1. Introduction
Tropical upwelling is an important component of the Brewer–Dobson circulation (BDC), strongly influencing temperature and tracer distributions in the tropical lower stratosphere (e.g., Holton et al. 1995; Plumb 2002; Shepherd 2007). The tropical BDC is mainly driven by momentum deposition from large- and small-scale waves, but the relative roles of different forcing regions and their contributions for variability on different time scales are topics of ongoing research (e.g., Randel et al. 2008; Taguchi 2009; Ueyama and Wallace 2010; Zhou et al. 2012; Ueyama et al. 2013). There is empirical evidence that wave activity in the extratropical winter stratosphere influences upwelling and temperatures in the tropical lower stratosphere (e.g., Fritz and Soules 1972; Randel 1993; Randel et al. 2002; Ueyama and Wallace 2010). This influence occurs as the extratropical waves exert a drag on the zonal mean circulation, inducing poleward mass flow, with mass continuity requiring downward flux at high latitudes and upwelling at low latitudes (e.g., Andrews et al. 1987). Theoretical considerations imply that, in the steady-state limit, these induced vertical motions are limited to the latitudinal extent of the forcing (cf. the “downward control principle”; Haynes et al. 1991), and this suggests that wave forcing needs to occur close to the tropics in order to produce realistic mean tropical upwelling (Plumb and Eluszkiewicz 1999; Semeniuk and Shepherd 2001). In contrast, for transient conditions, the influence of extratropical wave forcing can extend into the tropics through nonlocal effects (Garcia 1987; Haynes et al. 1991). The remote connection between high and low latitudes results from the coupled response of the atmospheric zonal wind, temperature, and meridional circulation to a transient wave drag (Garcia 1987).
There has been substantial interest in understanding the forcing responsible for the relatively large annual cycle in tropical upwelling, which in turn mainly drives the seasonality in temperature and chemical tracers in the tropical lower stratosphere (e.g., Yulaeva et al. 1994; Randel et al. 2007; Abalos et al. 2012, 2013). However, causality is difficult to untangle for the annual cycle, and the roles of different regions and types of wave forcing have been emphasized in a number of studies (including forcing from high latitudes, subtropics, and tropics). Based on the observed coherent temperature annual variations in the tropics and extratropics, some analyses have proposed a primary role of extratropical planetary waves (Yulaeva et al. 1994; Ueyama and Wallace 2010). However, as noted above, strictly high-latitude forcing is difficult to reconcile with theoretical expectations for time-mean tropical upwelling. Ueyama et al. (2013) recently suggested that the influence of the extratropical wave drag may involve the latitudinal progression of the forcing toward the tropics on time scales of about 10 days. Wave driving in the subtropics resulting from the dissipation of midlatitude baroclinic eddies has been emphasized by Taguchi (2009) and Chen and Sun (2011). Furthermore, several studies have highlighted an important role of equatorial planetary waves forced by convection in driving the seasonality in upwelling around the tropical tropopause (Boehm and Lee 2003; Kerr-Munslow and Norton 2006; Ryu and Lee 2010; Ortland and Alexander 2014). Randel et al. (2008) noted strong seasonal variations for wave forcing in the subtropics, resulting from the combined effects of extratropical and equatorial wave fluxes. The importance of subtropical wave drag has been also noticed in relation to the long-term trends in upwelling predicted by models (Butchart et al. 2006; Garcia and Randel 2008; Calvo and Garcia 2009; Garny et al. 2011; Shepherd and McLandress 2011).
