SEARCH

SEARCH BY CITATION

Keywords:

  • Interannual variablitly;
  • chemical modelling;
  • transport;
  • tropospheric ozone

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Model Description and Statistical Analysis
  5. 3. A 20-Year Climatology of the Tropospheric Ozone Distribution in the Middle East
  6. 4. Interannual Variations in Ozone Sources
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] We used the GEOS-Chem model to investigate the impact of interannual variations in transport on summertime ozone abundances (between 1987 and 2006) in the middle troposphere over the Middle East. We found that ozone abundances fluctuated interannually by about ±7% (or ±6 ppbv from the 20-year mean of ∼80 ppbv). In the 20-year mean, ozone transported from Asia and ozone produced locally were the dominant sources of ozone, accounting for 31% and 23%, respectively, of ozone abundances over the Middle East, with an interannual variability of ±30% and ±15%, respectively. We found that the interannual variations in the Asian and local sources were related to the strengths of the South Asian High and the Arabian anticyclone, respectively. In years when the Asian influence was weaker in the region, transport from other areas, such as North America, was enhanced. Consequently, variations in ozone transported from Asia were strongly anti-correlated with variations in ozone transported from North America, for example, with a correlation coefficient of r = −0.75. This trade-off between transport from Asia and other regions was found to be linked to the position and strength of the subtropical westerly jet over central Asia. When the westerly jet is displaced poleward, transport of ozone from Asia is enhanced and transport from North America and other regions in the Northern Hemisphere is diminished. In contrast, when the jet is displaced equatorward, transport of ozone from Asia is diminished and transport from North America and other regions in the Northern Hemisphere is enhanced. These results suggest that climate-related changes in the position of the westerly jet will have implications for the transport of pollution into the Middle East.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Model Description and Statistical Analysis
  5. 3. A 20-Year Climatology of the Tropospheric Ozone Distribution in the Middle East
  6. 4. Interannual Variations in Ozone Sources
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] Tropospheric ozone (O3) impacts our environment in different ways. It is a greenhouse gas in the middle and upper troposphere due to its absorption of infrared radiation. According to the Intergovernmental Panel on Climate Change (IPCC) [2007], tropospheric ozone is the third most important anthropogenic greenhouse gas, after CO2 and CH4. Ozone in the troposphere also acts indirectly as a cleansing agent because photolysis of ozone in the presence of water vapor is the dominant source of the hydroxyl radical (OH), the primary atmospheric oxidant [Logan et al., 1981; Thompson, 1992]. Tropospheric ozone varies on a range of temporal and spatial scales. Its distribution reflects a balance between in situ photochemical sources and sinks and atmospheric transport. In particular, long-range transport of pollution can significantly influence the distribution of ozone. This has been the focus of much attention since the 1990s [e.g., Dickerson et al., 1995; Jaffe et al., 1999; Berntsen et al., 1999; Yienger et al., 2000; Wild and Akimoto, 2001; Fiore et al., 2002; Lelieveld et al., 2002; Li et al., 2002; Cooper and Parrish, 2004; Lawrence, 2004; Duncan and Bey, 2004; Koumoutsaris et al., 2008]. Furthermore, only a few studies have examined the influence of long-range transport of pollution on ozone abundances over the Middle East and the Mediterranean region [e.g., Li et al., 2001; Lelieveld et al., 2002; Duncan et al., 2008]. We focus here on characterizing the impact of year-to-year variations in long-range transport on ozone abundances over the Middle East.

[3] The Middle East is of interest because this region is a nexus for transport of pollution. In the upper troposphere, the region is strongly influenced by transport of Asian pollution associated with the upper level Asian monsoon anticyclone system [Lawrence, 2004]. Previous studies [e.g., Kar et al., 2004; Park et al., 2004; Li et al., 2005; Randel and Park, 2006; Lawrence and Lelieveld, 2010] have shown that this anticyclone has a significant impact on the distribution of trace gases in the upper troposphere and lower stratosphere (UTLS). The influence of Asian pollution has also been observed in the middle troposphere in the eastern Mediterranean [Lelieveld et al., 2002]. Furthermore, European pollution is transported into the region at low altitudes in summer [Duncan and Bey, 2004; Duncan et al., 2008]. This has important implications for both air quality and climate in the region. In the climate context, Joiner et al. [2009] have shown that the outgoing long-wave radiation associated with tropospheric ozone is at a maximum in summer over North Africa and the Middle East, which they attributed to high surface temperatures, low humidity, and cloud-free conditions.

[4] It was first suggested by Li et al. [2001] that long-range transport of ozone and ozone precursors could contribute to a summertime buildup of ozone in the middle troposphere over the Middle East. Li et al. [2001] suggested that this so-called “Middle East ozone maximum” is linked to the large-scale subsidence in the region, associated with the summertime Asian monsoon. This summertime buildup of ozone has been observed by the Tropospheric Emission Spectrometer (TES) satellite instrument [Liu et al., 2009; Worden et al., 2009]. Liu et al. [2009] showed that transport of ozone from Asian and local ozone production in the Middle East are the predominant sources of the ozone, with each contributing about 30–35% to the ozone maximum. We are interested in better understanding how this Asian influence on the ozone abundances in the Middle East varies from year to year. The upper tropospheric anticyclone that forms in response to the thermal and orographic forcing of the Tibetan Plateau is shown in Figure 1a. This anticyclone, known as the South Asian High (SAH) or the Tibetan High (TH) [Yanai and Wu, 2006], has a significant impact on the atmospheric circulation in the Northern Hemisphere and on the weather and climate in eastern Asia [Krishnamurti et al., 1973; Ye and Wu, 1998; Qian et al., 2002; Zhang et al., 2005]. Along the southern edge of the SAH, the tropical easterly jet (between 100°E to 20°E) transports ozone and other pollutants westward from Asia. In the mid-troposphere (Figure 1b), a key feature is the Arabian anticyclone (or the Iranian High) over the Middle East [Zhou and Li, 2002; Ziv et al., 2004]. Its important role in trapping ozone at these altitudes was examined by Liu et al. [2009]. Using the GEOS-Chem global 3-dimensional chemical transport model (CTM), we examine here how interannual variations in the SAH and the Arabian anticyclone influenced the transport of ozone into the Middle East over the period 1987–2006.

image

Figure 1. The mean geopotential height (in m) overlaid with streamlines for July (1987–2006) at (a) 150 hPa, and (b) 400 hPa. Data source: NCEP/NCAR reanalysis [Kalnay et al., 1996].

