INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (2011) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.2424 Mechanisms of summertime precipitation variability in the Bolivian Altiplano: present and future Jeanne Thibeault, a * Anji Seth a and Guiling Wang b a Dept. of Geography, University of Connecticut, Storrs, Connecticut, CT 06269-4148, USA b Dept. of Civil and Environmental Engineering, University of Connecticut, Storrs, Connecticut, CT 06269-2037, USA ABSTRACT: Water availability is a major constraint to rural agriculture in the Bolivian Altiplano. The ability of medium and high-resolution CMIP3 models to simulate observed relationships between interannual Altiplano summer precipitation anomalies and large-scale patterns of precipitation, 200-hPa winds and 200-hPa geopotential height is examined. Altiplano precipitation anomalies are known to be related to upper-level wind anomalies, with westerly (easterly) anomalies associated with deficit (excess) rainfall. A majority of models are able to simulate an easterly/wet-westerly/dry relationship in response to changes in the zonal flow produced by fluctuations in tropical tropospheric temperatures that affect the meridional temperature gradient, consistent with observations. The 21st century evolution of these relationships under conditions of greenhouse warming is also examined. Results indicate that wet anomalies will continue to require moisture transport from the east. However, expected changes in mean circulation (e.g. tropical tropospheric warming) will likely influence precipitation variability. Models that better simulate Altiplano precipitation variability project an increase in 200-hPa westerly winds by 2050–2099, which are not favourable for precipitation in the Altiplano. The reduced frequency of summertime upper-level easterly wind anomalies may explain projections for less frequent summertime rainfall. Models that better simulate Altiplano precipitation variability project reduced summertime precipitation, consistent with projections for more frequent westerly wind anomalies. Summertime soil moisture reductions are expected from about 2020 onward due to reductions in summer rainfall, increases in the runoff ratio, higher evapotranspiration rates, and reduced precipitation frequency. These findings are consistent with expected large- scale changes in atmospheric circulation, providing confidence in the projections. These results have serious implications for water resources and food security in the Altiplano, but will require further testing with improved, higher-resolution models. Copyright  2011 Royal Meteorological Society KEY WORDS precipitation variability; coupled climate models; soil moisture; ENSO; WCRP-CMIP3 multi-model dataset; Altiplano; South America Received 18 August 2010; Revised 31 July 2011; Accepted 6 August 2011 1. Introduction The Bolivian Altiplano is a semi-arid region, located on a high plateau in the central Andes of South America. Water availability is a major constraint for agricul- ture, where approximately 50% of the rural popula- tion employs traditional farming methods, depending entirely on water supplied by spring and summer rain- fall (Garcia et al., 2007). Over 60% of annual pre- cipitation falls during the summer months (DJF) in association with the South American Monsoon (SAM) and interannual variations are large (e.g. Garreaud, 1999; Lenters and Cook, 1999; Garreaud and Aceituno, 2001; Garreaud et al., 2003). This research examines the ability of the medium and high-resolution cou- pled models from the World Climate Research Program (WCRP) Coupled Model Intercomparison Project version 3 (CMIP3) (Meehl et al., 2007) to simulate the observed relationships between Altiplano interannual precipitation * Correspondence to: Jeanne Thibeault, Dept. of Geography, University of Connecticut, CLAS Bldg., 215 Glenbrook Rd., U-4148, Storrs, CT 06269-2037, USA. E-mail: jeanne.thibeault@uconn.edu anomalies and large-scale atmospheric variables (i.e. precipitation, 200-hPa winds, and 200-hPa geopotential height) for summer (DJF). Because precipitation vari- ability is so important to agriculture and water resources in the Altiplano, this research also examines the evolu- tion of these relationships through the 21st century under conditions of greenhouse warming. A number of previous studies have investigated Alti- plano summertime precipitation variability and its rela- tionship to the atmospheric circulation (e.g. Garreaud, 1999; Lenters and Cook, 1999; Vuille, 1999; Gar- reaud and Aceituno, 2001; Falvey and Garreaud, 2005). On interannual timescales, precipitation variability is related to changes in the mid- and upper-level wind patterns; easterly flow favours moisture transport from the lowlands east of the Altiplano, while westerly flow inhibits moisture transport from the east. These changes in the zonal flow are associated with changes in the meridional temperature gradient between the tropics and mid-latitudes (Garreaud and Aceituno, 2001; Garreaud et al., 2003). Previous studies have shown a relationship between Altiplano precipitation anomalies and El Ni˜ no Southern Oscillation (ENSO) with wet (dry) summers Copyright  2011 Royal Meteorological Society