Observations suggest large variability in tropical upwelling on subseasonal time scales. Although the theory of transient driving of the meridional circulation is well understood, observations of the dynamical forcing mechanisms for subseasonal fluctuations in upwelling have been less well documented. The relation between transience in the tropical lower stratosphere and sporadic bursts of planetary wave activity in the stratospheric high latitudes has been assessed in some observational works (e.g., Yulaeva et al. 1994; Randel et al. 2002; Ueyama and Wallace 2010). Zhou et al. (2012) argued that extratropical and subtropical wave drag act cooperatively to drive upwelling on various time scales and highlighted the importance of including transient variability for determining the specific forcing latitudes. Using high-vertical-resolution GPS temperature measurements, Grise and Thompson (2013) analyzed the role of different forcing regions in driving transient variability in upwelling (inferred from temperature tendencies). They found that planetary waves in the extratropical and subtropical stratosphere mainly drive transient upwelling in the lower stratosphere (above about 70 hPa), while equatorial planetary waves and drag in the subtropical troposphere are important for upwelling around the tropopause.
In this work, the dynamical drivers of transient variability in upwelling are investigated using daily time series of tropical mean upwelling derived from momentum balance calculations based on European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) data. In contrast with the annual cycle, the distinctive signature of subseasonal fluctuations in upwelling allows isolating the specific forcing using linear correlations and regressions. Section 2 describes the data and analyses and illustrates the statistical signature of the response to an extratropical time-dependent wave drag, as observed in the reanalysis. The forcing regions relevant for transient variability of tropical upwelling in the lower stratosphere are identified through regression and composite analyses in sections 3a and 3b, respectively. In addition to the extratropical winter stratosphere, the subtropical upper troposphere is found to play an important role in driving tropical upwelling, and the remote driving of upwelling by wave drag in this region is explored in section 3c. The connection between the different forcing regions is investigated in section 3d. Distinct circulations and forcings associated with the shallow versus deep branches of the tropical Brewer–Dobson circulation are observed in the results, and this is further analyzed in section 3e. Finally, section 4 presents a summary and discussion of the main results in the context of previous works.
2. Data and analyses
Daily mean temperature and three-dimensional wind fields from ERA-Interim are used (Dee et al. 2011), obtained averaging the 6-hourly data, for the period 1979–2011. The data are archived in a 1.5° × 1.5° grid, on 37 pressure levels spanning from 1000 to 1 hPa. To isolate subseasonal variability, a high-pass filter is applied to all the fields, which retains fluctuations on time scales shorter than 90 days. This threshold is chosen to eliminate the influence of the seasonal cycle up to its fourth harmonic.


















As mentioned in the introduction, the zonal mean response of the atmosphere to a momentum forcing (DF) in the extratropical winter stratosphere, given by Eqs. (1)–(4), has been extensively studied in theoretical models (Garcia 1987; Haynes et al. 1991; Plumb and Eluszkiewicz 1999), and its relation to upwelling in the tropics is fairly well understood for transient wave forcing (e.g., for stratospheric sudden warming events; Dunkerton et al. 1981). Figure 1 illustrates this behavior as derived from one-point correlation cross-sections, calculated from ERA-Interim for subseasonal time scales in boreal winter [December–March (DJFM)]. The reference time series for these correlations is −DF [convergence of the Eliassen–Palm (EP) flux] at one point in the extratropical boreal stratosphere (54°N, 20 hPa), representative of a region with climatological EP flux convergence in DJFM, and Fig. 1 shows the correlations with different fields (DF,
One-point correlations of EP flux convergence (−DF) at 54°N, 20 hPa with (a) EP flux (vectors) and divergence (colors), (b) zonal mean wind tendency, (c) net forcing, and (d) temperature tendency (colors) and residual circulation (vectors) for DJFM. The solid and dashed contours show the threshold of 99% significant linear correlations (positive and negative, respectively). The seasonal-mean lapse-rate tropopause is shown in gray. The vertical arrows in (a),(d) indicate the length corresponding to a linear correlation of 1 (equal for horizontal and vertical components). All figures are based on ERA-Interim 1979–2011 data unless explicitly stated in the captions.
