1. Introduction
The Madden–Julian oscillation (MJO) (Madden and Julian 1972; Zhang 2005) is the dominant mode of intraseasonal oscillation (ISO) in the tropical troposphere. The MJO has important interactions with tropospheric phenomena such as El Niño–Southern Oscillation (ENSO) and monsoons (Tian et al. 2008; Waliser et al. 2003). The tropical diabatic heating variations associated with the MJO could lead to atmospheric circulation anomalies in both the tropics and extratropics (e.g., Matthews and Meredith 2004; Lin et al. 2009; Seo and Son 2012; Cassou 2008) and in turn excite planetary waves (PWs) in the middle and high latitudes (Ferranti et al. 1990; Seo and Son 2012). As suggested by previous studies, the MJO teleconnection is characterized by a Rossby wave train propagating from the heating source to higher latitudes (e.g., Matthews and Meredith 2004), which in turn results in anomalies over Asia, the North Pacific, North America, and the Atlantic (e.g., Higgins et al. 2000; Jones and Schemm 2000).
Although there have been fewer studies addressing the relationship between the MJO and the Antarctic region, the observational studies available indicate that there is evidence of a connection. Miller et al. (2003) indicates that both the Arctic Oscillation (AO) and the Antarctic Oscillation (AAO) have strong relationships with the MJO. The intraseasonal fluctuations in the AAO in austral summer have been related to MJO activity by Carvalho et al. (2005). Matthews and Meredith (2004) suggested that extratropical Rossby wave trains generated by MJO convection lead to an increase in the westerly winds around 60°S during the Southern Hemisphere winter.
The MJO also plays an important role in modulating the stratospheric circulation by triggering anomalous PW and gravity wave (GW) activity (Gill 1980; Schwartz et al. 2008; Zagar and Franzke 2015; Moss et al. 2016). As suggested by recent studies (e.g., Garfinkel et al. 2012, 2014; Zagar and Franzke 2015; Moss et al. 2016), the MJO affects the variability in the Northern Hemisphere (NH) stratosphere during the boreal wintertime. Zagar and Franzke (2015) proposed that a vertical inertia–gravity mode was responsible for the propagation of the MJO to the stratosphere. Moss et al. (2016) suggested a strong anticorrelation between gravity wave potential energy and the MJO eastward wind anomaly in the upper troposphere, which might be a result of the filtering of upward-propagating waves by the MJO winds. The variation of the Arctic polar vortex (Garfinkel 2012, 2014) and the ozone concentration in the high-latitude stratosphere (Li et al. 2013) are all affected by the tropospheric ISO. Recently, Garfinkel et al. (2012, 2014) suggested that the MJO has an influence on the NH stratospheric vortex of a magnitude comparable to that of the quasi-biennial oscillation (QBO) and ENSO.
Since there is a relationship between the MJO and the Southern Hemisphere high-latitude and midlatitude troposphere, a signature of the MJO extending into the middle atmosphere might be expected. However, up to now, the response of the Southern Hemisphere (SH) middle atmosphere to the MJO has not been well studied. The MJO is, on average, weaker during the austral winter than the boreal winter (Wang and Rui 1990; Hendon and Salby 1994). However, variability in the middle atmosphere is also weaker in the austral winter than in the boreal winter so the MJO-induced variation has the potential to have the same relative importance in both hemispheres. Variability in the austral winter and spring stratosphere is important because of the sensitivity of ozone hole development to the dynamical conditions during each winter.
The goal of this research is to investigate the influence of the MJO on the SH stratosphere and mesosphere during austral winter by using output from the specified-dynamics (SD) Whole Atmosphere Community Climate Model (WACCM). We also include the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) data for comparison. The analysis will show the effect of the MJO on PWs and GWs, as well as on the mean state, in order to better understand the mechanisms behind the relationship between the MJO and the SH middle atmosphere. Section 2 describes the reanalysis data and the model used in this study, as well as the approach to identifying MJO events. In section 3, we illustrate the SH stratospheric and mesospheric response to the MJO during the austral winters. Section 4 discusses the mechanism by which certain phases of the MJO affect the SH stratosphere and mesosphere. Section 5 provides a summary of the major findings of this study and discusses the implications.
