• ADVANCES IN ATMOSPHERIC SCIENCES, 2017, 34(12): 1415-1425
    doi: 10.1007/s00376-017-6321-1
    Variation in Brewer-Dobson Circulation During Three Sudden Stratospheric Major Warming Events in the 2000s
    Mengchu TAO1,2, Yi LIU1, Yuli ZHANG1,

    Abstract:

    As the strongest subseasonal atmospheric variability during boreal winter, three remarkable sudden stratospheric major warming (SSW) events in the 2000s are investigated in terms of the Brewer-Dobson circulation (BDC) response. Our study shows that the changes of cross-isentropic velocity during the SSWs are not only confined to the polar region, but also extend to the whole Northern Hemisphere: enhanced descent in the polar region, as well as enhanced ascent in the tropics. When the acceleration of the deep branch of the BDC descends to the middle stratosphere, its strength rapidly decreases over a period of one to two weeks. The acceleration of the deep branch of the BDC is driven by the enhanced planetary wave activity in the mid-to-high-latitude stratosphere. Different from the rapid response of the deep branch of the BDC, tropical upwelling in the lower stratosphere accelerates up to 20%-40% compared with the climatology, 20-30 days after the onset of the SSWs, and the acceleration lasts for one to three months. The enhancement of tropical upwelling is associated with the large-scale wave-breaking in the subtropics interacting with the midlatitude and tropical Quasi-Biennial Oscillation-related mean flow.

    Key words: sudden stratospheric major warming; Brewer-Dobson circulation; subtropical wave;
    摘要: 本文讨论了2000以来三次平流层爆发性增温(SSW)事件对Brewer-Dobson(BD)环流的影响. 结果表明, SSW期间穿越等熵面的垂直运动不只出现在极区, 而是在整个北半球, SSW后极区下沉和热带地区上升都有所增强. 位于平流层中高层的BD深环流上升支在SSW开始前后在中高纬度行星波的拖曳作用下明显增强, 持续时间在10天左右; 不同于BD深环流, 平流层低层的BD浅环流的热带上升支, 在SSW发生后约一个月左右才明显加强, 持续时间在1-3个月, 这是大尺度波在副热带地区的拖曳作用同副热带和赤道纬向风场相互作用的结果. 赤道纬向风的垂直结构(即QBO东西风位相)调节着波动破碎在副热带地区发生的位置.
    关键词: 平流层爆发性增温 ; BD环流 ; 副热带波动
    1. Introduction

    Sudden stratospheric major warming (SSW), characterized by a rapid temperature rise and strong wind disturbance in the polar stratosphere, has been identified as the most pronounced subseasonal dynamical phenomenon in the boreal winter atmosphere. Among the various definitions and classifications of SSW that have been proposed [reviewed by (Butler et al., 2015)], the current and most widely used definition of midwinter SSW was proposed as early as the 1970s by the World Meteorological Organization, and is described more explicitly in (Charlton and Polvani, 2007). The basic understanding regarding the dynamics of SSW is that enhanced planetary waves generate in the troposphere, propagate upwards into the stratosphere, break in the midlatitude stratosphere, and the wave forcing drives the polar temperature rise and slows the mean flow (Matsuno, 1971; Holton, 1976; Andrews et al., 1987).

    The enhanced wave activities during SSWs potentially influence the wave-driven hemispheric overturning circulation, i.e., the Brewer-Dobson circulation (BDC) (Dobson et al., 1929; Brewer, 1949). This mechanism of wave-driven mean meridional circulation is referred to as the "downward control" principle (Haynes et al., 1991) or extratropical "wave pump" (Holton et al., 1995). In fact, the complete structure of the BDC is more complicated. There are two hemispheric branches of the BDC in the stratosphere: the deep branch, which penetrates high into the upper stratosphere; and the shallow branch, which overturns in the lower stratosphere (e.g. Plumb, 2002; Bönisch et al., 2011). The connection and disconnection between the two BDC branches have been intensively discussed. Some studies emphasize that the extratropical wave drives the entire tropical upwelling (both branches), with an equatorward propagation of about 10 days (Ueyama and Wallace, 2010; Ueyama et al., 2013). Other works suggest that the equatorial wave is important for driving upwelling at the tropical tropopause (e.g., Kerr-Munslow and Norton, 2006; Ryu and Lee, 2010). Several works suggest an alternative dynamical mechanism is responsible for the shallow branch circulation: combined extratropical and equatorial wave drag in the subtropics (Bönisch et al., 2011; Garny et al., 2011) that remotely influences the tropical upwelling through the zonal wind tendency from subseasonal to decadal timescales (Randel et al., 2008; Shepherd and McLandress, 2011; Abalos et al., 2014).

    The SSW events that take place when the most pronounced enhancement of the planetary wave occurs on a subseasonal timescale can be regarded as a natural experiment in which we can study the response of the BDC to the variable dynamics. Most previous works on the association between the BDC and SSWs have focused on the poleward and descending motion of the BDC at high latitudes from the perspective of dynamical anomalies (Labitzke and Kunze, 2009; Manney et al., 2009a; Liu et al., 2014), or from tracer transport (Manney et al., 2005, 2009b; Tao et al., 2015a) or ozone chemistry (Konopka et al., 2005; Kuttippurath and Nikulin, 2012). Several recent studies have noticed an increased tropical upwelling response to SSWs based on evidence from lower-stratospheric water vapor. Extratropical cooling and drying related to SSWs was found as a subseasonal variation of tropical lower-stratospheric water vapor by a 35-year simulation (Tao et al., 2015b), and the extra-drying effect was confirmed by in-situ water vapor measurements (Evan et al., 2015). (Gómez-Escolar et al., 2014) proposed that SSWs induce strong tropical upwelling and extra cooling as a response to subtropical planetary wave breaking, which is modulated by the phases of the Quasi-Biennial Oscillation (QBO).

    Since the tropical BDC response to SSWs has rarely been discussed in detail, and the effect is important for troposphere-stratosphere transport of climate-relevant species like H2O and O3, we focus in this study on the BDC response at different altitudes based on three remarkable SSWs that occurred in the 2000s. Then, we discuss the association of the deep and shallow branches of the BDC with the evolution of the wave forcing. Following this introduction, section 2 introduces the data and method used in the study. Section 3 provides an overview of the dynamical background, including the wind, temperature, polar vortex and planetary wave propagation during the three warming events. In section 4, we diagnose the strength of upwelling in the shallow and deep branches of the BDC during the SSWs. In section 5, the relationship between the BDC and wave forcing is discussed based on the Eliassen-Palm (EP) flux and its divergence in an isentropic coordinate. A discussion and a summary of the main findings of our study are provided in section 6.

    2. Data and method

    To estimate the zonal-mean mass transport by the BDC, the widely used definition of the mean meridional residual circulation in log-pressure coordinates is defined as \begin{eqnarray} \overline{v}*&=&\overline{v}-\dfrac{1}{\rho_0}[(\rho_0\overline{v'\theta'})/\overline{\theta}_z]_z ,\ \ (1)\\ \overline{w}^*&=&\overline{w}+\dfrac{1}{a\cos \varphi}[(\cos\varphi\overline{v'\theta'})/\overline{\theta}_z]_\varphi , \ \ (2)\end{eqnarray} where overbars indicate the zonal mean. The items ρ, a, v, w represent density, radius, meridional and vertical velocity (Andrews et al., 1987).

    This study is performed in the isentropic coordinate, where φ and θ denote latitude and theta, respectively. The zonal mean residual circulation in the isentropic coordinate is defined as the isentropic mass density (σ) weighted mean meridional velocity (\(\overline{v}^*\)) and cross-isentropic velocity (\(\overline{w}^*\)): \begin{eqnarray} \overline{v}^*&=&\overline{\sigma v} ,\ \ (3)\\ \overline{w}^*&=&\overline{\sigma\dot{\theta}} , \ \ (4)\end{eqnarray} where the vertical velocity \(\dot\theta\) can be estimated by the total diabatic heating rates (\(\dot\theta=Q\)). The advantage of using the isentropic coordinate is that no additional transformation is required, compared to the formalism in log-pressure coordinates in (Andrews et al., 1987).

    Typically, 70 hPa is used as the upper-limit pressure level [∼500 K (20 km)-1] of the shallow branch of the BDC (e.g. Rosenlof, 1995, Bönisch et al., 2011, Abalos et al., 2014). Here, we use the total positive \(\overline{w}^*\) between 380 K and 500 K to quantify the upwelling of the shallow branch of the circulation, and the total positive \(\overline{w}^*\) between 600 K and 1500 K to quantify the upwelling of the deep branch.

