Sensitivity analysis of the potential impact of discrepancies in stratosphere–troposphere exchange on inferred sources and sinks of CO<sub>2</sub>
The upper troposphere and lower stratosphere (UTLS) represents a transition region between the more dynamically active troposphere and more stably stratified stratosphere. The region is characterized by strong gradients in the distribution of long-lived tracers, whose representation in models is sen...
| Main Authors: | , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-10-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/15/11773/2015/acp-15-11773-2015.pdf |
| Summary: | The upper troposphere and lower stratosphere (UTLS)
represents a transition region between the more dynamically active
troposphere and more stably stratified stratosphere. The region is
characterized by strong gradients in the distribution of long-lived tracers,
whose representation in models is sensitive to discrepancies in transport. We
evaluate the GEOS-Chem model in the UTLS using carbon dioxide (CO<sub>2</sub>) and
ozone (O<sub>3</sub>) observations from the HIAPER (The High-Performance
Instrumented Airborne Platform for Environmental Research) Pole-to-Pole
Observations (HIPPO) campaign in March 2010. GEOS-Chem CO<sub>2</sub>/O<sub>3</sub>
correlation suggests that there is a discrepancy in mixing across the
tropopause in the model, which results in an overestimate of CO<sub>2</sub> and an
underestimate of O<sub>3</sub> in the Arctic lower stratosphere. We assimilate
stratospheric O<sub>3</sub> data from the Optical Spectrograph and InfraRed Imager
System (OSIRIS) and use the assimilated O<sub>3</sub> fields together with the
HIPPO CO<sub>2</sub>/O<sub>3</sub> correlations to obtain an adjustment to the modeled
CO<sub>2</sub> profile in the Arctic UTLS (primarily between the 320 and 360 K
isentropic surfaces). The HIPPO-derived adjustment corresponds to a sink of
0.60 Pg C for March–August 2010 in the Arctic. Imposing this adjustment
results in a reduction in the CO<sub>2</sub> sinks inferred from GOSAT observations
for temperate North America, Europe, and tropical Asia of 19, 13, and
49 %, respectively. Conversely, the inversion increased the source of
CO<sub>2</sub> from tropical South America by 23 %. We find that the model also
underestimates CO<sub>2</sub> in the upper tropical and subtropical troposphere.
Correcting for the underestimate in the model relative to HIPPO in the
tropical upper troposphere leads to a reduction in the source from tropical
South America by 77 %, and produces an estimated sink for tropical Asia
that is only 19 % larger than the standard inversion (without the imposed
source and sink). Globally, the inversion with the Arctic and tropical
adjustment produces a sink of −6.64 Pg C, which is consistent with the
estimate of −6.65 Pg C in the standard inversion. However, the standard
inversion produces a stronger northern land sink by 0.98 Pg C to account
for the CO<sub>2</sub> overestimate in the high-latitude UTLS, suggesting that this
UTLS discrepancy can impact the latitudinal distribution of the inferred
sources and sinks. We find that doubling the model resolution from
4° × 5° to 2° × 2.5°
enhances the CO<sub>2</sub> vertical gradient in the high-latitude UTLS, and
reduces the overestimate in CO<sub>2</sub> in the extratropical lower stratosphere.
Our results illustrate that discrepancies in the CO<sub>2</sub> distribution in the
UTLS can affect CO<sub>2</sub> flux inversions and suggest the need for more
careful evaluation of model errors in the UTLS. |
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| ISSN: | 1680-7316 1680-7324 |