Unseasonal atmospheric river drives anomalous glacier accumulation in the ablation season of the subtropical Andes

Abstract. Climate change is associated with changes in the frequency and intensity of extreme weather events. These changes are impacting the mass balance of Andean glaciers, a phenomenon that requires further detailed investigation. Among these events, atmospheric rivers (ARs) play a significant role in influencing glacier mass balance, potentially leading to either accumulation or melting events. To assess the impact of ARs on Andean glaciers, we analysed an unseasonal event that occurred at the end of January 2021, marked by extreme rainfall, landslides, and flash floods in the lowlands, during the typically dry summer period. Satellite imagery and meteorological observations in the glaciated Maipo River basin and its Olivares River sub-basin (33° S) enabled the characterisation of this event and its basin-scale impacts. Moreover, a glacier mass balance model allows us to quantify the effects on the Olivares Alfa Glacier (4284 to 4988 m a.s.l.) over the context of the preceding six hydrological years. The significant water vapour transport by the AR led to substantial snow accumulation on the Maipo River glaciers, confirmed by the post-event snowline observed at 2463 m a.s.l. In the Olivares River sub-basin, the 0 °C isotherm dropped during the event to an elevation of 3250 m a.s.l., below the frontal zone of all glaciers in this sub-basin. The mass balance model for the Olivares Alfa Glacier during the dry 2020/21 hydrological year showed a trend toward negative values at the beginning of the ablation season, aligning with previous years and the prevailing mega-drought conditions. However, the AR event offset this trend, bringing the mass balance closer to equilibrium. This demonstrates that an unseasonal accumulation event can significantly counteract the broader seasonal trends affecting subtropical Andean glaciers. Our study sheds light on the impacts of extreme and unseasonal snow accumulation events on glacier mass balance in the high Andes, particularly those associated with ARs, a synoptic feature projected to become more common in a warming climate.