• Baldwin, M. P., and Coauthors, 2001: The quasi-biennial oscillation. Rev. Geophys., 39 , 179229.

  • Charlton, A. J., , L. M. Polvani, , J. Perlwitz, , F. Sassi, , E. Manzini, , K. Shibata, , S. Pawson, , J. E. Nielsen, , and D. Rind, 2007: A new look at stratospheric sudden warmings. Part II: Evaluation of numerical model simulations. J. Climate, 20 , 470488.

    • Search Google Scholar
    • Export Citation
  • Hamilton, K., 1995: Interannual variability in the Northern Hemisphere winter middle atmosphere in control and perturbed experiments with the GFDL SKYHI general circulation model. J. Atmos. Sci., 52 , 4466.

    • Search Google Scholar
    • Export Citation
  • Holton, J. R., , and H-C. Tan, 1980: The influence of the equatorial quasi-biennial oscillation on the global circulation at 50 mb. J. Atmos. Sci., 37 , 22002208.

    • Search Google Scholar
    • Export Citation
  • Holton, J. R., , P. H. Haynes, , M. E. McIntyre, , A. R. Douglass, , R. B. Rood, , and L. Pfister, 1995: Stratosphere-troposphere exchange. Rev. Geophys., 33 , 403439.

    • Search Google Scholar
    • Export Citation
  • Horel, J. D., , and J. M. Wallace, 1981: Planetary-scale atmospheric phenomena associated with the Southern Oscillation. Mon. Wea. Rev., 109 , 813829.

    • Search Google Scholar
    • Export Citation
  • Hoskins, B. J., , and D. J. Karoly, 1981: The steady linear response of a spherical atmosphere to thermal and orographic forcing. J. Atmos. Sci., 38 , 11791196.

    • Search Google Scholar
    • Export Citation
  • James, I. N., 2003: Hadley circulation. Encyclopedia of Atmospheric Sciences, J. R. Holton, J. A. Pyle, and J. A. Curry, Eds., Elsevier, 919–924.

    • Search Google Scholar
    • Export Citation
  • Kodera, K., 2006: Influence of stratospheric sudden warming on the equatorial troposphere. Geophys. Res. Lett., 33 .L06804, doi:10.1029/2005GL024510.

    • Search Google Scholar
    • Export Citation
  • Kodera, K., , H. Koide, , and H. Yoshimura, 1999: Northern Hemisphere winter circulation associated with the North Atlantic Oscillation and stratospheric polar-night jet. Geophys. Res. Lett., 26 , 443446.

    • Search Google Scholar
    • Export Citation
  • Kuang, Z., , and C. S. Bretherton, 2004: Convective influence on the heat balance of the tropical tropopause layer: A cloud-resolving model study. J. Atmos. Sci., 61 , 29192927.

    • Search Google Scholar
    • Export Citation
  • Labitzke, K. G., , and H. van Loon, 1999: The Stratosphere: Phenomena, History, and Relevance. Springer, 179 pp.

  • Manzini, E., , M. A. Giorgetta, , M. Esch, , L. Kornblueh, , and E. Roeckner, 2006: The influence of sea surface temperatures on the Northern winter stratosphere: Ensemble simulations with the MAECHAM5 model. J. Climate, 19 , 38633881.

    • Search Google Scholar
    • Export Citation
  • Newman, P. A., , E. R. Nash, , and J. E. Rosenfield, 2001: What controls the temperature of the Arctic stratosphere during the spring? J. Geophys. Res., 106 , D17. 1999920010.

    • Search Google Scholar
    • Export Citation
  • Oort, A. H., , and J. J. Yienger, 1996: Observed interannual variability in the Hadley circulation and its connection to ENSO. J. Climate, 9 , 27512767.

    • Search Google Scholar
    • Export Citation
  • Randel, W. J., , and P. A. Newman, 1998: The stratosphere in the Southern Hemisphere. Meteorology of the Southern Hemisphere, Meteor. Monogr., No. 27, Amer. Meteor. Soc., 243–282.

    • Search Google Scholar
    • Export Citation
  • Salby, M. L., , and P. F. Callaghan, 2002: Interannual changes of the stratospheric circulation: Relationship to ozone and tropospheric structure. J. Climate, 15 , 36733685.

    • Search Google Scholar
    • Export Citation
  • Salby, M. L., , and P. F. Callaghan, 2004: Interannual changes of the stratospheric circulation: Influence on the tropics and Southern Hemisphere. J. Climate, 17 , 952964.