Download figure to PowerPoint

[5] Previous studies with the general circulation models (GCMs) found that increasing greenhouse gases (GHGs) tends to produce a poleward and upward shift of the westerly jets in the Northern Hemisphere [Yin, 2005]. Zhou et al. [2009] showed, in an analysis based on four GCMs, that the SAH will be enhanced in summer if the sea surface temperatures over the Indian Ocean and the western Pacific increase in response to global warming. Therefore, better characterizing the interannual variability in transport of ozone to the Middle East is an important first step toward improving our understanding of the impact of climate-related changes in the circulation of the atmosphere on the distribution of ozone in the Middle East.

[6] We begin in Section 2 with a description of the GEOS-Chem model and statistical analysis methods used in this study. In Section 3 we examine the 20-year ozone climatology over the Middle East as simulated by the model. Then, in Section 4, we explore the interannual variations in ozone transported from Asia and in ozone produced locally in the Middle East. In this section we also examine the variability in transport of ozone from other source regions to the Middle East. We then summarize our results and present our conclusions in Section 5.

2. Model Description and Statistical Analysis

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Model Description and Statistical Analysis
  5. 3. A 20-Year Climatology of the Tropospheric Ozone Distribution in the Middle East
  6. 4. Interannual Variations in Ozone Sources
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

2.1. The GEOS-Chem Model

[7] The GEOS-Chem model [http://geos-chem.org] has been used extensively in studies of pollution transport [e.g., Fiore et al., 2002; Martin et al., 2002; Jaeglé et al., 2003; Li et al., 2005; Hudman et al., 2007; Zhang et al., 2008]. The model is driven by assimilated meteorological observations from the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System (GEOS-4) from the Global Modeling and Assimilation Office (GMAO). The model simulates detailed tropospheric O3-NOx-hydrocarbon chemistry, including the radiative and heterogeneous effects of aerosols. We employ version v7-02-04 of GEOS-Chem in this study. The horizontal resolution is 4° latitude by 5° longitude, degraded from the GEOS-4 native resolution of 1° latitude by 1.25° longitude. There are 30 vertical levels, with ∼17 levels in the troposphere from 1000 to 100 hPa. The advection, wet deposition, and dry deposition schemes are based on work by Lin and Rood [1996], Liu et al. [2001], and Wang et al. [1998], respectively. For moist convection, the model treats deep and shallow convection separately following the schemes of Zhang and McFarlane [1995] and Hack [1994].

[8] In this version of GEOS-Chem, the global photochemical source of tropospheric ozone is 5220 Tg O3 yr−1 [Wu et al., 2007], which is comparable to the ensemble average of ∼5660 Tg O3 yr−1 reported by Stevenson et al. [2006] from simulations from 21 global models. Anthropogenic emissions are from the Global Emissions Inventory Activity (GEIA) [Benkovitz et al., 1996], with emissions in the United States based on the Environmental Protection Agency (EPA) National Emission Inventory 1999 (NEI99) [Hudman et al., 2007]. Emissions from biofuel combustion and biomass burning are from Yevich and Logan [2003] and Duncan et al. [2003], respectively. Global NOx emissions from lightning are specified at 4.7 Tg N yr−1, based on the parameterization of Price and Rind [1992], in which lightning activity is determined using convective cloud top heights, and with the vertical distribution of the NOx emissions imposed according to Pickering et al. [1998]. These emission climatologies represented the standard configuration of v7-02-04 when this study was initiated.

[9] GEOS-Chem does not include an explicit treatment of stratospheric ozone chemistry. Instead, the linearized ozone scheme (Linoz) by McLinden et al. [2000] is used to simulate the distribution of ozone in the stratosphere. For the tagged ozone analysis described below, we use a specified synthetic ozone tracer (Synoz) [McLinden et al., 2000] in the stratosphere to impose an ozone flux boundary condition of 495 Tg O3/year at the tropopause, ensuring that the source of ozone from the stratosphere is not overestimated in the analysis. As described by Liu et al. [2009], a comparison of the simulated ozone distribution over the Middle East using the Linoz and Synoz schemes shows that these two schemes result in ozone differences in the region that are less than 3%.

[10] To investigate the interannual variations in ozone due to transport, a tagged ozone simulation is conducted from January 1986 to December 2006, using 1986 for model spin-up. In the tagged ozone simulation, the tropospheric ozone chemistry is linearized using archived production rates and loss frequencies of odd oxygen (Ox), with separate tracers for odd oxygen produced in different regions. This approach has been used by several previous studies [e.g., Li et al., 2001, 2002; Fiore et al., 2002; Koumoutsaris et al., 2008; Zhang et al., 2008] to quantify the regional sources of tropospheric ozone. In the analysis presented here odd oxygen is defined as Ox = O3 + O + NO2 + 2NO3 because NO2 and NO3 are short-lived precursors of ozone, which rapidly produce ozone once formed. Since ozone accounts for most of Ox, we refer to ozone instead of Ox for clarity. Daily odd oxygen production rates and loss frequencies are first generated and archived from a full chemistry model simulation. Then, GEOS-Chem is run again in a tagged ozone mode using the archived ozone production and loss data.

