[1] Using in situ measurements, we find a semiannual oscillation (SAO) in the midtropospheric and surface CO 2 . Chemistry transport models (2-D Caltech/JPL model, 3-D GEOS-Chem, and 3-D MOZART-2) are used to investigate possible sources for the SAO signal in the midtropospheric and surface CO 2 . From model sensitivity studies, it is revealed that the SAO signal in the midtropospheric CO 2 originates mainly from surface CO 2 with a small contribution from transport fields. It is also found that the source for the SAO signal in surface CO 2 is mostly related to the CO 2 exchange between the biosphere and the atmosphere. By comparing model CO 2 with in situ CO 2 measurements at the surface, we find that models are able to capture both annual and semiannual cycles well at the surface. Model simulations of the annual and semiannual cycles of CO 2 in the tropical middle troposphere agree reasonably well with aircraft measurements.

The impacts of El Niñ o-Southern Oscillation (ENSO) on the tropical total column ozone, the tropical tropopause pressure, and the 3.5-yr ozone signal in the midlatitude total column ozone were examined using the Goddard Earth Observing System Chemistry-Climate Model (GEOS CCM). Observed monthly mean sea surface temperature and sea ice between 1951 and 2004 were used as boundary conditions for the model. Since the model includes no solar cycle, quasi-biennial oscillation, or volcanic forcing, the ENSO signal was found to dominate the tropical total column ozone variability. Principal component analysis was applied to the detrended, deseasonalized, and low-pass filtered model outputs. The first mode of model total column ozone captured 63.8% of the total variance. The spatial pattern of this mode was similar to that in Total Ozone Mapping Spectrometer (TOMS) observations. There was also a clear ENSO signal in the tropical tropopause pressure in the GEOS CCM, which is related to the ENSO signal in the total column ozone. The regression coefficient between the model total column ozone and the model tropopause pressure was 0.71 Dobson units (DU) hPa 21 . The GEOS CCM was also used to investigate a possible mechanism for the 3.5-yr signal observed in the midlatitude total column ozone. The 3.5-yr signal in the GEOS CCM column ozone is similar to that in the observations, which suggests that a model with realistic ENSO can reproduce the 3.5-yr signal. Hence, it is likely that the 3.5-yr signal was caused by ENSO.

The authors investigate the influence of El Niñ o on midtropospheric CO 2 from the Atmospheric Infrared Sounder (AIRS) and the Model for Ozone and Related Chemical Tracers, version 2 (MOZART-2). AIRS midtropospheric CO 2 data are used to study the temporal and spatial variability of CO 2 in response to El Niñ o. CO 2 differences between the central and western Pacific Ocean correlate well with the Southern Oscillation index. To reveal the temporal and spatial variability of the El Niñ o signal in the AIRS midtropospheric CO 2 , a multiple regression method is applied to the CO 2 data from September 2002 to February 2011. There is more (less) midtropospheric CO 2 in the central Pacific and less (more) midtropospheric CO 2 in the western Pacific during El Niñ o (La Niñ a) events. Similar results are seen in the MOZART-2 convolved midtropospheric CO 2 , although the El Niñ o signal in the MOZART-2 is weaker than that in the AIRS data.

[1] Mid-tropospheric CO 2 retrieved from the Atmospheric Infrared Sounder (AIRS) was used to investigate CO 2 interannual variability over the Indo-Pacific region. A signal with periodicity around two years was found for the AIRS mid-tropospheric CO 2 for the first time, which is related to the Tropospheric Biennial Oscillation (TBO) associated with the strength of the monsoon. During a strong (weak) monsoon year, the Western Walker Circulation is strong (weak), resulting in enhanced (diminished) CO 2 transport from the surface to the mid-troposphere. As a result, there are positive (negative) CO 2 anomalies at mid-troposphere over the Indo-Pacific region. We simulated the influence of the TBO on the mid-tropospheric CO 2 over the IndoPacific region using the MOZART-2 model, and results were consistent with observations, although we found the TBO signal in the model CO 2 is to be smaller than that in the AIRS observations. Citation: Wang, J., X. Jiang, M. T.

Midtropospheric CO 2 retrievals from the Atmospheric Infrared Sounder (AIRS) were used to explore the influence of stratospheric sudden warming (SSW) on CO 2 in the middle to upper troposphere. To choose the SSW events that had strong coupling between the stratosphere and troposphere, the authors applied a principal component analysis to the NCEP/Department of Energy Global Reanalysis 2 (NCEP-2) geopotential height data at 17 pressure levels. Two events (April 2003 and March 2005) that have strong couplings between the stratosphere and troposphere were chosen to investigate the influence of SSW on AIRS midtropospheric CO 2 . The authors investigated the temporal and spatial variations of AIRS midtropospheric CO 2 before and after the SSW events and found that the midtropospheric CO 2 concentrations increased by 2-3 ppm within a few days after the SSW events. These results can be used to better understand how the chemical tracers respond to the large-scale dynamics in the high latitudes.