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| Titel |
Ensemble simulations of the role of the stratosphere in the attribution of northern extratropical tropospheric ozone variability |
| VerfasserIn |
P. Hess, D. Kinnison, Q. Tang |
| 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 ; 15, no. 5 ; Nr. 15, no. 5 (2015-03-04), S.2341-2365 |
| Datensatznummer |
250119487
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| Publikation (Nr.) |
 copernicus.org/acp-15-2341-2015.pdf |
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| Zusammenfassung |
| Despite the need to understand the impact of changes in emissions and climate
on tropospheric ozone, the attribution of tropospheric interannual ozone
variability to specific processes has proven difficult. Here, we analyze the
stratospheric contribution to tropospheric ozone variability and trends from
1953 to 2005 in the Northern Hemisphere (NH) mid-latitudes using four
ensemble simulations of the free running (FR) Whole Atmosphere Community
Climate Model (WACCM). The simulations are externally forced with observed
time-varying (1) sea-surface temperatures (SSTs), (2) greenhouse
gases (GHGs), (3) ozone depleting substances (ODS), (4) quasi-biennial
oscillation (QBO), (5) solar variability (SV) and (6) stratospheric sulfate
surface area density (SAD). A detailed representation of stratospheric
chemistry is simulated, including the ozone loss due to volcanic
eruptions and polar stratospheric clouds. In the troposphere, ozone
production is represented by CH4–NOx smog chemistry, where
surface chemical emissions remain interannually constant. Despite the
simplicity of its tropospheric chemistry,
at many NH measurement locations, the interannual ozone variability in the FR
WACCM simulations is significantly correlated with the measured interannual
variability. This suggests the importance of the external forcing applied in
these simulations in driving interannual ozone variability. The variability
and trend in the simulated 1953–2005 tropospheric ozone from 30 to
90° N at background surface measurement sites, 500 hPa measurement
sites and in the area average are largely explained on interannual timescales
by changes in the 30–90° N area averaged flux of ozone across the
100 hPa surface and changes in tropospheric methane concentrations. The
average sensitivity of tropospheric ozone to methane (percent change in ozone
to a percent change in methane) from 30 to 90° N is 0.17 at 500 hPa
and 0.21 at the surface; the average sensitivity of tropospheric ozone to the
100 hPa ozone flux (percent change in ozone to a percent change in the ozone
flux) from 30 to 90° N is 0.19 at 500 hPa and 0.11 at the surface.
The 30–90° N simulated downward residual velocity at 100 hPa
increased by 15% between 1953 and 2005. However, the impact of this on
the 30–90° N 100 hPa ozone flux is modulated by the long-term
changes in stratospheric ozone. The ozone flux decreases from 1965 to 1990
due to stratospheric ozone depletion, but increases again by approximately
7% from 1990 to 2005. The first empirical orthogonal function of
interannual ozone variability explains from 40% (at the surface) to over
80% (at 150 hPa) of the simulated ozone interannual variability from 30
to 90° N. This identified mode of ozone variability shows strong
stratosphere–troposphere coupling, demonstrating the importance of the
stratosphere in an attribution of tropospheric ozone variability. The
simulations, with no change in emissions, capture almost 50% of the
measured ozone change during the 1990s at a variety of locations. This
suggests that a large portion of the measured change is not due to changes in
emissions, but can be traced to changes in large-scale modes of ozone
variability. This emphasizes the difficulty in the attribution of ozone
changes, and the importance of natural variability in understanding the
trends and variability of ozone. We find little relation between the El
Niño–Southern Oscillation (ENSO) index and large-scale tropospheric
ozone variability over the long-term record. |
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