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    Composite anomalies of Hadley Centre SST (shading) for (left) El Niño and (right) La Niña events during (top) January, (middle) February, and (bottom) March. The composited NOAA OLR (contours) is also shown for reference. Positive and negative contours are represented by solid and dashed lines, respectively. (a),(c),(e) Only OLR values smaller than −5 W m−2 are shown. (b),(d),(f) Only OLR values greater than 5 W m−2 are shown. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test.

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    Composite anomalies of NIWA TOC for (a)–(c) El Niño and (d)–(f) La Niña events during (left) January, (center) February, and (right) March. (g)–(i) TOC differences between El Niño and La Niña events. The black rectangles in (a) represent the North Pacific, East Asia, the southern United States, and Europe, for which ozone profiles are averaged in Fig. 5. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test. The statistical significance of composite anomalies for El Niño events in (a)–(c) or La Niña events in (d)–(f) is calculated according to the difference between El Niño or La Niña and the climatology.

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    Differences between NIWA TOC variations related to N3I during El Niño years and those during La Niña years. The ozone variations related to N3I can be see text for more details. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test.

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    Differences in modeled TOC between runs R1 and R2 during (a) January, (b) February, and (c) March. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test. The black rectangles in (a) represent North Pacific, East Asia, the southern United States, and Europe, for which ozone profiles are averaged in Fig. 5.

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    Vertical distribution of differences in partial ozone column between El Niño and La Niña events during (left) January, (center) February, and (right) March. Note that the ozone partial columns are centered on the pressure levels. The ozone profiles derived from cohesive SBUV ozone data (red) and WACCM simulations (blue) are averaged over the regions indicated by the black rectangles in Figs. 2a and 4a, respectively. The regions include (a)–(c) East Asia, (d)–(f) the southern United States, (g)–(i) the North Pacific, and (j)–(l) Europe.

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    (left) Differences of relative vorticity at 200 hPa (color shaded) and thermal tropopause height (contours with 0.2-km interval; positive and negative contours are presented by the solid and dashed lines, respectively) between El Niño and La Niña events based on ERA-Interim data during (top) January, (middle) February, and (bottom) March. (right) Also shown are differences in the modeled thermal tropopause pressure between runs R1 and R2 during (b) January, (d) February, and (f) March. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test. Plots of the Rossby wave ray paths (thick solid lines) with zonal wavenumbers 2 (red) and 4 (black) starting at 15°N, 210°E following the wave for 12 days are also superimposed on the relative vorticity composite maps. The 1979–2009 climatological mean for each month derived from ERA-Interim and WACCM data are used as the basic state to derive the ray tracings for (left) and (right), respectively.

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    Differences in horizontal winds (vectors with scale of 10 m s−1) and pressure on the 370-K isentropic surface derived from (left) ERA-Interim data and (right) WACCM simulations between El Niño and La Niña events during (top) January, (middle) February, and (bottom) March. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test.

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    Differences in the zonal wind (contour interval: 5 m s−1) at 200 hPa and the occurrence frequency of RWB events (shading) on the 330-K isentropic surface derived from (left) ERA-Interim data and (right) WACCM simulations between El Niño and La Niña events during (top) January, (middle) February, and (bottom) March. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test. The frequency of RWB is defined as the number of days with negative meridional gradient in potential vorticity per month, as used by Hitchman and Huesmann (2007).

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    Composite longitude–height cross sections along 45°N of differences in geopotential height (contour interval: 30 m; dashed contours indicate negative values) and temperature (shading) derived from (left) ERA-Interim and (right) WACCM simulations between El Niño and La Niña events. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test.

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    Correlation coefficients between EP flux divergence at 10 hPa averaged over 45°–90°N and TOC tendency derived from (a) NIWA and (b) ERA-Interim data from November to May. The TOC tendency is defined as TOC differences between a given month and its preceding month (Nikulin and Karpechko 2005). Latitude–height cross sections of differences in (c),(d) EP flux divergence and (e),(f) BD circulation vertical velocity (Andrews et al. 1987) are derived from ERA-Interim data associated with (c),(e) El Niño and (d),(f) La Niña events during the period JFM. Climatological mean of EP flux divergence (contours with 0.4 m s−1 day−1 interval) and for the period 1979–2009 (contours with 0. 2 mm s−1 interval) are also shown in (c),(d) and (e),(f), respectively. Positive and negative contours are represented by solid and dashed lines, respectively. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test.

  • View in gallery

    Schematic diagram of the mechanisms through which El Niño events influence the midlatitude ozone. La Niña events have almost the opposite effects on TOC and meteorological fields (not shown). Green thick lines indicate North American and North Africa–Asia jet stream and yellow thick lines denote synoptic-scale RWB events. Curved arrows indicate planetary-scale anomalous horizontal circulations associated with ENSO events. Red and blue circles represent the falling and rising of tropopause height anomalies. In addition, red and blue thick arrow denotes downward and upward transports of ozone associated with tropopause changes, respectively. Black thin straight arrows represent wave-driven circulation due to RWB events.

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The Influence of ENSO on Northern Midlatitude Ozone during the Winter to Spring Transition

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  • 1 Key Laboratory for Semi-Arid Climate Change of the Ministry of Education, College of Atmospheric Sciences, Lanzhou University, Lanzhou, China
  • 2 College of Global Change and Earth System Science, Beijing Normal University, and Joint Center for Global Change Studies, Beijing, China
  • 3 Key Laboratory for Semi-Arid Climate Change of the Ministry of Education, College of Atmospheric Sciences, Lanzhou University, Lanzhou, China
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Abstract

The influence of El Niño–Southern Oscillation (ENSO) on northern midlatitude ozone during the period January–March (JFM) is investigated using various observations and a chemistry–climate model. The analysis reveals that, during El Niño events, there are noticeable anomalously high total ozone column (TOC) values over the North Pacific, the southern United States, northeastern Africa, and East Asia but anomalously low values in central Europe and over the North Atlantic. La Niña events have almost the opposite effects on TOC anomalies. The longitudinal dependence of midlatitude ozone anomalies associated with ENSO events during the period JFM is found to be related to planetary waves. Planetary waves excited by tropical convection propagate into the middle latitudes and give rise to longwave trains (Pacific–North American pattern) and shortwave trains along the North African–Asian jet. These wave trains affect ozone in the upper troposphere and lower stratosphere (UTLS) by modulating the midlatitude tropopause height and cause TOC anomalies by changing the vertical distributions of ozone. In addition, synoptic-scale Rossby wave breaking increases on the poleward flanks of the enhanced westerly jet during El Niño events, leading to a stronger eddy-driven meridional circulation in the UTLS and hence causing TOC increases over the North Pacific, the southern United States, northeastern Africa, and East Asia and vice versa for La Niña events. It is also found that the contribution of changes in Brewer–Dobson circulation due to anomalous planetary wave dissipation in the stratosphere during ENSO events to TOC changes in the middle latitudes for the period JFM is small, not more than 1 Dobson unit (DU) per month.