    The wave forcing is diagnosed by the divergence of EP flux (e.g., Eliassen, 1951; Plumb and Bell, 1982). EP flux vectors represent the propagation direction of wave energy. More specifically, EP flux divergence characterizes the forcing from eddy (or wave) to the zonal mean flow. Here, we use the EP flux formula in an isentropic coordinate, referring to the TEM formalism by (Andrews et al., 1987). The vertical and horizontal components of EP flux [F=(Fφ,Fθ) are given by \begin{eqnarray} F_\varphi&=&-a\cos\varphi\overline{(\sigma v)'u'} ,\ \ (5)\\ F_\theta&=&(\overline{p'M'_\lambda}/g)-a\cos\varphi\overline{(\sigma Q)'u'} , \ \ (6)\end{eqnarray} where M is the Montgomery streamfunction (M=CpT+gz), p is pressure, and a is the Earth's radius. The Montgomery streamfunction is the streamfunction for the geostrophic flow, which is equal to CpT+gz on an isentropic surface, where z is the height of the isentropic surface.

    The European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) dataset is used to study the dynamical background and to derive the eddy heat and momentum fluxes. The ERA-Interim data have a temporal resolution of 6 h and a horizontal resolution of 1.5°× 1.5°. Originally in a 37-layer pressure coordinate, the data are interpolated into a 60-layer hybrid coordinate between the pressure coordinate (below 300 hPa) and isentropic coordinate (above 300 hPa). The vertical cross-isentropic velocity \((\dot\theta=d\theta/dt)\) is equal to the ERA-Interim forecast total diabatic heating rate (Q) (Ploeger et al., 2010). The rate of theta change following a parcel is equal to the total diabatic heating rate, which contains the effects of all-sky radiative heating, latent heat release, and diffusive heating.

    The methodology to identify the onset of the SSWs is based on (Charlton and Polvani, 2007). The main criterion of this method to identify SSW events is the reversal of the zonal mean zonal wind at 10 hPa and 60°N during November-March [see (Charlton and Polvani, 2007) for more details]. In addition, the magnitude of an SSW event is measured by the mean temperature anomaly over the polar cap (60°-90°N) at 10 hPa (∆ T10, shown in Table 1), and the polar vortex edge is identified by the maximum potential vorticity meridional gradient combined with the maximum westerlies [further details in (Nash et al., 1996)].

    Table 1. Key information regarding the three SSW events selected in this study. The dominant wavenumber is determined by the amplitude of wave components averaged over 40°-70°N at 400 K through Fourier decomposition. The vortex geometry during the SSWs is determined by checking the daily potential vorticity map in the Northern Hemisphere. The QBO phase for each SSW is defined as the 30-day smoothed equatorial mean wind at θ=500 K (∼50 hPa), calculated for each SSW central date.
    Year Central date Magnitude ∆ T10 (K) Planetary wavenumber Polar vortex geometry QBO phase
    2003/04 5 Jan 2004 12.5 wavenumber-1 Displaced Easterly
    2005/06 21 Jan 2006 7.7 wavenumber-1 Displaced Easterly
    2008/09 24 Jan 2009 14.4 wavenumber-2 Split Westerly

    Table 1. Key information regarding the three SSW events selected in this study. The dominant wavenumber is determined by the amplitude of wave components averaged over 40°-70°N at 400 K through Fourier decomposition. The vortex geometry during the SSWs is determined by checking the daily potential vorticity map in the Northern Hemisphere. The QBO phase for each SSW is defined as the 30-day smoothed equatorial mean wind at θ=500 K (∼50 hPa), calculated for each SSW central date.

    The QBO is a major impact factor of tropical upwelling and tropical temperature in the stratosphere, inducing temperature variations of 4 K and 0.5 K in the lower stratosphere and around the tropopause, respectively (Baldwin et al., 2001; Xie et al., 2014). Although El ño-Southern Oscillation (ENSO) is an important tropospheric factor (Xie et al., 2012, 2014), (Konopka et al., 2016) suggested that, whilst there is a three-dimensional impact of ENSO on tropical upwelling, it is not as pronounced as that of the QBO in a zonal-mean sense. Thus, the QBO phase during the SSW period, and not the ENSO phase, is considered in this study. The QBO phase for each SSW event is defined as the 30-day smoothed equatorial mean wind at θ=500 K (∼ 50 hPa), calculated for each SSW central date.

    3. Dynamical background

    The 20-year de-seasonalized (referred to as the climatology from 1979 to 2013) records of averaged polar cap temperature and mean zonal wind around the North Pole (55°-65°N) are shown in Fig. 1. A 7-day smoothing is applied to both the temperature and wind data to remove the short-term variations, e.g., synoptic perturbation. We can see that SSW events are the most pronounced subseasonal variation during boreal winter. There are nine major SSWs during the 20 years. The black arrows mark the central dates of all the major SSWs, according to (Charlton and Polvani, 2007). Note that in this study we follow the recommendation of (Butler et al., 2015) to use the mean zonal wind from 55°-65°N, instead of the zonal wind at a specific latitude such as 60°N.

    Fig.1. De-seasonalized temperature (color shading) at the North Pole (60°-90°N) and zonal mean wind between 55°N and 65°N (positive: solid contours; negative: dashed contours; values larger than 10 m s-1 shown) during boreal winter (November-March) from 1990 to 2009. A 7-day running mean is applied to both the temperature and zonal wind. Black arrows point to the central dates of the SSWs (Charlton and Polvani, 2007).

    Among all the major SSWs, three events during the boreal winters of 2003/04, 2005/06 and 2008/09 are the most remarkable. Table 1 provides the essential information about these three remarkable SSWs, including their central dates, magnitudes of temperature rise, dominant planetary wavenumbers, polar vortex geometries, and QBO phases in the tropics. Firstly, the three SSWs experience the strongest temperature and wind variation, which can be quantified by the polar cap temperature increase listed in Table 1 as ∆ T10. Secondly, the perturbation of temperature and wind descend more deeply to the lower stratosphere and the upper troposphere during the three SSWs than in the other SSWs. Thirdly, all three SSWs are followed by strong polar vortex recovery, shown as the strong cooling and westerly anomalies after their occurrence. This polar warm and cold oscillation in boreal winter refers to the polar-night jet oscillation (e.g., Kuroda and Kodera, 2001; Hitchcock and Shepherd, 2013). This is the result of radiative relaxation, diabatic cooling in the polar region, as well as the suppression of planetary wave propagation into the high latitudes due to the formation of easterlies.

    The SSWs in 2003/04 and 2005/06 both correspond to an increasing wavenumber-1, whereas the warming event in 2008/09 is driven by a rapid increase in wavenumber-2 (Manney et al., 2005, 2008, 2009a, 2009b). The geometry of the polar vortex during the SSWs is closely associated with the dominant planetary wave: wavenumber-1 and wavenumber-2 usually lead to vortex-displacement and vortex-split events, respectively. Moreover, the SSWs in 2003/04 and 2005/06 both occur during an easterly QBO phase in the tropics, whereas the SSW in 2008/09 occurs during a westerly QBO phase. Therefore, the three cases include different wavenumber and QBO situations, and are thus representative of various dynamical backgrounds.

    Fig.2. De-seasonalized cross-isentropic velocity (\(\dot\theta\)) averaged over the North Pole (60°-90°N; left column) and over the tropics (30°S-30°N; right column), shown respectively for the 2003/04 (top), 2005/06 (middle) and 2008/09 (bottom) winters. Contours in the right-hand panels show the anomalies referring to the corresponding climatology in the same QBO phase. Black arrows indicate the central dates of the SSWs (Charlton and Polvani, 2007).

    4. Response of the BDC

    Aside from the intense change in wind and temperature during SSWs, a dramatic enhancement in vertical mass transport is also noticeable. Figure 2 shows the anomalies of the diabatic heating rate (\(\dot\theta\)) in the polar region (left-hand column) and tropical region (right-hand column). The anomalies of \(\dot\theta\) over the North Pole and tropics illustrate the evolution of the polar downwelling and tropical upwelling of the BDC, respectively. We can see that enhanced downwelling (negative anomalies) occurs in the upper stratosphere and propagates downwards gradually to 500 K after 10-15 days.

    The negative anomalies at upper levels (e.g., 1000 K) are only sustained for less than one month (only 10 days in the 2009 case), whereas in the lower stratosphere the intensified descent lasts longer than two months. The time scales are consistent with radiative relaxation time scales: 10 days in the upper stratosphere and around 100 days in the lower stratosphere (Mlynczak et al., 1999).

    Besides, the left-hand column shows the acceleration of polar descent (negative anomalies) is always followed by strong deceleration of polar descent (positive anomalies) in the upper stratosphere. This deceleration of polar descent corresponds to a strong polar vortex recovery due to radiative cooling. It is also worth mentioning that the negative anomaly during the 2008/09 SSW has the most intense amplitude (velocity decreasing to -10 K d-1 at 1000 K), the fastest recovery at 1000 K (only 10 days for 2008/09; one month for the other two events), and the quickest arrival in the lower stratosphere (<10 days for 2008/09; 15 days for the other two).

