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The O-18 signal in ice cores from the Quelccaya Ice Cap (QIC), Peru, corresponds with and has been used to reconstruct Nino region sea surface temperatures (SSTs), but the physical mechanisms that tie El Nino-Southern Oscillation (ENSO)-related equatorial Pacific SSTs to snow O-18 at 5,680 m in the Andes have not been fully established. We use a proxy system model to simulate how QIC snow O-18 varies by ENSO phase. The model accurately simulates higher and lower O-18 values during El Nino and La Nina, 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 O-18 values, and (iii) regional temperature. Most (more than two thirds) of the ENSO-related variability in the QIC O-18 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 O-18 values are affected by the strength of upstream convection associated with the SASM. Since convection over the Amazon is enhanced during La Nina, the water vapor over the western Amazon Basinwhich serves as moisture source for snowfall on QICis characterized by more negative O-18 values. In the forward model, higher initial water vapor -values during El Nino yield higher snow O-18 at the QIC. Our results clarify that the ENSO-related isotope signal on Quelccaya should not be interpreted as a simple temperature response.

Plain Language Summary The Quelccaya Ice Cap in the Andes Mountains of Peru is retreating because of global warming, and ice cores from Quelccaya are some of the best records that we have for climate from the last 2,000years. Quelccaya is the world's largest tropical ice cap, and as such it is an important regional water resource. The climate record from Quelccaya's ice core chemistry has long been tied to El Nino activity. However, it is not obvious how ocean water temperature in the equatorial Pacific alters the chemistry of snow that falls at 5,680m above sea level in the Andes Mountains. We used 15years of weather station data from the summit of Quelccaya, along with a climate model, to show that it is the intensity of rainfall over the Amazon rainforest that changes the chemistry of snow at Quelccaya. For example, the rainfall over the Amazon is more intense during La Nina, causing the water vapor over the western Amazon to be lighter. This lighter version of water vapor is then transported up to the height of Quelccaya and makes snow with a lower chemical signature.