2. Data and methods
a. ERA-Interim datasets
We examine the ERA-Interim dataset (Dee et al. 2011) for the years 1979–2016 to quantify the stratospheric temperature response to the MJO, and compare the response with that seen in the WACCM simulation. The ERA-Interim dataset is produced by a model that assimilates various conventional and satellite observations. The data used here cover the altitude range of 1000–1 hPa.
b. Specified-dynamics WACCM
Version 4 of WACCM (WACCM4) is a general circulation model, developed at the National Center for Atmospheric Research (NCAR), and is a fully coupled chemistry–climate model that is part of the Community Earth System Model (Garcia et al. 2007; Marsh et al. 2013; Hurrell et al. 2013). Details of the basic model and its performance have been descripted by Marsh et al. (2013). For this study, we used data from the simulations using a “specified dynamics” version of WACCM4 (SD-WACCM4) to investigate the response of middle atmosphere to the MJO. The dynamics and temperature in SD-WACCM4 are constrained by relaxing the horizontal winds and the temperature to those from the NASA Global Modeling and Assimilation Office Modern-Era Retrospective Analysis for Research and Applications (MERRA) (Rienecker et al. 2011) reanalysis data in the troposphere and stratosphere. (The SD-WACCM4 is nudged to MERRA up to 1 hPa and is free-running above 0.3 hPa.) In the nudging process, the meteorological fields are typically updated at intervals of 6 h; the variables are interpolated between the two bracketing times to get fields for nudging at intervening time steps. The nudging is applied at every model time step (i.e., every 30 min). The method to constrain the WACCM meteorological fields with the MERRA meteorological fields is described in Kunz et al. (2011). More details about the dynamical constraints in SD-WACCM4 and their impact on simulating the mesosphere are given by Smith et al. (2017). With the relaxation, the MJO characteristics and the responses to it in the troposphere and stratosphere in SD-WACCM4 follow those in the reanalysis meteorological fields. SD-WACCM4 is free-running in the mesosphere and lower thermosphere. This setup allows us to investigate the physical processes involved in the response of the mesosphere to observed MJO events.
The SD-WACCM4 simulation used in this study, which spans from 1979 to 2015, has a horizontal resolution of 1.9° × 2.5° (latitude by longitude) with 88 vertical levels (extending from the ground up to 150 km). The effect of GWs (orographic and nonorographic) is parameterized (Garcia et al. 2007; Richter et al. 2008, 2010) while PWs and other large-scale waves are resolved.
Daily values of GW forcing and Eliassen–Palm (EP) flux and its divergence due to PWs are calculated from the daily output of the model. The residual circulation using the transformed Eulerian mean (TEM) zonal momentum budget (Andrews et al. 1987) is also calculated daily.
c. Active MJO days
The MJO phase is identified with the real-time multivariate MJO (RMM) index (available at http://www.bom.gov.au/climate/mjo/graphics/rmm.74toRealtime.txt) using the methods of Wheeler and Hendon (2004). According to the amplitude and phase information obtained from RMM1 and RMM2, MJO events are divided into eight active phases that indicate the location of the convective activity. To focus on SH winter, events that occurred between the beginning of May and the end of September were used to construct the composites of the MJO. Dates before May and after September were excluded when the lagged influence of the MJO were evaluated. The “active MJO days” were identified as those for which the MJO amplitude exceeds 1.5 [as in Yoo et al. (2012)] for more than five consecutive days. Results are similar when the threshold is varied within the range of 1.0–2.0. An “independent MJO event” is identified when consecutive active MJO days last for at least five days and are separated by at least seven days from any other active MJO days with the same phase. According to the criteria, we identified 306 MJO phase 1 (P1) days and 230 MJO phase 5 (P5) days (phases are denoted likewise for other MJO phases) during the extended austral wintertime (May–September) from 1979 to 2015. NOAA interpolated outgoing longwave radiation (OLR) is used (Liebmann and Smith 1996) to show the convection anomalies during certain MJO phases. The convection pattern of the MJO P1 is generally characterized by enhanced convection (negative OLR anomalies) over the Indian Ocean and suppressed convection (positive OLR anomalies) over the western Pacific, which is roughly the opposite to that of the MJO P5 (Fig. 1). There are 62 and 51 independent MJO P1 and P5 events, respectively, identified in these winters.
Anomalies of NOAA OLR in MJO (a) P1 and (b) P5 during May–September. Only the anomalies significant at 95% levels are presented. The contour intervals are 3 W m−2.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
d. Analysis methods
A 10–100-day bandpass was employed to identify the intraseasonal variation from the data before they were used to generate the composites in this study. A Monte Carlo test was used to evaluate the statistical significance of the results of the composite for the MJO. A normalized time series with the same span as the reanalysis data was generated randomly; the random variables follow normal distribution in this analysis. Then, the composite for each MJO phase was computed using this series according to the active MJO days selected. By performing the Monte Carlo procedure 10 000 times, we produced 10 000 composite values for that MJO phase; the composite value for a certain MJO phase is compared with the values from the 10 000 calculations to determine the statistical significance. The area-weighted average used to focus on the polar region is from 65°S poleward.
