Seasonal and intraseasonal variability of precipitable water vapour in the Chajnantor plateau, Chile.

The Chajnantor plateau is located in the Atacama Desert at over 4800m above sea level. The plateau features a large fraction of days during the year where dry and stable atmospheric conditions predominate. The suitability of the type of astronomy observations routinely made at sites across the Chajnantor region at different times of the year depends on the variability of atmospheric water vapour. In this work, we study seasonal and intraseasonal variability associated with periods of large and low precipitable water vapour (PWV) in the Chajnantor region over 32 years. We establish large-scale conditions favourable for above- and below-normal PWV for each of the four seasons. We also explore whether intraseasonal PWV variations are related to the Madden-Julian Oscillation (MJO). Below-normal PWV in winter, spring, and autumn seasons was found associated with mostly zonal 500-hPa winds, which advect drier than normal air from the anticyclone over the eastern portions of the southeast (SE) Pacific Ocean. In summer, periods with below-normal PWV were found to have anomalously south-westerly winds at mid-levels, also indicating transport of anomalously dry air from the stable SE Pacific. In winter, spring, and autumn seasons, above-normal PWV was generally associated with north-westerly mid-tropospheric wind anomalies, indicating advection of above-normal PWV from the subtropical and tropical regions of the SE Pacific Ocean. Above-normal PWV conditions in summer were associated with nearly calm winds at 500 hPa. Months and phases of the MJO for the most extreme anomalies of PWV for each season agreed well with the seasonal anomalies. Furthermore, three independent reanalysis data sets generally agreed on MJO phases associated with largest positive and negative PWV anomalies. This agreement provides evidence for the first time that the MJO modulates the PWV content over the Atacama Desert. [ABSTRACT FROM AUTHOR]