[11] The regional definitions for the tagged ozone simulation are as follows: the Middle East (15°N–35°N, 30°E–60°E), Asia (5°N–60°N, 60°E–145°E), North America (15°N–70°N, 170°W–65°W), South America (55°S–15°N, 90°W–30°W), North Africa (15°N–35°N, 20°W–30°E), South Africa (35°S–15°N, 20°W–55°E), Europe (35°N–70°N, 15°W–60°E), and Australia (45°S–5°N, 90°E–160°E). Two additional regions are the stratosphere and the rest of the world (ROW), the latter including all of the troposphere outside the above-defined areas. We focus here mainly on the Middle East as a receptor region.

[12] Since the focus of this work is on the impact of transport on the ozone distribution, the meteorological fields are allowed to vary from year to year, but the ozone chemical production and loss rates are kept constant, similar to the approach applied by Li et al. [2002] and Liu et al. [2005]. For this purpose, the tagged ozone analysis from 1986 to 2006 is conducted using archived daily ozone production and loss data for 2005. To test the robustness of the simulation results, we conducted 20-year simulations with chemical rates appropriate for 1990 and 1995. These years were randomly selected for the comparison analysis. Ozone production and loss rates in 1990, 1995, and 2005 represent different chemical conditions, but we find that the differences in the net ozone production rates between the three years, and between each year and the 20-year mean production rates, were about an order of magnitude smaller than the net ozone production rates. Consequently, as described in Section 4.4, using the 1990, 1995, or 2005 rates have a negligible impact on our analysis of the interannual variations in ozone. Note as the years for chemistry inputs are randomly selected, there may be greater variability in production than those from the three sets of chemical fields used. In Section 4.4 we also show that the interplay between chemistry and transport is also an important source of interannual variability, although our focus here is on the variability associated only with transport.

2.2. Statistical Analysis

[13] The statistical analysis is conducted at a monthly time step. Monthly means for the ozone mixing ratio are calculated from the GEOS-Chem fields, which were archived every six hours. The anomaly of a variable in a given month for a year (e.g., anomaly of ozone mixing ratio or anomaly of wind speed) is calculated from the difference between the monthly mean of the variable in that year and the long-term mean of the variable in that month.

[14] For variables x and y, the best estimate of their population covariance can be calculated from their sample covariance

  • equation image

where X and Y are the long-term mean of x and y, respectively. n is the number of years (20 years for this study). xi and yi are monthly mean of x and y for year i, respectively. Therefore, σx2 and σy2, the variance of x and y, respectively, can also be obtained from equation (1) using x or y only for the two variables. Furthermore, the correlation coefficient r between x and y is calculated from

  • equation image

where σx and σy are the standard deviations of x and y, respectively. In our analysis, we usually normalize variable x first by

  • equation image

Consequently, σequation image = 1 and thus the resultant covariance σequation imagey has the same magnitude as σy since ∣r∣ ≤ 1. It also has the same unit as σy because equation image is dimensionless. For a three-dimensional domain, all the equations apply to each grid box.

[15] A significance level test is critical in climate statistical analyses as the test provides an objective criterion for rejecting or accepting a hypothesis [von Storch and Zwiers, 1999]. We use the Student's t-test to assess statistical significance of covariance or correlation coefficient between variables of interest [von Storch and Zwiers, 1999].

3. A 20-Year Climatology of the Tropospheric Ozone Distribution in the Middle East

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Model Description and Statistical Analysis
  5. 3. A 20-Year Climatology of the Tropospheric Ozone Distribution in the Middle East
  6. 4. Interannual Variations in Ozone Sources
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[16] Figure 2 shows the GEOS-Chem simulated ozone mixing ratio at 400 hPa in July averaged from 1987 to 2006. It shows spatially that ozone mixing ratios are low over the tropical oceans, whereas they are high over the Northern Hemisphere, over the biomass burning regions in the Southern Hemisphere, and across the Atlantic and the Indian oceans. Ozone mixing ratios are also high over eastern North America and over Eurasia. The high ozone abundances (up to 100 ppbv) over the Middle East and North Africa stand out in this 20-year mean. This distribution is similar to that shown by Liu et al. [2009] for summer 2005 and is consistent with TES observations in the region for 2005–2007 [Worden et al., 2009]. In agreement with Liu et al. [2009], we find that the ozone build up in the 20-year climatology is also located in the mid-troposphere and is at a maximum in summer. During the 20 years, ozone mixing ratio averaged over the Middle East in July at 400 hPa fluctuates by 7% about the mean (or ±6 ppbv).

image

Figure 2. GEOS-Chem simulated mean ozone mixing ratios (in ppbv) in July averaged from 1987 to 2006 at 400 hPa.

Download figure to PowerPoint

[17] The 20-year averaged ozone mixing ratio at 400 hPa over the Middle East and the ozone contributions from the different source regions are given in Table 1. This is also consistent with the annual analysis shown by Liu et al. [2009, Figure 8]. In the boreal summer, the Asian and local ozone sources provide the dominant contribution to the ozone mixing ratios over the Middle Eastern mid-troposphere. The ozone mixing ratio over the Middle East at 400 hPa averaged in summer is 77 ppbv, the Asian and Middle Eastern sources being 24 and 18 ppbv each, contributing about 31% and 23%, respectively, to the total ozone mixing ratio. The next largest contributions are from North Africa and South Africa (∼10% each), North America, the ROW, and the stratosphere (∼6% each). The other regions contribute little. In the boreal winter, ozone from outside the Middle East and Asia provides the dominant contribution to the total ozone mixing ratios.

Table 1. Seasonal Variation of Mean Ozone Mixing Ratios (in ppbv) in the Middle East at 400 hPa in Total and Contributions From Different Source Regions Over the 20 Years 1987–2006a
 DJFMAMJJASON
  • a

    In parentheses is the standard deviation (in ppbv). DJF denotes the mean of December, January, and February; MAM refers to March, April, and May; JJA refers to June, July, and August; SON refers to September, October, and November.

