Summertime Moisture Transport to the Southern South American Altiplano: Constraints from In Situ Measurements of Water Vapor Isotopic Composition

摘要

Austral summer transport of water vapor to the southern South American Altiplano is investigated using in situ measurements of water vapor isotopic composition collected from 1 November 2012 to 10 February 2013 on the Chajnantor Plateau in the Chilean Andes. Onset of the wet season in December was associated with an increase in mixing ratios from an average of 1500 ppmv during the winter dry season to 5400 ppmv in early December. Water vapor isotopes delta D and delta O-18 increased from dry season averages of -235 parts per thousand and -31 parts per thousand, respectively, to wet season averages of -142 parts per thousand and -17 parts per thousand, reaching as high as -70 parts per thousand and -17 parts per thousand, respectively. The highest water vapor delta values were close to those measured in coastal settings, suggesting little condensation during transport to the site. About 5% of the wet season data have d values that are lower than expected for Rayleigh distillation and are associated with high relative humidity (>75%), easterly winds, and periods of low outgoing longwave radiation over the Altiplano, consistent with moistening by deep convection. The remainder of the data have d values that are greater than expected for Rayleigh distillation, up to 250 parts per thousand above the Rayleigh curve. These data are consistent with mixing between very dry air and moist air from the boundary layer. The data show intraseasonal variability coherently linked to the position of the Bolivian high, with moist air associated with a southward displacement in the Bolivian high. The humidity over the southern Altiplano during the wet season reflects a balance among advective drying, advective moistening with little condensation, and convective moistening.