The thunderstorms called Pyrocumulonimbus (PyroCb) are a spectacular atmospheric phenomenon induced by intense wildfires. These violent storms, created and boosted by the fire energy, can generate powerful updrafts rising above the jet aircraft cruising altitudes, thereby sending smoke and ice into the stratosphere – the second layer of the atmosphere (~12 – 50 km). As no precipitation is washing out the fine smoke particles at such altitudes, they can reside for many months in the stratosphere. Moreover, the absorption of solar radiation by the black carbon produced by combustion, heats the smoke plumes thereby propelling them upwards. The impact of PyroCb on the global stratosphere has been deemed marginal until the Canadian wildfire event in August 2017, which caused planetary-scale stratospheric perturbation comparable to that of moderate volcanic eruptions. However, no one could have imagined how much stronger the stratospheric effects of wildfires can actually be until the Australian wildfire season 2019/20.
The Australian “Black Summer” was marked by exceptionally strong PyroCb activity in the south-east of the continent with 5.8 million hectares of forest burnt. The strongest PyroCb outbreak that occurred just on the New Year’s eve lofted a colossal cloud of smoke-ice mixture to 15 km altitude. Already two weeks after, it became clear from satellite observations that the magnitude of stratospheric perturbation from this single PyroCb event had tripled that of the record-breaking 2017 Canadian wildfires.
While the bush fires were still raging in Australia, we started to chase their smoke plumes in the stratosphere as they dispersed all across the Southern hemisphere. We looked at the data from several satellite-based atmospheric monitoring instruments, including the space-borne laser radar (lidar) CALIOP, which produces very high resolution slices of aerosols plumes. The CALIOP data were showing an unusual pattern: a thousand kilometer-large and 7 km-tall bubble of smoke that appeared to remain surprisingly compact whilst rising through the stratosphere.
The biggest surprise came in when we started looking at the meteorological data produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The ECMWF forecasting system determines the state of the atmosphere and its future evolution based on very high resolution numerical model supplied by a wealth of atmospheric measurements using ground-based, airborne and satellite sensors. The ECMWF analyzed wind fields were revealing an organized anticyclonic vortex that encompassed the rising smoke bubble. This vortex, created by the localized heating of the smoke cloud, kept the bubble confined by strong winds during its entire life . The whirling bubble contained not only the smoke particles but also several megatons of water and carbonaceous gases. Ozone concentrations were found to be very low inside the bubble, thereby creating a synoptic-scale ozone hole.
By mid-March, we were all confined in our homes because of the Covid lockdown, but we were still closely following the evolution of the confined vortex bubble, getting amazed that it was still rising and guessing how high it will climb. Among our team, the vortex got the nickname Koobor after the first koala who climbed to the top of the trees to escape his persecutors, as Australian aboriginal legend says. Only this time, the spirit of Koobor the koala has climbed all the way to the stratosphere to escape the fires.
The Koobor vortex survived three months, during which it traveled over 66,000 km and climbed up to a whopping 35 km altitude, as concluded by our study. The last time the aerosol particles were seen this high was after the major eruption of Mt Pinatubo in 1991. With that, the phenomenon of the smoke-charged vortex has never been observed in the atmosphere and is exclusive to the PyroCb. It is clear that the solar heating of black carbon was essential to maintain the vortex and provide the lifting force, however it is an awesome theoretical challenge to understand how it has self-organized and survived for such a long time despite external perturbations in the atmosphere. Another surprising discovery was that the Koobor vortex had a family – two baby vortices that developed from smaller smoke clouds and survived for many weeks.
The ability of smoke clouds to self-organize into confined structures lofting themselves to high altitudes acts to extend the residence time of solar-absorbing particles in the atmosphere, thereby prolonging their effects on Earth's climate. Using a radiative transfer model constrained with satellite observations, we showed that the planetary-scale blocking of sunlight by the Australian smoke was of the same order magnitude as the cumulative effect of moderate volcanic eruptions over the last three decades. Obviously, the atmospheric imprint of the wildfires that increase in frequency and severity should be carefully considered in the climate change studies.
Yet another vivid evidence of the increasing power of wildfires was not long to wait. Only eight months after the Australian event, the devastating wildfires have hit western U.S. producing the strongest PyroCb storms ever observed in U.S. and turning the Californian sky red.