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| Titel |
Equatorial transport as diagnosed from nitrous oxide variability |
| VerfasserIn |
P. Ricaud, J.-P. Pommereau, J.-L. Attié, E. Flochmoën, L. Amraoui, H. Teyssèdre, V.-H. Peuch, W. Feng, M. P. Chipperfield |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 9, no. 21 ; Nr. 9, no. 21 (2009-11-02), S.8173-8188 |
| Datensatznummer |
250007718
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| Publikation (Nr.) |
 copernicus.org/acp-9-8173-2009.pdf |
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| Zusammenfassung |
| 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). |
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