with distance from the ocean along wind streamlines flowing over forests, while it exponentially decreases downwind of the forests. These patterns are consistent with multiple mechanisms through which forests influence the transport of atmospheric moisture and precipitation production over the continent. We propose a conceptual explanation of this forest influence based on the atmospheric water balance. Our results imply that a major consequence of the degradation or loss of forests may be a disruption of these mechanisms, with widespread impacts on continental precipitation.

Plain Language Summary How and why precipitation varies within continents is fundamental for many natural and social phenomena and related decision making. Here we address these questions in South America, specifically studying the relationship between precipitation and the presence of the Amazon forests. We found that precipitation exponentially increases with distance following the direction of winds passing over the Amazon forests and transporting moisture from the Atlantic ocean. In contrast, precipitation decreases exponentially as these winds flow away from forests. We propose a conceptual explanation of this forests' influence, through multiple mechanisms, based on the atmospheric water balance. This implies that a major consequence of forest degradation or loss may be a disruption of these mechanisms, with widespread impacts on continental precipitation.