Growing importance of soil moisture anomalies for prediction of summer heatwaves in the Western Mediterranean

Land surface and the planetary boundary layer are linked by the water and energy cycles, and the effects of soil water-air coupling modulate near-surface temperatures. In particular, late spring soil moisture anomalies may uncover predictability to the system, and then contribute to predictions of extreme events such as heatwaves at the subseasonal to seasonal scale. In this study, we use a data-driven seasonal forecast for summer heat waves in the Western Mediterranean, and a downstream explainable artificial intelligence helps us separate the individual contribution of the predictors and quantify and rank them in terms of their relative importance. Soil moisture emerges as one of the heat wave predictors, along with SST in the North Atlantic and the background global warming. Results show that soil largely contributes to heatwaves predictability when it is dry at the beginning of the season, otherwise its importance is limited. In addition, soil moisture contribution substantially increases from the beginning of the 1990s, when the local warming quickly arises and summer precipitation declines sharply. When atmospheric patterns are favorable for the advection of hot and dry air, conditions for persisting and more intense heatwaves are supported by an interacting land surface. With little water available for evaporation, the increased atmospheric evaporative demand may not be met, therefore the lack of latent cooling in the atmosphere enables more intense and persistent heatwaves.