An idealized two-dimensional approach to study the impact of the West African monsoon on the meridional gradient of tropospheric ozone
Résumé
An idealized vertical-meridional zonally symmetrical version of the Méso-NH model is used to study the response of tropospheric ozone to the dynamics of the West African monsoon, as well as surface emissions and NOx-production by lightning (LNOx). An O3-NOx-VOC chemical scheme has been added to the dynamical model, including surface emissions and a parameterization of the LNOx production. The model shows that the ozone precursors emitted at the surface are uplifted by deep convection and then advected in the upper branches of the Hadley cells on both sides of the Inter Tropical Convergence Zone (ITCZ). The NOx produced by lightning promotes chemical ozone production in the middle and upper troposphere from the oxidation of CO and VOCs. The analysis of the convective and chemical tendencies shows that the ozone minimum at the ITCZ is induced by venting of ozone-poor air masses into the upper troposphere. The bi-dimensional model suffers from limitations due to the absence of exchange with the higher latitudes and ventilation in the zonal direction. Despite of these restrictions, sensitivity simulations show that the LNOx source and biogenic VOCs are necessary to create the meridional gradient of ozone observed by the Measurements of OZone and water vapor by in-service AIrbus airCraft (MOZAIC) aircrafts in the southern Hadley cell. The LNOx source is also required to maintain the meridional ozone gradient up to 24°N in the northern Hadley cell. The modeled meridional gradient of O3 in the upper troposphere ranges from 0.22 to 0.52 ppbv/deg without the LNOx source and from 0.60 to 1.08 ppbv/deg with the LNOx source, in the southern and the northern cells respectively.
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