Total60 (2.9)70 (3.1)77 (3.7)61 (3.0)
Asia6 (0.9)7 (1.2)24 (4.2)14 (3.4)
Middle East2 (0.3)5 (1.1)18 (2.8)9 (1.9)
North Africa3 (0.4)5 (0.6)8 (1.3)6 (1.0)
South Africa14 (2.8)16 (3.4)8 (1.3)12 (2.6)
North America5 (0.8)8 (1.1)5 (1.2)6 (1.0)
South America9 (1.5)6 (1.2)1 (0.3)2 (0.5)
Europe1 (0.2)1 (0.3)3 (0.6)2 (0.5)
Australia2 (0.3)1 (0.2)1 (0.1)1 (0.2)
Rest of the World9 (0.6)9 (0.8)5 (0.8)6 (0.6)
Stratosphere10 (1.9)11 (1.9)5 (0.9)3 (0.5)

[18] As shown in Table 1, the seasonal variations of the Asian and local ozone sources are larger than the variations in the total ozone mixing ratio over the Middle East. The ratio of the summer to winter (JJA/DJF) for the Asian and local ozone sources are 4.0 and 9.0, respectively, whereas the JJA/DJF ratio for the total ozone mixing ratio in the region is only 1.3. This reflects the seasonal compensation between the Asian and local ozone sources and the other source regions as discussed by Liu et al. [2009].

[19] The values of the standard deviation in Table 1 show that the variability in the Asian source is the largest in the boreal summer and fall, with a standard deviation of 3–4 ppbv, likely due to the Asian monsoon. Locally produced ozone also varies greatly in the boreal summer and fall with a standard deviation of 2–3 ppbv. The variability in ozone from all the source regions in the Northern Hemisphere and the ROW is larger in the boreal summer than in winter, whereas the variability in ozone from all the source regions in the Southern Hemisphere and from the stratosphere is smaller in the boreal summer than in winter.

4. Interannual Variations in Ozone Sources

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Model Description and Statistical Analysis
  5. 3. A 20-Year Climatology of the Tropospheric Ozone Distribution in the Middle East
  6. 4. Interannual Variations in Ozone Sources
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

4.1. Variations in Long-Range Transport of Ozone From Asia

4.1.1. Interannual Variability of Asian Ozone Source and Related Mechanisms

[20] Figure 3a shows the time series of the anomalies in Asian ozone in July averaged over the Middle East at 400 hPa, as simulated by GEOS-Chem. This is based on the 20-year simulation using 2005 chemical production rates and loss frequencies for ozone. The Asian ozone anomalies fluctuate by ±8 ppbv or ±30% from the 20-year mean of 28 ppbv. The two most extreme years are 1994 and 2002, with the largest positive and negative anomalies, respectively. Since the chemical production rates and loss frequencies are fixed from year to year, these anomalies are due solely to interannual variations on transport.

image

Figure 3. Anomalies of (a) Asian ozone and (b) locally produced ozone in July from 1987 to 2006, averaged over the Middle East at 400 hPa (in % on the left y axis and in ppbv on the right y axis).

Download figure to PowerPoint

[21] The distributions of the Asian ozone source in the mid-troposphere at 400 hPa, overlaid with the horizontal winds for the two extreme years, 1994 and 2002, are shown in Figures 4a and 4b. High ozone mixing ratios appear in two regions in Asia: one in the east over eastern China and the other in the west over South Asia. As shown in the wind field, the ozone in the eastern part is transported eastward to the Pacific ocean and then to North America, whereas the ozone in the western part, i.e., over South Asia, is transported westward to the Middle East along the tropical easterly jet (between 20°E–90°E and 15°N–25°N). The location of high ozone abundances in South Asia corresponds to the location of high ozone production, as shown by Li et al. [2001] and Liu et al. [2009]. Park et al. [2004] also showed a lightning-induced upper tropospheric NOx maximum over the Asian monsoon region from a MOZART model simulation. In the upper troposphere, a large portion of the lifted ozone over the eastern China is circulated southward and then westward around the SAH, as pointed out by Liu et al. [2002]. This transport pathway can also be seen in Figure 1a over 20°N–30°N.

image

Figure 4. Asian ozone (in ppbv) at 400 hPa in July for (a) 1994 and (b) 2002. Longitude-altitude cross section of Asian ozone (in ppbv) along 26°N in July for (c) 1994 and (d) 2002. All are overlaid with winds. The vertical wind velocity in Figures 4c and 4d is enlarged by 100 times for illustration purposes. White areas denote topography. The altitude 400 hPa is marked with a line in Figures 4c and 4d.

Download figure to PowerPoint

[22] A striking difference between the ozone distributions in 1994 and 2002 shown in Figures 4a and 4b is the absence of significant amounts of Asian ozone over the Arabian Peninsula in 2002. The vertical structure of the ozone distribution and the wind fields between the two years is compared in Figures 4c and 4d. The easterly jet in the subtropics is clearly stronger in 1994 than in 2002, in particular between 400 and 100 hPa and 80°E–30°E. The mean wind speed averaged over an area of 15°N–30°N and 50°E–90°E at 300 hPa is −6.2 m s−1 and −2.8 m s−1 for 1994 and 2002, respectively (the negative values indicate westward winds). As shown in Figures 4c and 4d, the peak in Asian ozone is further east in 2002 than in 1994, with much less ozone transported to the west, especially in the middle troposphere over the Middle East. The 20-year mean winds and the July wind anomalies at 300 hPa in 1994 and 2002 are shown in Figure 5. On average, easterlies prevail south of 20°N in July while westerlies dominate north of 30°N (Figure 5a). Between 20°N–35°N, extending from South Asia to the Middle East (80°E to 40°E), there are negative anomalies in zonal winds in 1994 (Figure 5b), indicating stronger than normal easterlies, whereas positive anomalies are present in the same region in 2002 (Figure 5c). Consequently, there is more Asian ozone transported to the Middle Eastern middle troposphere in 1994 than in 2002.

image

Figure 5. For July at 300 hPa: (a) wind climatology, (b) anomalies in 1994, and (c) anomalies in 2002. Unit: m s−1.

