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Seasonal and Interannual Variability of Mid-tropospheric CO2 from
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Atmospheric Infrared Sounder
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Xun Jiang1*, Moustafa T. Chahine2, Edward T. Olsen2, Luke L. Chen2,
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and Yuk L. Yung3
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1 Department of Earth & Atmospheric Sciences, University of Houston, USA
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2 Science Division, Jet Propulsion Laboratory, California Institute of Technology, USA
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3 Division of Geological and Planetary Sciences, California Institute of Technology,
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Pasadena, USA.
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* To whom all correspondence should be addressed. E-mail: xjiang4@mail.uh.edu
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To be Submitted to GRL, Oct 26 2009
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Abstract:
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[1] The Atmospheric Infrared Sounder offers an opportunity to investigate the
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variability of middle tropospheric CO2 over the entire globe. In this paper, we use AIRS
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data to examine the annual and interannual variability of CO2 and show that the annual
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cycle of AIRS CO2 agrees well with Matsueda aircraft measurements at 25N and 25S.
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We also find significant correlations between AIRS middle tropospheric CO2
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concentrations and large-scale atmospheric dynamics. During El Niño events, middle
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tropospheric CO2 over the central Pacific Ocean is enhanced whereas it is reduced over
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the western Pacific Ocean as a result of the change in the circulation. The variation of
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AIRS CO2 in the high latitudes of northern hemisphere is closely related to the strength
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of the northern hemispheric annular mode. These results will help us to better understand
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the large-scale dynamics and its influence on tracer in the polar region.
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1. Introduction
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[2] The increasing level of atmospheric CO2 has a significant influence on the global
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climate changes [Dickinson and Cicerone, 1986]. Observations show a global trend of
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CO2 of approximately 2 ppm/year [Keeling et al., 1995]. Superimposed upon this trend
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is an annual cycle resulting from the uptake and release of CO2 by vegetation whose
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amplitude is greatest in the northern hemisphere (NH). In addition to the trend and annual
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cycle, atmospheric CO2 also shows interannual variabilities. In this paper, we will
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investigate the impact of the transport and large-scale dynamics on the variability of CO2.
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[3] El Niño and Southern Oscillation (ENSO) is the most important large-scale climate
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inter-annual variability in the tropics. The atmospheric CO2 is influenced by the ENSO at
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the surface [Bacastow 1976; Bacastow et al., 1980]. During El Niño events, the
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atmospheric CO2 growth rate increases at surface stations [Keeling et al., 1995; Jones et
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al., 2001]. During La Niña events, the atmospheric CO2 growth rates decrease at the
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surface stations.
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[4] During El Niño events, ocean is a sink for CO2 [Feely et al., 1987; Inoue and
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Sugimuray 1992; Wong et al., 2003; Feely et al., 1997; Feely et al., 2006]. The upwelling
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of the cold nutrient-rich waters off the South American coast ceases during El Niño, and
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the cessation of upwelling will cause surface pCO2 concentrations decrease along the
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equator [Feely et al., 1987; 1997]. Meanwhile, tropical land becomes dryer and warmer
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during El Niño. As a result, the gross primary productivity decreases and the respiration
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increases over the land [Jones et al., 2001]. Thus terrestrial biosphere becomes a greater
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source of CO2 to the atmosphere during El Niño [Keeling et al., 1995; Francey et al.,
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1995]. The net effect from land and ocean is that the surface CO2 growth rate increases
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during El Niño events. Conversely, the surface CO2 growth rate decreases during La Niña
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events.
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[5] The focus of these previous studies was on the influence of climate variability on the
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CO2 concentration at a few stations near the surface. However, there are no previous
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investigation on understanding the influence of El Niño on the middle tropospheric CO2
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over a global domain. In this paper, we analyze the influence of ENSO on the middle
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tropospheric CO2 retrieved by the Atmospheric Infrared Sounder (AIRS). This global
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data set provides new insight on how ENSO influence the carbon interannual variations
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in the middle atmosphere and promotes our understanding of vertical transport and future
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climate change.
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[6] The global coverage of AIRS CO2 retrievals also allows us to investigate the
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influence of large-scale dynamics on the polar CO2 concentration. The polar region has
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profound significance for climate. The current general circulation models (GCMs) do not
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work well for the large-scale dynamics in the polar region, particularly in simulating the
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exchange between the stratosphere and troposphere in that region [Meloen et al. 2003;
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Jiang et al., 2009]. Therefore, observations are essential for understanding the large-scale
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dynamics in the polar region. However, most observations and measurements in the polar
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region suffer from limited coverage both in time and space. The global distributions of
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CO2 retrieved from AIRS offer a unique opportunity to study the large-scale dynamics in
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the polar region.
