On the Interpretation of the ENSO Signal Embedded in the Stable Isotopic Composition of Quelccaya Ice Cap, Peru
The δ18O signal in ice cores from the Quelccaya Ice Cap (QIC), Peru, corresponds with and has been used to reconstruct Niño region sea surface temperatures (SSTs), but the physical mechanisms that tie El Niño-Southern Oscillation (ENSO)-related equatorial Pacific SSTs to snow δ18O at 5,680 m in the Andes have not been fully established. We use a proxy system model to simulate how QIC snow δ18O varies by ENSO phase. The model accurately simulates higher and lower δ18O values during El Niño and La Niña, respectively. We then explore the relative roles of ENSO forcing on different components of the forward model: (i) the seasonality and amount of snow gain and loss at the QIC, (ii) the initial water vapor δ18O values, and (iii) regional temperature. Most (more than two thirds) of the ENSO-related variability in the QIC δ18O can be accounted for by ENSO's influence on South American summer monsoon (SASM) activity and the resulting change in the initial water vapor isotopic composition. The initial water vapor δ18O values are affected by the strength of upstream convection associated with the SASM. Since convection over the Amazon is enhanced during La Niña, the water vapor over the western Amazon Basin—which serves as moisture source for snowfall on QIC—is characterized by more negative δ18O values. In the forward model, higher initial water vapor δ-values during El Niño yield higher snow δ18O at the QIC. Our results clarify that the ENSO-related isotope signal on Quelccaya should not be interpreted as a simple temperature response.