3. Composite of SH polar atmosphere with respect to the MJO
a. Polar temperature
Figure 2 shows the influence of each MJO phase on temperature anomalies at the SH polar cap (65°S and poleward) as a function of time in the extended austral winters (May–September) from 1979 to 2015. The plots are for 10 hPa; note that SD-WACCM is nudged with MERRA at this level and therefore the SD-WACCM temperatures are almost identical to those of MERRA. To make sure that the signal seen in SD-WACCM is robust in different datasets, composites derived from ERA-Interim using the same method (Fig. 2a) are compared with those derived from SD-WACCM (Fig. 2b). The most significant positive anomalies (with a maximum of ~1 K) occur in the polar cap at a lag of 30 days after MJO P1 and 10 days after P5 in both datasets. This suggests that only certain MJO phases affect the variation of the SH polar temperature; for some other MJO phases there is no indication of a significant response in the polar stratosphere.
(a) Anomalies of polar cap temperature (K; 65°S and poleward) at 10 hPa derived from ERA-Interim data during different MJO phases in austral winter season (May–September) for 1979–2015. (b) As in (a), but for data from SD-WACCM nudged with MERRA reanalysis. (c) Anomalies of zonal mean temperature from SD-WACCM of the SH polar region (K; 65°S and poleward) composite over days 0–50 after MJO P1. (d) As in (c), but for the composite from 10 days before to 40 days after MJO P5. The white and light gray areas denote 95% and 90% significance, respectively, according to the Monte Carlo test.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
The lifetime for a complete MJO cycle is usually in the range of 30–60 days. The MJO phase 20–30 days after the peak of MJO P1 would be would be in the range P4–P8, which would have a pattern of anomalies nearly opposite to that of MJO P1. The phase at 0–10 days after MJO P5 falls into this window. In other words, the tropical pattern at 20–30 days after MJO P1 and that at 0–10 days after P5 will on average be similar although, because of the intermittency and variable period of the MJO, this will not always be the case. The MJO does not always progress smoothly from one phase to the next and is not “active” during most days (the active MJO days only account for 29.5% all calendar days). The positive and negative signals after the full cycle of MJO phases vary in magnitude but form a coherent pattern in time. A significant cooling is seen at a 20–30-day lag after MJO P7 and a 10–20-day lag after MJO P8 and P1.
More details of the response of the polar temperature to the MJO P1 and P5 are shown in Figs. 2c and 2d, respectively. The vertical ranges of these panels extend through the entire middle atmosphere. During MJO P1, a significant cooling is revealed between 0.1 and 10 hPa. At lags of 10–20 days after MJO P1, a significant warming is seen between 0.1 and 1 hPa; the warming propagates downward to 10 hPa at a lag of 30 days after MJO P1. Meanwhile, the polar temperature above 0.3 hPa is significantly cooler than normal at lags of 30–40 days after MJO P1 (Fig. 2c). In the composite of MJO P5, which is characterized by the suppression of convection in the Indian Ocean and enhancement of convection over the western Pacific, opposite to that of MJO P1, there is an anomalous warming in the lower mesosphere (0.1 hPa) and an anomalous cooling in the upper stratosphere (between 10 and 1 hPa) 10 days before MJO P5 (Fig. 2d). At a lag of 10 days after MJO P5, the polar temperature in the lower mesosphere becomes cooler than normal while the polar stratosphere is warmer. The downward propagation of the anomalous temperature after MJO P1 is weaker and slower than that after MJO P5. The temperature anomalies in the polar region of the mesosphere and the stratosphere alternate between warmer and cooler over a period of roughly 40–60 days for MJO P1 and about 40 days for the MJO P5. The positive temperature anomalies after MJO P1 at approximately 0.3 hPa last longer than that after MJO P5.
b. Evolution of the atmospheric response
To better understand the response of the SH stratosphere and mesosphere to the MJO, lag composites of the zonal mean temperature and the zonal wind in SD-WACCM after MJO P1 and P5 are presented in Figs. 3 and 4, respectively. During MJO P1 active days (lag 0; Fig. 3), there is an anomalous cooling in the SH stratosphere and lower mesosphere (10–0.1 hPa) and an anomalous warming in the middle and upper mesosphere (0.1–0.001 hPa) polar region, associated with stronger westerlies at the SH high latitudes (60°–90°S). At lags of 10–20 days, the temperature anomalies become less significant, while the anomalous westerlies at the SH midlatitude (30°–60°S) and the easterlies at the tropical region of the mesosphere become stronger. At lags of 30–40 days, the positive temperature anomalies propagate downward to the SH polar stratosphere and become much stronger: the maximum positive anomaly is approximately 1 K; the negative temperature anomalies dominate the SH mesosphere with a magnitude of approximately 1.5 K. The anomalous mesospheric temperature is associated with anomalous westerlies with a minimum of about −4 m s−1; this pattern is basically the opposite to that at a lag of 0. The evolution of the temperature and zonal wind anomalies at a lag of 30–40 days indicate a weakened SH polar vortex last for over 10 days in the stratosphere. In the tropical region, there is an anomalous warming in the lower mesosphere and a cooling in the upper stratosphere while the zonal wind is enhanced in the lower mesosphere and is suppressed in the upper stratosphere.
