Impact of Amazonian evapotranspiration on moisture transport and convection along the eastern flanks of the tropical Andes

Abstract

The Weather Research and Forecasting (WRF) model was used to investigate the impact of Amazonian evapotranspiration (ET) on moisture transport and convection along the eastern flanks of the Andes (EADS). To isolate the role of surface ET, quasi‐idealized simulations down to 1.2 km grid spacing were conducted, where over the Amazon lowlands (AMZL) and at every time step the surface sensible‐heat effects are identical to the realistic reference runs while surface latent heat fluxes are prevented from entering the atmosphere. The results show that, without surface ET, daily precipitation within the AMZL decreases by as much as ∼75%, but nearly doubles over the surrounding mountainous regions. This dramatic influence is attributed to a dipole structure of convergence–divergence anomalies over the AMZL, primarily due to the considerable cooling of the troposphere associated with suppressed convection. Further examination of moist static energy evolution indicates that the net decrease in convective available potential energy over the AMZL is due to the removal of surface ET that is only partially compensated by related regional circulation changes. Because of the concave shape of the Andean mountain range, enhanced low‐level divergence promotes air mass accumulation to the east of the central EADS. This perturbation becomes sufficiently strong around nightfall and produces significant eastward low‐level pressure gradient force, rendering stronger winds away from the Andes. Moisture convergence and convection over the EADS vary accordingly, strengthened in the day but attenuated at night. Nocturnal convective motion is, however, more widespread. Analytical solutions of simplified diagnostic equations of convective fraction suggest that reduction of lower troposphere evaporation is the driving mechanism. Additional exploratory experiments with varied surface ET magnitude demonstrate that the connection between the AMZL ET and EADS precipitation is robust.