Download figure to PowerPoint

[23] The covariance between the zonal winds and Asian ozone are shown in Figure 6. The Asian ozone anomalies in the Middle East are mostly correlated to zonal winds in the subtropical upper troposphere between 30°E–80°E (Figure 6a); the stronger the easterlies (more negative), the greater the transport of Asian ozone to the Middle East. Furthermore, the covariance between the easterlies and the geopotential heights at 150 hPa show a coherent structure across Eurasia, suggesting a teleconnection associated with the geopotential heights at the upper troposphere (Figure 6b). The tropical easterly jet is strongly correlated with geopotential heights at 150 hPa over central Asia, near 30°N–40°N and 40°E–80°E (Figure 6b). Higher pressures in this region are linked to stronger (more negative) easterlies in the subtropics. Hoskins and Wang [2006] described the close link between the SAH and the easterly jet. They pointed out that the Tibetan Plateau is a major factor determining the character of both the SAH and the easterly jet. As a result, the Asian ozone anomalies are positively correlated with the geopotential height at 150 hPa (Figure 7) and the largest covariance is located over the region in central Asia (30°N–40°N, 50°E–80°E), similar to Figure 6b.

image

Figure 6. (a) Covariance (in m s−1) between easterly winds and the Asian ozone anomaly (in Figure 3a) in a longitude-altitude cross section along 26°N. Brown areas denote topography. (b) Covariance (in m) between the geopotential height at 150 hPa and the easterly winds averaged over a region of 20°N–30°N and 30°E–80°E from 300 to 100 hPa (the boxed area in Figure 6a). Filled contours are statistically significant at the 90% level using the Student's t-test. Data source for the geopotential heights: NCEP/NCAR reanalysis [Kalnay et al., 1996]. See Section 2 for the statistical methods.

Download figure to PowerPoint

image

Figure 7. Covariance (in m) between the Asian ozone anomaly (in Figure 3a) and the geopotential height at 150 hPa. Filled contours are statistically significant at the 90% level using the Student's t-test. Data source for the geopotential heights: NCEP/NCAR reanalysis [Kalnay et al., 1996]. See Section 2 for the statistical method.

Download figure to PowerPoint

[24] The core with covariance at the 90% significant level in Figure 7 coincides with one of six centers in the upper troposphere where the interannual variations in the geopotential height are large [Ding and Wang, 2005]. Ding and Wang [2005] proposed that the six centers compose a circumglobal teleconnection (CGT) pattern in the summertime midlatitude circulation of the Northern Hemisphere linked to the Asian summer monsoon. This CGT is apparently reflected in climate anomalies in rainfall and surface temperature over western Europe, European Russia, northwest India, eastern Asia, and North America [Ding and Wang, 2005]. This study shows that this CGT is also linked to the interannual variation in transport of air pollution.

4.1.2. Relationships Between the Westward Transport of Asian Ozone and the Asian Summer Monsoon

[25] Webster et al. [1998] suggested three large-scale circulations associated with the Asian monsoon: a transverse monsoon circulation to the west of the monsoon region, a lateral monsoon circulation to the south, as well as a circulation to the east. The ascending regions of these circulations are all collocated over South Asia [Webster et al., 1998]. Ye and Wu [1998] found from the NCEP reanalysis data that the lateral circulation extends between 55°E and 140°E. They also suggested an additional circulation to the north between 40°N and 47°N. Ye and Wu [1998] found that the outflow of ascending air over the Tibetan Plateau is separated into two branches: an eastern branch that flows eastward and descends east of 170°E, and a western branch that flows westward and then descends over Afghanistan, Iran and Saudi Arabia. The transverse circulation to the west is responsible for the westward transport of the Asian pollution. When deep convection is intensified, the transverse circulation, and thus transport of the Asian outflow to the Middle East, is intensified. In the meantime, the lateral circulation to the south, which is related to the Indian summer monsoon, is likely to be strengthened as well.

[26] We use two independent monsoon indices to examine the correlation between the Asian monsoon and the Asian ozone anomalies over the Middle East, shown in Figure 3a. One index is the All-India Monsoon Rainfall (AIMR) anomalies over India for JJAS (June–July–August–September), based on the measurements from 306 weather stations [Parthasarathy et al., 1987, 1994]. The rainfall data are available from the Indian Institute of Tropical Meteorology (www.tropmet.res.in). The second index is the Webster-Yang monsoon index, which is circulation-based and is defined according to the strength of the vertical shear in the zonal wind between 850 and 200 hPa averaged over the region 40°E–110°E and 0°N–20°N [Webster and Yang, 1992]. The normalized Webster-Yang monsoon index is available from the Asia-Pacific Data-Research Center (APDRC) of the International Pacific Research Center (IPRC) (iprc.soest.hawaii.edu/users/ykaji/monsoon/seasonal-monidx.html). According to both indices, 1994 is a strong monsoon year while 2002 is a weak monsoon year. Figure 8 shows that the Asian ozone anomaly significantly correlates with the AIMR anomalies (with a correlation coefficient r = 0.67, p < 0.01) and the Webster-Yang monsoon index (with a correlation coefficient r = 0.66, p < 0.01). This suggests that the circulation in favor of the Asian monsoon also enhances transport of Asian ozone to the Middle East.

image

Figure 8. The Asian ozone anomaly in July at 400 hPa correlated to (a) the All Indian Monsoon Rainfall (AIMR) anomaly and (b) Webster-Yang monsoon index (see the text for the definitions and data sources). The correlation coefficient and the p-value in the Student's t-test are indicated by r and p, respectively. The data points for 1994 and 2002 are indicated.

