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.