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[7] The annular modes are the most important inter-annual variability in the polar
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region. Jiang et al. [2008a, 2008b] applied principal component analysis to the
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extratropical total column ozone from the combined Merged Ozone Data product and the
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European Center for Medium-Range Weather Forecasts assimilated ozone from Jan 1979
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to Aug 2002. In both hemispheres, the first two leading modes are nearly zonally
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symmetric and represent the connections to the annular modes. When the polar vortex is
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stronger (positive annular mode phase), there is less ozone transported to the polar region
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due to a weaker Brew-Dobson circulation. Similar results in extratropical column ozone
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are found in the Goddard Earth Observation System Chemistry-Climate Model (GEOS-
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CCM) [Jiang et al., 2008a; 2008b]. Since there is no complex chemistry for CO2 as that
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for O3, it is easier to use CO2 for investigating the large-scale dynamics in the polar
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region. In this paper, we will investigate the influence of the annular modes on AIRS
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middle tropospheric CO2.
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2. Data
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[8] AIRS is a cross-track scanning grating spectrometer with 2378 channels from 3.7 to
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15.4 m with a 13.5 km field of view at nadir [Aumann et al., 2003]. Chahine et al.
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[2005] found that the range 690-725 cm-1 is best for selecting the main channel set to
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retrieve the middle tropospheric CO2 mixing ratio. The mixing ratios of middle
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tropospheric CO2 are retrieved using the Vanish Partial Derivative Method (VPD)
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[Chahine et al., 2005; Chahine et al., 2008; Olsen et al., 2009]. The sensitivity function
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of AIRS middle tropospheric CO2 peaks between 500 hPa to 300 hPa. AIRS middle
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tropospheric CO2 is retrieved globally in the middle troposphere day and night under
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clear and cloudy conditions. Validation by comparison to in situ aircraft measurements
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and retrievals by land-based upward looking Fourier Transform Interferometers
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demonstrates that AIRS CO2 is accurate to 1-2 ppm between latitudes 30°S and 80°N
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[Chahine et al., 2008; Olsen et al., 2009]. The middle tropospheric CO2 retrieved via the
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VPD method captures the correct seasonal cycle compared with those from Matsueda
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[Chahine et al., 2005; Olsen et al., 2009].
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[9] In Section 3 of this paper, we compare the seasonal cycle for the AIRS CO2 with
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independent biweekly aircraft measurements over the western Pacific between Australia
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and Japan [Matsueda et al., 2002]. The latitudinal range of the Comprehensive
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Observation Network for Trace gases by AIrLiner (CONTRAIL) measurements at
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cruising altitude (above 10 km) is approximately from 35S to 35N and the longitudinal
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range is from 135E to 150E.
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3. Results and Discussions
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3.1 Annual Cycle of AIRS CO2
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[10] We compared the NH and southern hemisphere (SH) annual cycles of detrended
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AIRS retrieved CO2 and detrended CONTRAIL measurements over the period spanning
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September 2002 and December 2007. A 2 ppm/year trend has been removed from both
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data sets. The CONTRAIL CO2 data above 8 km altitude are averaged over 10° latitude
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ranges centered at 25°S and 25°N. AIRS data for the same dates as the CONTRAIL
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measurements have also been averaged over these latitude ranges. The annual cycle was
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calculated for each week from the detrended CO2 data. The annual cycles for AIRS and
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CONTRAIL CO2 at 25N and 25S are shown in Figure 1. Error bars are the standard
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deviations for CO2 in each month. The CONTRAIL annual cycle is depicted by the red
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line; the AIRS CO2 retrieval sampled in the same locations is depicted by the black line.
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The annual cycle amplitude and phase of AIRS CO2 agree very well with those of the in-
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situ aircraft data. The annual cycle amplitude is greater in the NH than in the SH due to
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the larger seasonal cycle from surface biosphere contribution in the NH.
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3.2 Interannual Variability of Tropical AIRS CO2
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[11] We will explore the interannual variability of tropical CO2 and the influence of
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ENSO on CO2 in this section. ENSO is the most important large-scale climate variability
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in the tropics, The Southern Oscillation Index (SOI), shown in Fig 2a, is an index for
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ENSO. It is defined as the monthly mean sea level pressure difference between Tahiti and
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Darwin. Negative SOI index corresponds to an El Niño event and positive SOI index
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corresponds to a La Niña event. We have derived the spatial distributions of AIRS CO2 in
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Feb 2005 (El Niño event; Negative SOI Index) and in Feb 2008 (La Niña event; Positive
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SOI index). Figs 2b and 2c are the spatial patterns of the detrended AIRS CO 2 for these
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two events. Detrending was accomplished by removing a 2 ppm/year trend from the data.
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[12] During an El Niño event, warm sea surface temperature (SST) anomalies appear in
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the central and eastern Pacific and cold anomalies appear in the western Pacific, and the
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convection will move eastward to the central Pacific [Gage and Reid, 1987]. As a result,
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the convection in the central Pacific will bring surface high concentration CO2 into
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middle troposphere. Fig. 2b shows that AIRS CO2 retrievals indicate that surface layer
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CO2 has been lifted into the middle troposphere over the central Pacific region during the
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El Niño event. Low concentration of middle tropospheric CO2 is seen in the western and
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eastern Pacific Ocean which is a result of sinking motions over these regions transporting
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air from high altitude to the middle troposphere.
