Downward propagation from the stratosphere to the troposphere: A comparison of the two hemispheres

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D24, 4780, doi: /2003jd004077, 2003 Downward propagation from the stratosphere to the troposphere: A comparison of the two hemispheres Rune G. Graversen and Bo Christiansen Climate and Research Division, Danish Meteorological Institute, Copenhagen, Denmark Received 15 August 2003; revised 5 September 2003; accepted 24 September 2003; published 23 December [1] The variability in the middle- and high-latitude stratosphere and troposphere on annual and intraannual timescales is investigated with focus on the differences between the two hemispheres. Results obtained with the National Centers for Environmental Prediction reanalysis data are compared to a general circulation model simulation. Although downward propagation of zonal mean zonal wind anomalies from the stratosphere to the troposphere are found on intraannual timescales in both hemispheres the two hemispheres show several differences. First, the intraannual variability is around 30% weaker in the southern hemisphere (SH) than in the northern hemisphere (NH). Second, in the SH the downward propagation is concentrated to the spring season while it is found during the whole winter half-year in the NH. Third, the coupling between the stratosphere and the troposphere is weaker and faster in the SH compared to the NH. Also, the annual cycle of the zonal mean zonal wind is quite different in the two hemispheres. In the SH the annual cycle is strong, shows a downward propagation with a timescale of two months, and includes higher annual harmonics of considerable strength. In the NH the annual cycle is weaker, shows no downward propagation, and is more sinusoidal. In both hemispheres the annual cycle and the intraannual variability are weaker in the model than in the reanalysis. The downward propagation in the model closely resembles that of the reanalysis both on intraannual timescales and in the annual cycle. In the SH the model underestimates the importance of the higher annual harmonics. INDEX TERMS: 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); 3362 Meteorology and Atmospheric Dynamics: Stratosphere/troposphere interactions; 3319 Meteorology and Atmospheric Dynamics: General circulation; KEYWORDS: Downward propagation, wave zonal wind interaction Citation: Graversen, R. G., and B. Christiansen, Downward propagation from the stratosphere to the troposphere: A comparison of the two hemispheres, J. Geophys. Res., 108(D24), 4780, doi: /2003jd004077, Introduction [2] The stratosphere in the SH differs from that of the NH mainly due to the weaker wave forcing from the troposphere [e.g., Leovy and Webster, 1976; Hartmann, 1976; Randel, 1988] which is related to the more zonal symmetric orography in the SH. The result is a more undisturbed SH winter stratosphere with a stronger vortex and a colder polar region. The climatology of the SH stratosphere and its dynamical connection to the troposphere has among others also been studied by, e.g., Labitzke and van Loon [1972], van Loon and Jenne [1972], Yamazaki and Mechoso [1985], Hartmann et al. [1984], Mechoso et al. [1985], Kodera et al. [1990], Waugh et al. [1999], and Kodera and Kuroda [2002]. Major warmings, where zonal mean easterlies are found below 10 hpa poleward of 60, occur in the NH several times per decade but were not observed in the SH [Labitzke and van Loon, 1972; Schoeberl, 1978; Labitzke, 1981] until September 2002 [Baldwin et al., 2003]. Another, probably related, difference between the two hemispheres is Copyright 2003 by the American Geophysical Union /03/2003JD the existence of a strong semi-annual component in the SH stratosphere [Dunkerton, 2000]. [3] The intraannual variability in the stratosphere at high northern latitudes is dominated by stratospheric vacillations which display a clear downward propagation. These stratospheric vacillations were discovered in studies of simplistic models [Holton and Mass, 1976] and were recently documented in general circulation models [Christiansen et al., 1997; Yamazaki and Shinya, 1999; Christiansen, 2000]. Additional interest in stratospheric vacillations raised with the introduction of the Arctic Oscillation (AO) [Thompson and Wallace, 1998] due to its deep vertical extent reaching into the stratosphere. The downward propagation of dynamical zonal mean quantities from the stratosphere to the troposphere has recently been identified and studied in reanalysis data [Baldwin and Dunkerton, 1999; Christiansen, 2001; Kuroda and Kodera, 2001] and model simulations [Christiansen, 2001]. While Baldwin and Dunkerton [1999] based their study on an index representing pressure pattern anomalies calculated with a Principal Component Analysis, Christiansen [2001] directly used the zonal mean zonal wind. The zonal mean anomalies, despite their precise definition, propagate from 10 hpa to the troposphere on a ACL 9-1