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
The statistical relations in Fig. 1 evidence the meridional circulation (Fig. 1d) induced by a localized extratropical forcing (Fig. 1a) in the reanalysis, which is consistent with theoretical models based on the TEM equations [Eqs. (1)–(4)]. In particular, the patterns of correlations in Fig. 1 demonstrate the nonlocal response to high-latitude time-dependent wave forcing, with the residual circulation acting to maintain thermal wind balance between the remotely induced zonal wind and temperature tendencies. A key aspect is that, from a diagnostic point of view, the remote response of
3. Forcing of subseasonal variability in upwelling
The focus of this work is to identify dynamical forcing associated with transient tropical upwelling, and this is done using correlations and regressions applied to
Time series of upwelling (mm s−1) averaged over 18°S–18°N at 70 hPa for the years 2008/09. Black: momentum balance calculation [Eq. (5)], red: thermodynamic estimate (see text for details). (top) Full time series and (bottom) high-pass-filtered series, including only time scales shorter than 90 days.
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
The time series of tropical upwelling at 70 hPa in Fig. 2 are linked to the global zonal mean residual circulation, and Fig. 3 shows the structure of the associated circulations [calculated using
Regression of the residual circulation (vectors; m s−1) and temperature tendency (colors; K day−1) on tropical upwelling averaged over 18°S–18°N at 70 hPa for (a) DJFM and (b) JJAS. The reference level for upwelling is indicated by the black horizontal bar. The seasonal-mean lapse-rate tropopause is shown in gray. The solid and dashed contours show the threshold of 99% significant linear correlations (positive and negative, respectively). The scale of the vectors is indicated for both vertical and horizontal components. Regression values in all figures are per unit standard deviation in the upwelling.
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
The strong signature of transience in the Brewer–Dobson circulation is also observed in satellite-derived ozone measurements. Figure 4 shows 70-hPa
Regression of the residual circulation (vectors; m s−1) and correlations with zonal mean ozone tendency (colors) from the Aura Microwave Limb Sounder (MLS) satellite instrument on tropical upwelling averaged over 18°S–18°N at 70 hPa for (a) DJFM and (b) JJAS for the period 2005–10. The scale of the vectors is indicated for both vertical and horizontal components.
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
a. Wave drag, wind response, and net forcing
The dynamical driving of transient variability in upwelling in the tropical lower stratosphere is investigated in this section by examining the linear regressions of the different components of the net forcing onto the time series of
Regression of (a) EP flux (vectors) and divergence (colors), (b) zonal wind tendency, and (c) net forcing onto upwelling at 70 hPa averaged over 18°S–18°N for DJFM. The scale of the EP flux vectors (m3 s−2) is indicated in (a) for both vertical and horizontal components. The regions A, B, and C in (a) denote centers of action for the EP flux divergence, as discussed in the text. The time series are filtered to remove time scales longer than 90 days. The reference level for upwelling is indicated by the black horizontal bar. The seasonal-mean lapse-rate tropopause is shown in gray. The solid and dashed contours show the threshold of 99% significant linear correlations (positive and negative, respectively). Units for all color-shaded terms are m s−1 day−1.
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
As in Fig. 5, but for JJAS.
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
Regressions onto the global EP flux (Figs. 5a and 6a) show that tropical upwelling in the lower stratosphere is highly correlated with vertical wave propagation and convergence in the extratropical winter stratosphere, primarily over latitudes poleward of 30°. Upwelling is also positively correlated with EP flux divergence in the subtropical upper troposphere (in both hemispheres during DJFM in Fig. 5a, only weakly in the summer hemisphere in JJAS, Fig. 6a), with quasi-horizontal EP flux vectors indicating wave propagation away from these regions. The corresponding regressions onto the zonal mean wind tendencies (Figs. 5b and 6b) show negative correlations (i.e., deceleration of the flow) in the stratosphere centered at the latitude of strongest wave drag in the winter extratropics and positive correlations (acceleration) in the subtropical upper troposphere, approximately mirroring the patterns in EP flux divergence. The extratropical
The temporal evolution of the regressions during DJFM is shown in Fig. 7, which represents lagged regressions onto
Lagged regressions as a function of latitude and time (days) of upwelling (m s−1 day−1) at 70 hPa averaged over 18°S–18°N with the forcing terms integrated in altitude [as in Eq. (5)] from 70 to 1 hPa for DJFM. The latitudinal boundaries for upwelling calculations (18°S–18°N) are indicated by gray lines. The solid and dashed contours show the threshold of 99% significant linear correlations (positive and negative, respectively).
