A4 Extreme Thermal Events

Topic

Extreme thermal events encompass two opposing phenomena that share the same dynamic origin within large-scale circulation: heatwaves and cold waves caused by polar air outbreaks. Heatwaves are periods of abnormally high temperatures lasting from days to weeks; among all extreme thermal events, they represent the meteorological hazard responsible for the highest direct mortality in many countries.

The primary dynamic mechanism driving heatwaves is subsidence occurring in the descending limb of a persistent high-pressure system—such as an atmospheric block or a stationary high-pressure ridge. This subsidence causes air to descend and compress, resulting in adiabatic warming that raises the potential temperature of the lower atmosphere. The rise in potential temperature dries out the ground beneath the persistent high-pressure system; dry soil, in turn, reduces evapotranspiration—a process that normally cools the air near the surface. This reduction in evapotranspiration leaves more energy available to further warm the lower atmosphere, reinforcing the very potential temperature that dried the soil initially and thereby closing a positive feedback loop between the soil and temperature. Furthermore, subsidence inhibits convection—which might otherwise cool the atmosphere—thereby eliminating the natural pathway for the accumulated heat to escape.

Within cities, this warming of the lower atmosphere is amplified by the urban heat island effect: cities experience reduced evapotranspiration because pavement seals the ground and prevents water evaporation; they absorb more solar radiation due to low albedo and "urban canyons" that trap radiation between buildings; and they store more heat in construction materials than in the vegetation or natural soil of the surrounding countryside. Due to the combination of these three factors, nighttime temperatures within the city can be five to eight degrees higher than in the surrounding countryside—a difference significant enough to prevent the human body from recovering thermally at night, a process that normally relies on temperatures dropping after dark.

In the context of climate change, heatwaves not only perpetuate this mechanism of subsidence and soil-temperature feedback but also increase in frequency, duration, and intensity across all regions, thereby magnifying the impact of heatwaves within the broader spectrum of extreme thermal events.

The second type of extreme thermal event—cold waves caused by polar air incursions—follows a different logic but stems from the same type of large-scale circulation disturbance: it occurs when the stratospheric polar vortex weakens, or when amplified Rossby waves manage to draw Arctic air and push it toward mid-latitudes. As this Arctic air descends to mid-latitudes, it arrives with a potential temperature ten to twenty degrees lower than the local climatological norm for that region; this stark difference is what causes destructive frosts in areas where such cold is unusual.

Paradoxically, what makes these Arctic air incursions more frequent is global warming itself: Arctic amplification causes the pole to warm two to four times faster than the global average, and this accelerated polar warming reduces the meridional temperature gradient between the Arctic and mid-latitudes. When the meridional temperature gradient narrows, the stratospheric polar vortex loses some of the strength that normally keeps it confined and weakens more easily; it is precisely this weakness that allows Arctic air to escape more frequently into mid-latitudes. Thus, extreme thermal events turn out to be two sides of the same climate system: heatwaves—driven by subsidence, soil-temperature feedback, and urban amplification—push temperatures well above normal and represent the leading cause of direct mortality among weather-related hazards; meanwhile, cold waves caused by polar air outbreaks—sustained by a weakening stratospheric polar vortex and exacerbated by Arctic amplification—bring destructive frosts to regions ill-prepared for such cold. Both extremes—heat and cold—demonstrate that extreme thermal events depend less on the planet's average energy content than on how that energy is distributed among the surface, the lower atmosphere, and the polar vortex at any given time.

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