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

Record-breaking heatwaves are increasing globally, necessitating understanding of regional drivers. In late April 2023, an exceptional spring heatwave struck the western Mediterranean, setting new temperature records in Portugal and Spain (reaching 36.9 °C and 38.8 °C respectively), and exceeding 40°C in parts of Morocco and Algeria. This event coincided with a severe, multi-year drought; Spanish reservoirs were 50% below average, Moroccan dam storage at 33%, and Tunisia's largest reservoir at only 16% capacity. This drought significantly impacted agriculture, resulting in over €636 million in Spanish government aid to farmers. The study hypothesizes that pre-existing drought amplified the heatwave by reducing evaporation and increasing surface sensible heat flux, creating a compound hazard. While drought's influence on summer heatwaves in temperate regions is documented, its role in spring heatwaves in Mediterranean climates, where soil moisture is usually higher, remains less understood. This study aims to (1) identify surface and synoptic characteristics of the heatwave; (2) quantify drought's influence on energy balance and air temperature; and (3) infer the likelihood of such an event under different soil moisture conditions.

Previous research has shown the impact of drought on heatwave intensification, particularly for events like the 2003, 2018, and 2019 European heatwaves and the 2010 Russian heatwave. Summer heatwaves in temperate regions are strongly influenced by soil moisture anomalies. However, in Mediterranean climates, summer heatwaves are often triggered by heat advection, with less influence from soil moisture due to naturally dry conditions. The role of land-atmosphere feedbacks in spring, when soil moisture is higher and droughts cause larger negative anomalies, is less clear. This study addresses this gap by examining the April 2023 heatwave.

The study analyzed observational data including: daily maximum temperature (Tx) time series from the Spanish meteorological service (AEMET); Standardized Precipitation Evapotranspiration Index (SPEI) data from the SPEI Spain Drought monitor; ERA5-Land reanalysis data for Tx, soil moisture, and other variables; and ERA5 reanalysis data for atmospheric variables. The soil moisture-temperature coupling metric (π) from Miralles et al. (2012) was used to quantify the influence of soil moisture on temperature. This metric combines temperature anomalies with an energy term representing the impact of soil moisture on energy balance partitioning. The study divided the heatwave into three phases: build-up, peak, and demise. A flow analogues technique was applied, comparing atmospheric patterns during the heatwave peak (26-28 April) to similar patterns in the past under both wet and dry soil moisture conditions. The difference in temperatures between dry and wet analogues was analyzed. Finally, a Generalized Extreme Value (GEV) distribution was used to assess the probability and magnitude of the observed maximum regional Tx under both wet and dry conditions. Sensitivity analyses were performed to check the robustness of the results with respect to time window size, seasonal bias and domain size.

The analysis revealed that a high-pressure system (subtropical ridge) and extremely low soil moisture levels contributed to the record-breaking temperatures. The Córdoba, Spain, maximum temperature reached 38.8 °C on April 27th, 4.8 °C above the previous record. The soil moisture-temperature coupling metric (π) showed a strong positive correlation during the heatwave peak, indicating that dry soil conditions significantly increased surface sensible heat flux, leading to higher temperatures. Using the flow analogue technique, the study found that similar atmospheric conditions in the past led to significantly higher temperatures when preceded by dry soil conditions compared to wet conditions. Specifically, the difference in temperature exceeded 1 °C over most of the Iberian Peninsula, reaching up to 3 °C near Córdoba. The extreme regional maximum Tx during the peak was 4.53 times more likely under dry soil conditions, and the maximum temperature would have been 2.19 °C lower if soils had been wet. These findings indicate that antecedent drought substantially amplified the heatwave. While the analogue results underestimate the observed peak temperature by 2.5 °C, the concurrent development of marine heatwaves in the eastern Atlantic and Mediterranean likely further contributed to the heatwave's intensity.

The findings highlight the significant role of antecedent drought in amplifying the April 2023 heatwave. While a subtropical ridge drove the warm air northward, the most extreme temperatures were observed in regions with the most severe drought and highest soil moisture-temperature coupling. The study demonstrates that semi-arid regions in spring can exhibit land-atmosphere coupling similar to temperate regions in summer, highlighting the importance of soil moisture as a predictor of spring heatwave risk in these regions. This methodology offers a novel approach for quantifying drought-induced temperature amplification, offering a less computationally expensive and potentially more robust alternative to climate model simulations. The results suggest that soil moisture could be an important variable for subseasonal forecasting of heatwaves in Mediterranean and other similar climate regions.

This study conclusively demonstrates the critical contribution of antecedent drought to the exceptional intensity of the April 2023 heatwave in the western Mediterranean. The combination of a subtropical ridge and severe soil moisture deficits led to record-breaking temperatures, particularly in the southern Iberian Peninsula. The methodology employed, combining flow analogues and extreme value analysis, provides a valuable tool for quantifying the impact of land-atmosphere interactions on heatwaves. Further research should investigate the role of marine heatwaves and refine subseasonal heatwave forecasting using soil moisture as a key predictor. The projected increase in drought conditions in the region necessitates focused mitigation and adaptation strategies.

The study's analogue approach relies on historical data, which might not fully capture the range of possible future atmospheric patterns. The analysis focuses on the Iberian Peninsula and surrounding areas, limiting generalizability to other regions. While the study accounts for the influence of soil moisture and atmospheric circulation, other factors could have contributed to the record-breaking temperatures, such as land use changes and urban heat island effects. The model slightly underestimates the observed peak temperature, suggesting that additional factors beyond soil moisture and atmospheric circulation played a role.