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
b. Composites of extreme events in tropical upwelling
The regression patterns in Figs. 3–7 show the statistical signatures of circulations linked to transient tropical upwelling in the lower stratosphere. In this section, these results are complemented with the direct assessment of the EP flux field and its divergence and the zonal mean flow tendency for composited events of extremes in upwelling (both positive and negative). Extreme events composites are constructed based on the multiyear record of
EP flux (vectors) and divergence (colors) composites for 5% (a) high, (b) low, and (c) difference (high minus low) extremes of upwelling at 70 hPa and averaged over 18°S–18°N for DJFM. The horizontal black bar indicates the location of tropical upwelling used to generate the composites. The seasonal-mean lapse-rate tropopause is shown in gray. The scale of the vectors (m3 s−2) is indicated in the panels for both vertical and horizontal components. Units for all color-shaded terms are m s−1 day−1.
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
The corresponding composites for the zonal mean wind tendencies are shown in Fig. 9. Since the climatological mean of the wind tendency is near zero, the anomalies coincide with the absolute values. During maximum upwelling events there is deceleration of the stratospheric winds over roughly 30°–60°N, coincident with the enhanced EP flux convergence in Fig. 8a, and this pattern extends vertically downward across the tropopause. There is also wind acceleration on the equatorward flank of the subtropical jets, extending with an equivalent barotropic structure over a deep layer from approximately 30 hPa to below 400 hPa. Because the stratospheric EP flux convergence is small at low latitudes, these
As in Fig. 8, but for zonal mean wind tendency (m s−1 day−1). Green contours represent the composited zonal mean winds for the corresponding upwelling extreme events (solid: positive values), with contour interval of 5 m s−1 (zero contour omitted).
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
c. Influence of wave forcing in the subtropical upper troposphere
Calculations of tropical upwelling using Eq. (5) involve integrating the net forcing above the level where upwelling is computed. Hence, it is not evident how the variability in upwelling at 70 hPa is affected by wave drag below that level (i.e., in the subtropical upper troposphere). To appreciate the mechanism through which this influence occurs, Fig. 10 shows one-point correlations of −DF at one point in the subtropical upper troposphere (24°N, 175 hPa) with different fields (as in Fig. 1). In this case, the wave forcing is localized in a relatively narrow vertical and latitudinal region (Fig. 10a), in contrast with the broader forcing region in Fig. 1a. The EP flux vectors suggest a strong contribution to the variability from extratropical waves and additionally some influence from near-equatorial latitudes (i.e., from equatorial planetary waves; Randel et al. 2008). The zonal wind response (Fig. 10b) shows deceleration collocated with the wave drag, extending higher in altitude than the forcing (together with accelerations to the north and south). The different vertical extent of DF and
As in Fig. 1, but one-point correlations of EP flux convergence (−DF) at 24°N, 175 hPa.
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
d. Relations of wave forcing between high and low latitudes and between hemispheres
The results of
First, the links between high- and low-latitude wave forcing are examined. Figure 11 shows a two-dimensional histogram of DF in the extratropical stratosphere (average over 36°–60°N, 20–10 hPa; region A; Fig 11a) and the subtropical upper troposphere (21°–33°N, 300–125 hPa; region B; Fig 11b). The black contours in Fig. 11 illustrate the distribution of all daily DJFM samples during 1979/80–2010/11 (121 days yr−1 × 32 yr = 3872 days), and the overall circular pattern of contours is evidence of weak correlation of wave forcing between regions A and B (i.e., enhanced wave drag in region A is not necessarily associated with reduced wave drag in region B). This is consistent with the direct calculation of time series correlation, which is statistically insignificant (~0.03). Figure 11 further includes red and blue dots indicating days with positive and negative extremes in
Two-dimensional distribution of wave forcing in the extratropical winter stratosphere (average over 20–10 hPa and 36°–60°N; region A in Fig. 5a) vs wave forcing in the subtropical upper troposphere (average over 300–125 hPa and 21°–33°N; region B in Fig. 5a) for DJFM. The axes are normalized to standard deviations in each quantity. Black contours show probability distribution of all DJFM days during 1979/80–2010/11 (contours shown are 1%, 5%, 20%, 50%, and 80%). Red and blue dots show days with 5% extreme maximum and minimum tropical upwelling. Larger circles indicate the mean of the distribution of all points (white), high extremes (red), and low extremes (blue).