    • Search Google Scholar
    • Export Citation
  • Salby, M. L., , and P. F. Callaghan, 2005: Interaction between the Brewer–Dobson circulation and the Hadley circulation. J. Climate, 18 , 43034316.

    • Search Google Scholar
    • Export Citation
  • Sassi, F., , D. Kinnison, , B. A. Boville, , R. R. Garcia, , and R. Roble, 2004: Effect of El Niño–Southern Oscillation on the dynamical, thermal, and chemical structure of the middle atmosphere. J. Geophys. Res., 109 .D17108, doi:10.1029/2003JD004434.

    • Search Google Scholar
    • Export Citation
  • Taguchi, M., , and D. L. Hartmann, 2006: Increased occurrence of stratospheric sudden warmings during El Niño as simulated by WACCM. J. Climate, 19 , 324332.

    • Search Google Scholar
    • Export Citation
  • van Loon, H., , and K. Labitzke, 1987: The Southern Oscillation. Part V: The anomalies in the lower stratosphere of the Northern Hemisphere in winter and a comparison with the quasi-biennial oscillation. Mon. Wea. Rev., 115 , 357369.

    • Search Google Scholar
    • Export Citation
  • Yulaeva, E., , and J. M. Wallace, 1994: The signature of ENSO in global temperature and precipitation fields derived from the microwave sounding unit. J. Climate, 7 , 17191736.

    • Search Google Scholar
    • Export Citation
  • View in gallery

    Correlations with FZI: (a) zonal mean temperature (contours and shadings) and RMMC (vectors), and (b) EP flux (vectors) and its divergence (contours and shadings). The results are for the JF means. Contour interval for the correlations of the temperature and EP flux divergence is 0.1. The correlation of the EP flux divergence is plotted only above 200 hPa. The correlation vectors are omitted where the correlations for both components are weaker than ±0.2. The reference vector below (b) denotes the magnitude of the correlation of 1. The black horizontal line in (b) denotes where the EOF analysis is applied to Fz.

  • View in gallery

    As in Fig. 1, but for correlations with CTI.

  • View in gallery

    As in Fig. 1, but for correlations with FZI′ = FZI − rCTI (r = 0.25: the correlation between FZI and CTI).

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Interannual Variations of the Stratosphere and Troposphere during Northern Winter as Simulated by WACCM

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  • 1 Department of Earth Sciences, Aichi University of Education, Kariya, Japan
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Abstract

This study explores interannual variations (IAVs) of the stratosphere and troposphere during Northern Hemisphere (NH) winter using a 50-yr simulation of Sassi et al. with the Whole Atmosphere Community Climate Model (WACCM). The simulation is forced with observed sea surface temperature (SST) and sea ice distributions from 1950 to 1999. The focus herein is on tropical tropospheric variations correlated with NH stratospheric variations and El Niño–Southern Oscillation (ENSO).

The discussed correlation analysis generally reproduces the following features as obtained in observational studies by Salby and Callaghan: an intensification of the Brewer–Dobson (BD) circulation driven by enhanced planetary wave (PW) drag in the NH stratosphere is accompanied by intensification of the Hadley circulation and anomalous warming of the tropical troposphere. It is further revealed that the tropical tropospheric warming is a reflection of the ENSO variability resulting from a positive correlation between the PW driving/BD circulation and ENSO, whereas the Hadley circulation does intensify with the BD circulation even when ENSO’s effects are removed.

Corresponding author address: Dr. Masakazu Taguchi, Department of Earth Sciences, Aichi University of Education, Hirosawa 1, Igaya-cho, Kariya 448-8542, Japan. Email: mtaguchi@auecc.aichi-edu.ac.jp

Abstract

This study explores interannual variations (IAVs) of the stratosphere and troposphere during Northern Hemisphere (NH) winter using a 50-yr simulation of Sassi et al. with the Whole Atmosphere Community Climate Model (WACCM). The simulation is forced with observed sea surface temperature (SST) and sea ice distributions from 1950 to 1999. The focus herein is on tropical tropospheric variations correlated with NH stratospheric variations and El Niño–Southern Oscillation (ENSO).

