The Unprecedented Late-Summer 2023 Heatwave in Southeastern South America: Attribution and future projection of similar events

In March 2023, southeastern South America (SESA) experienced a severe heatwave with its maximum intensity exceeding four standard deviations from the climatological mean. The timing of the occurrence was also unusual, as it occurred in the late summer. This study examines the contributing factors to the March 2023 SESA heatwave using a dynamical adjustment approach based on constructed atmospheric circulation analogs from the ERA5 reanalysis. Additionally, we assess changes in March heatwaves in the Coupled Model Intercomparison Project 6 (CMIP6) Shared Socioeconomic Pathways 3-7.0 climate simulations using the same method. The dynamical adjustment indicates that the largest contributors to the heatwave are circulation anomalies (on average 33%, 2.72°C) and thermodynamic effect (58%, 4.75°C), primarily linked to soil-temperature feedback. This result supports that extremely dry soil from the ongoing multi-year drought played a role in amplifying the heatwave intensity. The persistence of the circulation anomalies is also noticeable during the period. The contribution of the long-term temperature trend is 9% (0.78°C). In CMIP6 future simulations, the number of March heatwaves increases, but the relative frequency of March-2023-like dry-hot heatwaves decreases, largely due to projected increases in soil moisture. The contributions of the temperature trends and circulation anomalies are larger, while the thermodynamic effects related to soil-temperature feedback are reduced. The finding suggests that future March heatwaves are driven by increases in temperatures with reduced roles of soil moisture. However, uncertainty exists in future soil moisture projections, indicating the need for more understanding of changes in heatwaves in the region.