Dynamical Drivers of Free-Tropospheric Ozone Increases Over Southeast Asia

Positive trends in tropical free-tropospheric (FT) ozone are frequently ascribed to emissions growth, but less is known about the effects of changing dynamics and the possible fingerprints of climate change. In an update of our previous work (Thompson et al., 2021; JGR; https://doi.org/10.1029/2021JD034691; “T21”), we re-examine Southern Hemisphere Additional Ozonesondes (SHADOZ) ozone trends over tropical Southeast Asia, one of the most convectively active regions on Earth, now with 25 years of ozone profile data from 1998-2022. T21 posited that consistent, early-year positive FT ozone trends at several tropical SHADOZ stations may be related to decreases in convective activity. Our 25-year analysis of Kuala Lumpur and Watukosek (Java) SHADOZ stations finds that the +5 to +15% (+2 to +6 ppbv) per decade FT ozone trends from ~February-May (Figure 1a) are coincident with large increases in satellite IR brightness temperatures (T_b; 1b) and outgoing longwave radiation (OLR; 1d; both are indicators of convective activity), and decreases in convective precipitation amount from NASA’s MERRA-2 reanalysis (1c). Trends in ozone and dynamical indicators are generally weak throughout the rest of the year. All of these results suggest that dynamical influences, i.e., decreases in the intensity and frequency of convection, are a primary driver of FT ozone buildup in the early months of the year over Southeast Asia, with waning convection suppressing the typical lofting and redistribution of low, near-surface ozone throughout the tropical FT. The decrease in convective precipitation amounts likely also enables the accumulation of biomass burning emissions, as AIRS satellite FT carbon monoxide trends (2002-2022) also show a small peak in ~March-May. Finally, these results demonstrate the need for monthly or seasonally-resolved analyses, as opposed to annual means, for the robust attribution of observed ozone trends.