    The impact of the SSWs is not only confined to the polar region, but also extends to the tropics. Along with the intensified polar descent, tropical upwelling also enhances, which can be seen as the positive anomalies in the right-hand column of Fig. 2. Note that two anomalies are shown in the right-hand column: the anomalies referring to the climatology (de-seasonalized anomalies; color shading); and the anomalies referring to the easterly QBO (eQBO)/ westerly QBO (wQBO) climatology (de-seasonalized and de-QBO anomalies; black contours). Both quantities also indicate that the response of tropical upwelling to the SSWs starts simultaneously with the polar downwelling beginning to accelerate in the upper stratosphere. Disconnection between the deep and shallow BDC can be observed in the 2003/04 and 2005/06 cases: the descent of upwelling positive anomalies from the upper stratosphere pauses at around 500 K, but continues with a lag time in the lower stratosphere. The disconnection between the two branches is consistent with a previous work (Plumb, 2002; Bönisch et al., 2011), but also suggests a potential connection with a lagged effect due to latitudinal propagation (Ueyama and Wallace, 2010; Ueyama et al., 2013).

    Fig.3. De-seasonalized zonal mean cross-isentropic velocity (\(\dot\theta\)) on the 400 K isentropic surface, shown as color shading, for the 2003/04 (top), 2005/06 (middle) and 2008/09 (bottom) winters. Contours show the anomalies referring to the corresponding climatology in the same QBO phase. Black arrows indicate the central dates of the SSWs (Charlton and Polvani, 2007).

    The difference in the tropical and polar regional mean is not significant. Figure 3 shows the evolution of the two (de-seasonalized and de-QBO) anomalies along the latitudes on the 1000 K (∼ 10 hPa) and on the 450 K isentropic surface (∼ 80 hPa). The enhancement of upwelling on the 1000 K surface reaches a maximum around the central dates (10 days earlier than the central date for the 2003/04 case) and lasts for 10-20 days. The differences in the two (de-seasonalized and de-QBO) upwelling anomalies on the 1000 K surface (left-hand panels) are noticeable: although the de-seasonalized upwelling enhances roughly hemisphere-symmetrically after the SSW and is centered at equator, the positive anomalies referring to the QBO climatology shift more to the Southern Hemisphere. The asymmetry of the de-QBO anomalies is not significant for the upwelling in the lower stratosphere (right-hand panels). Enhanced upwelling of the shallow BDC in the Northern Hemisphere is also found following the three SSWs, but the peak of the positive anomalies happens 20-30 days after the SSWs, when the upwelling for the deep branch starts to decrease.

    Figure 4 sums up the BDC response to the SSWs by quantifying the strength of the deep branch upwelling anomalies (black lines) and shallow branch upwelling anomalies (gray lines). Here, we use the total positive (upward) vertical component of residual circulation \(\overline{w}^*\) over the regions 30°S-30°N between 600 K and 1500 K and 30°S-30°N between 380 K and 500 K, representing the deep and shallow branches of the BDC upwelling, respectively. These quantities are proportional to the upward mass fluxes crossing the isentropic surface [see Eq. (4)]. Note that the corresponding QBO climatology is used for calculating the relative differences. Thus, the values in Fig. 4 stand for the relative anomalies from the QBO climatology (units: %).

    We find that the deep branch upwelling quickly increases with large variability before the SSWs commence, whereas it decreases sharply 10 days afterwards. The increase in amplitude of the upwelling of the deep BDC reaches 20% for the 2003/04 and 2005/06 cases, and 30% for the 2008/09 case. The shallow branch upwelling remains almost constant, with some variations before the start of the SSWs, before slowly accelerating and reaching a peak within a month. The enhancement of the shallow BDC upwelling after the SSW cases reaches a maximum of 40% in the 2003/04 and 2005/06 cases, and 20% in the 2008/09 case. The contribution from the two hemispheres varies from case to case. Compared with the shallow branch upwelling in the Northern Hemisphere, the variation of upwelling in the Southern Hemisphere is not significant (Fig. 3).

    Fig.4. Variation in tropical upwelling relative differences from the corresponding QBO phase climatology (\(\dot{\overline{w}}^*\)), shown for the total shallow BDC (gray lines) and deep branch of the BDC (black lines). The total positive \(\dot{\overline{w}}^*\) over 30°S-30°N between 600 K and 1500 K denotes the deep branch upwelling. The total positive \(\dot{\overline{w}}^*\) over 30°S-30°N between 380 K and 500 K represents the shallow branch upwelling. Arrows mark the central dates of the SSWs. Dashed straight lines mark the peak of the black lines (strongest upwelling for deep BDC), and solid straight lines mark the peak of the grey lines (strongest upwelling for shallow BDC).

    5. Wave forcing

    Following discussion of the lagged evolution from the deep to shallow branch upwelling in the previous section, we next study the associated wave activities driving the circulation. Figure 5 shows the evolution of Northern Hemisphere EP flux divergence at selected locations, shown as "A" and "B" in Fig. 6, during the 45 days around the central date. Note that a five-day running mean has been applied to the quantities in Fig. 5. The black line represents the wave forcing in the mid-to-upper stratosphere. We can see that the black lines all reach a minimum (largest EP flux convergence) around the central date of the SSW, and all gradually increase afterwards. The gray lines clearly have a similar phase to the black lines shown in the deep BDC variation in Fig. 5. The correlation coefficients between the two quantities are about -0.6 in all three cases. The negative correlation between the evolution of equatorward upwelling and EP flux divergence indicates in-phase wave forcing (negative EP flux divergence) of the upwelling according to the "downward control" principle (Haynes et al., 1991).

    Similar to the lagged response from the deep to shallow BDC, we see a 20-40-day lagged evolution of the subtropical lower stratosphere (grey lines) to the pronounced subseasonal variation of EP flux divergence at high latitudes in the mid-to-upper stratosphere in all three cases. Although a small-scale wave, e.g., a synoptic wave or gravity wave, could have contributed to the wave drag of tropical upwelling, the subtropical planetary wave forcing (gray lines) in the lower stratosphere revealed by the ERA-Interim data shows a close association with the shallow branch BDC. The EP flux divergence gradually decreases before the central dates, and continuously decreases afterwards. The maximum wave forcing (minimum EP flux divergence) is found around 20-40 days after the central dates. The gray lines in Fig. 5 again illustrate a close evolution with the upwelling of the shallow BDC (gray lines) in Fig. 4. The correlation coefficients between the two quantities are about -0.5 for the selected cases, which suggests a relationship between subtropical wave forcing in the Northern Hemisphere and shallow tropical upwelling.

    Fig.5. Evolution of EP flux divergence during the three SSW events. Black lines show the EP flux divergence at (50°N, θ=800 K; marked "A" in Fig. 6). Gray lines show the EP flux divergence at (15°N, θ=450 K; marked "B" in Fig. 6). The central date of each SSW is marked by an arrow. Dashed lines mark the minimum of the black lines (strongest wave forcing), and the solid lines mark the minimum of the gray lines (strongest wave forcing). A five-day running mean is applied to the quantities.

    Fig.6. Latitude-height projections of EP flux divergence (color shading) averaged over 10 days around the central dates. Solid and dashed contours show the westerly and easterly zonal mean wind averaged over 45 days around the central dates. Regions where the correlation of the EP flux divergence with the variation of upwelling over the red-square region (left-hand column: 30°S-30°N, 600-1500 K; right-hand column: 30°S-30°N, 380-500 K) pass the 95% significance test are marked by yellow crosses (positive correlation) and blue circles (negative correlation).

    Fig.7. Mean evolution of total positive tropical upwelling (\(\dot{\overline{w}}^*\)) of the deep branch (30°S-30°N, 600-1500 K) and shallow branch (30°S-30°N, 380-500 K) for all SSWs in eQBO (red solid line) and wQBO years (blue solid line), and winters without SSW in eQBO (red dashed line) and wQBO (blue dashed line) phases. Zero days for winter with SSWs are the central dates of each SSW and are the mean central dates for SSWs in each QBO phase for the years without SSWs.

    To follow this method and to explore the relationship between the wave forcing and the two branches of the BDC and its relationship with zonal wind, we further apply correlation analysis to the variation in EP flux divergence with that of the shallow and deep BDC upwelling during 45 days around the central dates in Fig. 6. We analyze the correlation in the variation of EP flux divergence with the variation in the shallow and deep branch upwelling shown in Fig. 5 (the area used for calculating the BDC upwelling is indicated by the red square). Note that, before the correlation analysis, a five-day running mean is applied to the EP flux divergence shown in Fig. 6. The regions within which the correlation of EP flux divergence with the tropical upwelling is statistically significant (t-test; 0.95 significance level) are overlaid with orange crosses (positive correlation) or blue dots (negative correlation). Recall here that negative correlation (blue dots) suggests the corresponding wave forcing could be the wave drag to the correlated upwelling, according to the "downward control" principle, as shown in the relationship in Fig. 4 and Fig. 5. Positive correlation (orange crosses) indicates a lag time between the subseasonal variation of wave forcing and the correlated upwelling.