Corresponding author address: Wenshou Tian, College of Atmospheric Sciences, Lanzhou University, Tianshui South Road 222, Lanzhou, 730000 China. E-mail: wstian@lzu.edu.cn

Abstract

The influence of El Niño–Southern Oscillation (ENSO) on northern midlatitude ozone during the period January–March (JFM) is investigated using various observations and a chemistry–climate model. The analysis reveals that, during El Niño events, there are noticeable anomalously high total ozone column (TOC) values over the North Pacific, the southern United States, northeastern Africa, and East Asia but anomalously low values in central Europe and over the North Atlantic. La Niña events have almost the opposite effects on TOC anomalies. The longitudinal dependence of midlatitude ozone anomalies associated with ENSO events during the period JFM is found to be related to planetary waves. Planetary waves excited by tropical convection propagate into the middle latitudes and give rise to longwave trains (Pacific–North American pattern) and shortwave trains along the North African–Asian jet. These wave trains affect ozone in the upper troposphere and lower stratosphere (UTLS) by modulating the midlatitude tropopause height and cause TOC anomalies by changing the vertical distributions of ozone. In addition, synoptic-scale Rossby wave breaking increases on the poleward flanks of the enhanced westerly jet during El Niño events, leading to a stronger eddy-driven meridional circulation in the UTLS and hence causing TOC increases over the North Pacific, the southern United States, northeastern Africa, and East Asia and vice versa for La Niña events. It is also found that the contribution of changes in Brewer–Dobson circulation due to anomalous planetary wave dissipation in the stratosphere during ENSO events to TOC changes in the middle latitudes for the period JFM is small, not more than 1 Dobson unit (DU) per month.

Corresponding author address: Wenshou Tian, College of Atmospheric Sciences, Lanzhou University, Tianshui South Road 222, Lanzhou, 730000 China. E-mail: wstian@lzu.edu.cn

1. Introduction

Atmospheric ozone plays an important role in both the radiative heating budget (e.g., Forster and Shine 1997) and protecting life on Earth from harmful solar ultraviolet radiation (e.g., Kerr and McElroy 1993). In addition, both observational and modeling studies show that stratospheric ozone changes can exert a significant influence on the climate system, particularly for the Southern Hemisphere (SH) (e.g., Thompson and Solomon 2002; Son et al. 2010; Kang et al. 2011) and even the ocean in the SH (e.g., Cai 2006; Cagnazzo et al. 2013). Long-term ozone observations at middle latitudes show a significant decline since the 1980s because of the emissions of anthropogenic chlorofluorocarbons (CFCs) (WMO 2011) and a weak recovering signal in the past decade because of steadily decreasing CFCs in the atmosphere in response to the Montreal Protocol signed in 1987 (e.g., Angell and Free 2009; Krzyścin 2012; Zhang et al. 2014). Although long-term variations of ozone are dominated by ozone depletion substances, short-term and long-term dynamical variabilities also strongly influence the interannual and long-term variations of midlatitude ozone (e.g., Salby and Callaghan 1993; Fusco and Salby 1999; Appenzeller et al. 2000; Hood and Soukharev 2005). Previous studies reported that the contribution of dynamical processes to the long-term trend of total ozone column (TOC) ranges from ~20% up to 50% (e.g., Hadjinicolaou et al. 2002; Hood and Soukharev 2005; Harris et al. 2008). Quantifying the contribution of different dynamical processes to the interannual variability of ozone is crucial for understanding the midlatitude ozone trend.

El Niño–Southern Oscillation (ENSO) is an important source of interannual variability in the atmosphere and its influence on weather and climate systems as well as on atmospheric tracer distributions has been investigated extensively (e.g., Yulaeva and Wallace 1994; Trenberth 1997; Wang et al. 2000; Cagnazzo et al. 2009; Calvo et al. 2010; Xie et al. 2011, 2012). For ENSO’s effects on atmospheric tracer distributions, most previous studies have focused on tropical ozone and water vapor (e.g., Kayano 1997; Chandra et al. 1998; Randel et al. 2009; Hood et al. 2010; Oman et al. 2011; Xie et al. 2014) and some other studies focused on the influence of ENSO on ozone in the polar region (e.g., Cagnazzo et al. 2009; Zubiaurre and Calvo 2012). Cagnazzo et al. (2009) found from the simulations of 12 chemistry climate models that there are significant TOC increases over the NH polar region in responses to El Niño events. ENSO signals from the stratosphere can even reach to the troposphere in the polar region through stratosphere–troposphere coupling (e.g., Bell et al. 2009; Cagnazzo and Manzini 2009; Ineson and Scaife 2009). However, ENSO’s effects on midlatitude ozone fields are still an issue under debate. Some previous studies showed that ENSO’s impact on midlatitude ozone is significant. Bojkov (1987) analyzed Total Ozone Mapping Spectrometer (TOMS) data at several midlatitude stations and found that the El Niño event during 1983 caused more than 10% ozone deficiencies over these stations. Wang et al. (2011) reported a 3.5-yr cycle in the observed monthly mean TOC at some stations in Europe and North America and linked it to ENSO events. Mäder et al. (2007) argued that ENSO events have negligible influence on zonal-mean TOC in both the NH and SH middle latitudes. The insignificant ENSO signals in the zonal mean of midlatitude ozone may be masked by volcanic eruptions (Randel et al. 2009) and affected by the quasi-biennial oscillation (QBO) (van Loon and Labitzke 1987). Nevertheless, Rieder et al. (2013) found that ENSO events have a strong influence on ozone over the North and South Pacific, central Europe, and south Indian Ocean.

