Seasonal, interannual and decadal variability of tropospheric ozone in the North Atlantic: comparison of UM-UKCA and remote sensing observations for 2005–2018
Authors: Russo Maria Rosa , Kerridge Brian John , Abraham Nathan Luke , Keeble James , Latter Barry Graham , Siddans Richard , Weber James , Griffiths Paul Thomas , Pyle John Adrian , Archibald Alexander Thomas 
Published: June 2023

Abstract. Tropospheric ozone is an important component of the Earth system as it can
affect both climate and air quality. In this work, we use observed
tropospheric column ozone derived from the Ozone Monitoring Instrument (OMI)
and Microwave Limb Sounder (MLS) OMI-MLS, in addition to OMI ozone retrieved
in discrete vertical layers, and compare it to tropospheric ozone from
UM-UKCA simulations (which utilize the Unified Model, UM, coupled to UK
Chemistry and Aerosol, UKCA). Our aim is to investigate recent changes
(2005–2018) in tropospheric ozone in the North Atlantic region,
specifically its seasonal, interannual and decadal variability, and to
understand what factors are driving such changes. The model exhibits a large
positive bias (greater than 5 DU or ∼ 50 %) in the tropical
upper troposphere: through sensitivity experiments, time series correlation,
and comparison with the Lightning Imaging Sensor and Optical Transient Detector lightning flash dataset, the model positive
bias in the tropics is attributed to shortcomings in the convection and
lightning parameterizations, which overestimate lightning flashes in the
tropics relative to mid-latitudes. Use of OMI data, for which vertical
averaging kernels and a priori information are available, suggests that the
model negative bias (6–10 DU or ∼ 20 %) at mid-latitudes,
relative to OMI-MLS tropospheric column, could be the result of vertical
sampling. Ozone in the North Atlantic peaks in spring and early summer, with
generally good agreement between the modelled and observed seasonal cycle.
Recent trends in tropospheric ozone were investigated: whilst both
observational datasets indicate positive trends of ∼ 5 % and
∼ 10 % in North Atlantic ozone, the modelled ozone trends
are much closer to zero and have large uncertainties. North Atlantic ozone
interannual variability (IAV) in the model was found to be correlated to the
IAV of ozone transported to the North Atlantic from the stratosphere (R=0.77) and emission of NOx from lightning in the tropics (R=0.72). The
discrepancy between modelled and observed trends for 2005–2018 could be
linked to the model underestimating lower stratospheric ozone trends and
associated stratosphere to troposphere transport. Modelled tropospheric
ozone IAV is driven by IAV of tropical emissions of NOx from lightning and
IAV of ozone transport from the stratosphere; however, the modelled and
observed IAV differ. To understand the IAV discrepancy we investigated how
modelled ozone and its drivers respond to large-scale modes of variability.
Using OMI height-resolved data and model idealized tracers, we were able to
identify stratospheric transport of ozone into the troposphere as the main
driver of the dynamical response of North Atlantic ozone to the Arctic
Oscillation (AO) and the North Atlantic Oscillation (NAO). Finally, we found
that the modelled ozone IAV is too strongly correlated to the El Niño–Southern
Oscillation (ENSO) compared to observed ozone IAV. This is again linked to
shortcomings in the lightning flashes parameterization, which
underestimates (overestimates) lightning flash production in the tropics
during positive (negative) ENSO events.