    All three cases suggest that the deep branch of the BDC upwelling is mainly a response to the high-latitude EP flux convergence or extratropical planetary wave drag in the mid-to-upper stratosphere (blue dots in the left-hand panels of Fig. 6). An example is the EP flux divergence variation of location "A" shown in Fig. 6 (black lines). On the other hand, the wave drag correlated with the upwelling of the shallow BDC shows an almost opposite correlation with wave forcing: its acceleration is positively correlated with the high-latitude EP flux divergence in the mid-to-upper stratosphere (near "A") and negatively correlated to the EP flux divergence in the subtropics between 400 K and 500 K (near "B"). The positive correlation of extratropical wave drag indicates the lagged effect from the wave drag for the deep BDC to that for the shallow BDC. A significantly negatively correlated subtropical wave drag can be a remote forcing for enhanced shallow upwelling in the tropics (Garcia, 1987).

    The propagation and breaking of the wave interacts with the background zonal wind shown as contours in Fig. 6 (Dickinson, 1968). Before the SSWs, strong westerlies in the high latitudes favor the upward propagation of planetary waves more poleward, and breaks at high latitudes. After the SSWs, when the extratropical westerlies have been largely weakened or reversed to easterlies, further wave propagation into the stratosphere either breaks at the bottom of the extratropical easterlies, or breaks at the tropics close to the zero-wind line (Gómez-Escolar et al., 2014). As seen in the right-hand panels of Fig. 6, the significant correlation of subtropical wave drag is mainly located where the zonal wind is close to zero. As proposed by (Gómez-Escolar et al., 2014), the QBO phase is important for the location of the zero-wind line, and thus important to the breaking of waves driving the lower-stratospheric upwelling.

    6. Conclusion and discussion

    The three remarkable mid-winter SSW events studied here all show strong subseasonal temperature, wind and cross-isentropic velocity variability from the middle of winter to spring, throughout the Northern Hemispheric stratosphere. The response of the tropical upwelling of the shallow branch of the BDC must be discussed separately to the deep branch because they show a different tendency and a lagged enhancement after the SSWs. Our results suggest that the strength of the deep branch of the BDC reaches a peak around the central date of the SSW, and rapidly decreases afterwards. On the other hand, the shallow branch of the BDC gradually accelerates after the SSW central date, remaining for more than one month in all three cases.

    The result suggests a disconnection between the deep and shallow branches of the BDC, which show different variation after the SSW, but also a connection between the two branches in the form of a lagged enhancement from the deep branch to the shallow branch. This case study result is consistent with results from composite analysis by (Gómez-Escolar et al., 2014), in that the tropical BDC acceleration is a response to SSW events. Moreover, the modulation by the QBO of the shallow BDC upwelling is also confirmed in our present study: the 2008/09 (wQBO) case shows a weaker increase of upwelling in the lower stratosphere compared with the other two (eQBO) cases.

    We also test the result of increased upwelling as a response to SSWs in more cases. The evolutions of mean upwelling of the deep and shallow BDC during years with and without SSWs during each QBO phase are compared. For the detailed method of case sampling and the identification of central dates (Charlton and Polvani, 2007), refer to (Tao et al., 2015b). The result, shown in Fig. 7, includes all years with SSWs (1979-2013: eight wQBO cases and eight eQBO cases) and the years without SSWs (five eQBO cases and seven wQBO cases) in each QBO phase. Note here that the subseasonal variation is largely smoothed out by composition. Nevertheless, the composite still shows BDC enhancement due to SSWs for both branches, with a 20-30-day delay in both QBO phases, which supports the conclusion based on the case study.

    Further analysis of wave forcing shows the deep and shallow BDC responding to the extratropical planetary wave forcing and the subtropical planetary wave drag, respectively. The lagged and remote connection between the extratropical and subtropical wave drag can be explained by the variation in the zonal wind background, consistent with previous works (e.g., Dickinson, 1968; Garcia, 1987).

    The variations in dynamics in the tropical lower stratosphere influence the transport from the troposphere to the stratosphere. This work supports and completes the theory of extra drying at a cooler tropopause associated with SSW events, as suggested by the simulation in (Tao et al., 2015b) and from in-situ observations in (Evan et al., 2015), from the dynamical perspective. Furthermore, the complex interaction between the variation in temperature, chemical reaction rates, and radiation effect, needs further investigation.

