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Interannual Variability of Mid-tropospheric CO2 from Atmospheric
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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 mid-tropospheric CO2 over the entire globe. In this paper, we use AIRS data
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to examine the interannual variability of CO2 and find significant correlations between
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AIRS mid-tropospheric CO2 concentrations and large-scale atmospheric dynamics.
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During El Niño events, mid-tropospheric CO2 over the central Pacific Ocean is enhanced
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whereas it is reduced over the western Pacific Ocean as a result of the change in the
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Walker circulation. The variation of AIRS CO2 in the high latitudes of northern
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hemisphere is closely related to the strength of the northern hemispheric annular mode.
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These results contribute to a better understanding of the influence of large-scale dynamics
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on tracer distributions 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 global
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climate change [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 variability. In this paper, we will
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investigate the impact of the transport and large-scale dynamics on the interannual
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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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interannual 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 (La Niña) events, the
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atmospheric CO2 growth rate increases (decreases) at surface stations [Keeling et al.,
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1995; Jones et al., 2001]. The reasons are as follows.
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[4] During El Niño events, the 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 water off the South American coast ceases during El Niño,
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causing the surface pCO2 concentration to decrease along the equator [Feely et al., 1987;
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1997]. Meanwhile, the tropical land becomes dryer and warmer. As a result, the gross
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primary productivity decreases and respiration increases over the land [Jones et al.,
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2001]. Thus the terrestrial biosphere becomes a greater source of CO2 to the atmosphere
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during El Niño [Keeling et al., 1995; Francey et al., 1995]. The net effect from land and
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ocean is that the surface CO2 growth rate increases during El Niño events. Conversely,
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the surface CO2 growth rate decreases during La Niña events.
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[5] The focus of these previous studies is on the influence of climate variability on the
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CO2 concentration at a few stations near the surface. However, there is no previous
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investigation on understanding the influence of El Niño on the mid-tropospheric CO2 on
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a global scale. In this paper, we analyze the influence of ENSO on the mid-tropospheric
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CO2 retrieved by the Atmospheric Infrared Sounder (AIRS). This global dataset provides
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new insight on how ENSO influence the carbon interannual variability in the middle
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atmosphere and contributes to our understanding of the vertical transport of tracers
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between the surface and the mid-troposphere.
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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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perform well in the polar region, especially in simulating the exchange between the
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stratosphere and troposphere in that region [Meloen et al. 2003; Jiang et al., 2009].
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Therefore, observations are essential for understanding the large-scale dynamics in the
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polar region. However, most observations and measurements in the polar region suffer
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from limited coverage both in time and space. The global distribution of CO2 retrieved
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from AIRS offers a unique opportunity for studying the large-scale dynamics in the polar
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region.
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[7] The annular modes are the most important interannual variability in the polar region
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$$$[Thompson and Wallace papers here]$$$. Jiang et al. [2008a, 2008b] applied
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principal component analysis to the extratropical total column ozone from the combined
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Merged Ozone Data product and the European Center for Medium-Range Weather
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Forecasts assimilated ozone from Jan 1979 to Aug 2002. In both hemispheres, the first
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two leading modes are nearly zonally symmetric and are related to the annular modes.
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When the polar vortex is stronger (positive annular mode phase), there is less ozone
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transported to the polar region due to a weaker Brew-Dobson circulation. Similar results
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in extratropical column ozone are found in the Goddard Earth Observation System
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Chemistry-Climate Model (GEOS-CCM) [Jiang et al., 2008a; 2008b]. Since there is no
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chemical destruction of CO2, in contrast to O3, CO2 is a better tracer for investigating the
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large-scale dynamics in the polar region. In this paper, we will examine the influence of
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the northern hemispheric annular mode (see later discussion on its definition) on AIRS
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mid-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 mid-tropospheric CO2 mixing ratio. The mixing ratios of mid-tropospheric
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CO2 are retrieved using the Vanish Partial Derivative Method (VPD) [Chahine et al.,
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2005; Chahine et al., 2008; Olsen et al., 2009]. The sensitivity function of AIRS mid-
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tropospheric CO2 peaks between 500 hPa to 300 hPa. AIRS mid-tropospheric CO2 is
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retrieved globally in the middle troposphere day and night under clear and cloudy
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conditions. Validation by comparison to in situ aircraft measurements and retrievals by
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land-based upward looking Fourier Transform Interferometers demonstrates that AIRS
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CO2 is accurate to 1-2 ppm between latitudes 30°S and 80°N [Chahine et al., 2008; Olsen
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et al., 2009]. The mid-tropospheric CO2 retrieved via the VPD method captures the
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correct seasonal cycle compared with those from Matsueda [Chahine et al., 2005; Olsen
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et al., 2009].