The discussed correlation analysis generally reproduces the following features as obtained in observational studies by Salby and Callaghan: an intensification of the Brewer–Dobson (BD) circulation driven by enhanced planetary wave (PW) drag in the NH stratosphere is accompanied by intensification of the Hadley circulation and anomalous warming of the tropical troposphere. It is further revealed that the tropical tropospheric warming is a reflection of the ENSO variability resulting from a positive correlation between the PW driving/BD circulation and ENSO, whereas the Hadley circulation does intensify with the BD circulation even when ENSO’s effects are removed.

Corresponding author address: Dr. Masakazu Taguchi, Department of Earth Sciences, Aichi University of Education, Hirosawa 1, Igaya-cho, Kariya 448-8542, Japan. Email: mtaguchi@auecc.aichi-edu.ac.jp

1. Introduction

The meridional circulation in the tropical troposphere is characterized by the thermally driven Hadley circulation, whereas that in the stratosphere is known as the wave-driven Brewer–Dobson (BD) circulation (e.g., James 2003; Holton et al. 1995). In the stratosphere, the wave driving of the BD circulation is primarily induced by planetary waves (PWs). Because the zonally asymmetric surface conditions in the Northern Hemisphere (NH) are the dominant driver of PWs, the PW driving and hence the BD circulation are strongest in the NH during its winter.

Investigating interannual variations (IAVs) of the stratosphere and troposphere during NH winter, Salby and Callaghan (2004, hereafter SC04; 2005, hereafter SC05) showed that an intensification of the BD circulation is accompanied by intensification of the Hadley circulation and anomalous warming of the tropical troposphere. SC05 discussed that the coherent changes in the BD circulation and the Hadley circulation act to satisfy mass continuity in the troposphere and stratosphere. They also speculated that the tropical tropospheric warming is induced by diabatic (convective) warming close to the equator and adiabatic warming in the subtropics in association with the intensified Hadley circulation. These tropical tropospheric variations are the focus in this study. On a shorter intraseasonal time scale, Kodera (2006) made a composite analysis of stratospheric warmings during NH winter to find anomalous cooling in the tropical lower stratosphere and upper troposphere, accompanied by a seesaw of convective activity.

An important aspect, however, remains missing in the studies of SC04 and SC05 that may affect their arguments on global effects of ENSO. It is well known that ENSO dominates IAVs in the tropical troposphere, with overall warming and intensification of the Hadley circulation for El Niño conditions (e.g., Yulaeva and Wallace 1994; Oort and Yienger 1996). As for NH stratospheric effects, recent studies with general circulation models (GCMs) have shown that the polar vortex is more disturbed for El Niño conditions accompanied by enhancement of PW driving, and hence enhancement of the BD circulation (Hamilton 1995; Sassi et al. 2004; Taguchi and Hartmann 2006; Manzini et al. 2006). In particular, Sassi et al. (2004) made a composite analysis of three Whole Atmosphere Community Climate Model (WACCM) simulations forced with observed SSTs from 1950 to 2000, whereas Taguchi and Hartmann (2006) conducted experiments with WACCM using perpetual ENSO-like SST forcings. Taguchi and Hartmann attribute the enhanced PW driving in the stratosphere to a PW response in the troposphere induced by El Niño–like SST forcing: the PW response, which resembles the Pacific–North American pattern, is superimposed on stationary PWs to yield an enhanced PW of zonal wavenumber 1. The change in tropospheric PWs simulated in GCMs is consistent with both observational and theoretical results (Horel and Wallace 1981; Hoskins and Karoly 1981). Observational detections of stratospheric changes with ENSO will be more difficult because of the large variability and the various external factors in operation, such as the quasi-biennial oscillation (QBO), volcanic eruptions, and solar-cycle variability, in addition to ENSO (e.g., Baldwin et al. 2001; Labitzke and van Loon 1999).

A possibility thus seems to exist that the coherent changes in the NH winter stratosphere and the tropical troposphere observed by SC04 and SC05 are at least partly driven by ENSO. In other words, the correlations may not mean that the intensification of the BD circulation produces the changes in the tropical troposphere. Using a 50-yr WACCM simulation of Sassi et al. (2004), including ENSO forcing, this study seeks to explore IAVs in the tropical troposphere associated with those in the NH winter stratosphere and ENSO. Our results first verify the tropical tropospheric variations associated with an intensification of the BD circulation as observed by SC04 and SC05. Our further analysis, in which the effects of ENSO are taken into account, demonstrates that the tropical tropospheric warming is a reflection of ENSO’s effects, whereas the intensification of the Hadley circulation accompanies that of the BD circulation.