The mechanism by which ENSO can influence midlatitude ozone is also worthy of investigation. It has been suggested that the response of tropospheric ozone in the middle latitudes to ENSO is related to stratosphere–troposphere exchange processes (Zeng and Pyle 2005) and the transport of air pollution (Koumoutsaris et al. 2008). However, tropospheric ozone only makes a minor contribution to TOC, so these mechanisms cannot fully explain ENSO signals in midlatitude TOC. Some studies pointed out that ENSO events can modulate Brewer–Dobson circulation (BDC) by affecting the upward propagation of Rossby waves (e.g., García-Herrera et al. 2006; Cagnazzo et al. 2009). As a result, the poleward ozone transport associated with the anomalous circulation can influence the stratospheric ozone distribution at the middle and high latitudes of both hemispheres. Although this mechanism can reasonably explain the enhanced zonal-mean ozone in the extratropics, which factors cause zonal asymmetric ozone responses to ENSO are not fully understood.

Hitchman and Rogal (2010a) investigated the influence of ENSO on southern midlatitude ozone and pointed out that the maximum ozone response to ENSO over southern Australia is related to changes in planetary wave-1 structures in temperature and geopotential height. The wave activity in the atmosphere of the SH middle latitudes is different from that in the NH because of different land–sea distribution between the two hemispheres. We might therefore expect that response of northern midlatitude ozone to ENSO is different from that of southern midlatitude ozone. The present work attempts to explore the influence of ENSO on ozone at NH middle latitudes and the associated mechanisms. We focused on the ozone responses during the period January–March (JFM), when there is the largest interannual variability of ozone and strongest sea surface temperature (SST) anomalies associated with ENSO events. Data and methods are described in section 2. In section 3, the responses of midlatitude TOC to ENSO during the period JFM are described. The mechanisms responsible for the influence of ENSO on midlatitude ozone are analyzed in section 4, followed by a summary and conclusions in section 5.

2. Data and methods

The TOC dataset used here is the National Institute of Water and Atmospheric Research (NIWA) dataset that assimilates satellite measurements such as TOMS, Solar Backscatter Ultraviolet (SBUV), and Global Ozone Monitoring Experiment (GOME). More details about the dataset can be found in Bodeker et al. (2005). The NIWA dataset has a horizontal resolution of 1° latitude × 1.25° longitude and covers the time period from January 1979 to November 2009. To investigate vertical ozone variations associated with ENSO events, monthly mean ozone profiles for the period 1979–2009 derived from the SBUV (version 8.6) ozone data are used (Frith et al. 2014). The SBUV instruments infer ozone profiles from backscattered radiance measurements at different wavelengths in the ultraviolet radiation. The dataset contains layer ozone amount [Dobson unit (DU)] for 21 layers. The pressure at the bottom of SBUV layer L is 1013.25 × 10−(L−1)/5 hPa and each layer is ~3.2 km thick (Bhartia et al. 2013). The resolution in the lower stratosphere and troposphere is 10–15 km, with the lowest resolution in the tropics. Detailed information about SBUV ozone data can be found in Miller et al. (2002), McPeters et al. (2013), and Frith et al. (2014). Monthly mean geopotential height, relative vorticity, tropopause height, and temperature fields are derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) dataset with a resolution of 1.5° latitude × 1.5° longitude from 1979 to 2009.

To analyze the influence of ENSO events on midlatitude TOC, a composite analysis is performed with respect to the Niño-3 index (N3I) anomalies for the period 1981–2010, available from the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (http://www.cpc.ncep.noaa.gov/data/indices/ersst3b.nino.mth.81-10.ascii). Only strong, persistent El Niño and La Niña events were selected for this study. The criterion for such El Niño and La Niña events is JFM mean of N3I anomalies should be greater and less than 1 and −1 K, respectively. According to this criterion, four El Niño and four La Niña events (Table 1) are selected for analysis. It should be pointed out that the composite analysis results are not sensitive to a small change of the threshold for selecting ENSO events (e.g., 0.5 or 1 K). The warm and cold composite for a given field is calculated by averaging the detrended monthly mean field during warm and cold ENSO events minus the monthly mean climatology for the period 1979–2009, respectively. El Niño and La Niña composite anomalies of observed SST derived from Hadley Centre data and outgoing longwave radiation (OLR) from NOAA during the period JFM are shown in Fig. 1. As expected, there are warm SST anomalies over the eastern Pacific Ocean during El Niño events. Meanwhile, there are negative OLR anomalies, suggesting enhanced convection over the eastern Pacific Ocean. La Niña has the opposite effects to El Niño. The student’s T statistic is used to calculate the statistical probability that two sample populations have meaningfully distinct averages (Hitchman and Rogal 2010a). Here, X and Y are the sample averages, and are the corresponding variances, and N and M are the number of degrees of freedom associated with the two populations.

Table 1.

Samples of strong persistent (left) El Niño and (right) La Niña events from 1979 to 2009 analyzed in this study. The Niño-3 index (N3I) anomalies averaged over the whole period of each selected ENSO event and the N3I anomalies averaged over the mature phase (January–March) of each selected ENSO event are also shown. The composite mean of N3I anomaly averaged over El Niño and La Niña events is shown in the last row of the table.

Table 1.
Fig. 1.
Fig. 1.

Composite anomalies of Hadley Centre SST (shading) for (left) El Niño and (right) La Niña events during (top) January, (middle) February, and (bottom) March. The composited NOAA OLR (contours) is also shown for reference. Positive and negative contours are represented by solid and dashed lines, respectively. (a),(c),(e) Only OLR values smaller than −5 W m−2 are shown. (b),(d),(f) Only OLR values greater than 5 W m−2 are shown. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test.

