Tropical upper tropospheric trends in ozone and carbon monoxide (2005–2020): observational and model results

Abstract. We analyze tropical ozone (O₃) and carbon monoxide (CO) distributions in the upper troposphere (UT) and their temporal changes for 2005–2020 using Aura Microwave Limb Sounder (MLS) observations and chemistry climate models. The models are the Whole Atmosphere Community Climate Model (WACCM6) and two variants of the Community Atmosphere Model with Chemistry (CAM-chem), each variant using different anthropogenic emissions. Upper tropospheric trends and variability diagnostics are obtained from multiple linear regression analyses. We compare the model and MLS annual climatologies, focusing on 147 and 215 hPa pressure levels; climatological values generally fall within 10–20 % of each other, with both positive and negative differences for O₃, and with models generally underestimating observed CO. In the northern hemisphere tropics, we find significantly poorer model fits to the observed phasing of CO seasonal changes at 215 hPa than at 147 hPa. This discrepancy is much smaller for the comparison of modeled and Measurements of Pollution in the Troposphere (MOPITT) V9J CO columns. We also find that the sensitivity of UT CO to El Niño / Southern Oscillation (ENSO) is positive at all tropical longitudes, in contrast to the dipolar longitudinal structure that exists for UT O₃ ENSO sensitivity.

MLS O₃ has a zonal mean trend at 20° S–20° N of +0.39 ± 0.28 % yr^-1; CAM-chem and WACCM have similar trends, though the WACCM trend is somewhat smaller. Our analyses for specific latitude/longitude bins yield positive trends up to 1.4 % yr^-1 over Indonesia and East of that region, as well as over tropical Africa and the tropical Atlantic. We find broad similarities between the mapped MLS-derived UT O₃ trends and corresponding mapped trends of tropospheric column ozone. Positive tropical UT mapped O₃ trends are generally captured by the models, although in a more muted way. There is room for improvements in modeled tropical UT CO trends. Indeed, the MLS zonal mean CO trend is -0.25 ± 0.30 % yr^-1, whereas the corresponding modeled CO trends are near zero (0.0 ± 0.14 % yr^-1) when the anthropogenic emissions used in CAM-chem and WACCM are taken from Community Emissions Data System (CEDS) version 2. The non-CEDS version of CAM-chem yields CO UT trends of 0.22 ± 0.19 % yr^-1, in contrast to the negative MLS CO trends throughout the tropics. The negative MLS tropical UT CO trends for 2005–2020 agree with (but tend to be smaller in magnitude than) previously published total column CO trends. Decreasing CO emissions from anthropogenic and biomass burning sources have previously been suggested as the main causes of tropospheric CO decreases, although significant regional emission trend variations exist.