Abstract

High temperature extremes accompanied by drought have led to serious ramifications for environmental and socio-economic systems. Thus, improving the predictability of heat-wave events is a high priority. One key to achieving this is to better understand land-atmosphere interactions. Recent studies have documented a hypersensitive regime in the soil moisture-temperature relationship when soil dries below a critical low threshold, air temperatures increase at a greater rate as soil moisture declines. In this study, we explore the mechanisms linking low soil moisture to high air temperatures. From in-situ observations, we confirm that the hypersensitive regime acts throughout the chain of energy processes from land to atmosphere. A simple energy-balance model indicates that the cause of the hypersensitive regime is the dramatic drop in evaporative cooling that occurs when soil moisture dries to the permanent wilting point, below which latent heat flux almost ceases. Precisely how a model represents the relationship between evapotranspiration and soil moisture is found to be essential to describe the occurrence of hypersensitive regime. Thus, we advocate that climate models should ensure a realistic representation of land-atmosphere interactions to obtain reliable forecasts of extremes and climate projections, aiding the assessment of climate vulnerability and adaptation.