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    Extremely low water vapour concentrations (as low as 1.5 ppmv) in the tropical tropopause layer (TTL) were observed by in situ measurements during the Airborne Tropical TRopopause Experiment (ATTREX) winter 2013 deployment in February 2013. The January 2013 tropical (15°N–15°S) mean value of Microwave Limb Sounder (MLS) water vapour satellite data at 82 hPa (2.3 ppmv) was one of the lowest during the instrument record (2004–2013). The relationship between a cooling of the tropical tropopause, a sudden stratospheric warming (SSW) event and convective activity in the western Pacific is investigated using satellite data and reanalysis meteorological products to elucidate the likely origin of those extremely low water vapour concentrations. A major midwinter SSW developed on 6 January 2013. Stratospheric polar temperatures increased by 6530 K in a matter of days and temperatures in the tropical upper troposphere and lower stratosphere (UTLS) dropped at the same time. As a result of the easterly shear phase of the Quasi-Biennial Oscillation and the SSW, the tropical tropopause in January 2013 was anomalously cold (zonal mean of 187 K) and elevated (85 hPa). The tropical cold point tropopause (CPT) temperature and water vapour concentration at 82 hPa decreased by about 2 K and 1.5 ppmv respectively within the first 15 days of January; the water vapour change was likely a result of dehydration associated with the rapid cooling of the tropical CPT during that period.
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    Abstract Major Stratospheric Sudden Warmings (SSWs) are characterized by a reversal of the zonal mean zonal wind and an anomalous warming in the polar stratosphere that proceeds downward to the lower stratosphere. In the tropical stratosphere, a downward propagating cooling is observed. However, the strong modulation of tropical winds and temperatures by the quasi-biennial oscillation (QBO) renders accurate characterization of the tropical response to SSWs challenging. A novel metric based on temperature variations relative to the central date of the SSW using ERA-Interim data is presented. It filters most of the temperature structure related to the phase of the QBO and provides proper characterization of the SSW cooling amplitude and downward propagation tropical signal. Using this new metric, a large SSW-related cooling is detected in the tropical upper stratosphere that occurs almost simultaneously with the polar cap warming. The tropical cooling weakens as it propagates downward, reaching the lower stratosphere in a few days. Substantial differences are found in the response to SSWs depending on the QBO phase. Similar to what is observed in the polar stratosphere, tropical SSW-associated temperatures persist longer during the west QBO phase at levels above about 40 hPa, suggesting that the signal is mainly controlled by changes in the residual mean meridional circulation associated with SSWs. Conversely, in the lower stratosphere, around 5070 hPa, enhanced cooling occurs only during QBO east phase. This behavior seems to be driven by anomalous subtropical wave breaking related to changes in the zero-wind line position with the QBO phase.
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    The recovery of the Arctic polar vortex following stratospheric sudden warmings is found to take upward of 3 months in a particular subset of cases, termed here polar-night jet oscillation (PJO) events. The anomalous zonal-mean circulation above the pole during this recovery is characterized by a persistently warm lower stratosphere, and above this a cold midstratosphere and anomalously high stratopause, which descends as the event unfolds. Composites of these events in the Canadian Middle Atmosphere Model show the persistence of the lower-stratospheric anomaly is a result of strongly suppressed wave driving and weak radiative cooling at these heights. The upper-stratospheric and lower-mesospheric anomalies are driven immediately following the warming by anomalous planetary-scale eddies, following which, anomalous parameterized nonorographic and orographic gravity waves play an important role. These details are found to be robust for PJO events (as opposed to sudden warmings in general) in that many details of individual PJO events match the composite mean.A zonal-mean quasigeostrophic model on the sphere is shown to reproduce the response to the thermal and mechanical forcings produced during a PJO event. The former is well approximated by Newtonian cooling. The response can thus be considered as a transient approach to the steady-state, downward control limit. In this context, the time scale of the lower-stratospheric anomaly is determined by the transient, radiative response to the extended absence of wave driving. The extent to which the dynamics of the wave-driven descent of the stratopause can be considered analogous to the descending phases of the quasi-biennial oscillation (QBO) is also discussed.
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    Abstract Based on simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) for the period 1979-2013, with model transport driven by the ECMWF ERA-Interim reanalysis, we discuss the impact of the El Ni09o Southern Oscillation (ENSO) on the variability of the dynamics, water vapor, ozone and mean age of air (AoA) in the tropical lower stratosphere during boreal winter. Our zonally-resolved analysis at the 390 K potential temperature level reveals that not only (deseasonalized) ENSO-related temperature anomalies are confined to the tropical Pacific (180 - 300° E) but also anomalous wave propagation and breaking, as quantified in terms of the Eliassen-Palm (EP) flux divergence, with strongest local contribution during the La Ni09a phase. This anomaly is coherent with respective anomalies of water vapor ( ± 0.5 ppmv) and ozone ( ± 100 ppbv) derived from CLaMS being in excellent agreement with the Aura Microwave Limb Sounder observations. Thus, during El Ni09o a more zonally symmetric wave forcing drives a deep branch of the Brewer-Dobson (BD) circulation. During La Ni09a this forcing increases at lower levels (≈390K) over the tropical Pacific, likely influencing the shallow branch of the BD circulation. In agreement with previous studies, wet (dry) and young (old) tape-recorder anomalies propagate upwards in the subsequent months following El Ni09o (La Ni09a). Using CLaMS these anomalies are found to be around +0.3 (-0.2) ppmv and -4 (+4) months for water vapor and AoA, respectively. The AoA ENSO anomaly is more strongly affected by the residual circulation (≈2/3) than by eddy mixing (≈1/3).
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    The spatial and temporal characteristics of the month-to-month variability of the polar night jet and its relationship with tropospheric circulation is investigated for both the Northern and Southern Hemispheres. The variabilities of the hemispheres have many common characteristics of the Polar Night Jet Oscillation (PJO). These common characteristics include the following: (1) the anomalous zonal-mean zonal winds shift poleward and downward; (2) the anomalous polar temperatures propagate downward from the stratopause to the upper troposphere; and (3) they are made through a wave-mean flow interaction with mainly the planetary wave of zonal wave number one. Annular modes associated with the PJOs appear in both hemispheres when the zones of maximum polar temperature anomaly descend to the lowermost stratosphere and upper troposphere. The major difference in the PJOs of the two hemispheres is found in their temporal characteristics. In the Southern Hemisphere, the phase of the PJO is closely locked to the annual cycle, while in the Northern Hemisphere it exhibits quasiperiodic variability with its envelope controlled by the annual cycle. The origin of the differences between the PJOs is discussed based on the theory of the wave-mean flow interaction.
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    [23] Kuttippurath J., G. Nikulin, 2012: A comparative study of the major sudden stratospheric warmings in the arctic winters 2003/2004-2009/2010.Atmospheric Chemistry and Physics,12,8115-8129,doi: 10.5194/acp-12-8115-2012.
    We present an analysis of the major sudden stratospheric warmings (SSWs) in the Arctic winters 2003/04–2009/10. There were 6 major SSWs (major warmings [MWs]) in 6 out of the 7 winters, in which the MWs of 2003/04, 2005/06, and 2008/09 were in January and those of 2006/07, 2007/08, and 2009/10 were in February. Although the winter 2009/10 was relatively cold from mid-December to mid-January, strong wave 1 activity led to a MW in early February, for which the largest momentum flux among the winters was estimated at 60° N/10 hPa, about 450 m2 s 2. The strongest MW, however, was observed in 2008/09 and the weakest in 2006/07. The MW in 2008/09 was triggered by intense wave 2 activity and was a vortex split event. In contrast, strong wave 1 activity led to the MWs of other winters and were vortex displacement events. Large amounts of Eliassen-Palm (EP) and wave 1/2 EP fluxes (about 2–4 ×105 kg s 2) are estimated shortly before the MWs at 100 hPa averaged over 45–75° N in all winters, suggesting profound tropospheric forcing for the MWs. We observe an increase in the occurrence of MWs (~1.1 MWs/winter) in recent years (1998/99–2009/10), as there were 13 MWs in the 12 Arctic winters, although the long-term average (1957/58–2009/10) of the frequency stays around its historical value (~0.7 MWs/winter), consistent with the findings of previous studies. An analysis of the chemical ozone loss in the past 17 Arctic winters (1993/94–2009/10) suggests that the loss is inversely proportional to the intensity and timing of MWs in each winter, where early (December–January) MWs lead to minimal ozone loss. Therefore, this high frequency of MWs in recent Arctic winters has significant implications for stratospheric ozone trends in the northern hemisphere.
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    [24] Labitzke K., M. Kunze, 2009: On the remarkable arctic winter in 2008/2009. J. Geophys. Res., 114,D00I02, doi: 10.1029/ 2009JD012273.
    It is well known that the interannual variability of the stratospheric winters over the Arctic is very large. On the basis of data for more than 60 winters, this variability has been studied with the aim of understanding and possibly forecasting the type of the coming winter, in the stratosphere and also in the troposphere. Today, there is general agreement that the variability of the stratospheric circulation during the Arctic winters is influenced by different forcing mechanisms: by the tropospheric planetary waves which penetrate into the stratosphere, by the Quasi-Biennial Oscillation (QBO) and the Southern Oscillation (SO) in the tropics which influence the stratospheric polar vortex, and by the 11-year sunspot cycle which interacts with the QBO and probably also with the SO. For the winter 2008/2009, all of the known signals pointed to a stable, cold stratospheric polar vortex throughout the winter, but in the real atmosphere a major midwinter warming developed in January and February with record-breaking temperatures. The synoptics of this winter will be discussed in the context of all of the above-mentioned forcing mechanisms.
    DOI:10.1029/2009JD012273      URL     [Cited within:1]