Composite of (left) zonal mean temperature and (right) zonal wind anomalies from SD-WACCM lagging MJO P1 by 0–40 days in the austral winter season (May–September). The contour intervals are 0.3 K for temperature and 1 m s−1 for zonal wind. The light gray and white areas indicate results are significant at 90% and 95% level, respectively, according to the Monte Carlo test.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
As in Fig. 3, but for the composite of anomalies lagging MJO P5.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
In the composite of the MJO P5, shown in Fig. 3, the temperature anomalies at zero lag are not significant in the polar region while the positive anomalies at 10–0.1 hPa and negative anomalies between 0.1 and 0.01 hPa are significant at the midlatitudes (~30°–60°S). In the tropical region, the temperature anomalies are opposite to those at the midlatitudes. At a lag of 10 days after MJO P5, the anomalous temperatures in the SH polar region (10–0.1 hPa) are significantly positive in the stratosphere with a maximum of approximately 1.5 K between 10 and 1 hPa and are significantly negative in the mesosphere with a minimum of approximately −1.5 K between 0.1 and 0.01 hPa. The wind anomalies indicate a suppression of westerlies in the SH midlatitude and an enhancement in the tropical mesosphere. This anomalous pattern of the temperature and zonal wind lagging MJO P5 by 10 days is similar with that lagging MJO P1 by 30 days. However, compare to the anomalous pattern in the composite of MJO P1, which lasts for over 10 days, the anomalous temperature and the zonal wind after the MJO P5 become marginally significant at the SH polar region sooner, at a lag of 20 days. In the tropical region, the temperature anomalies become negative at 0.01 hPa and positive at 0.1 hPa, which is opposite to that at a lag of 0. At a lag of 30 days, the temperature anomalies are significant in the SH polar region; they indicate warming in the mesosphere and cooling in the stratosphere, opposite to anomalies at 10-day lag. The anomalies in the tropical region and most of the zonal wind anomalies disappear. At a lag of 40 days, neither the temperature nor zonal wind anomalies in the SH is significant. The poleward and downward propagations in both composites are similar to the dominant features of the stratospheric/mesospheric variability (e.g., Kuroda and Kodera 2001).
As discussed above, the most significant positive temperature anomalies in the stratosphere and negative temperature anomalies in the mesosphere of the SH polar region are found at a lag of 30–40 days after the MJO P1, associated with the negative anomalous westerlies (anomalous easterly) in the midlatitude of the SH stratosphere and mesosphere. A similar pattern of the anomalous temperature and zonal wind is seen at a lag of 10 days in the composite of the MJO P5, although it disappears quickly.
Since the most significant responses in the SH stratosphere are seen beginning 30 days after phase 1 and 10 days after phase 5, our discussion of the mechanism by which the MJO modulates the SH middle atmosphere will focus on these two periods. Although the dates of these two periods may have overlaps, only 12.3% of the individual days included in the composites for MJO P1 are the same as those for MJO P5. Although the patterns at these two periods are, on average, roughly similar, there is no one-to-one correspondence between these two periods because of the intermittency of the MJO and its variable period.
4. Physical mechanisms
a. Tropospheric teleconnection
To understand the mechanisms by which the MJO modulates the atmosphere in the SH, Fig. 5 shows the tropospheric teleconnections in SD-WACCM output associated with MJO P1 and P5. Since the responses of the austral middle atmosphere are most significant during about 30 days after MJO P1 (Fig. 3) and about 10 after MJO P5 (Fig. 4), we focus on the variations during and before these two periods. During 10–30 days after the MJO P1, as revealed by the 500-hPa geopotential height anomalies (Figs. 5a–d), there are two wave trains in the SH that emanate from the Indian and the Pacific Ocean to the SH polar region as indicated by the thick dashed lines in Figs. 5a,b. One wave train extends from the negative anomalies between 30° and 60°S in the Indian Ocean to the positive anomalies to the south of Australia at around 60°S and the negative anomalies centered at 60°–90°S, 150°–120°W. The other connects the negative anomaly at the subtropical Pacific (which is not significant during the period lagging MJO P1 by 16–20 days; Fig. 5b), the positive anomalies at the midlatitude (30°–60°S) of the southern Pacific, and the negative anomalies near the polar region (60°–90°S, 150°–120°W). The geopotential anomalies are positive but not significant over the SH polar region at lags of 11–15 days (Fig. 5a) and become negative at lags of 16–30 days (Figs. 5b–d).