Download figure to PowerPoint

[27] The two extreme years for transport of ozone from Asia are also associated with extreme anomalies in rainfall over India. In 1994 there was significantly greater rainfall than average, whereas in 2002 there was a drought [Chaudhari et al., 2008]. According to Chaudhari et al. [2008], July 2002 had the lowest rainfall recorded in the previous 130 years. Chaudhari et al. [2008] found that 1994 was associated with a negative outgoing longwave radiation (OLR) anomaly over South Asia, in contrast to a positive OLR anomaly in 2002, suggesting stronger than normal convection in 1994 and weaker than normal convection in 2002.

4.2. Transport-Related Variations in Locally Produced Ozone in the Middle East

[28] The anomalies in locally produced ozone are shown in Figure 3b. We define locally produced ozone as ozone produced within the region between 15°N–35°N and 30°E–60°E. Compared with Asian ozone (Figure 3a), locally produced ozone varies less year-to-year in the mid-troposphere in summer, with a range of variability within ±3 ppbv or ±15% from the mean. Unlike Asian ozone, the local ozone anomalies show no extrema in 1994 and 2002. As discussed by Liu et al. [2009], the confinement of locally produced ozone in the mid-troposphere of the Middle East is a result of the presence of the anticyclone over the region. A comparison of the anomalies in the geopotential heights at 400 hPa in July between years with positive local ozone anomalies and years with negative local ozone anomalies reveals differences in geopotential heights of about ±5 m over the Arabian Peninsula (not shown). The covariance between locally produced ozone anomalies and the geopotential heights over the 20-year period at 400 hPa is shown in Figure 9. The areas with largest covariances (∼10 m) are located over the northeast of the Arabian Peninsula, within the anticyclone (shown in Figure 1b), where the correlation coefficient between the two variables reaches ∼0.8.

image

Figure 9. Covariance (in m) between the locally produced ozone anomaly (in Figure 3b) and the geopotential heights at 400 hPa. Filled contours are statistically significant at the 90% level using the Student's t-test. Data source for the geopotential heights: NCEP/NCAR reanalysis [Kalnay et al., 1996].

Download figure to PowerPoint

[29] In the GEOS-Chem simulation, the chemically produced ozone is the same throughout the 20 years. It is the atmospheric circulations that control how the locally produced ozone is distributed. The simulated 20-year mean of this ozone is localized over the Middle East (not shown; see Figure 9a of Liu et al. [2009] for a case in 2005), reflecting the confinement by the anticyclone in the mid-troposphere. Nevertheless, the spatial distribution of this ozone varies from year to year. For example, 2003 was a year with a maximum anomaly, indicating that locally produced ozone was mostly confined in the region in the 20-year period. This extreme was associated with a strong Iranian High, which was split into two and resulted in two high ozone centers over the Middle East. In the cases of 1988 and 1996 with negative extrema in ozone anomalies, the extreme in 1996 was due to the weakest Iranian High in the 20-year period and the high ozone center was shifted into the southwestern part of the Middle East, while in 1988, a large amount of locally produced ozone was transported out to the northeast of the anticyclone by the westerlies and out to the southwest by the easterlies.

4.3. Variability in Long-Range Transport of Ozone From Other Regions

[30] Although the ozone produced locally and the ozone transported from Asia predominantly contribute to the total ozone mixing ratios over the Middle East in summer, we find that the interannual variations in the ozone mixing ratios in the Middle East are only partially explained by the variations of these two large sources. For example, in Figure 10, the correlation coefficient is 0.54 between the ozone anomalies and the sum of Asian and locally produced ozone anomalies, suggesting that the interannual variations in ozone in the Middle East is also likely to be influenced by ozone transported from other regions.

image

Figure 10. Correlation between the total ozone anomaly and the sum of Asian and locally produced ozone anomalies in the Middle East in July at 400 hPa. The correlation coefficient and the p-value in the Student's t-test are indicated by r and p, respectively. The 1:1 line is indicated by a dashed line.

Download figure to PowerPoint

[31] As shown by Liu et al. [2009], and in Table 1, in the middle troposphere of the Middle East there is a seasonal trade-off between ozone produced in Asia and the Middle East and ozone produced outside these regions, with the total contribution of ozone from outside of the Middle East and Asia at a minimum in summer. We find that there is a similar trade-off in the year-to-year variations between ozone from Asia and the Middle East and ozone from other regions. For example, the year-to-year variations in ozone transported from North America are strongly anti-correlated with the Asian ozone anomalies over the Middle East, with a correlation coefficient of r = −0.75 in July at 400 hPa. Similarly, the correlation between the ROW anomalies and the Asian anomalies in July is r = −0.87. The contributions from both of these regions were a maximum in 2002 and a minimum in 1994, when the Asian source was at a minimum and a maximum, respectively. Therefore, although the locally produced ozone and ozone transported from Asia are the dominant contributions to ozone abundances over the Middle East, transport of ozone from other regions is important in the context of driving the interannual variations in ozone in the Middle East.

[32] According to Liu et al. [2009], much of the ozone transported from North America to the Middle East originates in the North American boundary layer and is exported to the upper troposphere over the North Atlantic, where it is transported along the subtropical westerly jet and descends into the Middle East, near the Mediterranean. Shown in Figure 11 is a latitude-altitude cross section of the zonal wind along at 40°E (across the Middle East). In 1994, when the North American ozone contribution was at a minimum, the subtropical westerly jet was stronger and shifted further north, to between 40°N–50°N, compared to 2002, when the North American ozone contribution was at a maximum and the jet was weaker and located between 30°N–45°N. We find that when the westerly jet was further south like in 2002, the Arabian anticyclone circulated the North American ozone, descending from the upper troposphere, around the eastern flank of the anticyclone and into the Middle East region. In contrast, when the westerly jet was further north like in 1994, the North American ozone contribution was confined north of the Middle East region.

image

Figure 11. Zonal winds (in m s−1) in a latitude-altitude cross section in July at 40°E in (a) 1994 and (b) 2002. The latitude of 35°N is marked with a line. Brown areas denote topography.