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

To probe the possible mechanisms involved in ENSO effects on northern midlatitude ozone, two time-slice simulations from the Whole Atmosphere Community Climate Model, version 3 (WACCM3), are performed. WACCM3 has 66 vertical levels extending from the ground to 4.5 × 10−6 hPa (~145-km geometric altitude), and the model’s vertical resolution is 1.1–1.4 km near the tropopause region. WACCM3 has been extensively evaluated against various satellite datasets and has a good performance in simulating the stratospheric chemistry and dynamics (e.g., Eyring et al. 2006). Details of the model version used here can be found in Garcia et al. (2007). Both time-slice simulations presented in this paper were performed at a resolution of 1.9° latitude × 2.5° longitude, with interactive chemistry enabled and without QBO forcing. The two runs (R1 and R2) were conducted using the same greenhouse gas emissions and mixing ratio lower boundary conditions for CFCs but with different SST forcing. Both of them are run with SSTs that are the composites of observed SST associated with ENSO events only in the tropical Pacific Ocean (15°S–15°N, 135°–280°E). SSTs over the other regions are the observed monthly mean climatology for the time period from 1979 to 2009. The SST used in R1 is the composite of observed tropical Pacific SST associated with El Niño events for the period 1979–2009. In experiment R2, SST is as in R1, except that the tropical Pacific SST represents the composites of observed SST associated with La Niña events during this period. It should be pointed out that the threshold of SST anomaly for El Niño and La Niña events in the numerical experiments is 0.5 and −0.5 K, respectively, in order to cover more ENSO events in the simulations and make a more generalized conclusion about the influence of ENSO events on midlatitude ozone than using a threshold of 1 K. Both experiments were run for 25 yr with the first 5 yr excluded for model “spinup”; the remaining 20 yr of data are used for the analysis.

Finally, we performed Rossby wave ray tracing on the 200-hPa pressure level to analyze the effect of planetary waves on the tropopause height. The wave ray tracing is calculated based on the solution of the linearized nondivergent, barotropic, and quasigeostrophic vorticity equation. Following Karoly (1983), the equation for the perturbation of streamfunction Ψ′ can be written as
e1
where is the Mercator projection of the time-mean zonal and meridional wind; θ is latitude; and k and l are the zonal and meridional wavenumbers, respectively. The expression is the time-mean quasigeostrophic potential vorticity. Equation (1) is solved using the Wentzel–Kramers–Brillouin (WKB) approximation and the dispersion relation is obtained as
e2
where ω is angular frequency. Integrating the group velocity (ug, υg) and the changing wavenumbers Eqs. (3)(6) under the constraints of dispersion relation Eq. (2), the location (x, y) of a Rossby wave ray path is computed, which shows how Rossby wave propagates,
e3
e4
e5
e6

3. ENSO signals in northern midlatitude ozone

Figure 2 shows the composite anomalies of TOC associated with El Niño and La Niña events in NIWA data. For El Niño events, there are positive TOC anomalies over the North Pacific, northeastern Africa, East Asia, and the southern United States and significant negative anomalies in northeastern Canada and the North Atlantic–European area. The patterns over the North Pacific and North American continents associated with El Niño events resembling the Pacific–North American (PNA) pattern (Horel and Wallace 1981; Wallace and Gutzler 1981). In particular, note that the four positive anomalous centers associated with El Niño events constitute circumglobal wave trains in the middle latitudes. La Niña events have almost the opposite effect on TOC anomalies. However, the magnitudes of TOC anomalies during La Niña events are weaker than those during El Niño events, even disappearing in Asia and the southern United States during January and February; this might be related to the weaker SST anomalies in the selected La Niña events (Fig. 1 and Table 1). Consistently, Bojkov (1987) reported large TOC deficiencies over northern and central Europe (e.g., at Arosa, Oslo, Belsk, and Hohenpeissenberg) in January–March 1983, when ENSO was in its warm phase, while during 1985, when ENSO was in its cold phase, large TOC deficiencies occurred at most North American stations (e.g., Boulder and Nashville). The spatial pattern of our composited TOC anomalies associated with ENSO events is also consistent with Fig. 2 of Rieder et al. (2013), highlighting the strong influence of ENSO on ozone over the North Pacific and central Europe. Previous studies have reported that the PNA-like pattern associated with El Niño events results from the quasi-stationary Rossby wave response (Trenberth et al. 1998) and synoptic eddy and flow interaction (Liu and Alexander 2007). Therefore, the PNA-like pattern in Fig. 2 may be related to planetary waves and will be further discussed later.

Fig. 2.
Fig. 2.

Composite anomalies of NIWA TOC for (a)–(c) El Niño and (d)–(f) La Niña events during (left) January, (center) February, and (right) March. (g)–(i) TOC differences between El Niño and La Niña events. The black rectangles in (a) represent the North Pacific, East Asia, the southern United States, and Europe, for which ozone profiles are averaged in Fig. 5. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test. The statistical significance of composite anomalies for El Niño events in (a)–(c) or La Niña events in (d)–(f) is calculated according to the difference between El Niño or La Niña and the climatology.

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

To further clarify the importance of ENSO events on modulating interannual variations of northern midlatitude ozone, a multiple linear regression (MLR) analysis is performed on the TOC time series for the period 1979–2009. The MLR is a useful tool for attributing ozone variations to dynamical, radiative, and chemical effects (e.g., Steinbrecht et al. 2003; Stolarski et al. 2006; Dhomse et al. 2006; Zhang et al. 2014). The MLR model is applied individually to the data for each calendar month and can be represented as follows:
eq1
where t is the time in 1-yr increments, is the long-term mean for each calendar month, is the contribution coefficient of proxy x, and represents the residual time series. The selected proxies include the equivalent effective stratospheric chlorine (EESC), the QBO index at 10 and 30 hPa measured at Singapore (QBO10 and QBO30), the stratospheric aerosol 550-nm optical depth (aero), the solar cycle effect represented by the F10.7 index of 10.7 cm solar flux (solar), and the N3I. The ozone variations related to one proxy variable (e.g., EESC, QBO, N3I, solar, and aero) are calculated using the time series of the proxy index multiplied by the fitting coefficient for the proxy variable () derived from the MLR. The differences between ozone variations related to N3I during El Niño years and those during La Niña years are shown in Fig. 3. The geographical distribution of ozone differences related to N3I agrees well with that of the composited anomalies of TOC shown in Fig. 2: positive ozone variations associated with N3I correspond to positive TOC anomalies over the North Pacific, southern Eurasia, and the southern United States, but negative ozone variations due to N3I correspond to negative TOC anomalies over the North Atlantic region associated with El Niño events and vice versa for La Niña events. Note that the similar patterns of TOC response to ENSO in Figs. 2 and 3 suggest that our statistical extraction of ENSO-induced interannual variability of ozone at middle latitudes is reliable.
Fig. 3.
Fig. 3.