    [25] Liu C. X., B. J. Tian, K.-F. Li, G. L. Manney, N. J. Livesey, Y. L. Yung, and D. E. Waliser, 2014: Northern hemisphere mid-winter vortex-displacement and vortex-split stratospheric sudden warmings: Influence of the madden-Julian oscillation and quasi-biennial oscillation.J. Geophys. Res.,119,12 599-12 620,doi: 10.1002/2014JD021876.
    Abstract We investigate the connection between the equatorial Madden-Julian Oscillation (MJO) and different types of the Northern Hemisphere mid-winter major stratospheric sudden warmings (SSWs), i.e., vortex-displacement and vortex-split SSWs. The MJO-SSW relationship for vortex-split SSWs is stronger than that for vortex-displacement SSWs, as a result of the stronger and more coherent eastward propagating MJOs before vortex-split SSWs than those before vortex-displacement SSWs. Composite analysis indicates that both the intensity and propagation features of MJO may influence the MJO-related circulation pattern at high latitudes and the type of SSWs. A pronounced Quasi-Biennial Oscillation (QBO) dependence is found for vortex-displacement and vortex-split SSWs, with vortex-displacement (-split) SSWs occurring preferentially in easterly (westerly) QBO phases. The lagged composites suggest that the MJO-related anomalies in the Arctic are very likely initiated when the MJO-related convection is active over the equatorial Indian Ocean (around the MJO phase 3). Further analysis suggests that the QBO may modulate the MJO-related wave disturbances via its influence on the upper tropospheric subtropical jet. As a result, the MJO-related circulation pattern in the Arctic tends to be wave number-one/wave number-two ~25–3065days following phase 3 (i.e., approximately phases 7–8, when the MJO-related convection is active over the western Pacific) during easterly/westerly QBO phases, which resembles the circulation pattern associated with vortex-displacement/vortex-split SSWs.
    DOI:10.1002/2014JD021876      URL     [Cited within:1]
    [26] Manney G. L., M. L. Santee, N. J. Livesey, L. Froidevaux, W. G. Read, H. C. Pumphrey, J. W. Waters, and S. Pawson, 2005: EOS Microwave Limb Sounder observations of the Antarctic polar vortex breakup in 2004. Geophys. Res. Lett., 32,L12811, doi: 10.1029/2005GL022823.
    New observations from the Microwave Limb Sounder (MLS) on NASA's Aura satellite give a detailed picture of the spring Antarctic polar vortex breakup throughout the stratosphere, with the first daily global HCl profiles providing an unprecedentedly clear view of transport in the lower stratosphere. Poleward transport at progressively lower levels, filamentation, and mixing are detailed in MLS HCl, NO, HO, and Oas the 2004 Antarctic vortex broke up from the top down in early October through late December. Improved MLS HO data show the subvortex, below the tropical tropopause, breaking up almost simultaneously with the lower stratospheric vortex in December. Vortex remnants persisted in MLS tracers for over a month after the breakup in the midstratosphere, but no more than a week in the lower stratosphere. MLS observations show diabatic descent continuing throughout November, but weak ascent after late October in the lower stratospheric vortex core. Our results extend previous observational transport studies and show consistency with mixing and vortex evolution in meteorological analyses, and with model studies.
    DOI:10.1029/2005GL022823      URL     [Cited within:2]
    [27] Manney, G. L., Coauthors, 2008: The evolution of the stratopause during the 2006 major warming: Satellite data andassimilated meteorological analyses. J. Geophys. Res., 113,D11115, doi: 10.1029/2007JD009097.
    Microwave Limb Sounder and Sounding of the Atmosphere with Broadband Emission Radiometry data provide the first opportunity to characterize the four-dimensional stratopause evolution throughout the life-cycle of a major stratospheric sudden warming (SSW). The polar stratopause, usually higher than that at midlatitudes, dropped by 30 km and warmed during development of a major ave 1 SSW in January 2006, with accompanying mesospheric cooling. When the polar vortex broke down, the stratopause cooled and became ill-defined, with a nearly isothermal stratosphere. After the polar vortex started to recover in the upper stratosphere/lower mesosphere (USLM), a cool stratopause reformed above 75 km, then dropped and warmed; both the mesosphere above and the stratosphere below cooled at this time. The polar stratopause remained separated from that at midlatitudes across the core of the polar night jet. In the early stages of the SSW, the strongly tilted (westward with increasing altitude) polar vortex extended into the mesosphere, and enclosed a secondary temperature maximum extending westward and slightly equatorward from the highest altitude part of the polar stratopause over the cool stratopause near the vortex edge. The temperature evolution in the USLM resulted in strongly enhanced radiative cooling in the mesosphere during the recovery from the SSW, but significantly reduced radiative cooling in the upper stratosphere. Assimilated meteorological analyses from the European Centre for Medium-Range weather Forecasts (ECMWF) and Goddard Earth Observing System Version 5.0.1 (GEOS-5), which are not constrained by data at polar stratopause altitudes and have model tops near 80 km, could not capture the secondary temperature maximum or the high stratopause after the SSW; they also misrepresent polar temperature structure during and after the stratopause breakdown, leading to large biases in their radiative heating rates. ECMWF analyses represent the stratospheric temperature structure more accurately, suggesting a better representation of vertical motion; GEOS-5 analyses more faithfully describe stratopause level wind and wave amplitudes. The high-quality satellite temperature data used here provide the first daily, global, multiannual data sets suitable for assessing and, eventually, improving representation of the USLM in models and assimilation systems.
    DOI:10.1029/2007JD009097      URL     [Cited within:]
    [28] Manney, G. L., Coauthors, 2009a: Satellite observations and modeling of transport in the upper troposphere through the lower mesosphere during the 2006 major stratospheric sudden warming.Atmospheric Chemistry and Physics,9,4775-4795,doi: 10.5194/acp-9-4775-2009.
    An unusually strong and prolonged stratospheric sudden warming (SSW) in January 2006 was the first major SSW for which globally distributed long-lived trace gas data are available covering the upper troposphere through the lower mesosphere. We use Aura Microwave Limb Sounder (MLS), Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) data, the SLIMCAT Chemistry Transport Model (CTM), and assimilated meteorological analyses to provide a comprehensive picture of transport during this event. The upper tropospheric ridge that triggered the SSW was associated with an elevated tropopause and layering in trace gas profiles in conjunction with stratospheric and tropospheric intrusions. Anomalous poleward transport (with corresponding quasi-isentropic troposphere-to-stratosphere exchange at the lowest levels studied) in the region over the ridge extended well into the lower stratosphere. In the middle and upper stratosphere, the breakdown of the polar vortex transport barrier was seen in a signature of rapid, widespread mixing in trace gases, including CO, Hlt;subgt;2lt;/subgt;O, CHlt;subgt;4lt;/subgt; and Nlt;subgt;2lt;/subgt;O. The vortex broke down slightly later and more slowly in the lower than in the middle stratosphere. In the middle and lower stratosphere, small remnants with trace gas values characteristic of the pre-SSW vortex lingered through the weak and slow recovery of the vortex. The upper stratospheric vortex quickly reformed, and, as enhanced diabatic descent set in, CO descended into this strong vortex, echoing the fall vortex development. Trace gas evolution in the SLIMCAT CTM agrees well with that in the satellite trace gas data from the upper troposphere through the middle stratosphere. In the upper stratosphere and lower mesosphere, the SLIMCAT simulation does not capture the strong descent of mesospheric CO and Hlt;subgt;2lt;/subgt;O values into the reformed vortex; this poor CTM performance in the upper stratosphere and lower mesosphere results primarily from biases in the diabatic descent in assimilated analyses.
    DOI:10.1080/0458063X.1996.10392327      URL     [Cited within:1]
    [29] Manney, G. L., Coauthors, 2009b: Aura microwave limb sounder observations of dynamics and transport during the record-breaking 2009 arctic stratospheric major warming. Geophys. Res. Lett., 36,L12815, doi: 10.1029/2009GL 038586.
    A major stratospheric sudden warming (SSW) in January 2009 was the strongest and most prolonged on record. Aura Microwave Limb Sounder (MLS) observations are used to provide an overview of dynamics and transport during the 2009 SSW, and to compare with the intense, long-lasting SSW in January 2006. The Arctic polar vortex split during the 2009 SSW, whereas the 2006 SSW was a vortex displacement event. Winds reversed to easterly more rapidly and reverted to westerly more slowly in 2009 than in 2006. More mixing of trace gases out of the vortex during the decay of the vortex fragments, and less before the fulfillment of major SSW criteria, was seen in 2009 than in 2006; persistent well-defined fragments of vortex and anticyclone air were more prevalent in 2009. The 2009 SSW had a more profound impact on the lower stratosphere than any previously observed SSW, with no significant recovery of the vortex in that region. The stratopause breakdown and subsequent reformation at very high altitude, accompanied by enhanced descent into a rapidly strengthening upper stratospheric vortex, were similar in 2009 and 2006. Many differences between 2006 and 2009 appear to be related to the different character of the SSWs in the two years.
    DOI:10.1029/2009GL038586      URL     [Cited within:1]
    [30] Matsuno T., 1971: A dynamical model of the stratospheric sudden warming.J. Atmos. Sci.,28,1479-1494,doi: 10.1175/1520-0469(1971)028<1479:ADMOTS>2.0.CO;2.
    The dynamics of the stratosphere sudden warming phenomenon is discussed in terms of the interaction of vertically propagating planetary waves with zonal winds. If global-scale disturbances are generated in the troposphere, they propagate upward into the stratosphere, where the waves act to decelerate the polar night jet through the induction of a meridional circulation. Thus, the distortion and the break-down of the polar vortex occur. If the disturbance is intense and persists, the westerly jet may eventually disappear and an easterly wind may replace it. Then `critical layer interaction' takes place. Further intensification of the easterly wind and rapid warming of the polar air are expected to occur as well as weakening of the disturbance. The model is verified by numerical integrations of the adiabatic-geostrophic potential vorticity equation. Computed results possess features similar to those observed in sudden warming phenomena.
    DOI:10.1175/1520-0469(1971)0282.0.CO;2      URL     [Cited within:1]
    [31] Mlynczak M. G., C. J. Mertens, R. R. Garcia, and R. W. Portmann, 1999: A detailed evaluation of the stratospheric heat budget: 2.Global radiation balance and diabatic circulations. J. Geophys. Res.,104,6039-6066,doi: 10.1029/1998JD200099.