2. Simulation and analysis

This study makes use of one (1950–99) of the three WACCM simulations performed by Sassi et al. (2004). The model has T63 horizontal resolution and 66 levels from the surface to about 110 km. This simulation is useful for studying IAVs of the stratosphere and troposphere in relation to ENSO, because ENSO is the dominant tropical forcing of the simulation. The model does not simulate the QBO or include other external forcings.

We focus on IAVs of January and February (JF) mean states when the variations are large in the simulation. It is noted that the IAVs in the simulation are generally smaller in magnitude than those in observations. The month of December is not included in the IAVs of interest, because the model bias is conspicuous in December. It is verified that using December–February (DJF) means instead of JF means hardly affects our following argument (not shown). The model bias is consistent with the result of Charlton et al. (2007): stratospheric sudden warmings are simulated less frequently in six GCMs, including WACCM, compared to an observed climatology, although the reason remains an open question.

To extract a dominant mode of IAVs of the NH extratropical stratosphere, an empirical orthogonal function (EOF) analysis is applied to latitudinal distributions (20°–90°N) of the vertical component of the Eliassen– Palm (EP) flux (Fz) at 100 hPa. The wave activity entering the stratosphere yields the (resolved) wave drag in the stratosphere that primarily drives the BD circulation, so that the variability of Fz in the lower stratosphere determines that of the stratospheric circulation well (e.g., Newman et al. 2001). The leading EOF of Fz, which explains a large fraction (70.5%) of the total variance, represents either an increase or decrease of PW activity entering the stratosphere (Fig. 1b). The principal component (referred to as FZI) is used for our correlation analysis of some dynamical fields. In the analysis, we simply use JF mean temperature, whereas SC04 and SC05 used wintertime temperature tendency because it is directly related to the adiabatic warming/cooling in the thermodynamic equation. However, using the temperature hardly affects our argument because the temperature is highly correlated with its tendency (Fig. 2 of Salby and Callaghan 2002).

The ENSO variability is represented here by the JF mean “cold-tongue index” (CTI), or SST anomalies averaged over an eastern tropical Pacific region of 6°N–6°S, 90°–180°W. The CTI is also used for the correlation analysis. Our analysis uses correlations, instead of regressions, to represent the strength of coherent variations associated with the indices. Note that FZI and CTI have a positive correlation of 0.25. Our simple Monte Carlo simulation, in which two groups of 50 random numbers are generated 10 000 times, shows that about 10% (5%) of all trials yield correlations over +0.18 (+0.24). The simulation thus suggests that correlations over +0.18 and +0.24 can be regarded as statistically significant at the 90% and 95% confidence limits, respectively. It is noted that, whereas the correlation between FZI and CTI is marginally significant, ENSO accounts for only about 10% of the interannual variance of FZI.

3. Results

The IAVs in the stratosphere and troposphere correlated with FZI (Fig. 1) reproduce the results by SC04 and SC05 well. The correlation of the zonal mean temperature exhibits a dipole pattern in the middle and lower stratosphere, consisting of positive values (anomalous warming) in the NH extratropics and negative values (anomalous cooling) at lower latitudes and in the Southern Hemisphere. The warming and cooling are associated with anomalous downwelling and upwelling, respectively, as seen in the correlations of the residual mean meridional circulation (RMMC) with FZI. The formulation of the RMMC is given, for example, by Randel and Newman (1998). The RMMC can be used as a good measure of the BD circulation. The anomalous RMMC is driven by enhanced PW drag, because the EP flux divergence is negatively correlated in a large part of the NH stratosphere. The enhanced PW drag is induced by enhanced PW activity in the lower stratosphere extracted by the EOF analysis. The increase of PW activity can be traced into the troposphere. The structure of the temperature correlation is somewhat different in the extratropical troposphere from the observations (Fig. 2 of SC05): in the model result, positive correlations extend from the extratropical stratosphere down to the surface around 60°N.

It is important to note that the tropical troposphere exhibits anomalous warming, with positive correlations over 0.2 in places. A hint of an intensification of the Hadley circulation is also noticeable in anomalous upwelling between the equator and 10°S (with positive correlations over +0.4) and anomalous downwelling northward of it. These tropical tropospheric variations are qualitatively consistent with the image of SC05. There are also some differences of the model results from the observations (Fig. 2 of SC05). The observed correlations of the tropical tropospheric temperature are generally much stronger. The observed temperature correlation has an isolated local maximum (exceeding +0.5) around 20°N near the surface, whereas the model result shows interspersed local maxima in the tropical/subtropical troposphere.