2 ACL 9-2 GRAVERSEN AND CHRISTIANSEN: DOWNWARD PROPAGATION timescale of several weeks. The downward propagation is related to the AO in the way that the AO index becomes positive when a positive downward propagating anomaly reaches the surface. [4] The downward propagation in the NH stratosphere is a consequence of non-linear interactions between large scale waves and the zonal mean flow [Christiansen, 2001]. Experiments with both simple models [Plumb, 1989; Yoden, 1990; Scott and Haynes, 2002] and general circulation models [Christiansen, 1999; Scaife and James, 2000] indicate that the stratospheric variability depends critically on the strength of the wave forcing from the troposphere. For example, for weak wave forcing the stratospheric variability is very weak and shows no downward propagation. For larger wave forcing the timescale of the downward propagation depends on the strength of the forcing [Christiansen, 1999]. [5] Most previous studies of the downward propagation on intraannual time scales have focused on middle and high northern latitudes, in particular 60 N. In the SH the annual cycle (the climatology) has been the subject of considerable previous research as mentioned above, whereas the intraannual variability and the possible downward propagation have attracted less attention. However, recently Kuroda and Kodera [2001] showed downward propagation of SH polar temperature anomalies. Because of the different wave forcing in the two hemispheres, a comparison of the downward propagation is of interest and may potentially increase our understanding of the stratosphere-troposphere coupling. [6] Weather forecast models have been compared to observations and verified on weather timescales shorter than 10 days where initial conditions dominate. Verifying atmospheric general circulation models on climate timescales where forcings and boundary condition change is notoriously difficult. Between these extremes, the daily weather and the climate, the variability on intraseasonal timescales is to a large extent internally generated in the atmosphere and therefore offers an independent opportunity to test the models. Furthermore, a good model representation of downward propagation from the stratosphere to the troposphere on intraannual timescales may have potential for extended range forecasting as anomalies can be detected in the stratosphere days before they reach the surface [Baldwin and Dunkerton, 2001; Christiansen, 2001; Kuroda, 2002; Thompson et al., 2002]. In the NH an excellent agreement of intraannual variability between model and reanalysis was reported in Christiansen [2001]. [7] In this paper we study the annual cycle and the variability on intraannual timescales with focus on the differences between the SH and the NH. We present a quantitative description of the variability in the meridional plane divided into the annual harmonic, the higher annual harmonics, and the intraannual variability. We study the duration, vertical extent, and strength of the downward propagation of intraannual variability in the SH. To our knowledge these issues have not been considered comprehensively before in the literature. Results from the reanalysis data are compared to results from a general circulation model experiment. [8] The data, the model, and the numerical procedures are briefly discussed in section 2. The annual cycle and the intraannual variability are studied in section 3.1 with focus on the reanalysis and in section 3.2 we compare with the model. In section 4 the downward propagation of both the annual cycle and the intraannual variability is treated. We close the paper with a conclusion in section Model and Data [9] AsinChristiansen [2001] we use the daily National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis [Kalnay et al., 1996] pressure level data set from 1958 to 1999 and a 42 years simulation with the general circulation model ARPEGE cycle 14 [Déqué etal., 1994]. [10] In Christiansen [2001] the intraannual variability included timescales shorter than 350 and longer than 30 days. However, as the higher harmonics of the annual cycle and especially the semi-annual component have a considerable effect in the SH some care should be taken in the definitions. The annual cycle is calculated as the long term mean of each calendar day. Hence it includes the sum of the annual harmonic (the harmonic component with a period of one year) and higher annual harmonic components (the harmonic components with periods of 1/2, 1/3,... year). In this paper the