We computed the budget of stratospheric N2O variations, separating the contributions from transport (the BDC) and from chemistry. The impact of the BDC on the N2O rate of change was further separated into advection (transport by long-range currents) and mixing (by large-scale eddies). We focused on the contributions from the vertical advection and the horizontal mixing, as these are the most important causes of N2O variations in the stratosphere.
The impact of the two processes depends largely on the latitude. N2O is produced in the troposphere and enters the stratosphere in the Tropics. The vertical advection driven by the BDC increases the N2O abundance across the entire tropical stratosphere. This generates meridional gradients in the N2O abundance in the stratosphere: in the Tropics there is a large amount of N2O, but in the middle latitudes N2O is nearly absent. Here comes into play the horizontal mixing, homogenizing the N2O concentrations between the Tropics and middle latitudes. In the polar regions, the vertical advection changes sign and transports N2O downwards towards the tropopause.
We evaluated and compared the roles of vertical advection and horizontal mixing in several datasets:
- the free-running Chemistry-Climate Model CESM/WACCM;
- the BASCOE Chemistry-Transport Model driven by four different reanalyses of atmospheric dynamics: ERA-Interim, JRA-55, MERRA and MERRA-2;
- BRAM2, a reanalysis by the BASCOE assimilation system of the chemical observations from the NASA instrument Aura/MLS.
Evaluation of the modelled vertical advection
These datasets agree very well in the Northern Hemisphere, and well in the Southern Hemisphere except above the Antarctic region, where WACCM and JRA-55 differ from the other datasets. The inter-annual variability of the vertical advection term is characterized by large differences between the different datasets in the mid-stratospheric tropical regions, due to the Quasi-Biennial Oscillation of zonal winds and its impact on the tropical upwelling from the troposphere.
Evaluation of the modelled horizontal mixing
In the Northern Hemisphere, the horizontal mixing contribution is weaker in WACCM than in the reanalyses during winter. But it is above the Antarctic that one notes the largest differences in this horizontal mixing term, also during winter hence in the polar vortex. According to the reanalyses, the horizontal mixing plays a major role in that region, while this process plays alsmot no role in WACCM.
The horizontal mixing term is most variable in the polar regions in the middle stratosphere. In the Antarctic Spring, this is related to the variability of the vortex breakup dates, whereas in the Arctic winter, it is associated to the highly variable polar vortex.
This study highlighted the regions of the stratosphere where a state-of-the-art climate model such as WACCM requires further improvement, and provided reference data to this aim. These reference datasets are derived from leading reanalyses of satellite observations, including the BRAM2 reanalysis of stratospheric chemistry that was created by BIRA-IASB.
Minganti, D., Chabrillat, S., Christophe, Y., Errera, Q., Abalos, M., Prignon, M., Kinnison, D.E., and Mahieu, E. (2020). Climatological impact of the Brewer-Dobson circulation on the N2O budget in WACCM, a chemical reanalysis and a CTM driven by four dynamical reanalyses. Atmospheric Chemistry and Physics, 20(21), 12609-12631. https://doi.org/10.5194/acp-20-12609-2020