Anomalies in geopotential height from SD-WACCM at 500 hPa in the extended austral winter during a composite of (a) 11–15, (b) 16–20, (c) 21–25, and (d) 26–30 days after MJO P1 and (e) 10–6 and (f) 5–1 days before and (g) 1–5 and (h) 6–10 days after MJO P5. The light gray and white areas indicate results are significant at the 90% and 95% level, respectively, according to the Monte Carlo test. The dashed black lines guide the eye for the two wave trains from the tropics to high latitudes in the SH.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
These two wave trains are strongest (with a minimum of ~20 m in the SH polar region) during 16–20 days lagging MJO P1 and become weaker (with a minimum of ~10 m in the SH polar region) during 20–30 days lagging MJO P5. These extratropical Rossby wave trains are consistent with the wave trains induced by anomalous MJO convection as suggested by Matthews and Meredith (2004). The anomalous geopotential patterns during 10–0 days (Figs. 5e,f) before MJO P5 are out of phase with those during 10–30 days after MJO P1. During 1–10 days after the MJO P5, the anomalous geopotential patterns become into phase with those during MJO P1 with a maximum of approximately 20 m at midlatitude and a minimum of approximately 15 m in the polar region. Although the anomalies during 1–10 days after P5 are more intense, the variations associated with the wave trains for P1 last longer and are more coherent. Similar wave trains are also seen in the 200-hPa geopotential height anomalies (not shown).
b. Stratospheric response in the SH
As suggested by the geopotential height anomalies, PW perturbations associated with the MJO convection emanate from the tropics and extend to the SH polar latitudes in the troposphere. A response in the polar middle atmosphere after MJO P1 and P5 is also expected. Figure 6 shows the anomalous EP flux and its divergence from SD-WACCM after MJO P1 (Figs. 6a–c) and P5 (Figs. 6d–f) in the SH stratosphere. Recall that the SD-WACCM horizontal winds and temperatures are nudged to MERRA below 1 hPa and are free-running above about 0.3 hPa. The anomalies in the upward propagation and the dissipation (indicated by the EP flux divergence) of the PWs are not significant during the first 10 days after MJO P1 (Fig. 6a). The upward propagation and the dissipation are weakened in the middle and high latitudes of the SH upper stratosphere but enhanced in the lower stratosphere at the lags of 10–20 days. At lags of 20–30 days after MJO P1, the upward propagation and the dissipation are significantly enhanced in the midlatitudes (~60°S) through the depth of the SH stratosphere (100–1 hPa). The enhancement of the PWs (first enhanced in the lower stratosphere and then extending to the higher altitudes) is consistent with the variations in the tropospheric geopotential height (Figs. 5a–d). During the period 10–0 days before MJO P5 (Fig. 5d), the PW anomalies are not significant. At lags of 0–10 days (Fig. 5e), the upward propagation and dissipation of the PWs are significantly enhanced from 100 to 3 hPa in stratosphere; the anomalies are less intense in the high altitudes as compared to those 20–30 days after MJO P1 (Fig. 6c). The EP flux divergence is larger in P5 than P1 but peaks at a lower level. The PW anomalies of MJO P5 become marginally significant more quickly, at lags of 10–20 days.
Composite anomalies of EP flux (m s−2 day−1; vectors, the horizontal and vertical components are weighted in the different ways to make them comparable) and EP flux divergence (m s−1 day−1; contours) in the SH stratosphere (100–0.1 hPa) during (a) 0–10, (b) 10–20, (c) 20–30 days after MJO P1 and (d) 0–10 days before, (e) 0–10 days after, and (f) 10–20 days after MJO P5. Significant anomalies above 90% (95%) confidence level are denoted by the white (light gray) areas according to the Monte Carlo test. Red contours denote positive divergence, and blue contours denote negative divergence.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
As shown in Fig. 7, the MJO-related stratospheric Brewer–Dobson (BD) circulation is represented by the residual mean meridional circulation anomalies. At lags of 0–10 days after MJO P1 (Fig. 7a), there is no clear variation of the BD circulation. At lags of 10–20 days (Fig. 7b), the residual circulation anomalies are characterized by weaker downwelling in the polar region and upwelling in the tropical region, which suggests that the BD circulation is suppressed. During the 20–30 days following the MJO P1, the BD circulation is significantly strengthened as seen by an enhanced upwelling in the polar region, a stronger downwelling in the tropical region and a significant anomalous meridional flow from the tropics to the South Pole. This enhancement is consistent with the enhanced upward propagation and dissipation of the PWs (Fig. 6c). In the composite of MJO P5, the most significant signals are seen at lags of 0–10 days; these indicate an enhanced BD circulation. The circulation changes are responsible for the positive temperature anomalies in the SH stratospheric polar region (Figs. 3 and 4).