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[13] In contrast, during the La Niña event (Positive SOI index) the SST is warmer in the
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western Pacific and its associated convection is stronger. Fig. 2c shows that AIRS CO2
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retrievals indicate that CO2 has been lifted into the middle troposphere over the western
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Pacific during the La Niña event. Lower middle tropospheric CO2 is seen in the eastern
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Pacific Ocean during the La Niña event as a result of transport of low CO2 from high
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altitude to the middle troposphere. Fig. 2d is a map of the difference between El Niño and
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La Niña middle tropospheric CO2. The difference, (El Niño - La Niña), of AIRS middle
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tropospheric CO2 is about 1  2 ppm over the central Pacific and -1 to -2 ppm over the
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western Pacific.
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3.3 Impact of Polar Vortex on AIRS CO2
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[14] The annular modes are the most important climate variability in the high latitudes.
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To investigate the influence of the polar vortex on AIRS middle tropospheric CO2, we
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averaged the detrended AIRS polar CO2 north of 60°N from November to April, for the
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years 2003 through 2007. The result is shown as solid line in Fig 3a. The strength of the
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polar vortex is characterized by the Arctic Oscillation (AO) index. The AO index is
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defined as the leading time series for the sea-level pressure anomalies within November
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to April from 20N to 90N [Thompson and Wallace, 1998]. The detrended AO index is
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shown as dashed line in Fig 3a. Pearson’s correlation coefficient for the detrended AIRS
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polar CO2 and detrended AO index is -0.74, and the corresponding significance level is
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7%. The significance statistic for correlations was generated by a Monte Carlo method
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[Press et al., 1992; Jiang et al., 2004].
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[15] Fig 3a indicates that there are strong polar vortices in 2005 and 2007, i.e., for those
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years the AO index is positive. When the polar vortex is strong, there is less horizontal
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mixing of air between the mid-latitudes and high-latitudes. As a result, the transport of air
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containing a high concentration of CO2 from the mid-latitudes into the polar region is
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weakened. During those years the concentration of AIRS CO2 in the polar region should
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therefore be relatively low. Fig 3b shows that the average of AIRS polar CO2 from
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November to April in 2005 and 2007 was reduced.
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[16] On the other hand, Fig 3a indicates that the polar vortices are weak in 2006 and
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2008, for the AO index is negative in those years. For the case of a weak polar vortex, the
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horizontal mixing is strong and mid-latitude air containing a high concentration of CO2
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can be transported into the polar region. Fig. 3c shows that the AIRS polar CO2 was
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enhanced from November to April in 2006 and 2008. The difference of AIRS polar CO2
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between the strong polar vortex and weak polar vortex years is shown in Fig. 3d. There is
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less CO2 at the polar region for the strong polar vortex years. The minimum value for the
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difference can be 2-3 ppm. Student-t test is used to calculate the statistical significance of
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the difference for CO2 concentration in the strong and weak vortex years. The result is
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statistically significant when t is larger than a certain value t0. The number of degrees of
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freedom for the CO2 difference between two groups is equal to 2. Given the number of
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degrees of freedom, t0 can be found from the t distribution table. t0 with a 10%
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significance level is 2.9. When the t-value in Fig. 3e is larger than 2.9, the results is
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within 10% significance level.
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4. Conclusions
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[17] AIRS mid-tropospheric CO2 retrievals have been used to investigate the annual
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cycle and interannual variability of CO2 in the middle troposphere for the first time. The
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comparison between
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demonstrates that the AIRS CO2 measurements in the middle troposphere capture correct
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seasonal cycle of CO2 in the tropics. Globally distributed CO2 retrievals offer a unique
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opportunity to explore the relationships between the middle tropospheric CO2
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concentration and large-scale atmospheric processes. Our analysis suggests that the
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influences of El Niño events and polar vortex on the CO2 concentration are apparent in
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the AIRS data. During El Niño, middle tropospheric CO2 is enhanced in central Pacific
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Ocean and diminished in the western Pacific Ocean. In the polar region, middle
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tropospheric CO2 is diminished if the polar vortex is strong and enhanced if it is weak.
the AIRS retrieved
CO2 and the in-situ observations
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Acknowledgement
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This work was carried out at the Jet Propulsion Laboratory, California Institute of
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Technology, under contract with the National Aeronautics and Space Administration.
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Figure 1: CO2 annual cycle at 25 N and 25 S. Red line is Matsueda CO2. Black line is
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AIRS CO2.
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Figure 2: (a) Southern Oscillation Index, (b) AIRS CO2 in Feb 2005, (c) AIRS CO2 in
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Feb 2008, and (d) AIRS CO2 difference between Feb 2005 and Feb 2008. Units are ppm.
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Figure 3: (a) Detrended AIRS CO2 (solid line) averaged from 60N to 90N and
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detrended Arctic Oscillation index (dash line). (b) AIRS CO2 averaged in 2005 and 2007,
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which are two strong polar vortex years. (c) AIRS CO2 averaged in 2006 and 2008,
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which are two weak polar vortex years. (d) Difference of AIRS CO2 for strong and weak
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polar vortex years. (e) t-value for the CO2 difference.
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