As expected from the positive correlation between FZI and CTI, the IAVs related to FZI share some features with those related to CTI (Fig. 2). The latter features include intensified PW drag and hence BD circulation in the NH stratosphere, induced by enhanced PW activity in the lower stratosphere. These features are also obtained in Sassi et al. (2004) and Taguchi and Hartmann (2006). In the tropics, the IAVs correlated with the CTI show more pronounced signals in both stratosphere and troposphere, including warming of the tropical troposphere and intensification of the Hadley circulation. The EP flux and its divergence exhibit strong but complicated signals in the low and midlatitudes of both hemispheres, which are probably contributed by large-scale equatorial and subtropical waves.

The positive correlation between FZI and CTI, which results in some similarity between Figs. 1 and 2, suggests that the tropical tropospheric variations in Fig. 1 may partially reflect the effects of ENSO, as argued in the introduction. The coherent variability of ENSO can be removed from FZI by defining another index FZI′, with a linear regression as FZI′ = FZI − rCTI. Here, the FZI and CTI indices are both normalized, and r = 0.25 is the correlation between them. Note that FZI′ has no correlation with CTI, while it retains a major part of the FZI variability with a correlation of 0.97. FZI′ is used to extract tropical tropospheric variations that are correlated with the NH extratropical PW drag and BD circulation but are not correlated with ENSO. Such a technique is used, for example, in Kodera et al. (1999), who analyzed tropospheric and stratospheric variations associated with the North Atlantic Oscillation and stratospheric polar-night jet oscillation.

When the ENSO variability is thus removed, the stratospheric variations retain basic features, including the intensification of the BD circulation (Fig. 3). On the other hand, the tropical troposphere exhibits some noticeable changes: it shows almost no temperature correlation with FZI′ whereas the intensification of the Hadley circulation is still noticeable. The positive correlations of the vertical velocity with FZI′ exceed +0.3, although they are weaker than those with FZI. These results demonstrate that the tropical tropospheric warming in Fig. 1 is a reflection of ENSO, while the intensification of the Hadley circulation is associated with that of the BD circulation as well as with ENSO. Such features are also partly obtained in another 50-yr WACCM simulation forced with a climatological SST condition (conducted by the author), where the ENSO variability is not included: accompanied by the intensified BD circulation, the tropical troposphere exhibits slightly negative temperature correlations together with a hint of intensification of the Hadley circulation, as seen in Fig. 3a (not shown).

One may wonder how the correlations with FZI′ (Fig. 3), including the intensified Hadley circulation and almost no temperature changes, are consistent with each other. The intensified Hadley circulation should reflect an increase in either intensity or area (or both) of deep convection, provided that the model includes a reasonable expression of convection. These convective changes may well result in an increase in latent heating by convection. However, the intensified Hadley circulation also implies an increase in adiabatic cooling where anomalous upward motion takes place. The refined correlations can thus be possible under the two competing effects.

4. Summary and discussion

Using a 50-yr WACCM simulation forced with observed SSTs from 1950 to 1999, this study has investigated IAVs in the stratosphere and troposphere during NH winter, with a focus on tropical tropospheric variations correlated with NH stratospheric variations and ENSO. We have verified that an intensification of the BD circulation is accompanied by that of the Hadley circulation, together with anomalous warming of the tropical troposphere, in good agreement with SC04 and SC05. It is further argued that, because the strength of the BD circulation is somewhat correlated with ENSO, the tropical tropospheric variations may at least partly reflect the effects of ENSO. Our analysis reveals that the tropical tropospheric warming is a reflection of the effects of ENSO, whereas the Hadley circulation does intensify with the BD circulation even when ENSO’s effects are removed.

As far as the simulation is a good representation of the real world, this study thus suggests that one should be careful in interpreting tropical tropospheric variations that seem correlated with an index because they are inevitably dominated by the ENSO signature. This study gives a simple demonstration of such an example of tropical tropospheric variations correlated with the strength of the BD circulation. The present analysis of the simulation has shown that the temperature correlations in the tropical troposphere with the BD circulation arise because ENSO affects both of the BD circulation and the tropical troposphere.