Composite of residual circulation anomalies during (a) 0–10, (b) 10–20, and (c) 20–30 days after MJO P1 and (d) 0–10 days before, (e) 0–10 days after, and (f) 10–20 days after MJO P5 in the stratosphere and lower mesosphere (100–0.1 hPa). The light gray and white shading indicate the 90% and 95% significance level, respectively.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
Figure 8 shows anomaly patterns to explore more details of the thermal changes in the stratosphere. Figures 8a and 8c compare the variation of momentum flux (
(a) Evolution of the anomalies of the zonal mean zonal wind (cyan dashed line) and the momentum flux (black solid line) averaged between 30° and 60°S at 3 hPa for MJO P1. Asterisks represent significance at the 95% level. (b) Evolution of the anomalies of the SH polar temperature (red dashed line) and the eddy heat flux (black solid line) averaged between 60° and 90°S at 3 hPa for MJO P1. (c),(d) As in (a),(b), but for days before and after P5.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
Anomalies in the geopotential height (m) at 200 hPa in the extended austral winter during a composite of (a)–(c) 20–30 days after MJO P1 and (d)–(f) 0–10 days after MJO P5 with WN1, WN2, and WN3. The shading indicates the anomalies while the contours indicate the climatology; the thick black contours indicate zero anomaly.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
Consistent with the geopotential height anomalies, the WN1 and WN2 anomalies (as indicated by the blue and green lines in Fig. 10) are responsible for most of the response in the vertical component of the extratropical (40°–80°S averaged) EP flux at 200 hPa following MJO P1 (Fig. 9a). MJO P1 leads to suppressed upward propagation of PWs (the black line) 10 days after and enhanced upward propagation of PWs about 20–30 days later. The zonal mean zonal wind anomalies at 200 hPa become positive beginning about 15 days after MJO P1, which would contribute to additional upward propagation of the PWs. The most significant positive anomalies are found between 15 and 25 days after MJO P1. The WN2 anomalies increase at a lag of approximately 15 days after MJO P1 and then return to normal, whereas the WN1 counterpart becomes positive about 20 days after the MJO P1. In the stratosphere (Fig. 10b), the anomalous westerlies are also enhanced beginning about 15 days after MJO P1. The most significant positive anomalies of the EP flux are found between 20 and 30 days after MJO P1, which is approximately 5 days later than the peak in the troposphere. The WN1 component of EP flux anomalies reaches a peak 20–30 days after MJO P1; this is much stronger than the WN2 counterpart and basically matches the peak of temperature anomalies presented in Fig. 2c. The WN2 component is enhanced about 15 days after MJO P1 and is then suppressed.
(a) Evolution of the anomalies of the vertical component of EP flux averaged between 40° and 80°S at 100 hPa and zonal mean zonal wind anomalies response (indicated by the dashed cyan line) to MJO P1. Asterisks represent significance at the 95% level. The black line represents the vertical component of total EP flux (m2 s−1 day−1), and the red line represents the summed EP flux by WN1 and WN2. The blue and green lines represent the WN1 and WN2 components, respectively. (b) As in (a), but for 30 hPa. (c),(d) As in (a),(b), but for days before and after P5.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
During the MJO P5, the zonal mean zonal wind anomalies are less significant near the tropopause (Fig. 10c). As a result, although the geopotential height anomalies are more intense during 0–10 days after MJO P5 (Fig. 9d), the total EP flux anomalies are comparable to those after MJO P1 (Fig. 10c). Moreover, the enhancement of EP flux beginning 5 days after MJO P5 only lasts for about 10 days, which is much shorter than that after MJO P1. The WN1 components account for the most variation of the total EP flux. In the stratosphere (Fig. 10d), the variation of the zonal mean zonal wind anomalies are not significant. Both the WN1 and WN2 components of the vertical EP flux anomalies are strengthened during the first 10 days after MJO P5, matching the peak of temperature anomalies presented in Fig. 2d, although the WN1 anomalies contribute most.
