Tropical Dynamics Shaping Decadal Air Quality in Subtropical East Asia

Abstract Although research has reduced uncertainty in the range and magnitude of aerosol climate effects, it remains unclear to what extent decadal changes in dynamical patterns influence aerosols and their accumulation potential. The present study explored how leading dynamic conditions in the tropics affect aerosol interannual variability over subtropical East Asia each March, when the annual pollutant level is highest. We examined tropical convective activity, regardless of seasonality, and evaluated its time-lagged influence poleward on East Asian seasonal air quality the following year. Nonseasonal atmospheric dynamics in the tropical upper troposphere can be characterized as divergent circulation over the Maritime Continent, which alters planetary circulation to a zonal wave-like pattern. In the 21st century, years of increased convective activity over the Maritime Continent have been observed to strengthen the boreal Hadley cell and shift the East Asian westerly jet stream farther poleward, which clears the air over subtropical East Asia the following March. This precursor mechanism may be evaluated with greater confidence when only seasonal decomposition is considered. Moreover, decadal tendencies were found to influence the tropical leading control of air quality in East Asia. This monsoon-Hadley circulation led to narrower variability in air quality in March across the years, with less interference from decadal dynamics. Although mean-state constraints may indicate that climatological change over time modulates decadal changes in regional and synoptic pollutant levels, the aforementioned findings call into question conventional understandings of the role of aerosols in climate variability. Graphical Abstract It is explored how leading dynamic conditions in the tropics limit aerosol interannual variability over subtropical East Asia each March, when the annual pollutant level is highest. Years with stronger convective activity over the Maritime Continent, as evidenced by nonseasonal enhancements in divergent circulation in the upper troposphere, resulted in the boreal Hadley cell strengthening and the midlatitude westerly jet stream shifting poleward over the East Asian sector. The resulting increase to the lower-tropospheric northerly flow may have cleared pollutants from the air the following year. Meanwhile, stronger trade winds generated by stronger Pacific anticyclones supported the decadal nonseasonal tendency of stronger divergent circulation in the upper troposphere across the Asia–Australia sector. During decades without excessive interference from tropical dynamics, the leading adjustment of nonseasonal tropical dynamics was followed by better seasonal air conditions over subtropical East Asia. Conversely, when faced with increasing tropical interference, nonseasonal dynamical adjustments to air conditions were only moderate.