The role of boundary‐layer friction in modulating midlatitude heatwaves

Heatwaves are among the most harmful extreme events, impacting human health, ecosystems, and infrastructure. Heatwaves are driven and modulated by a range of dynamical and thermodynamic factors, but their relative importance for the characteristics of these extremes, such as intensity, frequency, and location, for example, is not fully resolved. Dynamical models allow for a better understanding of the physical drivers of heatwaves, and idealised models in particular allow us to discriminate between the effects of different components of the climate system on heatwaves. One such factor is the boundary-layer friction associated with different surface types, which is known to influence the dynamics of the atmosphere. The geography and land cover of continental surfaces, as well as local characteristics in surface roughness, can influence the atmospheric circulation. In this study, we use an idealised general circulation model to conduct sensitivity experiments testing the role of changes in local and global boundary-layer friction for midlatitude heatwaves. The number of heatwave days and events is found to be larger for areas where the jet stream is strongly decelerated as a consequence of a local increase in boundary-layer friction. This effect strengthens when increasing the areas of modified friction. Changes in boundary-layer friction applied over areas with a geographical location and extent similar to those of continental areas of the Northern Hemisphere result in heatwave day differences of up to 20% between different locations at the same latitudes. These findings highlight how the extent, surface properties, and geographic position of continental and smaller land masses influence heatwave occurrence through their frictional effects on atmospheric dynamics.