Download figure to PowerPoint

[33] In the Northern Hemisphere, the ROW region includes the Northern Hemispheric Pacific and Atlantic Oceans. It is ozone produced in these regions (downwind of Asia and North America) that is transported to the Middle East through the subtropical westerlies. Little ozone in the Southern Hemisphere of the ROW is transported to the Northern Hemisphere in the boreal summer (similarly South American and Australian ozone abundances are small in the Middle East in the boreal summer as shown in Table 1). Similar to the case for North America, we find that when the jet shifts to the north in 1994, less ozone from the ROW is transported through the westerly jet to the Middle East. Conversely, when the jet shifts to the south in 2002, more ozone from the ROW is transported into the Middle East.

[34] The subtropical westerly jet over Asia exhibits seasonal variations in strength and its meridional shift. In summer, the core of the jet over Asia is generally located near 200 hPa with wind speeds around 20–40 m s−1, compared with 50–70 m s−1 in winter [Zhang et al., 2006; Schiemann et al., 2009]. From winter to summer, the jet transits northward [Zhang et al., 2006; Schiemann et al., 2009]. Schiemann et al. [2009] found that during the northward transition, the jet intensity and its latitudinal location vary greatly from year to year. As shown in Figure 4a, in 1994 the jet was characterized with strong cyclonic flow over Europe and the equatorward component of the flow contributed to the stronger westerly jet in central Asia. In 2002, the cyclonic flow over Europe was weaker (see Figure 4b) and extended into Siberia, which contributed to a weaker westerly jet that was shifted further south over the Middle East.

[35] Our analysis suggests a strong connection between the trade-off in transport from the different source regions and the meridional location and strength of the subtropical westerly jet. The Asian ozone anomalies are negatively correlated with zonal winds at 200 hPa over a region of 20°N–40°N and 30°E–90°E from South Asia to the Middle East at the 90% significance level (Figure 12a). In contrast, over almost the same latitudes, positive correlations of 0.5–0.8 are found between zonal winds and the North American and ROW ozone anomalies (Figures 12b and 12c), although the details are slightly different. The American and ROW ozone anomalies are more sensitive to zonal wind over the Middle East (30°E–60°E), while the Asian ozone anomalies are more sensitive to zonal wind from Asia to the Middle East (60°E–80°E). A more southward shift of the westerly jet is linked to stronger westerly flow over the Middle East, which favors transport of ozone from North America and ROW to the region. The transport of ozone from these regions into the Middle East is through strong descent over North Africa and the eastern Mediterranean. When the westerly jet is displaced north, the transport of this ozone down into the Middle East is restricted.

image

Figure 12. Correlation between zonal winds at 200 hPa and averaged ozone anomalies over the Middle East from (a) Asia, (b) North America, and (c) ROW. Filled areas are statistically significant at the 90% level using the Student's t-test. All values are for July.

Download figure to PowerPoint

[36] In their analysis, Liu et al. [2009] found that transport from the stratosphere does not contribute significantly to the ozone maximum over the Middle East. Similarly, we find that the interannual variations in transport of ozone from the stratosphere is not significantly correlated with variations in ozone in the Middle Eastern mid-troposphere (r = 0.26). In addition, the stratospheric ozone contribution is only weakly anti-correlated with the Asian contribution in the Middle East (r = −0.30).

[37] Previous studies have related long-range transport of pollution with the state of the climate system through the use of climate indices, such as El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). It was found that ENSO is linked to year-to-year variability of Asian outflow toward North America and Europe [Liu et al., 2003, 2005; Koumoutsaris et al., 2008]. Liu et al. [2003] found that the Asian outflow in spring is strong in the upper troposphere under La Niña conditions while Koumoutsaris et al. [2008] found that El Niño conditions in winter favor the export of Asia pollutants toward Europe in the spring of the subsequent year. Moreover, Liu et al. [2005] suggested that the correlation between ENSO and Asian outflow toward North America is seasonally dependent: it is strong in winter but weak in summer with spring and fall as intermediate cases. In our analysis we found no significant correlation between ENSO (using the Southern Oscillation Index (SOI)) and the westward transport of Asian ozone to the Middle East in summer. The SOI index in July is also insignificantly correlated with the ozone contributions from North America and the ROW. Lin and Lu [2009] suggested that the subtropical westerly jet stream in summer may be correlated with ENSO in the previous winter. We explored possible links between the SOI index in the previous winter and Asian or American ozone anomaly over the Middle East in summer and found only weak correlations (r ≈ ±0.3) that are not significant (p > 0.1).

[38] The NAO index is found to be closely related to interannual variations in pollution transport from North America to Europe [Li et al., 2002] and from Europe to other continents [Duncan and Bey, 2004]. A positive NAO phase means a strong north-south pressure gradient over the Atlantic Ocean and this condition usually enhances the transport of North American boundary layer pollutants to Europe [Li et al., 2002]. However, it seems that after American ozone reaches Europe, its transport to the Middle Eastern mid-troposphere is not closely linked to NAO as we found only a weak correlation between the NAO and North American ozone over the Middle East.

4.4. Sensitivity to Linearization of the Ozone Chemistry

[39] To assess the potential impact of our use of chemical fields for 2005 to linearize the ozone chemistry, we repeated the analysis using chemical fields for 1990 and 1995. We found that the differences in the chemical fields had a small impact on the estimated anomalies. For example, the magnitude of the Asian ozone anomalies in 1994 were 26.7% over the Middle East using the 2005 chemistry fields, whereas they were 29.2% and 29.5% using the 1990 and 1995 chemistry fields, respectively. In 2002, the Asian ozone anomalies were −26.6% with the 2005 chemical rates, compared with −27.6% and −27.0% using the 1990 and 1995 chemical rates, respectively. Similarly, the differences in ozone produced over the Middle East were small. Similarly, the differences in the anomalies of locally produced ozone with different chemistry inputs were small. For example, the largest difference between the anomalies in locally produced ozone using 2005 and 1990 chemical inputs was 1.9%.

