Luke L. Chen

Interannual variability of mid‐tropospheric CO2 from Atmospheric Infrared Sounder

Atmospheric Infrared Sounder (AIRS) offers a unique opportunity to investigate the variability of mid-tropospheric CO_2 over the entire globe. In this paper, we use AIRS data to examine the interannual variability of CO_2 and find significant correlations between AIRS mid-tropospheric CO_2 and large-scale atmospheric dynamics. During El Nino events, mid-tropospheric CO_2 over the central Pacific Ocean is enhanced whereas it is reduced over the western Pacific Ocean as a result of the change in the Walker circulation. The variation of AIRS CO_2 in the high latitudes of the northern hemisphere is closely related to the strength of the northern hemispheric annular mode. These results contribute to a better understanding of the influence of large-scale dynamics on tracer distributions.

Simulation of upper tropospheric CO2 from chemistry and transport models

The California Institute of Technology/Jet Propulsion Laboratory two-dimensional (2-D), three-dimensional (3-D) GEOS-Chem, and 3-D MOZART-2 chemistry and transport models (CTMs), driven respectively by NCEP2, GEOS-4, and NCEP1 reanalysis data, have been used to simulate upper tropospheric CO2 from 2000 to 2004. Model results of CO2 mixing ratios agree well with monthly mean aircraft observations at altitudes between 8 and 13 km (Matsueda et al., 2002) in the tropics. The upper tropospheric CO2 seasonal cycle phases are well captured by the CTMs. Model results have smaller seasonal cycle amplitudes in the Southern Hemisphere compared with those in the Northern Hemisphere, which are consistent with the aircraft data. Some discrepancies are evident between the model and aircraft data in the midlatitudes, where models tend to underestimate the amplitude of CO2 seasonal cycle. Comparison of the simulated vertical profiles of CO2 between the different models reveals that the convection in the 3-D models is likely too weak in boreal winter and spring. Model sensitivity studies suggest that convection mass flux is important for the correct simulation of upper tropospheric CO2.

CO2 semiannual oscillation in the middle troposphere and at the surface

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.

Influence of Stratospheric Sudden Warming on AIRS Midtropospheric CO2

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.

Influence of El Niño on Midtropospheric CO2 from Atmospheric Infrared Sounder and Model

AbstractThe authors investigate the influence of El Nino on midtropospheric CO2 from the Atmospheric Infrared Sounder (AIRS) and the Model for Ozone and Related Chemical Tracers, version 2 (MOZART-2). AIRS midtropospheric CO2 data are used to study the temporal and spatial variability of CO2 in response to El Nino. CO2 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 Nino signal in the AIRS midtropospheric CO2, a multiple regression method is applied to the CO2 data from September 2002 to February 2011. There is more (less) midtropospheric CO2 in the central Pacific and less (more) midtropospheric CO2 in the western Pacific during El Nino (La Nina) events. Similar results are seen in the MOZART-2 convolved midtropospheric CO2, although the El Nino signal in the MOZART-2 is weaker than that in the AIRS data.

The influence of tropospheric biennial oscillation on mid-tropospheric CO2

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) CO2 anomalies at mid-troposphere over the Indo-Pacific region. We simulated the influence of the TBO on the mid-tropospheric CO_2 over the Indo-Pacific 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.