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3. Results and Discussions
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In order to study the interannual variability of CO2, we must first remove the seasonal
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cycle. This is done in section 3.1, where we compare the seasonal cycle for the AIRS
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CO2 with independent biweekly aircraft measurements over the western Pacific between
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Australia and Japan [Matsueda et al., 2002]. The interannual variability of CO2 in the
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tropics and the polar region is discussed in sections 3.2 and 3.3, respectively.
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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 [Matsueda et al., 2002]. A 2 ppm/year trend has
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been removed from both datasets. The CONTRAIL CO2 data above 8 km altitude are
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averaged over 10° latitude ranges centered at 25°S and 25°N. AIRS data for the same
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dates as the CONTRAIL measurements have also been averaged over these latitude
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ranges. The annual cycle was calculated for each week from the detrended CO2 data. The
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annual cycles for AIRS and CONTRAIL CO2 at 25N and 25S are shown in Figure 1.
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Error bars are the standard deviations for CO2 in each month. The CONTRAIL annual
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cycle is represented by the red line; the AIRS CO2 retrieval sampled in the same locations
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is depicted by the black line. The amplitude and phase in the annual cycle of AIRS CO2
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agree very well with those of CONTRAIL. The amplitude of the annual cycle is greater
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in the NH than in the SH due to the larger seasonal cycle from surface biosphere
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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 (positive) SOI index corresponds to an El Niño (La Niña) event. We
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have derived the spatial distributions of AIRS CO2 in Feb 2005 (El Niño event; Negative
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SOI Index) and in Feb 2008 (La Niña event; Positive SOI index). Figs 2b and 2c are the
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spatial patterns of the detrended AIRS CO2 for these two events. Detrending was
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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
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result,convection in the central Pacific brings surface high concentration CO2 into middle
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troposphere. In Fig. 2b shows, AIRS CO2 data indicate that surface layer CO2 has been
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lifted into the middle troposphere over the central Pacific region during the El Niño
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event. Low concentration of mid-tropospheric CO2 is seen in the western and eastern
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Pacific Ocean, where there is sinking motion.
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[13] In contrast, during a La Niña event (Positive SOI index) the SST is warmer in the
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western Pacific and its associated convection is stronger. In Fig. 2c, AIRS CO2 data
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suggest that CO2 has been lifted into the middle troposphere over the western Pacific.
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Lower mid-tropospheric CO2 is seen in the eastern Pacific Ocean as a result of transport
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of low CO2 from high altitude to the middle troposphere. Fig. 2d is a map of the
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difference between El Niño and La Niña mid-tropospheric CO2. The difference, (El Niño
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- La Niña), of AIRS mid-tropospheric CO2 is about 1  2 ppm over the central Pacific
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and -1 to -2 ppm over the 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 mid-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 northern annua mode (NAM) index. The NAM index
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is defined as the leading time series for the sea-level pressure anomalies within
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November to April from 20N to 90N [Thompson and Wallace, 1998]. The detrended
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NAM index is shown by the dashed line in Fig 3a. Pearson’s correlation coefficient for
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the detrended AIRS polar CO2 and detrended NAM index is -0.74, and the corresponding
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significance level is 7%. The significance statistic for correlations was generated by a
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Monte Carlo method [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 NAM 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; the NAM 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 higher concentrations 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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The results obtained here are similar to those obtained for stratospheric O3 in the
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northern polar vortex [Jiang et al. 2008a], where the principal compenent PC1 of O3
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is correlated with the NAM index at 100 hPa with a correlation coefficient of -0.53.
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The underlying physics is the same, as a stronger vortex inhibits transport of O3
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from
the
lower
latitudes
into
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polar
region.
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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 the
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correct seasonal cycle of CO2 in the tropics. Global CO2 retrievals offer a unique
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opportunity to explore the relationships between the mid-tropospheric CO2 concentration
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and large-scale atmospheric processes. Our analysis suggests that the influences of El
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Niño events and polar vortex on the CO2 concentration are apparent in the AIRS data.
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During El Niño, mid-tropospheric CO2 is enhanced in central Pacific Ocean and
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diminished in the western Pacific Ocean. In the polar region, mid-tropospheric CO2 is
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diminished if the polar vortex is strong and enhanced if it is weak. It remains a challenge
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for GCMs to simulate these results.
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: CONTRAIL. Black line: AIRS.
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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 NAM index (dash line, inverted). (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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