The School of Earth and Atmospheric Sciences Presents Dr. Lucas Vargas Zeppetello, Harvard University & University of California Berkeley 

The Physics of Heat Waves : Simple Models to Understand Temperature Variability in a Warming World

We analyze observations and develop a hierarchy of models to understand heat waves—long-lived, high temperature anomalies—and extremely high daily temperatures during summertime in the continental extratropics. Throughout the extratropics, heatwaves are disproportionately clustered in periods with drier than normal soils.

Furthermore, in many locations the temperature skewness switches from negative on daily time scales to positive on monthly time scales (or shifts from positive on daily time scales to higher positive values on monthly time scales) in ways that cannot be explained by averaging alone. These observations motivate a hierarchy of models of the surface energy and moisture budgets that we use to illuminate the physics responsible for daily and monthly averaged temperature variability. Shortwave radiation fluctuations drive much of the variance and the negative skewness found in daily temperature observations. 

On longer time scales, precipitation-induced soil moisture anomalies are important for temperature variability and account for the shift toward positive skewness in monthly averaged temperature. Our results demonstrate that long-lived heat waves are due to (i) the residence time of soil moisture anomalies and (ii) a nonlinear feedback between temperature and evapotranspiration via the impact of temperature on vapor pressure deficit. For most climates, these two processes give rise to infrequent, long-lived heat waves in response to randomly distributed precipitation forcing. Combined with our results concerning high-frequency variability, extremely hot days are seen to be state-independent filigree driven by shortwave variability acting on top of longer-lived, moisture-driven heat waves.