Adiabatic versus diabatic transport contributions to the ozone budget in the northern hemispheric upper troposphere and lower stratosphere

Abstract. Ozone in the extratropical lowermost stratosphere (LMS) is important for the local radiative balance and contributes to the tropospheric and near-surface ozone burden via stratosphere–troposphere exchange. Here, we aim to deepen our understanding of the transport contributions to LMS ozone in the Northern Hemisphere by studying the ozone budget in isentropic coordinates, which allows for a clean distinction of adiabatic and diabatic transport contributions. This is done by analyzing 20 years of ERA5 reanalysis output on model levels and a free-running simulation using the EMAC chemistry–climate model. Our analysis confirms that the ozone tendencies in the extratropical LMS at high latitudes are dominated by diabatic mean flow advection (associated with downwelling within the Brewer–Dobson circulation) and quasi-horizontal adiabatic eddy mixing due to planetary- and medium-scale Rossby waves. These transport contributions are somewhat weaker during summer compared to winter, although seasonality is found to be weaker in the LMS compared to higher altitudes. Horizontal mean flow advection is found to be relevant near the tropopause and just above the subtropical jet core. Notably, vertical (i.e., diabatic) eddy ozone transport is found to be important near the tropopause. While the adiabatic eddy ozone fluxes in the LMS are consistent with diffusive, down-gradient eddy transport, the vertical eddy ozone transport also features up-gradient regions, which by itself would act to reinforce the background ozone gradients near the tropopause. Closer analysis reveals that this is due to long-wave radiative damping of planetary waves, which acts to dampen the down-gradient horizontal eddy transport.