Differences between NIWA TOC variations related to N3I during El Niño years and those during La Niña years. The ozone variations related to N3I can be see text for more details. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test.

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

The analysis of relative contributions of ENSO events to the interannual variance of TOC, represented by two standard deviations (2σ) of the ozone time series for the period 1979–2009 indicates that there is large interannual variance due to ENSO over the North Pacific (not shown). In January, the contribution of ENSO to TOC variance over the North Pacific is only 20%, whereas its contribution in February is the strongest (about 50%). The contribution of ENSO to the interannual variance of TOC over central Europe, northeastern Africa, East Asia, and the southern United States can also be up to 30%. Overall, the regions with large ENSO-induced ozone interannual variance are collocated with the large composite ozone anomalies shown in Fig. 2. Previous studies have shown that different meteorological factors can account for 20%–50% of the interannual variance of TOC in the northern middle latitudes (e.g., Hood and Soukharev 2005; Ossó et al. 2011); ENSO-induced interannual ozone variance obtained here also falls in this range.

Figure 4 shows the TOC differences between runs R1 and R2 (see section 2 for the details of these two model simulations), which represent the differences between El Niño and La Niña events during the period JFM in WACCM. Similar to the composited results from NIWA data, there are higher TOC values over the North Pacific and lower TOC in northeastern Canada and the North Atlantic area during El Niño events than during La Niña events. There is also a negative TOC anomaly over Europe during El Niño events relative to La Niña events for the period JFM. The PNA-like pattern in NIWA TOC is also noticeable in WACCM simulations. Note that the positive TOC anomalies over northeastern Africa and East Asia in January and February are also well reproduced, although the simulated TOC anomalies in March are of opposite sign to those in the observations. Overall, the patterns of simulated TOC anomalies in the northern middle latitudes are similar to those obtained from NIWA data, further confirming ENSO's effects on northern midlatitude ozone.

Fig. 4.
Fig. 4.

Differences in modeled TOC between runs R1 and R2 during (a) January, (b) February, and (c) March. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test. The black rectangles in (a) represent North Pacific, East Asia, the southern United States, and Europe, for which ozone profiles are averaged in Fig. 5.

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

Figure 5 shows the vertical distribution of differences in partial column ozone between El Niño and La Niña events derived from SBUV ozone data and modeled ozone averaged over East Asia, the southern United States, North Pacific, and Europe. The simulated vertical distributions of El Niño minus La Niña differences are in reasonable agreement with those of SBUV ozone data over the North Pacific, East Asia, and the southern United States, and both indicate that ozone concentrations in the whole stratosphere increase during El Niño events compared with those during La Niña events, in accordance with the higher TOC over these regions during El Niño events shown in Figs. 2 and 4. Note particularly that the ozone changes below the middle stratosphere associated with ENSO events contribute the most to the TOC anomalies. The maximum differences in partial column ozone occur at 100 hPa over the North Pacific, and it can be estimated that ozone differences in the layer between 300 and 70 hPa contribute 59% and 62% of the TOC differences over the North Pacific between El Niño and La Niña events during the period JFM, according to SBUV data and WACCM simulations, respectively. In contrast, there are ozone differences over Europe of opposite sign to those over the North Pacific, consistent with lower TOC over Europe during El Niño events than during La Niña events, and the maximum values of negative differences also occur between 70 and 150 hPa. It should be pointed out that the modeled TOC decreases in the UTLS during El Niño events relative to that during La Niña events over Europe are smaller than those in SBUV observations. It is possible that other factors may also have impacts on ozone over Europe but are not well simulated in WACCM3. The magnitude of the ozone differences over the North Pacific between El Niño and La Niña events derived from WACCM simulations is larger than that over the other three regions, corresponding to the strongest ozone response to ENSO over the Pacific Ocean. This feature is accompanied by the larger discrepancies between observed and modeled TOC over the North Pacific than the other regions.

Fig. 5.
Fig. 5.

Vertical distribution of differences in partial ozone column between El Niño and La Niña events during (left) January, (center) February, and (right) March. Note that the ozone partial columns are centered on the pressure levels. The ozone profiles derived from cohesive SBUV ozone data (red) and WACCM simulations (blue) are averaged over the regions indicated by the black rectangles in Figs. 2a and 4a, respectively. The regions include (a)–(c) East Asia, (d)–(f) the southern United States, (g)–(i) the North Pacific, and (j)–(l) Europe.

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

4. How ENSO affects midlatitude ozone

Previous studies have reported that tropopause variations have a large impact on TOC (e.g., Varotsos et al. 2004; Tian et al. 2008); therefore, it is necessary to investigate the differences in thermal tropopause height during ENSO events. Figures 6a, 6c, and 6e show the differences of thermal tropopause height anomalies derived from the ERA-Interim between El Niño and La Niña events. Anomalously low tropopause heights are associated with El Niño events over the North Pacific, the southern United States, eastern China, and northeastern Africa but anomalously high heights over the North Atlantic and northern Eurasia. La Niña events have opposite effects to El Niño events. The spatial patterns of the midlatitude tropopause height anomalies are similar to those of the upper-tropospheric geopotential height associated with ENSO events, as reported in previous studies (e.g., Trenberth 1997). Note that the tropopause height anomalies are almost in antiphase with the TOC anomalies (Fig. 2): negative height anomalies correspond to positive TOC anomalies and vice versa for positive height anomalies. Wang et al. (2011) reported that the regression coefficient of tropopause pressure on the TOC simulated by the Goddard Earth Observing System–Chemistry (GEOS-Chem) model is 0.71 DU hPa−1 during ENSO events. Tung and Yang (1988) gave an analytic estimate of the TOC change as a function of change in tropopause height: TOC decreases by ~7% for each 0.5-km increase in the height of the tropopause. In the present study, TOC changes accompanied by an change in tropopause pressure at a rate of 1 DU hPa−1, on the same order of magnitude as that estimated by Wang et al. (2011) and Tung and Yang (1988). In addition, the patterns and magnitude of thermal tropopause height differences between El Niño and La Niña events in reanalysis are pretty similar to those in WACCM (Figs. 6b, 6d, and 6f), providing further evidence that WACCM performs well in capturing the influence of ENSO on the midlatitude tropopause and TOC.

