Index Geophysics

Sensitivity analysis of the potential impact of discrepancies in stratosphere–troposphere exchange on inferred sources and sinks of CO2

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 (CO₂) and ozone (O₃) observations from the HIAPER (The High-Performance Instrumented Airborne Platform for Environmental Research) Pole-to-Pole Observations (HIPPO) campaign in March 2010. GEOS-Chem CO₂/O₃ correlation suggests that there is a discrepancy in mixing across the tropopause in the model, which results in an overestimate of CO₂ and an underestimate of O₃ in the Arctic lower stratosphere. We assimilate stratospheric O₃ data from the Optical Spectrograph and InfraRed Imager System (OSIRIS) and use the assimilated O₃ fields together with the HIPPO CO₂/O₃ correlations to obtain an adjustment to the modeled CO₂ 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 CO₂ 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 CO₂ from tropical South America by 23 %. We find that the model also underestimates CO₂ 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 CO₂ 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 CO₂ vertical gradient in the high-latitude UTLS, and reduces the overestimate in CO₂ in the extratropical lower stratosphere. Our results illustrate that discrepancies in the CO₂ distribution in the UTLS can affect CO₂ flux inversions and suggest the need for more careful evaluation of model errors in the UTLS.

Deng, F.

Jones, D. B. A.

Walker, T. W.

Keller, M.

Bowman, K. W.

Henze, D. K.

Nassar, R.

Kort, E. A.

Wofsy, S. C.

Walker, K. A.

Bourassa, A. E.

Degenstein, D. A.

2018-09-10

Text

application/pdf

10.5194/acp-15-11773-2015

https://acp.copernicus.org/articles/15/11773/2015/

eISSN: 1680-7324

eng