Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 141: 3325 – 3343, October 2015 B DOI:10.1002/qj.2615 Impact of Amazonian evapotranspiration on moisture transport and convection along the eastern flanks of the tropical Andes Xiaoming Sun and Ana P. Barros* Pratt School of Engineering, Duke University, Durham, NC, USA *Correspondence to: A. P. Barros, Pratt School of Engineering, Duke University, Box 90287, 2457 CIEMAS Fitzpatrick Bldg., 101 Science Drive, Durham, NC 27708, USA. E-mail: ana.barros@duke.edu The Weather Research and Forecasting (WRF) model was used to investigate the impact of Amazonian evapotranspiration (ET) on moisture transport and convection along the east- ern 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 influ- ence 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 con- vection. 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 con- cave 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, ren- dering stronger winds away from the Andes. Moisture convergence and convection over the EADS vary accordingly, strengthened in the day but attenuated at night. Nocturnal convec- tive motion is, however, more widespread. Analytical solutions of simplified diagnostic equa- tions 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. Key Words: evapotranspiration; moisture transport; convection; Andes; Amazon Received 24 April 2014; Revised 13 April 2015; Accepted 23 June 2015; Published online in Wiley Online Library 09 September 2015 1. Introduction Harbouring the most extensive rainforest, the Amazon basin represents the largest source of terrestrial evapotranspiration (ET) on the planet (Jung et al., 2010, their Fig. 1a). In this region, surface moisture flux, and the associated latent heat, can be crucial to atmospheric convection, since the temperature gradient is generally weak in the Tropics and diabatic heating is the primary convective energy source. The dominant role of diabatic heating was elucidated by Gill (1980), who demonstrated that symmetric heating about the Equator in the maritime subcontinent can generate low-level westerly inflows into the heated area via westward propagating Rossby waves, and low- level easterlies over the Pacific through eastward propagating Kelvin waves. Upon meridional integration, Gill’s solution to the east of the heating source resembles the Walker cell, while the zonally averaged flow bears typical features of Hadley-type circulations. The success of Gill’s model motivated subsequent analytical and numerical studies for South America (e.g. Silva Dias et al., 1983; DeMaria, 1985; Figueroa et al., 1995). Using specified heating profiles, these authors consistently reported a well simulated Bolivian High (BH), a prominent feature of summertime upper-level circulations over South America, although the Andes topography needs to be incorporated to reproduce the Northeast (‘Nordeste’ in Portuguese) Low and to improve the fidelity of the simulated South Atlantic convergence zone (SACZ) (Figueroa et al., 1995). Admittedly, studies with a priori prescribed heating profiles do not fully describe regional atmospheric dynamics because feedbacks to the forcing are excluded. The results presented by Lenters and Cook (1997), however, further corroborate that condensational heating, which can be modified by topography and is not necessarily confined to c  2015 Royal Meteorological Society