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
Figure 12 shows a similar diagnostic comparing wave forcing in the subtropics of each hemisphere—that is, DF averaged over 300–125 hPa for 21°–33°N (region B) and for 21°–33°S (region C). Again the overall two-dimensional distribution suggests little correlation between wave forcing in each hemisphere (as also inferred from Fig. 10a). Positive extreme tropical upwelling events (red dots in Fig. 12) occur when DF is less negative or positive in both hemispheres, and the opposite extremes occur when DF is more negative in both hemispheres. While there is no general correlation between hemispheres, events that occur simultaneously lead to tropical upwelling extremes and appear as symmetric patterns in statistical correlations. Equivalent conclusions are found when comparing
As in Fig. 11, but for wave forcing in the subtropical upper troposphere in the NH vs the SH (averages over 300–125 hPa and 21°–33°N and 21°–33°S, respectively; regions B and C in Fig. 5a).
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
e. Shallow and deep branches of the residual circulation
Analyses of circulation statistics and tracer behavior suggest that the stratospheric Brewer–Dobson circulation can be described by deep and shallow branches, which can evolve in different ways (e.g., Plumb 2002; Birner and Bönisch 2011). This characteristic is also found in the analyses of subseasonal variability in this work (see Figs. 3 and 4). Figure 13 illustrates the transition from shallow to deep residual circulation in ERA-Interim data derived by regressing global circulation onto the tropical upwelling at different altitudes. The statistical signature of the lower branch can be identified in correlations with
As in Fig. 3a, but for tropical upwelling computed at (a) 20, (b) 50, and (c) 100 hPa, as indicated by the black bars.
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
The transition of the residual circulation from the lower to the middle stratosphere is also reflected in distinctive patterns of forcing terms influencing upwelling at different levels. Figure 14 shows the regression of the EP flux onto upwelling at the different levels, showing a transition in the dominant forcing associated with upwelling variability at different levels. Near 20 hPa, upwelling is most affected by forcing in the winter extratropical stratosphere (Fig. 14a), while the lower branch of the BDC is most influenced by transient wave forcing in the subtropical upper troposphere and lower stratosphere (Fig. 14c). Note that the shallow circulation (Fig. 14c) is negatively correlated with DF in the subtropical lower stratosphere near and above the level of upwelling, while positive correlations are found just below (as discussed in section 3c). This behavior is consistent with the results of Grise and Thompson (2013), as will be further discussed in the next section. Finally, note that extratropical wave drag is also correlated with upwelling at 100 hPa, which could be linked to the connection with the deep branch seen in Fig. 13c.
As in Fig. 5a, but for tropical upwelling computed at (a) 20, (b) 50, and (c) 100 hPa, as indicated by the black bars.
