Evaluating the Contribution of Land-Atmosphere Coupling to Heat Extremes in CMIP5 Models

Land-atmosphere coupling can amplify heat extremes under declining soil moisture. Here we evaluate this coupling in 25 Coupled Model Intercomparison Project Phase 5 models using flux tower observations over Europe and North America. We compared heat extremes (2.5% of the hottest days of the year) and the evaporative fraction (EF; a measure of land surface dryness) on the day the heat extremes occurred. We found a negative relationship between the magnitude of heat extremes and EF in both models and observations in transitional regions, with the hottest temperatures occurring during the driest days, with a similar but less certain relationship in dry regions. Surprisingly, many models also showed an amplification of heat extremes by low EF in wet regions, a finding not supported by observations. Many models may therefore overamplify heat extremes over wet regions by overestimating the strength of land-atmosphere coupling, with consequences for future projections of heat extremes. Plain Language Summary Hot days are expected to become more frequent with climate change. Heat extremes can be exacerbated by dry conditions due to a lack of evaporative cooling. This has been shown to be an important mechanism for future amplification of heat extremes in several regions. We evaluated how well global climate models can simulate these interactions between hot and dry conditions under current climate conditions. We showed that many models overestimate this interaction in wet regions and consequently overamplify heat extremes. Our study points to an area of necessary model improvement to increase confidence in future projections of heat extremes.