Abstract. The stratospheric ozone layer plays a key role in atmospheric thermal
structure and circulation. Although stratospheric ozone distribution is
sensitive to changes in trace gases concentrations and climate, the
modifications of stratospheric ozone are not usually considered in climate
studies at geological timescales. Here, we evaluate the potential role of
stratospheric ozone chemistry in the case of the Eocene hot conditions.
Using a chemistry–climate model, we show that the structure of the ozone
layer is significantly different under these conditions (4×CO2 climate
and high concentrations of tropospheric N2O and CH4). The total
column ozone (TCO) remains more or less unchanged in the tropics whereas it
is found to be enhanced at mid- and high latitudes. These ozone changes are
related to the stratospheric cooling and an acceleration of stratospheric
Brewer–Dobson circulation simulated under Eocene climate. As a consequence,
the meridional distribution of the TCO appears to be modified, showing
particularly pronounced midlatitude maxima and a steeper negative poleward
gradient from these maxima. These anomalies are consistent with changes in
the seasonal evolution of the polar vortex during winter, especially in the
Northern Hemisphere, found to be mainly driven by seasonal changes in
planetary wave activity and stratospheric wave-drag. Compared to a
preindustrial atmospheric composition, the changes in local ozone
concentration reach up to 40 % for zonal annual mean and affect
temperature by a few kelvins in the middle stratosphere. As inter-model differences in simulating deep-past temperatures are
quite high, the consideration of atmospheric chemistry, which is
computationally demanding in Earth system models, may seem superfluous.
However, our results suggest that using stratospheric ozone calculated by
the model (and hence more physically consistent with Eocene conditions)
instead of the commonly specified preindustrial ozone distribution could
change the simulated global surface air temperature by as much as 14 %. This
error is of the same order as the effect of non-CO2 boundary conditions
(topography, bathymetry, solar constant and vegetation). Moreover, the
results highlight the sensitivity of stratospheric ozone to hot climate
conditions. Since the climate sensitivity to stratospheric ozone feedback
largely differs between models, it must be better constrained not only for
deep-past conditions but also for future climates.