Abstract
Abstract. We show that a fire plume injected into the lower stratosphere at high northern latitudes during the Canadian wildfire event in August 2017 partly reached the tropics. The transport to the tropics was mediated by the anticyclonic flow of the Asian monsoon circulation. The fire plume reached the Asian monsoon area in late August/early September, when the Asian monsoon anticyclone (AMA) was still in place. While there is no evidence of mixing into the center of the AMA, we show that a substantial part of the fire plume is entrained into the anticyclonic flow at the AMA edge and is transported from the extratropics to the tropics, and possibly the Southern Hemisphere particularly following the north–south flow on the eastern side of the AMA. In the tropics the fire plume is lifted by ∼5 km in 7 months. Inside the AMA we find evidence of the Asian tropopause aerosol layer (ATAL) in August, doubling background aerosol conditions with a calculated top of the atmosphere shortwave radiative forcing of −0.05 W m−2. The regional climate impact of the fire signal in the wider Asian monsoon area in September exceeds the impact of the ATAL by a factor of 2–4 and compares to that of a plume coming from an advected moderate volcanic eruption. The stratospheric, trans-continental transport of this plume to the tropics and the related regional climate impact point to the importance of long-range dynamical interconnections of pollution sources.
Highlights
During the 2017 summer season, historically severe wildfires appeared in western Canada and in the northwestern United States
The general impact on the radiative balance and climate of aerosol plumes from wildfires in the lowermost stratosphere has been discussed in Ditas et al (2018); they found that the global average direct radiative forcing at the top of the atmosphere (TOA) of biomass burning aerosols from wildfires may reach −0.20 W m−2
The circulation of the Asian monsoon anticyclone (AMA) has effectively transported Canadian fire plume air from northern latitudes (∼ 40◦ N) around its eastern flank into the tropics, where the air has further been lifted with the ascending branch of the Brewer–Dobson circulation (BDC)
Summary
During the 2017 summer season, historically severe wildfires appeared in western Canada and in the northwestern United States. During the year of the Sarychev eruption, which occurred at the beginning of the Asian monsoon season, a negative anomaly inside the AMA has been shown, while for all other years, there is a positive aerosol anomaly (ATAL) (Vernier et al, 2011). Both observations (for the Sarychev and Nabro eruption) support a transport barrier that separates “inside” and “outside”.
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