Cold air incursions, [delta]18O variability, and monsoon dynamics associated with snow days at Quelccaya Ice Cap, Peru

Quelccaya Ice Cap in the Andes of Peru contains an annually resolved [delta]18O record covering the past 1800years; yet atmospheric dynamics associated with snow deposition and [delta]18O variability at this site are poorly understood. Here we make use of 10years of snow pit and short core [delta]18O data and hourly snow-height measurements obtained by an automated weather station deployed at the ice cap's summit to analyze linkages between snowfall, [delta]18O, and the South American summer monsoon (SASM). Snow accumulation peaks in December and is negative May-September. Snow [delta]18O values decrease gradually through austral summer from about -17 to -24[per thousand]. Surface snow [delta]18O is altered after deposition during austral winter from about -24 to -15[per thousand]. More than 70% of the total snow accumulation is tied to convection along the leading edge of cold air incursions of midlatitude air advected equatorward from southern South America. Snowfall amplitude at Quelccaya Ice Cap varies systematically with regional precipitation, atmospheric dynamics, midtroposphere humidity, and water vapor [delta]D. Strongest snowfall gains correspond with positive precipitation anomalies over the western Amazon Basin, increased humidity, and lowered water vapor [delta]D values, consistent with the "amount effect." We discuss ventilation of the monsoon, modulated by midlatitude cold air advection, as potentially diagnostic of the relationship between SASM dynamics and Quelccaya snowfall. Results will serve as a basis for development of a comprehensive isotopic forward model to reconstruct past monsoon dynamics using the ice core [delta]18O record. Key Points Quelccaya snowfall linked to South American monsoon activity Most of Quelccaya snowfall is triggered by cold air incursions of midlatitude air Quelccaya seasonal [delta]18O records summer precipitation and winter alteration at surface