DECEMBER 2001: Global Model Analysis of TOMS and In-situ Observations of Tropical Troposphric Ozone

The figure shows the ozone column enhancement in the GEOS-CHEM model from lightning emissions, as determined by difference from a simulation without lightning emissions. Distributions of tropical tropospheric ozone columns (TTOCs) from TOMS , the GEOS-CHEM model, and sondes exhibit a persistent wave-1 amplitude of 13-17 DU with a zonal maximum in TTOCs over the Atlantic and a minimum over the Pacific throughout the year. Upper tropospheric ozone production from lightning NOx, combined with the large-scale Walker circulation, largely explains the wave-1 pattern in the model. We find that the influence of lightning on TTOCs is strongest over subsiding regions such as the tropical Atlantic and weakest over upwelling regions such as the tropical Pacific. The combination of upper tropospheric ozone production from lightning NOx and strong subsidence over the southern tropical Atlantic as part of the Walker circulation also contributes to the "tropical Atlantic paradox" [Thompson et al., 2000], i.e. the north-south gradient reversal in ozone over the tropical Atlantic during the northern African biomass burning season in December-February as observed by TTOCs and ozonesondes from ship cruises. This work was led by Randall Martin. A correction for the TOMS retrieval efficiency and a full account is given in Martin et al. [2001].