Surface flux contributions to Mediterranean heatwaves: a new Lagrangian diagnostic

The increasing frequency of Mediterranean heatwaves is associated with widespread impacts on human health, agricultural productivity, and infrastructure. Previous studies have shown that large-scale circulation patterns, such as persistent ridges and atmospheric blocking, play a key role in triggering heatwaves, along with subsidence and warm-air advection. However, the intensity and persistence of these events depends not only on the advection of heat and moisture but also on the heat and moisture supplied by turbulent surface fluxes into the advected air mass. Sensible and latent heat fluxes modify air-mass temperature and humidity (and thus equivalent potential temperature) along transport pathways to the heatwave region. These flux contributions, and their relative importance for heatwave anomalies, remain uncertain.

In this study, the contribution of surface sensible and latent heat fluxes to near-surface moisture and temperature anomalies during heatwaves is quantified using a new Lagrangian framework that combines backward air-mass trajectories from the FLEXPART particle dispersion model with surface fluxes from ERA5 reanalysis data. Surface flux contributions to the moist static energy are estimated by coupling them with near-surface residence times of air parcels arriving in the heatwave region. The approach is first validated by showing that moist static energy at the heatwave location can be reproduced by the sum of the particle initial conditions (i.e., most static energy at trajectory termination points) and the surface flux contributions accumulated over the Lagrangian tracking period. Following this validation, surface flux contributions can be split into latent and sensible heat flux contributions and mapped geographically.

The method is then applied to two recent Mediterranean heatwaves to assess the relative roles of sensible and latent heat fluxes and to identify the dominant land and sea source regions. Overall, this framework provides a direct and physically consistent way to attribute the moist static energy associated with heatwaves to surface fluxes, offering new insights into the processes that build and maintain Mediterranean heatwaves.