Citation: Journal of the Atmospheric Sciences 71, 9; 10.1175/JAS-D-13-0366.1
4. Summary and discussion
Upwelling across the tropical tropopause is a fundamental part of the global stratospheric circulation. It cannot be observed directly but can be inferred from thermodynamic or momentum calculations, or obtained from meteorological reanalyses, and these three estimates agree well for levels in the lower stratosphere (Abalos et al. 2012). Tropical upwelling is primarily driven by dynamical forcing (with temperatures and radiative balances responding to this forcing), which is embedded within the calculations of upwelling based on momentum balance
Regression analysis and composites of extreme tropical upwelling (
The nonlocal response to transient localized wave drag (DF) consists of two meridional circulation cells of opposite sense: one above and one below the forcing (the lower mass circulation cell is stronger because of the higher density). These extend outside the horizontal scale of the forcing and are linked to tendencies of zonal wind
The regression and composited patterns of DF based on
The statistical relationships between derived tropical upwelling in the lower stratosphere and global temperature and circulation reveal the shallow and deep branches of the Brewer–Dobson circulation (e.g., Fig. 3). The upper branch of the BDC is most strongly correlated to tropical upwelling at levels above the lower stratosphere, highlighting a broad-scale deep circulation connecting the tropics and the winter polar stratosphere (Fig. 13a). Variability in the deep branch is primarily linked to wave forcing in the high-latitude winter stratosphere (Fig. 14a). In contrast, transient upwelling near the tropopause (~100 or 70 hPa) is strongly correlated with downwelling circulation cells near the tropopause level in the subtropics (~25°–40°N/S), and these are reflected in zonal average temperature (Fig. 13c) and ozone tendencies (Fig. 4). This behavior is primarily linked to variability in wave forcing in the subtropical upper troposphere (Fig. 14c), wherein stronger (weaker)-than-average wave forcing affects the overturning circulation cells above the forcing, leading to reduced (enhanced) upwelling over the tropical tropopause and corresponding sinking in the subtropics (Fig. 10d). Note that the shallow branch of the BDC is also intensified by stronger convergence in the subtropical lower stratosphere (Fig. 14c). Hence, the exact altitude at which wave dissipation occurs in the subtropics is crucial for the variability of the shallow branch of the BDC, as the drag at slightly different levels can have opposite effects on upwelling near the tropopause. These results are consistent with the calculations of Grise and Thompson (2013), inferred from high-vertical-resolution GPS temperature observations. They show that wave drag in the subtropical lower stratosphere is associated with cooling near the tropical tropopause (~100–70 hPa), while subtropical tropospheric forcing leads to warming. Our analyses demonstrate the dynamical mechanism underlying these relationships, in terms of the momentum balance equation. Specifically, as follows from Eq. (5), subtropical EP flux convergence between 100 and 70 hPa drives stronger upwelling near the tropopause, while the wave drag at lower levels weakens the upwelling through the induced remote response in the zonal wind, as illustrated in Fig. 10.
The statistical analyses in the present work suggest an overall smooth transition between the two branches of the BDC, with upwelling at 100 hPa not fully decoupled from the deep circulation and the shallow branch reaching up to about 50 hPa (Fig. 13), consistently with the picture provided by the trajectory analysis of Birner and Bönisch (2011). Our results, based on time series of tropical upwelling, are also in general agreement with those obtained by Ueyama et al. (2013), based on temperature tendencies, which highlight distinct variability in circulation (and links to high-latitude wave forcing) in the tropics between 100 and 70 hPa and above. However, the latter work suggests a more abrupt disconnection between the shallow and the deep branches of the Brewer Dobson circulation than our results.
Acknowledgments
We thank Cameron Homeyer for facilitating access to ERA-Interim data used here. We are very grateful to Thomas Birner, Rolando Garcia, Rei Ueyama, Mike Wallace, and two anonymous referees for comments and suggestions, which have significantly improved the paper. This work was partially supported under the NASA Aura Science Program. M.A. also acknowledges the Spanish projects CGL2008-06295 and CGL2012-34997 and NCAR for hosting her visits.
REFERENCES
Abalos, M., W. J. Randel, and E. Serrano, 2012: Variability in upwelling across the tropical tropopause and correlations with tracers in the lower stratosphere. Atmos. Chem. Phys., 12, 11 505–11 517, doi:10.5194/acp-12-11505-2012.
Abalos, M., W. J. Randel, D. E. Kinnison, and E. Serrano, 2013: Quantifying tracer transport in the tropical lower stratosphere using WACCM. Atmos. Chem. Phys., 13, 10 591–10 607, doi:10.5194/acp-13-10591-2013.