Fig. 6.
Fig. 6.

(left) Differences of relative vorticity at 200 hPa (color shaded) and thermal tropopause height (contours with 0.2-km interval; positive and negative contours are presented by the solid and dashed lines, respectively) between El Niño and La Niña events based on ERA-Interim data during (top) January, (middle) February, and (bottom) March. (right) Also shown are differences in the modeled thermal tropopause pressure between runs R1 and R2 during (b) January, (d) February, and (f) March. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test. Plots of the Rossby wave ray paths (thick solid lines) with zonal wavenumbers 2 (red) and 4 (black) starting at 15°N, 210°E following the wave for 12 days are also superimposed on the relative vorticity composite maps. The 1979–2009 climatological mean for each month derived from ERA-Interim and WACCM data are used as the basic state to derive the ray tracings for (left) and (right), respectively.

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

It is apparent from the above analysis that planetary wave trains are clearly visible in both midlatitude TOC and tropopause pressure fields and have a zonal wavelength of 60°–100° in longitude, corresponding to the typical wavelength of planetary Rossby waves (Holton et al. 2003). Stephenson and Royer (1995) also found from the TOMS ozone data and general circulation model (GCM) simulations noticeable stationary Rossby waves in the TOC fields; however, they did not discuss underlying mechanisms associated with those wave trains in the TOC data. To depict Rossby waves, the differences in composited relative vorticity at 200 hPa between El Niño and La Niña events are shown in Fig. 6. As expected, in the eastern tropical Pacific Ocean, negative vorticity anomalies in the upper troposphere correspond to the anomalous enhanced convection associated with El Niño events and vice versa for La Niña events. At middle latitudes, vorticity anomalies exhibit evident PNA-like patterns with strong positive anomalies over the North Pacific, negative anomalies over northern Canada, and positive anomalies over the southeastern United States during El Niño events. For La Niña events, the PNA-like patterns are not evident and the responses of vorticity over the North Pacific are confined more closely to the equator than those for El Niño events in January and February, which might be related to the weaker tropical SST anomalies in the selected La Niña events (Table 1). These composited vorticity anomalies are in accordance with the corresponding results from WACCM simulations (Fig. 6, right). It is known that the PNA patterns result from long Rossby waves (waves 1–3) (Hoskins and Karoly 1981; Chen 2002). The ray paths for the Rossby wave with zonal wavenumber 2 seen initially at 200 hPa from ERA-Interim data and reaching to Canada suggest a clear PNA-like pattern. Rossby waves with wavenumbers 1 and 3 show similar paths with that of wave 2. However, Rossby waves with wavenumbers 1–3 in the model simulations are reflected by the North American jet stream (not shown), possibly because of too strong of a westerly jet over the North American continent in the model simulations.

In addition, the shortwave trains forced by an anomalous anticyclone over the tropical upper troposphere propagate into middle latitudes during El Niño events and the opposite for La Niña events. Ray paths for the Rossby wave with zonal wavenumber 4 initiated at 200 hPa over the eastern equatorial Pacific are also shown in Fig. 6. Rossby waves with wavenumbers 5 and 6 show similar paths with that of wave 4. Note that the rays move initially northeastward and are then reflected off the jet over North America and enter the North African–Asian jet west of Africa. The rays remain confined in the jet and propagate eastward over central and eastern Asia, resulting in the vorticity anomalies over northeastern Africa and eastern Asia. The path of the Rossby wave rays is consistent with that in Fig. 7 of Shaman and Tziperman (2005) and is also confirmed by the WACCM simulations (Fig. 6, right). Note that the shortwave trains are independent of the PNA wave trains, although the rays propagating along the westerly jet can also reach the North Pacific and the southern United States (Chen 2002).

It was demonstrated in the above analysis that the tropopause changes and overall vertical shifts of ozone profiles along the PNA wave trains and shortwave trains are associated with ENSO events. In addition, the impact of ENSO events on midlatitude TOC is also related to transport processes that directly change ozone concentrations in the UTLS. Figure 7 shows the differences of horizontal winds on the 370-K isentropic surface (~150 hPa) derived from ERA-Interim data between El Niño and La Niña events. The pressure differences on the 370-K isentropic surface are overlaid as an indicator of the differences in vertical motion in the lowermost stratosphere between the two ENSO phases. Note that, during El Niño events, there are significant anomalous cyclonic flows over the North Pacific and anticyclonic flows over the eastern tropical Pacific Ocean. Meanwhile, the pressure on the 370-K isentropic surface shows positive anomalies over the midlatitude Pacific and negative anomalies over the tropical Pacific, suggesting large-scale downward and upward motion, respectively. There is also anomalous cyclonic flow and subsidence on the 370-K isentropic surface over East Asia and the southern United States. In contrast, there are negative pressure anomalies, with an uplifting of the isentropic surface, and an anticyclonic flow over northeastern Europe and the North Atlantic during winter and spring. Previous studies found that the convergence (divergence) associated with cyclonic (anticyclonic) flow above the tropopause can cause downward (upward) motion and hence lead to an increase (decrease) in the ozone concentration of the lowest stratosphere and changes in TOC (e.g., Salby and Callaghan 1993; Appenzeller et al. 2000). It should be pointed out that the anomalous planetary-scale circulations in the UTLS are dynamically related to Rossby wave trains excited by ENSO events. The corresponding results from the WACCM simulations also show the anomalous cyclonic circulations in January and February over the North Pacific, northeastern Africa, and East Asia during El Niño events compared with La Niña events. In contrast to the results from reanalysis data, the cyclonic flows over northeastern Africa and East Asia in March do not appear in the model simulations; instead, a strong anticyclonic flow occurs over the Asian continent. This discrepancy between reanalyzed and simulated circulations in March might explain why the simulated TOC anomalies are of the opposite sign to those in NIWA data (Fig. 4c).