[40] We also compared the tagged ozone simulation with a full chemistry run over the 20-year period to assess the potential contribution of nonlinearity in the ozone chemistry to the year-to-year variations in ozone. We found that the largest differences in the ozone abundances in the tagged ozone run, relative to the full chemistry simulation, was 3.8% in July 1992. For 1994 and 2002, the years with the extrema in the Asian ozone contribution to the Middle East, the tagged ozone run produced ozone abundances that were lower than the full chemistry run by −0.7% and −0.3%, respectively. Figure 13 shows the correlation between the ozone anomalies over the Middle East obtained with the tagged ozone simulation (using the 2005 chemistry fields) and the full chemistry run. A significant correlation of r = 0.82 is found between the two sets of anomalies. We also obtained correlations of r = 0.79 and r = 0.83 for the ozone anomalies based on the 1990 and 1995 chemistry fields, respectively (not shown). This suggests that in these simulations, transport can explain over 60% of the interannual variability in the ozone mixing ratios in the Middle Eastern mid-troposphere in summer.

image

Figure 13. Comparison of ozone anomalies in July at 400 hPa between a full chemistry simulation and a tagged ozone simulation using 2005 chemistry input. The 1:1 line is indicated by a dashed line. The correlation coefficient and the p-value in the Student's t-test are indicated by r and p, respectively.

Download figure to PowerPoint

[41] Although our focus in this paper is on the impact of interannual variations in transport on ozone abundances over the Middle East, our analysis suggests that the nonlinear interplay between chemistry and transport could also provide a significant contribution to interannual variations in tropospheric ozone over the Middle East, and should be further explored to better understand the underlying mechanisms driving these variations.

5. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Model Description and Statistical Analysis
  5. 3. A 20-Year Climatology of the Tropospheric Ozone Distribution in the Middle East
  6. 4. Interannual Variations in Ozone Sources
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[42] We used the GEOS-Chem model to examine the impact of interannual variations between 1987 to 2006 in atmospheric transport on the summertime ozone abundance over the Middle East. We found that in the 20-year mean, transport of ozone from Asia and locally produced ozone are the dominant sources of ozone in the middle troposphere over the Middle East in summer, consistent with the analysis of Liu et al. [2009] for 2005. We found that the year-to-year variations in the locally produced ozone were about ±3 ppbv (or ∼15%) about the 20-year mean. We found that these variations were correlated with the strength of the Arabian anticyclone, which acts to isolate the middle troposphere over the Middle East.

[43] The Asian contribution to ozone abundances in the Middle East mid-troposphere fluctuated by ±8 ppbv (or ±30%) about the mean during the 20 years, with a maximum contribution in 1994 and a minimum in 2002. These two extreme years were associated with extreme precipitation anomalies in India, with a severe drought in July 2002 and greater than average rainfall in July 1994. Our analysis suggests that the interannual variability in transport of ozone from Asia to the Middle East is linked to the strength of the SAH, through its influence on the intensities of easterlies in the subtropical upper troposphere. In particular, we found that variability in transport from Asia was strongly correlated with variations in the geopotential heights at 150 hPa over central Asia (to the northwest of the SAH). This area coincides with one of six centers in the upper troposphere that compose a circumglobal teleconnection (CGT) pattern in the summertime midlatitude circulation of the Northern Hemisphere [Ding and Wang, 2005]. This study also suggests that the variation of the Asian ozone source is closely correlated with the strength of the summer monsoon in South Asia, as indicated by two monsoon indices.

[44] Our analysis shows that interannual variations in transport of ozone from Asia to the Middle East are strongly anti-correlated with variations in transport from other regions in the Northern Hemisphere. We found that this trade-off in transport into the Middle East was linked to the position of the subtropical westerly jet. When the westerly jet is displaced further north, transport of ozone from Asia is enhanced and transport from North America and other regions in the Northern Hemisphere is diminished. In contrast, a southern shift of the westerly jet will weaken the Asian influence and enhance the influence of North America and other regions. Studies [e.g., Yin, 2005] have shown that increasing atmospheric abundances of greenhouse gases could produce a poleward shift in the subtropical westerly jet in the Northern Hemisphere. Our results suggest that such climate-related changes in the subtropical jet could have significant implications for transport of pollution into the Middle East. A northward displacement of the jet combined with increasing anthropogenic emissions in Asia could result in increasing ozone abundances over the Middle East, with implications for an increased local radiative forcing of the climate system.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Model Description and Statistical Analysis
  5. 3. A 20-Year Climatology of the Tropospheric Ozone Distribution in the Middle East
  6. 4. Interannual Variations in Ozone Sources
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[45] This work was supported by funding from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canadian Foundation for Climate and Atmospheric Sciences. Liu thanks NSERC, Environment Canada, and Canadian Space Agency for their financial support. We are grateful to Kimberly Strong and Paul Kushner for their valuable suggestions and to Hongyu Liu and the other two reviewers for their constructive comments. We acknowledge the NCEP/NCAR reanalysis data from the NOAA Physical Sciences Division, the Indian precipitation data from the Indian Institute of Meteorology, and the normalized Webster-Yang monsoon index from the Asia-Pacific Data-Research Center. The GEOS-Chem model is managed by the Atmospheric Chemistry Modeling Group at Harvard University with support from the NASA Atmospheric Chemistry Modeling and Analysis Program (ACMAP).

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Model Description and Statistical Analysis
  5. 3. A 20-Year Climatology of the Tropospheric Ozone Distribution in the Middle East
  6. 4. Interannual Variations in Ozone Sources
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Model Description and Statistical Analysis
  5. 3. A 20-Year Climatology of the Tropospheric Ozone Distribution in the Middle East
  6. 4. Interannual Variations in Ozone Sources
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
jgrd17362-sup-0001-t01.txtplain text document1KTab-delimited Table 1.

Please note: Wiley Blackwell is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.