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Equatorial transport from the troposphere to the stratosphere as diagnosed from nitrous oxide variability

Philippe Ricaud(1), Jean-Pierre Pommereau(2), Jean-Luc Attié(1), Laaziz El Amraoui(3), Hubert Teyssèdre(3), Vincent-Henri Peuch(3), Wuhu Feng(4), Martyn Chipperfield(4), Marco Matricardi(5) and Peter Schluessel(6)

(1) Laboratoire d'Aérologie, 14, Avenue Edouard Belin, 31400 Toulouse, France
(2) LATMOS, Verrière, Paris, France
(3) CNRM, Toulouse, 31 Toulouse, France
(4) School of Environment, Leeds, Leeds, United Kingdom
(5) ECMWF, Shinfield Park, RG2 9AX Reading, United Kingdom
(6) EUMETSAT, , Darmstadt, Germany

Abstract

The mechanisms of transport at different time scales in the equatorial (10°S-10°N) stratosphere and troposphere are investigated using different nitrous oxide (N2O) data sources. We are making use of the stratospheric N2O measurements from the space-borne ODIN Sub-Millimetre Radiometer instrument (November 2001-June 2005), and of the tropospheric total columns of N2O derived from radiance spectra as measured by the Infrared Atmospheric Sounding Interferometer (IASI) instrument aboard the MetOp-A platform and distributed by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) during the March-May (MAM) 2008 period. Measurements are compared to the simulations of the three-dimensional Chemistry Transport Models MOCAGE and SLIMCAT. The N2O variations (annual, semi-annual and quasi-biennal oscillations) in the mid-to-upper stratosphere at levels above 32 hPa are shown to be generally captured by the models. In the lower stratosphere (LS), below 32 hPa, the observed variations are shown to be principally seasonal with peak amplitude at 400 K (~19 km), and are totally missed by the models. 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 but, as subgrid process, not incorporated 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. IASI tropospheric N2O equatorial measurements also show a maximum over Africa (4.96 10-3 kg m-2) and a minimum over South America (4.86 10-3 kg m-2) in very good agreement with the outputs from MOCAGE despite the fact that emissions of N2O are more intense over America than over Africa. A difference between the two data sets is observed above the Western Pacific (110°E-150°E) with a marked minimum in IASI compared to MOCAGE. Based on equatorial total columns of N2O measured by IASI, our results are consistent with the fact that Africa is a zone of convergence of airmasses coming from different convective regions whilst Western Pacific behaves more like a divergence zone.

 

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