Fig. 7.
Fig. 7.

Differences in horizontal winds (vectors with scale of 10 m s−1) and pressure on the 370-K isentropic surface derived from (left) ERA-Interim data and (right) WACCM simulations between El Niño and La Niña events during (top) January, (middle) February, and (bottom) March. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test.

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

The above results indicate that ENSO-induced anomalous planetary-scale circulation can influence the midlatitude TOC on monthly time scales. In addition, the eddy-driven meridional circulation associated with synoptic-scale Rossby wave breaking (RWB) anomalies during ENSO events can also lead to ozone changes (e.g., Isotta et al. 2008; Hitchman and Rogal 2010b). Figure 8 shows the differences of zonal wind at 200 hPa and RWB frequencies on the 330-K isentropic surface between El Niño and La Niña events. The 200-hPa zonal wind field has westerly anomalies over the North Pacific, the southern United States, the Arabian Peninsula, and South China, implying that the jet streams over these regions are enhanced during El Niño events. This feature has also been noted by Koumoutsaris et al. (2008). There are more RWB events to the north of the enhanced westerly jet during El Niño events than during La Niña events. Previous studies have noted that RWB along the poleward flanks of the upper-tropospheric westerly jets forces a poleward and downward flow that brings ozone-rich air into the lowermost stratosphere (Haynes et al. 1991; Manney et al. 2011) and hence change TOC in this region. Note that more RWB corresponds closely to the positive TOC anomalies over the North Pacific, northeastern Africa, East Asia, and the southern United States associated with El Niño events, consistent with the location of the springtime ozone maximum over the south of Australia in the presence of strong RWB (Hitchman and Rogal 2010b). The La Niña event has the opposite effects to the El Niño event. In addition, both TOC and RWB anomalies over the North Pacific show a slight westward shift from January to March, further confirming their close linkage. However, this linkage is not supported over Europe and the Atlantic Ocean, where there is more RWB but lower TOC, suggesting that other factors also play a role in influencing the TOC over these two regions. The simulated RWB and zonal wind anomalies (Fig. 8, right) are in good agreement with the reanalysis data, although the anomalies are weaker in the simulations than in the reanalysis data because of the weaker composited SSTs in our numerical experiments (see section 2).

Fig. 8.
Fig. 8.

Differences in the zonal wind (contour interval: 5 m s−1) at 200 hPa and the occurrence frequency of RWB events (shading) on the 330-K isentropic surface derived from (left) ERA-Interim data and (right) WACCM simulations between El Niño and La Niña events during (top) January, (middle) February, and (bottom) March. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test. The frequency of RWB is defined as the number of days with negative meridional gradient in potential vorticity per month, as used by Hitchman and Huesmann (2007).

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

As discussed in the previous section, ozone changes caused by the dynamical processes associated with ENSO mainly occur in the UTLS. Note also that both ozone variations and thermal structure changes caused by ENSO events are mainly dynamically driven. On monthly time scales, the photochemical processes and radiative feedbacks play a less important role in modulating midlatitude ozone in the UTLS than dynamical processes do (e.g., Solomon et al. 1985; Randel and Cobb 1994). Figure 9 shows the composited longitude–height cross sections of differences in geopotential height and the temperature between El Niño and La Niña events at 45°N. In the UTLS, the geopotential height is lower over the North Pacific (210°E) but higher over northeastern Canada (280°E) during El Niño events than during La Niña events, with an amplification of the Aleutian low and the Canadian high, as reported previously (e.g., Garfinkel and Hartmann 2007; Zubiaurre and Calvo 2012). The height anomalies constitute a barotropic wave-2 structure in the UTLS, resulting in the significant vertically integrated ozone column anomalies shown in Fig. 2. Note in particular that the temperature anomalies in the lower stratosphere (100 hPa) are nearly out of phase with those in the middle troposphere (500 hPa) and that the negative geopotential height anomalies in the UTLS are accompanied above by warm temperature anomalies but below by cold temperature anomalies and vice versa for positive height anomalies, in accordance with potential vorticity theory (Hoskins et al. 1985). Recall that there is a good correlation between the temperature anomalies in the lower stratosphere and the TOC anomalies shown in Fig. 2. This correlation mainly results from downward transport of ozone-rich air accompanied by adiabatic descent and warming or upward transport of ozone-poor air associated with adiabatic ascent and cooling (Hitchman and Rogal 2010a). These results are confirmed by WACCM simulations (Fig. 9, right).

Fig. 9.
Fig. 9.

Composite longitude–height cross sections along 45°N of differences in geopotential height (contour interval: 30 m; dashed contours indicate negative values) and temperature (shading) derived from (left) ERA-Interim and (right) WACCM simulations between El Niño and La Niña events. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test.

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

In the above analysis, we have mainly focused on the influence of ENSO events on zonal asymmetric TOC changes. Previous studies pointed out that ENSO events can influence the meridional transport of the stratospheric ozone through modulating planetary wave dissipation in the stratosphere and BDC (e.g., García-Herrera et al. 2006; Cagnazzo et al. 2009). Therefore, it is worth examining correlations between TOC changes and anomalous wave dissipation associated with ENSO events. Figure 10 shows the seasonal evolution of the correlation coefficients between vertical EP flux divergence at 10 hPa averaged over 45°–90°N and TOC tendency derived from NIWA and ERA-Interim. The TOC tendency is defined as a TOC difference between a given month and its preceding month (Nikulin and Karpechko 2005). We can see that the coefficients are positive equatorward of 45°N but are negative poleward of 45°N during the period JFM. In general, the increased convergence of planetary wave dissipation in the upper stratosphere over middle and high latitudes tends to enhance BDC and hence more ozone from tropics being transported to middle and high latitudes. In accordance with previous studies (e.g., Cagnazzo et al. 2009; Calvo et al. 2010), our analysis also indicates that the EP flux convergence and downwelling over the polar region is enhanced during El Niño events compared to La Niña events (Figs. 10c–f). Based on the correlation coefficients between EP flux divergence and TOC tendency, it can be estimated from NIWA data that a TOC increase of about 0.74 DU month−1 between 45° and 60°N and a decrease of about −0.28 DU month−1 between 30° and 45°N are resulted from enhanced planetary wave dispassion and BDC for the period JFM during El Niño events compared to La Niña events.