intraannual timescales are obtained by first removing the annual cycle and then applying a filter based on the Fast Fourier transform that removes frequencies corresponding to timescales longer than 350 and shorter than 30 days. Defined in this way the intraannual timescales do not include the higher annual harmonics of the annual cycle. As we will see the higher annual harmonic components will be small in the NH but considerable in the SH. [11] A measure of the wave forcing from the troposphere on the stratosphere is given by the vertical component of the Eliassen-Palm flux [Eliassen and Palm, 1961] at the tropopause level. In the quasi-geostrophic approximation the vertical component is F z = r 0 cos ffrh 1 N 2 v 0 T 0, where v is the meridional wind, T is the temperature, f is the Coriolis parameter, N 2 is the Brunt-Väisälä frequency, R is the gas constant, f is the latitude, r 0 is the density of air, H is the scale height, and overline and prime denote the zonal mean and the deviation from the zonal mean, respectively [Andrews et al., 1987]. [12] The AO and its Antarctic counterpart the Antarctic Oscillation (AAO) are defined as the leading modes found in a Principal Component Analysis of the surface pressure. The Empirical Orthogonal Functions (EOFs) are calculated from data poleward of 20 after each data point has been weighted with the square-root of the area it represents. [13] The NCEP/NCAR reanalysis in the earlier years is less reliable in the SH than in the NH due to the more sparse data coverage. The results reported here are based on all 42 years but we have tested that no significant differences are found if only the period from 1979 is included. 3. Annual Cycle and Intraannual Variability [14] In this section we present some results regarding differences in the annual cycle and in the intraannual variability between the two hemispheres. The downward propagation will be studied in section NCEP Reanalysis [15] The annual development of the zonal mean zonal wind, i.e., the long term mean of each calendar day, is

3 GRAVERSEN AND CHRISTIANSEN: DOWNWARD PROPAGATION Figure 1. The annual cycle (composite year) of the zonal mean zonal wind at 10 hpa. Upper and lower panels are the NCEP reanalysis and the model, respectively. shown in Figure 1 for 10 hpa. In the NH high latitudes the zonal mean zonal wind is quite symmetric around its maximum in the beginning of January. In the SH the maximum is delayed 1 2 months from winter solstice and the following deceleration is much stronger than the preceding acceleration. This deceleration is related to the spring breakdown of the polar vortex. The amplitude of the annual cycle is more than twice as large in the SH than in the NH. These observations have been described several times before [e.g., Hirota et al., 1983; Randel, 1988; Shiotani et al., 1993]. [16] The stratospheric dynamics at extra-tropical latitudes are basically determined by the competition between radiative damping and wave forcing from the troposphere. The wave forcing from the troposphere is given by the vertical component of the Eliassen-Palm flux at the tropopause, FzT. Figure 2 shows the annual cycle of FzT/r0 at 150 hpa integrated over the NH and SH (latitudes north of 10 N and south of 10 S). In the SH winter the maximum wave forcing is more than 35% weaker than in the NH winter, which is the accepted explanation for the stronger zonal winds in the SH stratosphere [e.g., Randel, 1988]. In the NH strong wave forcing occurs in the entire winter season while in the SH the strongest wave forcing is restricted to October November. This is also the period when the zonal mean zonal wind decays fast in agreement with the generally decelerating effect of the wave forcing. The period of strongest wave forcing in the SH has also been identified by Yamazaki and Mechoso [1985]. [17] The different variability in the two hemispheres are reflected in Figure 3, which shows the power spectrum of the zonal mean zonal wind at 10 hpa as function of latitude. The annual harmonic component is strongest in the SH in agreement with the discussion above. Furthermore, in the SH clear spectral peaks are located at the higher harmonics corresponding to periods of 1/2, 1/3, and 1/4 years. The ACL 9-3 Figure 2. The annual cycle of the vertical component FzT/ r0 [m2s 1] of the Eliassen-Palm flux at 150 hpa integrated over the regions north of 10 N (thick curves) and south of 10 S (thin curves). Solid curves are the NCEP reanalysis, dashed curves are the model. This quantity measures the tropospheric wave forcing on the stratosphere. Figure 3. The power-spectrum of the zonal mean zonal wind at 10 hpa as function of latitude. Upper and lower panels are the NCEP reanalysis and the model, respectively.