Andrews, D. G., J. R. Holton, and C. B. Leovy, 1987: Middle Atmosphere Dynamics.International Geophysics Series, Vol. 40, Academic Press, 489 pp.
Birner, T., and H. Bönisch, 2011: Residual circulation trajectories and transit times into the extratropical lowermost stratosphere. Atmos. Chem. Phys., 11, 817–827, doi:10.5194/acp-11-817-2011.
Boehm, M. T., and S. Lee, 2003: The implications of tropical Rossby waves for tropical tropopause cirrus formation and for the equatorial upwelling of the Brewer–Dobson circulation. J. Atmos. Sci., 60, 247–261, doi:10.1175/1520-0469(2003)060<0247:TIOTRW>2.0.CO;2.
Butchart, N., and Coauthors, 2006: Simulations of anthropogenic change in the strength of the Brewer–Dobson circulation. Climate Dyn., 27, 727–741, doi:10.1007/s00382-006-0162-4.
Calvo, N., and R. R. Garcia, 2009: Wave forcing of the tropical upwelling in the lower stratosphere under increasing concentrations of greenhouse gases. J. Atmos. Sci., 66, 3184–3196, doi:10.1175/2009JAS3085.1.
Chen, G., and L. Sun, 2011: Mechanisms of the tropical upwelling branch of the Brewer–Dobson circulation: The role of extratropical waves. J. Atmos. Sci., 68, 2878–2892, doi:10.1175/JAS-D-11-044.1.
Dee, D. P., and Coauthors, 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553–597, doi:10.1002/qj.828.
Dunkerton, T. J., C.-P. F. Hsu, and M. E. McIntyre, 1981: Some Eulerian and Lagrangian diagnostics for a model stratospheric warming. J. Atmos. Sci., 38, 819–843, doi:10.1175/1520-0469(1981)038<0819:SEALDF>2.0.CO;2.
Fritz, S., and S. D. Soules, 1972: Planetary variations of stratospheric temperatures. Mon. Wea. Rev., 100, 582–589, doi:10.1175/1520-0493(1972)100<0582:PVOST>2.3.CO;2.
Garcia, R. R., 1987: On the mean meridional circulation of the middle atmosphere. J. Atmos. Sci., 44, 3599–3609, doi:10.1175/1520-0469(1987)044<3599:OTMMCO>2.0.CO;2.
Garcia, R. R., and W. J. Randel, 2008: Acceleration of the Brewer–Dobson circulation due to increases in greenhouse gases. J. Atmos. Sci., 65, 2731–2739, doi:10.1175/2008JAS2712.1.
Garny, H., M. Dameris, W. J. Randel, G. E. Bodeker, and R. Deckert, 2011: Dynamically forced increase of tropical upwelling in the lower stratosphere. J. Atmos. Sci., 68, 1214–1233, doi:10.1175/2011JAS3701.1.
Grise, K. M., and D. J. Thompson, 2013: On the signatures of equatorial and extratropical wave forcing in tropical tropopause layer temperatures. J. Atmos. Sci., 70, 1084–1102, doi:10.1175/JAS-D-12-0163.1.
Haynes, P. H., C. J. Marks, M. E. McIntyre, T. G. Shepherd, and K. P. Shine, 1991: On the “downward control” of extratropical diabatic circulations by eddy-induced mean zonal forces. J. Atmos. Sci., 48, 651–678, doi:10.1175/1520-0469(1991)048<0651:OTCOED>2.0.CO;2.
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, 403–439, doi:10.1029/95RG02097.
Kerr-Munslow, A. M., and W. A. Norton, 2006: Tropical wave driving of the annual cycle in tropical tropopause temperatures. Part I: ECMWF analyses. J. Atmos. Sci., 63, 1410–1419, doi:10.1175/JAS3697.1.
North, G. R., T. L. Bell, R. F. Cahalan, and F. J. Mohen, 1982: Sampling errors in the estimation of empirical orthogonal functions. Mon. Wea. Rev., 110, 699–706, doi:10.1175/1520-0493(1982)110<0699:SEITEO>2.0.CO;2.
