First steps toward development of a stable isotope forward model for tropical ice cores: cold air incursions and snow days at Quelccaya Ice Cap, Peru

Abstract:

We are working towards a forward-model reconstruction of the South American Summer Monsoon (SASM) for the last millennium from the Quelccaya Ice Cap (QIC) d¹⁸O record. QIC receives precipitation almost exclusively during the SASM season. Initial efforts focus on dynamics that yield precipitation at this receding tropical ice cap, and how they relate to the hydrogen and oxygen stable isotopes.

We present over a decade of daily snow-height change observations from the summit of QIC. Accumulation of snow (~2 m yr^-1) at the summit occurs October through April, peaking in December. Net monthly snow-height change is negative May through September, though positive snow height change days do occur throughout the year.

Snow height change time-series are used to develop d¹⁸O age-models for annual snow collected in vertical profiles near the summit of QIC since 2003. Snow d¹⁸O decreases during austral summer from about -17 to -24 per mil VSMOW. Post-depositional alteration of late summer snow during austral winter elevates d¹⁸O from about -24 to about -15 per mil VSMOW.

Timing of 90^thpercentile positive snow-height change events at QIC corresponds with regional precipitation and outgoing longwave radiation (OLR) anomalies that are dynamically triggered by cold air incursions propagating from the midlatitudes east of the Andes into the Amazon Basin. Precipitation and OLR anomalies migrate northwest in about 2-3 days from near Rio de la Plata to central Peru. The convective anomalies are the result of southerly horizontal wind anomalies in the lower troposphere that advect cold extratropical air equatorward.

Composite analysis of satellite measurements shows that cold air incursions are associated with negative water vapor dD (~ -40 per mil) anomalies at QIC. We expect that snow stable isotope values from QIC are thus not only records of the deep overturning component of the monsoon circulation but also of synoptic scale monsoon disturbances. Cold air incursions into the South American subtropics account for positive snow-height change anomalies and negative stable isotope anomalies at QIC.