4 ACL 9-4 GRAVERSEN AND CHRISTIANSEN: DOWNWARD PROPAGATION Figure 4. The standard deviation of the zonal mean zonal wind [ms 1 ] as function of latitude and pressure [hpa] for the annual harmonic (upper panels), higher annual harmonics (middle panels), and intraannual timescales (lower panels). Left and right panels are the NCEP reanalysis and the model, respectively. semi-annual cycle in the SH stratosphere has also been identified by Dunkerton [2000]. In the NH, north of 40 N, the power on intraannual timescales is smeared out with no clear peaks at the higher harmonics. The two hemispheres resemble each other in the region between 40 N and 40 S where the intraannual variability is weak. In the equatorial region strong inter-annual variability related to the Quasi-Biennial Oscillation (QBO) is seen at timescales around 2 3 years. [18] The differences in the frequency distribution of the variability in the two hemispheres are further substantiated in Figure 4, which shows the standard deviation of the zonal mean zonal wind in the meridional plane including only the annual harmonic component, only the higher annual harmonic components, and only the intraannual variability. In the SH stratosphere the intraannual variability is about 30% weaker than in the NH. The higher harmonics carry a large portion of the variability on timescales between 30 and 350 days in the SH where more than 50% of this variability is locked to the annual cycle in form of the higher harmonics. In the NH this number is only about 20% in agreement with the absence of higher harmonics in Figure 3. This difference between the hemispheres is also found to a weaker extent in the troposphere. Here 30 35% of the variability is locked in the SH and 20 25% in the NH. [19] Kuroda and Kodera [2001] also described locking of variability to the annual cycle in the SH stratosphere. However, in their study the locking seems to be related to inter-annual variability and not to the higher annual harmonics described in the present paper. They found a strong biennial signal in a leading component of the zonal mean zonal wind. We see no significant two year component anywhere in the stratosphere in the zonal mean zonal wind data used here General Circulation Model [20] In the general circulation model the annual cycle is also strongest in the SH but the model underestimates the amplitude in both hemispheres by about 25% (Figures 1 and 4). The delay of the SH maximum after the solstice is well represented in the model, but the asymmetry between the acceleration and deceleration is much less pronounced in the model compared to the reanalysis. [21] A consistent difference is found in the SH wave forcing which is more symmetric around its winter maximum than in the reanalysis (Figure 2). In the NH the wave forcing in the model more closely resembles that in the reanalysis, although the wave forcing is in general weaker in the model than in the reanalysis in both hemispheres. As the wave forcing drives the zonal mean zonal wind away

5 GRAVERSEN AND CHRISTIANSEN: DOWNWARD PROPAGATION ACL 9-5 Figure 5. The annual cycle (composite year) of the zonal mean zonal wind at 60 N (left) and 60 S (right). Upper and lower panels are the NCEP reanalysis and the model, respectively. from the radiative equilibrium, a weaker annual cycle in the wave forcing would be expected to lead to a stronger annual cycle in the zonal mean zonal wind. As a consequence, the weaker annual cycle in the zonal mean zonal wind found in the model must be related to other factors than the large scale wave forcing such as deficiencies in the radiative heating or gravity waves. [22] From the power spectrum (Figure 3) we find that the latitudinal variations in the SH of the annual harmonic component in the model compare well with the reanalysis. In the NH the reanalysis has two maxima at 25 and 65 N while the model has a single maximum at 40 N. [23] The intraannual variability in both hemispheres is well captured in the model (Figure 4) although the strength is somewhat underestimated. The model also underestimates the higher annual harmonics in the SH by up to 50%. [24] As seen from Figure 3 there is almost no inter-annual variability in the model. This is, as mentioned in Christiansen [2001], partly because the boundary conditions are repeated from year to year and partly because the model does not simulate a QBO. 4. Downward Propagation [25] In this section we focus on the downward propagation in both the annual cycle and in the intraannual variability NCEP Reanalysis [26] The annual cycle of the zonal mean zonal wind is shown in Figure 5 as function of time and pressure for 60 S and 60 N. At 60 S a maximum is found at 10 hpa in August. For lower altitudes the maximum is delayed. This delay increases with decreasing height and at the surface the maximum is found in October. Thus at 60 S the annual cycle propagates downward on a timescale of around 2 months. Figure 1 indicates that the downward propagation is associated with a poleward propagation. This poleward and downward propagation has previously been studied by, e.g., Hartmann [1976], Leovy and Webster [1976], Hirota et al. [1983], Hartmann et al. [1984], Yamazaki and Mechoso [1985], and Kuroda and Kodera [1998]. A corresponding propagation of the annual cycle is not found in the NH as is apparent from Figure 5. [27] Figure 6 shows the zonal mean zonal wind at 60 Sas function of time and pressure for the years Only the intraannual timescales are included. It is clear from the figure that the variability is largest in the late winter/ spring season, September November, and that it is mostly vertically coherent with the same sign in the full extent of the stratosphere and often also in the troposphere. Comparing with the similar plot for 60 N [Christiansen, 2001, Figure 2] reveals some important differences in addition to the general weaker variability in the SH compared to the NH discussed in section 3. [28] We first note that the downward propagation in the SH is much less obvious compared to the NH. In the NH an anomaly often (almost every winter) begins at the top of the reanalysis (10 hpa) and propagates downward. For the 6 years in Figure 6 downward propagation is only clear in the late winter of [29] Figure 7 summarizes the downward propagation at 60 S based on all 42 years of data by presenting the correlation between the zonal mean zonal wind at 10 hpa

6 ACL 9-6 GRAVERSEN AND CHRISTIANSEN: DOWNWARD PROPAGATION Figure 6. The zonal mean zonal wind at 60 S [m/s] as function of time and pressure for six consecutive years. The data are filtered so that intraannual timescales between 30 and 350 days are retained. The higher harmonics of the annual cycle have also been removed. Upper and lower panels are the NCEP reanalysis and the model, respectively.

