Why Does Atmospheric Radiative Heating Weaken Midlatitude Cyclones?

Recent work has indicated that atmospheric radiative heating reduces the kinetic energy of large‐scale eddies in the midlatitudes. However, a physical mechanism that connects radiation to the midlatitude eddy kinetic energy is still uncertain. Using a high‐resolution general circulation model we perform an experiment in which the radiative cooling profile at each model time step is overwritten with the climatological mean, computed from a control simulation. This approach separates the mean and transient effects of radiative heating on the extratropical circulation. We find that, when radiative heating is fixed, the globally‐averaged eddy kinetic energy is enhanced by ∼6%. We show that thermal radiation dampens temperature anomalies near the surface and tropopause in low‐pressure systems, destroying eddy available potential energy and eddy kinetic energy. We identify this as a possible mechanism by which atmospheric radiative heating weakens midlatitude cyclones. Plain Language Summary The atmosphere can be differentially heated through the absorption of visible and infrared radiation by water vapor and clouds. This process of radiative heating is thought to promote the formation of tropical cyclones, but its impact on midlatitude cyclones—storms characterized by the passage of warm and cold fronts—is less clear. Here we show that, unlike tropical cyclones, radiative heating weakens midlatitude storms. We argue that radiative cooling in the warm sector, and radiative heating in the cold sector of midlatitude cyclones, reduces the temperature gradient which is critical for the development of these weather systems. This study underscores an essential difference between tropical and midlatitude storms: in the tropics, temperature gradients are small, but at higher latitudes, temperature gradients are the drivers of storm formation. Key Points When radiative interactions are suppressed, the global‐mean eddy kinetic energy increases by ∼6% Radiative cooling coincident with positive temperature anomalies destroys eddy available potential energy and eddy kinetic energy The energetic perspective provides a useful way to understand the effect of radiative heating on the general circulation