    We present a detailed evaluation of radiative heating, radiative cooling, net heating, global radiation balance, radiative relaxation times, and diabatic circulations in the stratosphere using temperature and minor constituent data provided by instruments on the Upper Atmosphere Research Satellite (UARS) between 1991 and 1993 and by the limb infrared monitor of the stratosphere (LIMS) instrument which operated on the Nimbus-7 spacecraft in 1978-1979. Included in the calculations are heating due to absorption of solar radiation from ultraviolet through near-infrared wavelengths and radiative cooling due to emission by carbon dioxide, water vapor, and ozone from 0 to 3000 cm( - 3.3 m). Infrared radiative effects of Pinatubo aerosols are also considered in some detail. In general, we find the stratosphere to be in a state of global mean radiative equilibrium on monthly timescales to within the uncertainty of the satellite-provided measurements. Radiative relaxation times are found to be larger in the lower stratosphere during UARS than LIMS because of the presence of Pinatubo aerosols. The meridional circulations in the upper stratosphere as diagnosed from the calculated fields of net heating are generally stronger in the UARS period than during the LIMS period, while the lower stratosphere meridional circulations are stronger during the LIMS period. A climatology of these calculations is available to the community via a World Wide Web interface described herein.
    DOI:10.1029/1998JD200099      URL     [Cited within:1]
    [32] Nash E. R., P. A. Newman, J. E. Rosenfield, and M. R. Schoeberl, 1996: An objective determination of the polar vortex using Ertel's potential vorticity.J. Geophys. Res.,101,9471-9478,doi: 10.1029/96JD00066.
    We have developed objective criteria for choosing the location of the northern hemisphere polar vortex boundary region and the onset and breakup dates of the vortex. By determining the distribution of Ertel's potential vorticity (Epv) on equivalent latitudes, we define the vortex edge as the location of maximum gradient of Epv constrained by the location of the maximum wind jet calculated along Epv isolines. We define the vortex boundary region to be at the local maximum convex and concave curvature in the Epv distribution surrounding the edge. We have determined that the onset and breakup dates of the vortex on the 450 K isentropic surface occur when the maximum wind speed calculated along Epv isolines rises above and falls below approximately 15.2 m s. We use 1992-1993 as a test case to study the onset and breakup periods, and we find that the increase of polar vortex Epv values is associated with the dominance of the term in the potential vorticity equation involving the movement of air through the surface due to the diabatic circulation. We also find that the decrease is associated with the dominance of the term involving radiatively induced changes in the stability of the atmosphere.
    DOI:10.1029/96JD00066      URL     [Cited within:1]
    [33] Ploeger F., P. Konopka, G. Günther J.-U. Groo\ss, and R. Müller, 2010: Impact of the vertical velocity scheme on modeling transport in the tropical tropopause layer. J. Geophys. Res., 115,D03301, doi: 10.1029/2009JD012023.
    To assess the impact of the vertical velocity scheme on modeling transport in the tropical tropopause layer (TTL), 3 month backward trajectories are initialized in the TTL for boreal winter and summer 2002. The calculations are done in either a kinematic scenario with pressure tendency as the vertical velocity or in a diabatic scenario with cross-isentropic velocity deduced from various diabatic heating rates due to radiation (clear sky, all sky) and latent, diffusive and turbulent heating. This work provides a guideline for assessing the sensitivity of trajectory and chemical transport model (CTM) results on the choice of the vertical velocity scheme. We find that many transport characteristics, such as time scales, pathways and dispersion, crucially depend on the vertical velocity scheme. The strongest tropical upwelling results from the operational European Centre for Medium-Range Weather Forecasts kinematic scenario with the time scale for ascending from 340 to 400 K of 1 month. For the ERA-Interim kinematic and total diabatic scenarios, this time scale is about 2 months, and for the all-sky scenario it is as long as 2.5 months. In a diabatic scenario, the whole TTL exhibits mean upward motion, whereas in a kinematic scenario, regions of subsidence occur in the upper TTL. However, some transport characteristics robustly emerge from the different scenarios, such as an enhancement of residence times between 350 and 380 K and a strong impact of meridional in-mixing from the extratropics on the composition of the TTL. Moreover, an increase of meridionally transported air from the summer hemisphere into the TTL (maximum for boreal summer) is found as an invariant feature among all the scenarios.Pl枚ger, F.; Konopka, P.; G nther, G.; Groo , J.-U.; M ller, R.
    DOI:10.1029/2009JD012023      URL     [Cited within:1]
    [34] Plumb R. A., 2002: Stratospheric transport.J. Meteor. Soc. Japan,80,793-809,doi: 10.2151/jmsj.80.793.
    DOI:10.2151/jmsj.80.793      URL     [Cited within:2]
    [35] Plumb R. A., R. C. Bell, 1982: A model of the quasi-biennial oscillation on an equatorial beta-plane.Quart. J. Roy. Meteor. Soc.,108,335-352,doi: 10.1002/qj.49710845604.
    Abstract An equatorial beta-plane model of equatorial wave-mean flow interaction in the lower stratosphere is described. Kelvin and mixed Rossby-gravity waves generated by a specified forcing on the lower boundary radiate upward through the model domain where they are subject to thermal and mechanical dissipation. The wave-mean flow feedback problem is solved on the assumption that changes in the zonal mean state occur slowly enough for the wave field to be steady, locally in time. A quasi-biennial oscillation develops in the model with a period of about 1000 days. A novel feature of this model is the incorporation of latitudinal structure and of thermal and meridional wind components of the oscillation. The influence of the meridional circulation on the structure of the zonal wind oscillation is discussed in some detail.
    DOI:10.1002/qj.49710845604      URL     [Cited within:1]
    [36] Rand el, W. J., R. Garcia, F. Wu, 2008: Dynamical balances and tropical stratospheric upwelling.J. Atmos. Sci.,65,3584-3595,doi: 10.1175/2008JAS2756.1.
    The dynamical balances associated with upwelling in the tropical lower stratosphere are investigated based on climatological 40-yr ECMWF Re-Analysis (ERA-40) and NCEP-NCAR reanalysis data. Zonal mean upwelling is calculated from momentum balance and continuity ("downward control"), and these estimates in the deep tropics are found to be in reasonable agreement with stratospheric upwelling calculated from thermodynamic balance (and also with vertical velocity obtained from ERA-40). The detailed momentum balances associated with the dynamical upwelling are investigated, particularly the contributions to climatological Eliassen-Palm (EP) flux divergence in the subtropics. Results show that the equatorward extension of extratropical waves (baroclinic eddies and, in the NH, quasi-stationary planetary waves) contribute a large component of the subtropical wave driving at 100 hPa. Additionally, there is a significant contribution to subtropical forcing from equatorial planetary waves, which exhibit a strong seasonal cycle (a reversal in phase) in response to latitudinal migration of tropical convection. The observed balances suggest that the strong annual cycle in upwelling across the tropical tropopause is forced by subtropical horizontal eddy momentum flux convergence associated with waves originating in both the tropics and extratropics.
    DOI:10.1175/2008JAS2756.1      URL     [Cited within:1]
    [37] Rosenlof K. H., 1995: Seasonal cycle of the residual mean meridional circulation in the stratosphere.J. Geophys. Res.,100,5173-5191,doi: 10.1029/94JD03122.
    The transformed Eulerian-mean (TEM) residual circulation is used to study the zonally averaged transport of mass in the stratosphere. The residual circulation is estimated from heating rates computed with a radiative transfer model using data from the Upper Atmosphere Research Satellite (UARS) as inputs. An annual cycle exists in the resulting circulation in the lower stratosphere, with a larger net upward mass flux across a pressure surface in the tropics during northern hemisphere winter than during northern hemisphere summer. The annual cycle in upward tropical mass flux follows the annual cycle in downward mass flux across a pressure surface in the northern hemisphere extratropics. It is argued that the annual cycle in zonal momentum forcing in the northern hemisphere stratosphere is controlling mass flux across a pressure surface in the lower stratosphere both in the tropics and in the northern hemisphere extratropics.
    DOI:10.1029/94JD03122      URL     [Cited within:1]
    [38] Ryu J.-H., 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.
    An initial-value problem is employed with a GCM to investigate the role of the convectively driven Rossby and Kelvin waves for tropopause transition layer (TTL) upwelling in the tropics. The convective heating is mimicked with a prescribed heating field, and the Lagrangian upwelling is identified by examining the evolution of passive tracer fields whose initial distribution is identical to the initial heating field. This study shows that an overturning circulation, induced by the tropical Rossby waves, is capable of generating the TTL upwelling. Even when the heating is placed in the eastern Pacific, the TTL upwelling occurs only over the western tropical Pacific, indicating that the background flow plays a crucial role. The results from a Rossby wave source analysis suggest that a key feature of the background flow is the strong absolute vorticity gradient associated with the Asian subtropical jet. In addition, static stability is relatively weak over the western Pacific, suggesting that this may also contribute to the TTL upwelling in that region. The background flow also modulates the internal Kelvin waves in such a manner that the coldest region in the TTL (resembling the observed "cold trap") occurs over the western tropical Pacific. As a consequence, the upwelling air, induced by the meridional momentum flux of the Rossby wave, passes through the cold trap generated by the Kelvin wave. Since in reality the background flow is shaped by the convective heating, the climatological western tropical Pacific heating is ultimately responsible for both the TTL upwelling and the cold trap; however, both processes are realized indirectly through its impact on the background flow and the generation of the tropical waves.
    DOI:10.1175/2010JAS3434.1      URL     [Cited within:1]
    [39] Shepherd T. G., 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.
    Climate models consistently predict a strengthened Brewer–Dobson circulation in response to greenhouse gas (GHG)-induced climate change. Although the predicted circulation changes are clearly the result of changes in stratospheric wave drag, the mechanism behind the wave-drag changes remains unclear. Here, simulations from a chemistry–climate model are analyzed to show that the changes in resolved wave drag are largely explainable in terms of a simple and robust dynamical mechanism, namely changes in the location of critical layers within the subtropical lower stratosphere, which are known from observations to control the spatial distribution of Rossby wave breaking. In particular, the strengthening of the upper flanks of the subtropical jets that is robustly expected from GHG-induced tropospheric warming pushes the critical layers (and the associated regions of wave drag) upward, allowing more wave activity to penetrate into the subtropical lower stratosphere. Because the subtropics represent the critical region for wave driving of the Brewer–Dobson circulation, the circulation is thereby strengthened. Transient planetary-scale waves and synoptic-scale waves generated by baroclinic instability are both found to play a crucial role in this process. Changes in stationary planetary wave drag are not so important because they largely occur away from subtropical latitudes.
