Impact of Evapotranspiration on Dry Season Climate in the Amazon Forest*

Moisture recycling can be an important source of rainfall over the Amazon forest, but this process relies heavily upon the ability of plants to access soil moisture. Evapotranspiration (ET) in the Amazon is often maintained or even enhanced during the dry season, when net radiation is high. However, ecosystem models often over predict the dry season water stress. The authors removed unrealistic water stress in an ecosystem model [the Simple Biosphere Model, version 3 (SiB3)] and examined the impacts of enhanced ET on the dry season climate when coupled to a GCM. The ‘‘stressed’’ model experiences dry season water stress and limitations on ET, while the ‘‘unstressed’’ model has enhanced root water access and exhibits strong drought tolerance. During the dry season in the southeastern Amazon, SiB3 unstressed has significantly higher latent heat flux (LH) and lower sensible heat flux (SH) than SiB3 stressed. There are two competing impacts on the climate in SiB3 unstressed: cooling resulting from lower SH andmoistening resulting from higher LH. During the average dry season, the cooling plays a larger role and the atmosphere is more statically stable, resulting in less precipitation than in SiB3 stressed. During dry season droughts, significantly higher LH in SiB3 unstressed is a necessary but not sufficient condition for stronger precipitation. Themoistening effect of LH dominates when the Bowen ratio (BR 5 SH/LH) is .1.0 in SiB3 stressed and precipitation is up to 26% higher in SiB3 unstressed. An implication of this analysis is that forest conservation could enable theAmazon to copewith drying conditions in the future.