Why Wildfire Smoke Drives One-Way Swirling
Several years ago, atmospheric scientists observed a surprising new phenomenon arising from summer wildfires. Smoke from the fires was injected into the stratosphere and coalesced into blobs, which were heated by sunlight and driven upward in swirling airflows. But contrary to predictions for heat-driven vortices, these swirls only rotated in one direction. Researchers now explain why, using a model that includes heating effects and wind shear [1, 2]. Their findings, presented at the American Meteorological Society’s Annual Meeting last month, offer insights into the broader atmospheric impacts of wildfires.
The smoke-filled vortices have been detected in satellite imagery during several wildfire outbreaks—notably Canadian fires in 2017 and Australian fires in 2020. The swirling motion appears to originate in the stratosphere, some 15 km above the ground, driven by sunlight being absorbed by smoke particles and heating them up. The vortices carry these particles and associated gases to higher-than-expected altitudes (up to 35 km), where they may significantly alter the chemical composition of the upper atmosphere.
Heat-driven vortices are themselves not a surprise. They naturally form in a rotating fluid with a localized heat source. Columns of fluid rise and fall in response to this heating, and this motion stretches and narrows the columns, causing them to rotate faster. In the atmosphere the direction of the rotation depends on how the heating varies vertically and on which way the Coriolis force is oriented. For a stationary heat source, the heating decreases above the source, and this negative gradient produces an “anticyclone” vortex (clockwise flow in the Northern Hemisphere). At the same time, a positive gradient below the source produces an opposite spinning “cyclone” vortex (counterclockwise).
This cyclone–anticyclone pairing, or “dipole vortex,” was expected for the case of wildfires, but all the observed smoke-filled vortices have been anticyclones. To explain these single-direction vortices, Kasturi Shah and Peter Haynes, fluid dynamics researchers at the University of Cambridge in the UK, initially focused on the fact that the heat source in this case is not stationary—the heat from the sunlight-absorbing smoke particles causes them to move upward (self-lofting). In 2024, they showed with fluid dynamics simulations that an upward-moving heat source produces an asymmetric dipole vortex [1]. The sharper heating gradient at the top of the rising source drives a strong anticyclone, while a smoother gradient beneath the source creates a relatively weak cyclone that trails below. This asymmetry, which was also found by a separate team [3], is key to explaining the smoke-driven dynamics, but the question remained: Why aren’t weaker, lower-level cyclones observed in nature?
Shah and Haynes have now filled in the last missing piece: Summertime westward winds provide a vertical shear that selects which vortex survives. The team’s modeling shows that anticyclones from upward-moving sources are fundamentally more robust to vertical shear than the corresponding cyclones. As the researchers increased the shear rate in their simulations, they found that the lower-level cyclone deformed, leaving just the anticyclone.
In addition to the shear rate, the researchers investigated the heating rate, as dictated by the density of smoke. A sufficiently strong heating rate establishes vertical velocities quickly enough for a smoke-filled blob to self-loft. Too weak, however, and the entire structure is likely to be torn apart before vortices can develop. Only strong wildfires that inject sufficient heat-producing smoke into the stratosphere can generate long-lived vortices. “For stratospheric shear rates and heating rates of wildfire smoke in the stratosphere, only the anticyclone survives,” Haynes explains.
Noboru Nakamura, a fluid dynamics researcher at the University of Chicago, says that the work by Shah and Haynes “meticulously lays out the constraints” for a rising heat source to preferentially reinforce anticyclonic vorticity at the source’s leading edge. The theory is informed entirely by observable elements in a combination that’s specific to the stratosphere’s environment, Nakamura says, making this problem is “a fascinating test bed for meshing theory, modeling, and observation.”
Incorporating wildfire-driven dynamics in large-scale climate models—by making, for example, local heating rates proportional to the amount of wildfire smoke—will improve representations of how these vortices rise and strengthen. This broader picture, in turn, will help describe how long smoke particles and their trapped gases reside in the atmosphere, where they can have an impact on the transport of solar radiation, as well as on the chemical makeup in the stratosphere.
–Rachel Berkowitz
Rachel Berkowitz is a Corresponding Editor for Physics Magazine based in Vancouver, Canada.
References
- K. Shah and P. H. Haynes, “How heating tracers drive self-lofting long-lived stratospheric anticyclones: Simple dynamical models,” Weather Clim. Dynam. 5, 559 (2024).
- K. Shah and P. Haynes, “Dynamics of smoke-filled stratospheric vortices: An example of wildfire-induced circulation patterns,” AMS Ann. Meet. abstract (2026).
- A. Podglajen et al., “Dynamics of diabatically forced anticyclonic plumes in the stratosphere,” Q. J. R. Meteorol. Soc. 150, 1538 (2024).