When one seeks to apply the present argument to the real world, it will be, however, much more complicated, at least in a few respects. First, the present analysis may exaggerate the refinement of the correlations of the tropical troposphere (especially temperature) with the BD circulation by removing ENSO’s effects. The exaggeration can occur because the correlation of tropical tropospheric temperatures with the BD circulation (or FZI) is much weaker in the model than in the observations, whereas the correlation of the BD circulation with ENSO is stronger in the model. These model differences may reflect that the real atmosphere is subject to various external forcings, including ENSO, as mentioned in the introduction. In particular, IAVs of the NH winter stratosphere are strongly affected by QBO (e.g., Holton and Tan 1980). It is also suggested that NH stratospheric effects of ENSO are difficult to separate from those of QBO because the phases of QBO and ENSO tend to coincide (van Loon and Labitzke 1987).

Second, WACCM, like other GCMs, leaves some uncertainty in processes that are not represented explicitly and hence are incorporated only through parameterizations. Such processes include convection, which SC05 suggest plays a role in the warming of the tropical troposphere close to the equator. Kodera (2006) also suggested that tropical convective activity is sensitive to changes in the lower stratosphere/upper troposphere induced by a change in the BD circulation. These suggestions seem to agree with the modeling study with a cloud resolving model (Kuang and Bretherton 2004). It is speculated that tropical convective changes in response to a change in the BD circulation may trigger reorganization of the tropical tropospheric circulation. Further investigations will be useful to better understand the nature of coupled IAVs in the stratosphere and troposphere as well as dynamics of the coupling.

Acknowledgments

The author thanks Dr. Sassi and collaborators, who made the simulation available while the author stayed at University of Washington.

REFERENCES

  • Baldwin, M. P., and Coauthors, 2001: The quasi-biennial oscillation. Rev. Geophys., 39 , 179229.

  • Charlton, A. J., , L. M. Polvani, , J. Perlwitz, , F. Sassi, , E. Manzini, , K. Shibata, , S. Pawson, , J. E. Nielsen, , and D. Rind, 2007: A new look at stratospheric sudden warmings. Part II: Evaluation of numerical model simulations. J. Climate, 20 , 470488.

    • Search Google Scholar
    • Export Citation
  • Hamilton, K., 1995: Interannual variability in the Northern Hemisphere winter middle atmosphere in control and perturbed experiments with the GFDL SKYHI general circulation model. J. Atmos. Sci., 52 , 4466.

    • Search Google Scholar
    • Export Citation
  • Holton, J. R., , and H-C. Tan, 1980: The influence of the equatorial quasi-biennial oscillation on the global circulation at 50 mb. J. Atmos. Sci., 37 , 22002208.

    • Search Google Scholar
    • Export Citation
  • Holton, J. R., , P. H. Haynes, , M. E. McIntyre, , A. R. Douglass, , R. B. Rood, , and L. Pfister, 1995: Stratosphere-troposphere exchange. Rev. Geophys., 33 , 403439.

    • Search Google Scholar
    • Export Citation
  • Horel, J. D., , and J. M. Wallace, 1981: Planetary-scale atmospheric phenomena associated with the Southern Oscillation. Mon. Wea. Rev., 109 , 813829.

    • Search Google Scholar
    • Export Citation
  • Hoskins, B. J., , and D. J. Karoly, 1981: The steady linear response of a spherical atmosphere to thermal and orographic forcing. J. Atmos. Sci., 38 , 11791196.

    • Search Google Scholar
    • Export Citation
  • James, I. N., 2003: Hadley circulation. Encyclopedia of Atmospheric Sciences, J. R. Holton, J. A. Pyle, and J. A. Curry, Eds., Elsevier, 919–924.

    • Search Google Scholar
    • Export Citation
  • Kodera, K., 2006: Influence of stratospheric sudden warming on the equatorial troposphere. Geophys. Res. Lett., 33 .L06804, doi:10.1029/2005GL024510.

    • Search Google Scholar
    • Export Citation
  • Kodera, K., , H. Koide, , and H. Yoshimura, 1999: Northern Hemisphere winter circulation associated with the North Atlantic Oscillation and stratospheric polar-night jet. Geophys. Res. Lett., 26 , 443446.

