Mechanisms of summertime precipitation variability in the Bolivian Altiplano: present and future

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 20502099, 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 (c) 2011 Royal Meteorological Society