7 GRAVERSEN AND CHRISTIANSEN: DOWNWARD PROPAGATION ACL 9-7 a maximum correlation of 0.13 is found when the zonal wind lags the PC with 3 days. This should be compared with a maximum correlation of 0.24 found for a lag of 14 days in the NH. [31] Another difference between the downward propagation in the two hemispheres is found in the timing of the events. Considering all 42 years of data we find that the Figure 7. Correlations of the zonal mean zonal wind at 60 S with that at 10 hpa as function of pressure and time lag. Data are filtered so that only intraannual timescales between 30 and 350 days are retained. The higher harmonics of the annual cycle have also been removed. Light and dark shading identify regions where the correlations are significantly different from zero on 95 and 99% levels. Upper and lower panels are the NCEP reanalysis and the model, respectively. and other heights as function of time-lag. At 10 hpa positive correlations are distributed symmetrically around lag 0 while positive correlations in the troposphere are shifted toward positive lags. At the surface positive correlations are found for lags between 15 and 70 days. The correlations are significantly different from zero at the surface at the 99% level. The significance levels are calculated with a Monte Carlo approach as described in Christiansen [2001]. Compared to the NH the correlations at the surface are weaker. In the NH the correlations at the surface have a maximum of 0.21 at day 13, while in the SH a maximum of 0.17 is found already after 4 days. [30] The zonal mean zonal wind at 60 is in both hemispheres closely related to the phase of the leading EOF of surface pressure. The leading EOF of the intraannual variability in the SH sea level pressure explains 19% of the variance. The leading EOF is dominated by an annular structure (Figure 8) [Karoly, 1990] less disturbed than in the NH [Thompson and Wallace, 2000]. The correlation of the leading PC and the zonal mean zonal wind at 60 at the surface is 0.89 in the SH and 0.79 in the NH. The correlations in the SH between the leading PC and the zonal mean zonal wind at 60 S (Figure 9) describes only a weak downward propagation. At 10 hpa Figure 8. The leading empirical orthogonal function of the surface pressure in the NCEP reanalysis (top) and the model (bottom). These modes explain 19 and 21% of the total variance, respectively. The scaling is arbitrary. Data are filtered so that only intraannual timescales between 30 and 350 days are retained. The higher harmonics of the annual cycle have also been removed.

8 ACL 9-8 GRAVERSEN AND CHRISTIANSEN: DOWNWARD PROPAGATION 0.17 at a lag of 25 days. In addition, the propagation from 0.1 to 10 hpa takes 5 days. The leading EOF of the surface pressure explains 21% of the variance and closely resemble the leading EOF of the reanalysis, with a distinct annular structure with an almost circular node at 60 S. Figure 9. Correlations between the zonal mean zonal wind at 60 N and the normalized first principal component of the surface pressure. Data are filtered so that only intraannual timescales between 30 and 350 days are retained. The higher harmonics of the annual cycle have also been removed. Light and dark shading identify regions where the correlations are significantly different from zero on 95 and 99% levels. Upper and lower panels are the NCEP reanalysis and the model, respectively. downward propagating events in the NH are distributed almost homogeneously over the winter half-year. In contrast, in the SH the downward propagating events seem to be concentrated in the late winter and spring. This season coincides with the largest wave forcing from the troposphere and with the fast deceleration of the SH stratospheric jet General Circulation Model [32] The downward propagation of the modelled annual cycle in the SH resembles closely that of the reanalysis as can be seen from Figure 5. In the model the downward propagation is also present in the upper layers and it takes around 1 month for the annual cycle to propagate from 0.1 to 10 hpa. Also at 60 N the model and reanalysis agree well. At this latitude the model shows no downward propagation of the annual cycle neither below nor above 10 hpa. [33] The downward propagation at intraannual timescales is represented in the model (Figures 6, 7 and 9), but with an overestimation of both the timescale and the strength of the coupling between the troposphere and stratosphere. For example, the correlations between the zonal mean zonal wind at 10 hpa and that of the surface show a maximum of 5. Conclusions [34] We have studied the annual cycle and the variability on intraseasonal timescales in the zonal mean zonal wind. We have focused mainly on the SH and the differences between the SH and the NH. Results from 42 years of the NCEP reanalysis have been compared with results from a 42 years long GCM experiment. [35] The strength of the