Unprecedented 2023 Winter Heatwave in the Andes

In August 2023, South America experienced one of the most extreme heatwave events ever recorded, marking the warmest start to August in 117 years with temperature anomalies 10–20°C above the seasonal average. The heatwave impacted parts of Chile, northern Argentina, and southwestern Brazil, with temperatures in the Chilean Andes surpassing 38°C. Remote sensing images revealed widespread snowmelt across the Andes, and observations also suggested significant impacts on hydrological patterns, including spikes in winter runoff, affecting downstream water availability. Snow and glaciers in the Andes are essential reservoirs that sustain water supplies for millions of people. Winter plays a pivotal role in snowpack formation, which is crucial for the long-term stability of these resources. Extreme heatwave events during winter disrupt these processes, accelerating glacier retreat and snowmelt. This not only threatens water availability for agriculture, hydropower, and human consumption but also alters critical hydrological and ecological systems.

This study performs the first dedicated attribution on the 2023 unprecedented winter heatwave in the Andes, with a novel multi-method approach, combining 1) circulation analogue method, 2) statistical attribution method, and 3) physical-based storyline approach through a set of convective-permitting scale (4-km resolution) regional model simulations (CPRCM) over the Andes, to unpack the roles played by the blocking anticyclone, developing El Niño in the Pacific Ocean that year, and anthropogenic greenhouse gas emissions. The study leverages existing multiple global model simulations from Coupled Model Intercomparison Project Phase 6, a set of model simulations from the Large Ensemble Community Project, and the CPRCM simulations run in-house. Here we present initial findings from this attribution study on the attributed contributions from the different drivers to the spatial extent, intensity, and duration of the heatwave, as well as results on the changing frequency of occurrences between the current and pre-industrial climates using the large ensembles.

As global temperatures continue to rise, extreme winter heatwaves like this are projected to become more frequent, with profound consequences for snowpack dynamics and glacier stability in the Andes. This extraordinary winter heatwave serves as a stark reminder of the accelerating effects of climate change and the urgent need for adaptive strategies to protect critical Andean water systems.