Ortland, D. A., and M. J. Alexander, 2014: The residual-mean circulation in the tropical tropopause layer driven by tropical waves. J. Atmos. Sci., 71, 1305–1322, doi:10.1175/JAS-D-13-0100.1.
Plumb, R. A., 2002: Stratospheric transport. J. Meteor. Soc. Japan, 80, 793–809, doi:10.2151/jmsj.80.793.
Plumb, R. A., and J. Eluszkiewicz, 1999: The Brewer–Dobson circulation: Dynamics of the tropical upwelling. J. Atmos. Sci., 56, 868–890, doi:10.1175/1520-0469(1999)056<0868:TBDCDO>2.0.CO;2.
Randel, W. J., 1993: Global variations of zonal mean ozone during stratospheric warming events. J. Atmos. Sci., 50, 3308–3321, doi:10.1175/1520-0469(1993)050<3308:GVOZMO>2.0.CO;2.
Randel, W. J., R. R. Garcia, and F. Wu, 2002: Time-dependent upwelling in the tropical lower stratosphere estimated from the zonal-mean momentum budget. J. Atmos. Sci., 59, 2141–2152, doi:10.1175/1520-0469(2002)059<2141:TDUITT>2.0.CO;2.
Randel, W. J., M. Park, F. Wu, and N. Livesey, 2007: A large annual cycle in ozone above the tropical tropopause linked to the Brewer–Dobson circulation. J. Atmos. Sci., 64, 4479–4488, doi:10.1175/2007JAS2409.1.
Randel, W. J., R. R. Garcia, and F. Wu, 2008: Dynamical balances and tropical stratospheric upwelling. J. Atmos. Sci., 65, 3584–3595, doi:10.1175/2008JAS2756.1.
Ryu, J.-H., and S. Lee, 2010: Effect of tropical waves on the tropical tropopause transition layer upwelling. J. Atmos. Sci., 67, 3130–3148, doi:10.1175/2010JAS3434.1.
Semeniuk, K., and T. G. Shepherd, 2001: Mechanisms for tropical upwelling in the stratosphere. J. Atmos. Sci., 58, 3097–3115, doi:10.1175/1520-0469(2001)058<3097:MFTUIT>2.0.CO;2.
Shepherd, T. G., 2007: Transport in the middle atmosphere. J. Meteor. Soc. Japan, 85, 165–191, doi:10.2151/jmsj.85B.165.
Shepherd, T. G., and C. McLandress, 2011: A robust mechanism for strengthening of the Brewer–Dobson circulation in response to climate change: Critical-layer control of subtropical wave breaking. J. Atmos. Sci., 68, 784–797, doi:10.1175/2010JAS3608.1.
Taguchi, M., 2009: Wave driving in the tropical lower stratosphere as simulated by WACCM. Part I: Annual cycle. J. Atmos. Sci., 66, 2029–2043, doi:10.1175/2009JAS2854.1.
Ueyama, R., and J. M. Wallace, 2010: To what extent does high-latitude wave forcing drive tropical upwelling in the Brewer–Dobson circulation? J. Atmos. Sci., 67, 1232–1246, doi:10.1175/2009JAS3216.1.
Ueyama, R., E. P. Gerber, J. M. Wallace, and D. M. W. Frierson, 2013: The role of high-latitude waves in the intraseasonal to seasonal variability of tropical upwelling in the Brewer–Dobson circulation. J. Atmos. Sci., 70, 1631–1648, doi:10.1175/JAS-D-12-0174.1.
Yulaeva, E., J. R. Holton, and J. M. Wallace, 1994: On the cause of the annual cycle in tropical lower-stratospheric temperature. J. Atmos. Sci., 51, 169–174, doi:10.1175/1520-0469(1994)051<0169:OTCOTA>2.0.CO;2.
Zhou, T., M. A. Geller, and W. Lin, 2012: An observational study on the latitudes where wave forcing drives Brewer–Dobson upwelling. J. Atmos. Sci., 69, 1916–1935, doi:10.1175/JAS-D-11-0197.1.