Equatorial transport as diagnosed from nitrous oxide variability

Abstract

Abstract. The mechanisms of transport on annual, semi-annual and quasi-biennial time scales in the equatorial (10° S–10° N) stratosphere are investigated using the nitrous oxide (N2O) measurements of the space-borne ODIN Sub-Millimetre Radiometer instrument from November 2001 to June 2005, and the simulations of the three-dimensional Chemistry Transport Models MOCAGE and SLIMCAT. Both models are forced with analyses from the European Centre for Medium-range Weather Forecasts, but the vertical transport is derived either from the forcing analyses by solving the continuity equation (MOCAGE), or from diabatic heating rates using a radiation scheme (SLIMCAT). The N2O variations in the mid-to-upper stratosphere at levels above 32 hPa are shown to be generally captured by the models though significant differences appear with the observations as well as between the models, attributed to the difficulty of capturing correctly the slow vertical velocities of the Brewer-Dobson circulation. In the lower stratosphere (LS), below 32 hPa, the variations are shown to be principally seasonal with peak amplitude at 400 K (~19 km), and are totally missed by the models. The decrease in diabatic radiative heating in the LS during the Northern Hemisphere summer is found to be out of phase by one month and far too small to explain the observed N2O seasonal cycle. The proposed explanation for this annual variation is a combination of i) the annual cycle of tropopause height of 1 km amplitude, ii) the convective overshooting above 400 K peaking in May and absent in the models, and iii) an annual cycle of 15 ppbv amplitude of the N2O concentration at the tropopause, but for which no confirmation exists in the upper troposphere in the absence of global-scale measurements. The present study indicates i) a significant contribution of deep convective overshooting on the chemical composition of the LS at global scale up to 500 K, ii) a preferred region for that over the African continent, and iii) a maximum impact in May when the overshoot intensity is the largest and horizontal winds are the slowest.