The winter stratosphere response to MJO is owing to anomalous propagation and dissipation of PWs in the high-latitude stratosphere. This enhances the winter stratosphere residual meridional circulation and causes anomalous polar warming, similar to the stratosphere responses to ENSO as previously studied (Hurwitz et al. 2011a,b; Zubiaurre and Calvo 2012; Li et al. 2013).
c. Mesosphere
As shown in Figs. 3 and 4, there are also significant temperature anomalies in the mesosphere of the SH polar region, which have opposite sign to those in the stratosphere. Planetary Rossby waves in the mesosphere are much weaker than those in the stratosphere and not likely to be responsible for the temperature anomalies there. Thus, the gravity wave drag variations associated with the MJO phases are examined to understand their contribution to the MJO signal in the mesospheric circulation. Gravity waves, which are parameterized in SD-WACCM, are known to drive much of the dynamics of the mesosphere. Figures 11a and 11b depict the composite of the averaged gravity wave drag in the SH (between 30° and 80°S) for lags of MJO P1 and P5, respectively. During the periods of about 30 days after MJO P1 (Fig. 11a) and about 10 days after MJO P5 (Fig. 11b), there are significant anomalous reductions of westward wave forcing in the SH mesosphere, with peaks located between 0.1 and 0.01 hPa (~65–80 km).
(a) Composite of gravity wave drag anomalies (m s−1 day−1) in SH (between 30° and 80°S) mesosphere (1–0.0001 hPa; ~50–110 km) for 0–50 days after MJO P1. (b) As in (a), but for composite from 10 days before to 40 days after MJO P5. (c) Composite of zonal mean zonal wind anomalies (m s−1) in SH (between 30° and 80°S) stratosphere and mesosphere (100–0.001 hPa; ~15–95 km) for 0–50 days after MJO P1. (d) As in (c), but for the composite from 10 days before to 40 days after MJO P5.The white and light gray areas denote 95% and 90% significance, respectively, according to the Monte Carlo test.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
The GW evolution can be understood as the effect of the filtering by the stratospheric winds. At 20–30 day lag behind MJO P1and 0–10 day lag behind MJO P5, the stratospheric and lower mesospheric westerly zonal winds are weaker (Figs. 11c,d). Because of these wind differences, the removal of upward-propagating eastward GW by critical-level filtering decreases and therefore the phase speed range of eastward gravity waves that can penetrate to the mesosphere is broader. When these waves dissipate in the mesosphere, the eastward momentum forcing in the mesosphere partially counteracts the dominant westward forcing. As a result, the net westward gravity wave forcing is reduced from its climatological values during these periods. Thus, the eastward gravity wave forcing anomalies in the SH mesosphere associated with MJO weaken the wave driving of the mesospheric residual meridional circulation. The associated decrease in the downwelling leads to cooling of the polar mesosphere. The mechanism in the mesosphere is quite similar to a recent study of mesospheric response to ENSO by Li et al. (2013, 2016). Both mechanisms are linked to anomalous gravity waves filtering by the anomalous stratospheric zonal wind, leading to anomalous mesospheric residual meridional circulation thus anomalous polar mesospheric temperature.
In the middle and high latitudes of the mesosphere, we clearly see a significantly enhanced upwelling in the anomalies of the meridional residual circulation composite (Fig. 12) in the SH. The enhanced upwelling in the SH polar region extends from approximately 65 (0.1 hPa) to over 100 km (0.001 hPa) at 20–30 day lag behind MJO P1 (Fig. 12a), while the enhancement of upwelling at 0–10 day lag behind MJO P5 (Fig. 12b) is confined to between approximately 65 and approximately 80 km (0.1 and 0.01 hPa). A weaker but significant anomalous circulation is revealed in the midlatitude (~30°S) and the tropical mesosphere, with downwelling in the tropics and upwelling in the SH midlatitudes. The residual circulation anomalies indicate that adiabatic temperature variation due to downwelling/upwelling is the dominant contributor to the mesospheric temperature response to the MJO P1 and P5.
Composite of residual circulation anomalies during (a) 20–30 days after MJO P1 and (b) 0–10 days after MJO P5 in the mesosphere (1–0.0001 hPa; ~50–110 km). The light gray and white shading indicate 90% and 95% significance levels, respectively.
Citation: Journal of Climate 30, 20; 10.1175/JCLI-D-17-0063.1
5. Discussion and summary
In this study, we investigated the connection between the austral wintertime stratosphere/mesosphere and the MJO phases using SD-WACCM. The response of polar cap temperatures to the MJO in both ERA-Interim and SD-WACCM data suggests that a significant warming occurs in the SH polar cap when convection over Indian Ocean is enhanced while that over the western Pacific is suppressed. The temperature anomalies in the SH polar region of the mesosphere and the stratosphere have opposite signs and alternate between warmer or cooler over a period of approximately 40 days. The good agreement between two sets of reanalysis data (MERRA and ERA-Interim) suggests that the SH polar signal associated with the MJO is robust.
