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 from November 2001 to June 2005, and the simulations of the three-dimensional chemical transport models MOCAGE and SLIMCAT. Both models are forced with analyses from the European Centre for Medium-range Weather Forecast, 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 generally well 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 upwelling associated with the Brewer-Dobson circulation. However, in the lower stratosphere, below 32 hPa, the observed variations are shown to be mainly seasonal with peak amplitude at 400–450 K (~17.5–19 km), totally missed by the models. The minimum N2O in June, out of phase by two months with the known minimum seasonal upwelling associated with the Brewer-Dobson circulation and moreover amplified over the Western Pacific compared to Africa is incompatible with the seasonal change of upwelling evoked to explain the O3 annual cycle in the same altitude range (Randel et al., 2007). Unless the 1.5 ppbv amplitude of N2O annual cycle in the upper troposphere is totally wrong, the explanation of the observed N2O annual cycle of 15 ppbv in the lower stratosphere requires another mechanism. A possible candidate for that might be the existence of a downward time-averaged mass flux above specific regions, as shown by Sherwood (2000) over Indonesia, required for compensating the energy sink resulting from the deep overshooting of cold and heavy air at high altitude over intense convective areas. But, since global models do currently not capture this subsidence, it must be recognised that a full explanation of the observations cannot be provided for the moment. However, the coincidence of the peak contrast between the Western Pacific and Africa with the maximum overshooting volume in May reported by the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar, suggests a strong influence of deep convection on the chemical composition of the tropical lower stratosphere up to 500 K (21 km).
Highlights
Nitrous oxide (N2O) is an excellent tracer of atmospheric vertical transport since its sources are located in the troposphere where its lifetime is around 100 years
At lower altitude in the tropical upper troposphere-lower stratosphere (UTLS), several studies based on satellite observations of O3, N2O, HCl, H2O, HF and CH4 from Upper Atmosphere Research Satellite (UARS)/Halogen Occultation Experiment (HALOE) and AURA/Microwave Limb Sounder (MLS), together with ozonesondes, have shown the influence of the annual oscillation (AO) and the quasi-biennial oscillation (QBO) (Randel et al, 2007; Schoeberl et al, 2008), whilst Gettelman et al (2004) explored the impact of the Asian monsoon during the June–August period
As an extension of the previous study, here we investigate how N2O behaves over a five-year period in the entire equatorial stratosphere including the Tropical Tropopause Layer (TTL), the region of intermediate lapse rate extending from the level of zero net radiative heating (LZH) (∼14 km, ∼150 hPa) to the level of stratospheric lapse rate (∼18.5 km, ∼70 hPa)
Summary
Nitrous oxide (N2O) is an excellent tracer of atmospheric vertical transport since its sources are located in the troposphere (soils, wetlands, biomass burning and industrial emissions) where its lifetime is around 100 years. Ricaud et al.: Equatorial transport as diagnosed from N2O variability oscillation (SAO), as shown by Randel et al (1994) from two years of Cryogenic Limb Array Etalon Spectrometer (CLAES) observations This behaviour was recently confirmed by Jin et al (2009) from the measurements of the Microwave Limb Sounder (MLS) on the AURA platform, the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) and the Sub-Millimetre Radiometer (SMR) aboard ODIN. At lower altitude in the tropical upper troposphere-lower stratosphere (UTLS), several studies based on satellite observations of O3, N2O, HCl, H2O, HF and CH4 from UARS/HALOE and AURA/MLS, together with ozonesondes, have shown the influence of the annual oscillation (AO) and the QBO (Randel et al, 2007; Schoeberl et al, 2008), whilst Gettelman et al (2004) explored the impact of the Asian monsoon during the June–August period.
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