Record-shattering 2023 Spring heatwave in western Mediterranean amplified by long-term drought

The western Mediterranean region experienced an exceptional and unprecedented early heatwave in April 2023. By shattering historical temperature records, especially in the Iberian Peninsula and northwestern Africa, this extreme offers a stark illustration of a drought-heatwave compound event. Here, we investigate the soil moisture-temperature interactions that underpinned this event, using the most up-to-date observations and a robust statistical analysis. Our results reveal that soil moisture deficit preconditions, concurring with a strong subtropical ridge as a synoptic driver, had a key contribution to the amplification and duration of this record-breaking heatwave. Specifically, we estimate that the most extreme temperature records would have been 4.53 times less likely and 2.19 °C lower had the soils been wet. These findings indicate that soil moisture content may be a crucial variable for seasonal forecasting of early HW in this region and in other Mediterranean climate regimes that are already suffering an increment in the frequency of compound drought-heatwave events.