During MJO P1, there is a cooling in the SH polar stratosphere and a warming in the mesosphere. The mesospheric response is associated with stronger westerlies at high latitudes (60°–90°S) and weaker westerlies in the tropical region of the mesosphere. The temperature anomalies propagate downward and become marginally significant in the polar region 10–20 days after MJO P1. The most significant positive temperature anomalies in the stratosphere and negative temperature anomalies in the mesosphere of the SH polar region are found at a lag of 30 days, associated with the negative anomalous westerlies in the midlatitude of the SH stratosphere and mesosphere. The pattern at 30-day lag is basically opposite to that at a lag of 0. In the composite of MJO P5, the most significant temperature and zonal wind anomalies are seen at a lag of 10 days after MJO P5; they have a similar pattern to the anomalies at a lag of 30 days after MJO P1.
In the troposphere, the 500-hPa geopotential height anomalies indicate two wave trains emanating from the Indian and Pacific Oceans to the SH polar regions 10–30 days after MJO P1 and 0–10 days after MJO P5, respectively. This is consistent with the analysis of Matthews and Meredith (2004) showing that anomalous MJO convection forces a Rossby wave train that extends into the extratropical troposphere with a 10-day lag of their MJO index. The WN1 component is the most important of the geopotential height anomalies induced by the MJO convection at the top of troposphere (200 hPa). The most significant positive anomalous vertical component of the extratropical (40°–80°S averaged) EP flux at 100 hPa are found 15–35 days after MJO P1 and 5–10 days after MJO P5; the WN1 and WN2 anomalous EP flux are responsible for the most of the total EP flux anomaly while the WN1 component is more important.
As a response to the tropospheric PW perturbation, the upward propagation and dissipation of planetary Rossby waves in the middle and high latitudes of the SH stratosphere are significantly enhanced on 20–30 days after MJO P1 or 0–10 days after MJO P5. After MJO P1, the most significant positive anomalies of EP Flux are found at the lags of 20 and 30 days. The WN1 component is much stronger than the WN2 counterpart. On the other hand, the enhancement of the vertical EP flux anomalies is most evident at 0–10 day lag after MJO P5. Both the WN1 and the WN2 components are enhanced while the WN1 anomalies contribute most of the PW anomalies. As a result, the Brewer–Dobson (BD) circulation, as revealed by the residual mean meridional circulation, is significantly strengthened during both periods; downwelling associated with the stronger circulation leads to positive temperature anomalies in stratospheric SH polar region. The adiabatic heat induced by the downwelling of the residual circulation in the SH polar region in turn leads to the significantly positive temperature anomalies in the stratosphere at a lag of 30 days for MJO P1 and at a lag of 10 days for MJO P5.
Although the zonal mean temperature and zonal wind signals at 30-day-lag MJO P1 are similar to that at 10-day-lag MJO P5, there are still some differences in the responses to MJO P1 and P5. The time scale for the cycle of temperature anomalies after MJO P1 (~60 days) is a little longer that that after MJO P5 (~40 days), and the positive temperature anomalies after MJO P1 are located a little higher in altitude than those after MJO P5. In the troposphere, the geopotential height anomalies are more coherent and last longer after MJO P1 (Figs. 5a–d), whereas they are more intense during 0–10 days after MJO P5 (Figs. 5g,h). The westerly anomalies are strengthened in the upper troposphere and the lower stratosphere beginning 10–20 days after MJO P1, which would benefit upward propagation of the PWs. On the other hand, the zonal mean zonal wind anomalies after MJO P5 are not significant. Consistent with these differences, the PWs after MJO P1 propagate to higher altitude with a comparable magnitude with that after MJO P5 although the geopotential height anomalies after MJO P5 are more intense (Fig. 9d).
In the SH mesosphere, the enhancement of the dissipation and breaking of GWs propagating in the SH mid- and high-latitude mesosphere weakens the mesospheric residual circulation. The enhanced upwelling in the middle and high latitudes of the mesosphere is clearly seen in the residual circulation at 20–30 day lag after MJO P1 and at 0–10 day lag after MJO P5, resulting in the adiabatic temperature variation in the mesospheric.
Acknowledgments
This work was funded by the National Natural Science Foundation of China (Grants 41274150, 41225017, and 41404118), by the National Science Fund for Creative Research Groups (41421063), and by the Key Research Programs of the Chinese Academy of Sciences (KZZD-EW-01-1). We thank ECMWF for monthly mean reanalysis data. The National Center for Atmospheric Research (NCAR) is sponsored by the National Science Foundation. WACCM is a component of the Community Earth System Model (CESM), which is supported by the National Science Foundation (NSF) and the Office of Science of the U.S. Department of Energy. CESM and the files needed to run it are available from NCAR (http://www.cesm.ucar.edu/models/current.html). Computing resources were provided by NCAR’s Computational and Information Systems Laboratory (CISL). We thank Doug Kinnison from NCAR for providing the WACCM output used in this manuscript.
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