Reinterpreting Precipitation Stable Water Isotope Variability in the Andean Western Cordillera Due To Sub-Seasonal Moisture Source Changes and Sub-Cloud Evaporation

Precipitation in the arid Central Andean Western Cordillera is strongly sensitive to large-scale atmospheric circulation patterns which transport moisture from different source regions. As a result, the stable water isotopes of precipitation in this region have been difficult to interpret. We analyzed daily rainfall in the southern Peruvian city of Arequipa for water stable isotopes during the 2019 monsoon season to study sub-seasonal precipitation dynamics. At 2,300 masl elevation, most rain was sourced from easterly moisture transported over the Altiplano, and was isotopically enriched (delta O-18 = -5.2 parts per thousand), likely due to sub-cloud evaporation. This was contrary to literature expectations that enriched isotopic values indicate Pacific moisture. During a 2-week period of northerly moisture transport and intense rainfall, we observed rain stable water isotopes shifted to depleted values (delta O-18 = -10.3 parts per thousand). This study provides new context for interpreting time-varying and elevation-varying moisture source influence in the Western Cordillera.