Fig. 10.
Fig. 10.

Correlation coefficients between EP flux divergence at 10 hPa averaged over 45°–90°N and TOC tendency derived from (a) NIWA and (b) ERA-Interim data from November to May. The TOC tendency is defined as TOC differences between a given month and its preceding month (Nikulin and Karpechko 2005). Latitude–height cross sections of differences in (c),(d) EP flux divergence and (e),(f) BD circulation vertical velocity (Andrews et al. 1987) are derived from ERA-Interim data associated with (c),(e) El Niño and (d),(f) La Niña events during the period JFM. Climatological mean of EP flux divergence (contours with 0.4 m s−1 day−1 interval) and for the period 1979–2009 (contours with 0. 2 mm s−1 interval) are also shown in (c),(d) and (e),(f), respectively. Positive and negative contours are represented by solid and dashed lines, respectively. The dotted regions are statistically significant at the 99% confidence level according to Student’s t test.

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

5. Conclusions and summary

Using various observations and a chemistry–climate model (WACCM3), the influence of ENSO events on northern midlatitude ozone during the months of JFM and the underlying mechanisms are analyzed in this study. During El Niño events, TOC in JFM is higher than the climatological mean over the North Pacific and the southern United States but lower than normal over northeastern North America, a spatial pattern resembling the PNA pattern. An NAO-like pattern in TOC anomalies is also evident during the warm phase of ENSO, with higher TOC over the southern parts of the Atlantic Ocean and lower TOC for the northern parts of the Atlantic Ocean. Higher TOC anomalies associated with El Niño events are also observed over northeastern Africa and East Asia. Of particular interest is the fact that the four high TOC centers associated with El Niño events constitute circumglobal wave trains in the middle latitudes. La Niña events have almost the opposite effects on TOC anomalies, but the magnitudes of TOC anomalies during La Niña events are weaker than during El Niño events, possibly because of the weaker SST anomalies in the selected La Niña events. The relative contribution of ENSO to the interannual variation of midlatitude TOC during JFM is found to range from 20% to 30% and can be up to 50% over the North Pacific. Analysis of ozone vertical distributions over the North Pacific, East Asia, the southern United States, and Europe indicates that the maximum response of the ozone column to ENSO occurs in the UTLS region.

The longitudinal dependence of midlatitude ozone response to ENSO during JFM can be mainly attributed to dynamical processes influencing ozone concentrations in the UTLS, including wave-induced tropopause height changes and associated large-scale circulations as well as wave-driven anomalous circulations because of synoptic-scale RWB events. At monthly time scales, the chemical processes and radiative feedbacks have a small impact on ozone in the UTLS. An integrated picture of how El Niño events impacting on midlatitude ozone is shown in Fig. 11. First, El Niño events tend to enhance convection and wave activity over the eastern Pacific and two types of barotropic Rossby wave trains are excited and then propagate into middle latitudes: that is, longwave trains (PNA pattern) and shortwave trains along the North African–Asian jet. Then, these barotropic Rossby waves modulate the midlatitude tropopause height and change the vertical structure of ozone profiles. Meanwhile, the barotropic wave structure of geopotential height anomalies in the UTLS favors the generation of significant TOC anomalies. Descending of the tropopause height can induce downward transport of ozone, thereby giving rise to TOC increase, and the lifted tropopause causes TOC decrease. Second, there are more synoptic-scale RWB events on the poleward flanks of the enhanced westerly jet over the North Pacific and the southern United States during El Niño events, and the occurrence frequency of RWB events also increases over northeastern Africa and East Asia. Enhanced eddy-driven poleward meridional circulation associated with more RWB anomalies during El Niño events leads to ozone increases. The effects of La Niña events on midlatitude ozone are opposite to those of El Niño events in the above-mentioned processes. Finally, our results show that the contribution of changes in BDC due to anomalous planetary wave dissipation in the upper stratosphere to TOC tendency anomalies between 30° and 60°N for the period JFM is small, not more than 1 DU month−1. One possible reason is that most of the anomalous downwelling BDC associated with ENSO events occurs in the polar region rather than in the middle latitudes.

Fig. 11.
Fig. 11.

Schematic diagram of the mechanisms through which El Niño events influence the midlatitude ozone. La Niña events have almost the opposite effects on TOC and meteorological fields (not shown). Green thick lines indicate North American and North Africa–Asia jet stream and yellow thick lines denote synoptic-scale RWB events. Curved arrows indicate planetary-scale anomalous horizontal circulations associated with ENSO events. Red and blue circles represent the falling and rising of tropopause height anomalies. In addition, red and blue thick arrow denotes downward and upward transports of ozone associated with tropopause changes, respectively. Black thin straight arrows represent wave-driven circulation due to RWB events.

Citation: Journal of Climate 28, 12; 10.1175/JCLI-D-14-00615.1

Our composite results are further supported by the WACCM simulations forced by SST anomalies associated with El Niño and La Niña events. Both the observed data and the model simulations confirm that ENSO signals in midlatitude TOC and dynamical circulation anomalies in the UTLS are mainly caused by external ENSO-type SST anomalies rather than by spontaneous and internally generated variability of the climate system during ENSO years.

Acknowledgments

This work is supported by the 973 Program (2014CB441202) and the National Science Foundation of China (41225018, 41175042). The authors would also like to thank Mrs. Stacey Frith for her nice help with providing SBUV ozone data. Also thanks Dr. Greg Bodeker for providing the NIWA TOC data. Codes for Rossby wave ray tracing support provided by Dr. Jeff Shaman are highly appreciated. Comments and suggestions from the anonymous reviewers and the editor were valuable in improving the quality of this paper. We thank the scientific teams for ECMWF, NCEP, NASA, NOAA, and Hadley Centre data. We would also thank NCAR for providing WACCM3 model. Finally, the authors acknowledged the computing support provided by the College of Atmospheric Sciences, Lanzhou University.

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