total stratospheric variability on timescales between 10 and 350 days is not very different in the two hemispheres. However, in the SH stratosphere the majority of this variability is locked to the annual cycle in form of higher annual harmonics in contrast to the NH where the higher annual harmonics are much weaker. In the SH the higher annual harmonics give the annual cycle an asymmetric development with stronger spring decelerations than autumn accelerations. This difference between the NH and the SH is reflected in the wave forcing from the troposphere which has a more well defined maximum in spring in the SH than in the NH. As reported before the amplitude of the annual cycle is larger and the wave forcing from the troposphere consistently weaker in the SH compared to the NH. [36] In the SH downward propagation is found both in the annual cycle and in the variability on intraannual timescales. In the NH downward propagation is only found in the intraannual timescales and not in the annual cycle. The downward propagation of the annual cycle in the SH is associated with a poleward propagation [e.g., Kuroda and Kodera, 1998] and takes about 2 months from 10 hpa to the lower troposphere. The downward propagation in the intraannual timescales differs in details between the two hemispheres. In the SH the downward propagation is concentrated to the spring season while it is distributed throughout the whole winter half-year in the NH. We also found that the coupling between the stratosphere and the troposphere is weaker and faster in the SH compared to the NH. [37] As described before variability on timescales less than 30 days has been removed from the data. We have checked that retaining the high frequency variability has no significant effect on the conclusions about the strength and timescales of the downward propagation. Note that the timescales of the vertical propagation are determined from the vertical distribution of the phases of the Fourier modes [Christiansen, 2001]. [38] With respect to the downward propagation the general circulation model compares well to the reanalysis in both hemispheres. In the model the propagation from 0.1 hpa to 10 hpa takes one month for the annual cycle and about 5 days for the intraannual variability. The largest deficiencies of the model are the underestimation of the amplitude of the annual cycle in both hemispheres and the too weak higher annual harmonics in the SH. The weaker annual cycle could be explained by stronger wave forcing from the troposphere, which is consistent with the explana-

9 GRAVERSEN AND CHRISTIANSEN: DOWNWARD PROPAGATION ACL 9-9 Figure 10. The vertical component of the Eliassen-Palm flux at 60 S as function of time and pressure for the same six years as in Figure 6. The data are filtered so that intraannual timescales between 30 and 350 days are retained. The higher harmonics of the annual cycle have also been removed. Upper and lower panels are the NCEP reanalysis and the model, respectively.

10 ACL 9-10 GRAVERSEN AND CHRISTIANSEN: DOWNWARD PROPAGATION tion of the weaker annual cycle in the NH compared to the SH. However, as the wave forcing in the model is weaker than in the reanalysis the explanation must be found elsewhere, probably in the radiative heating giving a too weak meridional temperature gradient or in an excessive gravity wave forcing. We note that no major warmings were found in the SH in the 42 years simulation in agreement with the reanalysis. The only observed major warming yet in the SH was in 2002, which was not included in the present analysis for consistency with Christiansen [2001]. [39] Christiansen [2001] showed that downward propagation of zonal mean zonal wind anomalies at 60 N is driven by the vertical component of the Eliassen-Palm flux and that the downward propagation of zonal wind anomalies was accompanied by a corresponding downward propagation of the wave forcing. Figure 10 shows the vertical component of the Eliassen-Palm flux at 60 S as function of time and pressure for the same period as in Figure 6. Comparing these figures we find a clear negative correlation between the acceleration of the zonal mean zonal wind and the vertical component of the Eliassen-Palm flux. In the SH we do not observe downward propagation of the wave forcing in contrast to what was found in the NH. This negative result might be related to the weaker and faster downward propagation in the SH. [40] Acknowledgments. This work was supported by the Danish Climate Center. The NCEP Reanalysis data were provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, USA, from their Web site at References Andrews, D. G., J. R. Holton, and C. B. Leovy, Middle Atmosphere Dynamics, 489 pp., Academic, San Diego, Calif., Baldwin, M. P., and T. J. Dunkerton, Propagation of the Arctic Oscillation from the stratosphere to the troposphere, J. Geophys. 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