    DOI:10.1175/2010JAS3608.1      URL     [Cited within:1]
    [40] Tao M., P. Konopka, F. Ploeger, J.-U. Groo\ss, R. Müller, C. M. Volk, K. A. Walker, and M. Riese, 2015a: Impact of the 2009 major sudden stratospheric warming on the composition of the stratosphere.Atmospheric Chemistry and Physics,15,8695-8715,doi: 10.5194/acp-15-8695-2015.
    In a case study of a remarkable major sudden stratospheric warming (SSW) during the boreal winter 2008/09, we investigate how transport and mixing triggered by this event affected the composition of the entire stratosphere in the Northern Hemisphere. We simulate this event with the Chemical Lagrangian Model of the Stratosphere (CLaMS), both with optimized mixing parameters and with no mixing, i.e. with transport occurring only along the Lagrangian trajectories. The results are investigated by using tracer-tracer correlations and by applying the transformed Eulerian-mean formalism. The CLaMS simulation of NO and O, and in particular of the O-NO tracer correlations with optimized mixing parameters, shows good agreement with the Aura Microwave Limb Sounder (MLS) data. The spatial distribution of mixing intensity in CLaMS correlates fairly well with the Eliassen-Palm flux convergence. This correlation illustrates how planetary waves drive mixing. By comparing simulations with and without mixing, we find that after the SSW, poleward transport of air increases, not only across the vortex edge but also across the subtropical transport barrier. Moreover, the SSW event, at the same time, accelerates polar descent and tropical ascent of the Brewer-Dobson circulation. The accelerated ascent in the tropics and descent at high latitudes first occurs in the upper stratosphere and then propagates downward to the lower stratosphere. This downward propagation takes over 1 month from the potential temperature level of 1000 to 400 K.
    DOI:10.5194/acpd-15-4383-2015      URL     [Cited within:1]
    [41] Tao M. C., P. Konopka, F. Ploeger, M. Riese, R. Müller, and C. M. Volk, 2015b: Impact of stratospheric major warmings and the quasi-biennial oscillation on the variability of stratospheric water vapor.Geophys. Res. Lett.,42,4599-4607,doi: 10.1002/2015GL064443.
    Based on simulations with the Chemical Lagrangian Model of the Stratosphere for the 1979-2013 period, driven by the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis, we analyze the impact of the quasi-biennial oscillation (QBO) and of Major Stratospheric Warmings (MWs) on the amount of water vapor entering the stratosphere during boreal winter. The amplitude of HO variation related to the QBO amounts to 0.5 ppmv. The additional effect of MWs reaches its maximum about 2-4 weeks after the central date of the MW and strongly depends on the QBO phase. Whereas during the easterly QBO phase there is a pronounced drying of about 0.3 ppmv about 3 weeks after the MW, the impact of the MW during the westerly QBO phase is smaller (about 0.2 ppmv) and more diffusely spread over time. We suggest that the MW-associated enhanced dehydration combined with a higher frequency of MWs after the year 2000 may have contributed to the lower stratospheric water vapor after 2000.
    DOI:10.1002/2015GL064443      URL     [Cited within:3]
    [42] Ueyama R., 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.
    The causes of the annual cycle and nonseasonal variability in the globally-averaged, equator-to-pole Brewer-Dobson circulation (BDC: defined here as the equatorially-symmetric component of the Lagrangian-mean meridional circulation) are investigated based on zonally-averaged, lower-stratospheric temperature data from satellite-borne microwave sounders (MSU/AMSU). Time-varying vertical velocities in the BDC are inferred from departures of the meridional temperature profiles from the respective radiative equilibrium temperature profiles. Equatorward of ~45℃N/S, the annual-mean profile of lower-stratospheric temperature and the seasonal and nonseasonal variations about it project almost exclusively onto the equatorially-symmetric component. The climatological-mean annual cycle accounts for nearly 90% of the month-to-month variance of the equatorially-symmetric component of the temperature field; Jan/Feb is colder than Jul/Aug equatorward of ~45℃N/S and warmer than Jul/Aug poleward of that latitude. The equator-to-subpolar temperature contrast roughly doubles from Jul/Aug to Jan/Feb, implying an approximate doubling of the strength of the BDC. The nonseasonal variability is dominated by a similar pattern. Tropical upwelling in the BDC, as inferred from of the temperature field, varies in response to variations in eddy heat fluxes at high latitudes with comparable strength on the intraseasonal and interannual time scales; it does not appear to be correlated with equatorial tropospheric planetary wave activity or with variations in wave forcing in subtropical lower stratosphere. It is concluded that high-latitude wave forcing plays an important role in modulating tropical upwelling in the BDC across a wide range of frequencies.
    DOI:10.1175/2009JAS3216.1      URL     [Cited within:]
    [43] Ueyama R., E. P. Gerber, J. M. Wallace, and D. M. 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.
    The forcing of tropical upwelling in the Brewer-Dobson circulation (BDC) on intraseasonal to seasonal time scales is investigated in integrations of an idealized general circulation model, ECMWF Interim ReAnalysis, and lower-stratospheric temperature measurements from the (Advanced) Microwave Sounding Unit, with a focus on the extended boreal winter season. Enhanced poleward eddy heat fluxes in the high latitudes (45 degrees-90 degrees N) at the 100-hPa level are associated with anomalous tropical cooling and anomalous warming on the poleward side of the polar night jet at the 70-hPa level and above. In both the model and the observations, planetary waves entering the stratosphere at high latitudes propagate equatorward to the subtropics and tropics at levels above 70 hPa over an approximately 10-day period, exerting a force at sufficiently low latitudes to modulate the tropical upwelling in the upper branch of the BDC, even on time scales longer than the radiative relaxation time scale of the lower stratosphere. To the extent that they force the BDC via downward as opposed to sideways control, planetary waves originating in high latitudes contribute to the seasonally varying climatological mean and the interannual variability of tropical upwelling at the 70-hPa level and above. Their influence upon the strength of the tropical upwelling, however, diminishes rapidly with depth below 70 hPa. In particular, tropical upwelling at the cold-point tropopause, near 100 hPa, appears to be modulated by variations in the strength of the lower branch of the BDC.
    DOI:10.1175/JAS-D-12-0174.1      URL     [Cited within:]
    [44] Xie F., J. Li, W. Tian, J. Feng, and Y. Huo, 2012: Signals of El ño Modoki in the tropical tropopause layer and stratosphere.Atmospheric Chemistry and Physics,12,5259-5273,doi: 10.5194/acp-12-5259-2012.
    The effects of El Ni o Modoki events on the tropical tropopause layer (TTL) and on the stratosphere were investigated using European Center for Medium Range Weather Forecasting (ECMWF) reanalysis data, oceanic El Ni o indices, and general climate model outputs. El Ni o Modoki events tend to depress convective activities in the western and eastern Pacific but enhance convective activities in the central and northern Pacific. Consequently, during El Ni o Modoki events, negative water vapor anomalies occur in the western and eastern Pacific upper troposphere, whereas there are positive anomalies in the central and northern Pacific upper troposphere. The spatial patterns of the outgoing longwave radiation (OLR) and upper tropospheric water vapor anomalies exhibit a tripolar form. The empirical orthogonal function (EOF) analysis of the OLR and upper tropospheric water vapor anomalies reveals that canonical El Ni o events are associated with the leading mode of the EOF, while El Ni o Modoki events correspond to the second mode. The composite analysis based on ERA-interim data indicate that El Ni o Modoki events have a reverse effect on middle-high latitudes stratosphere, as compared with the effect of typical El Ni o events, i.e., the northern polar vortex is stronger and colder but the southern polar vortex is weaker and warmer during El Ni o Modoki events. According to the simulation' results, we found that the reverse effect on the middle-high latitudes stratosphere is resulted from a complicated interaction between quasi-biennial oscillation (QBO) signal of east phase and El Ni o Modoki signal. This interaction is not a simply linear overlay of QBO signal and El Ni o Modoki signal in the stratosphere, it is El Ni o Modoki that leads to different tropospheric zonal wind anomalies with QBO forcing from that caused by typical El Ni o, thus, the planetary wave propagation from troposphere to the stratosphere during El Ni o Modoki events is different from that during canonical El Ni o events. However, when QBO is in its west phase, El Ni o Modoki events have the same effect on middle-high latitudes stratosphere as the typical El Ni o events. Our simulations also suggest that canonical El Ni o and El Ni o Modoki activities actually have the same influence on the middle-high latitudes stratosphere when in the absence of QBO forcing.
    DOI:10.5194/acpd-12-3619-2012      URL     [Cited within:1]
    [45] Xie F., J. P. Li, W. S. Tian, J. K. Zhang, and C. Sun, 2014: The relative impacts of El ño Modoki, Canonical El ño, and QBO on tropical ozone changes since the 1980s. Environmental Research Letters, 9,064020, doi: 10.1088/1748-9326/9/6/ 064020.
    Some studies showed that since the 1980s Modoki activity—a different sea surface temperature anomaly pattern from canonical El Ni09o-Southern Oscillation (ENSO) in the tropics—has been increasing in frequency. In the light of an analysis of the observations and simulations, we found that Modoki, as a new driver of global climate change, can modulate the tropical upwelling that significantly affects mid-lower stratospheric ozone. As a result, it has an important impact on the variations of tropical total column ozone (TCO), alongside quasi-biennial oscillation or canonical ENSO. Our results suggest that, in the context of future global warming, Modoki activity may continue to be a primary driver of tropical TCO changes. Besides, it is possible can serve as a predictor of tropical TCO variations since Modoki events precede tropical ozone changes.
    DOI:10.1088/1748-9326/9/6/064020      URL     [Cited within:2]
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    Key words
    sudden stratospheric major warming
    Brewer-Dobson circulation
    subtropical wave

    Authors
    Mengchu TAO
    Yi LIU
    Yuli ZHANG