    • Search Google Scholar
    • Export Citation
  • Kuang, Z., , and C. S. Bretherton, 2004: Convective influence on the heat balance of the tropical tropopause layer: A cloud-resolving model study. J. Atmos. Sci., 61 , 29192927.

    • Search Google Scholar
    • Export Citation
  • Labitzke, K. G., , and H. van Loon, 1999: The Stratosphere: Phenomena, History, and Relevance. Springer, 179 pp.

  • Manzini, E., , M. A. Giorgetta, , M. Esch, , L. Kornblueh, , and E. Roeckner, 2006: The influence of sea surface temperatures on the Northern winter stratosphere: Ensemble simulations with the MAECHAM5 model. J. Climate, 19 , 38633881.

    • Search Google Scholar
    • Export Citation
  • Newman, P. A., , E. R. Nash, , and J. E. Rosenfield, 2001: What controls the temperature of the Arctic stratosphere during the spring? J. Geophys. Res., 106 , D17. 1999920010.

    • Search Google Scholar
    • Export Citation
  • Oort, A. H., , and J. J. Yienger, 1996: Observed interannual variability in the Hadley circulation and its connection to ENSO. J. Climate, 9 , 27512767.

    • Search Google Scholar
    • Export Citation
  • Randel, W. J., , and P. A. Newman, 1998: The stratosphere in the Southern Hemisphere. Meteorology of the Southern Hemisphere, Meteor. Monogr., No. 27, Amer. Meteor. Soc., 243–282.

    • Search Google Scholar
    • Export Citation
  • Salby, M. L., , and P. F. Callaghan, 2002: Interannual changes of the stratospheric circulation: Relationship to ozone and tropospheric structure. J. Climate, 15 , 36733685.

    • Search Google Scholar
    • Export Citation
  • Salby, M. L., , and P. F. Callaghan, 2004: Interannual changes of the stratospheric circulation: Influence on the tropics and Southern Hemisphere. J. Climate, 17 , 952964.

    • Search Google Scholar
    • Export Citation
  • Salby, M. L., , and P. F. Callaghan, 2005: Interaction between the Brewer–Dobson circulation and the Hadley circulation. J. Climate, 18 , 43034316.

    • Search Google Scholar
    • Export Citation
  • Sassi, F., , D. Kinnison, , B. A. Boville, , R. R. Garcia, , and R. Roble, 2004: Effect of El Niño–Southern Oscillation on the dynamical, thermal, and chemical structure of the middle atmosphere. J. Geophys. Res., 109 .D17108, doi:10.1029/2003JD004434.

    • Search Google Scholar
    • Export Citation
  • Taguchi, M., , and D. L. Hartmann, 2006: Increased occurrence of stratospheric sudden warmings during El Niño as simulated by WACCM. J. Climate, 19 , 324332.

    • Search Google Scholar
    • Export Citation
  • van Loon, H., , and K. Labitzke, 1987: The Southern Oscillation. Part V: The anomalies in the lower stratosphere of the Northern Hemisphere in winter and a comparison with the quasi-biennial oscillation. Mon. Wea. Rev., 115 , 357369.

    • Search Google Scholar
    • Export Citation
  • Yulaeva, E., , and J. M. Wallace, 1994: The signature of ENSO in global temperature and precipitation fields derived from the microwave sounding unit. J. Climate, 7 , 17191736.

    • Search Google Scholar
    • Export Citation
Fig. 1.
Fig. 1.

Correlations with FZI: (a) zonal mean temperature (contours and shadings) and RMMC (vectors), and (b) EP flux (vectors) and its divergence (contours and shadings). The results are for the JF means. Contour interval for the correlations of the temperature and EP flux divergence is 0.1. The correlation of the EP flux divergence is plotted only above 200 hPa. The correlation vectors are omitted where the correlations for both components are weaker than ±0.2. The reference vector below (b) denotes the magnitude of the correlation of 1. The black horizontal line in (b) denotes where the EOF analysis is applied to Fz.

Citation: Journal of Climate 21, 10; 10.1175/2007JCLI1744.1

Fig. 2.
Fig. 2.

As in Fig. 1, but for correlations with CTI.

Citation: Journal of Climate 21, 10; 10.1175/2007JCLI1744.1

Fig. 3.
Fig. 3.

As in Fig. 1, but for correlations with FZI′ = FZI − rCTI (r = 0.25: the correlation between FZI and CTI).

Citation: Journal of Climate 21, 10; 10.1175/2007JCLI1744.1

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