September 2019 Antarctic Sudden Stratospheric Warming: Quasi‐6‐Day Wave Burst and Ionospheric Effects

An exceptionally strong stationary planetary wave with Zonal Wavenumber 1 led to a sudden stratospheric warming (SSW) in the Southern Hemisphere in September 2019. Ionospheric data from European Space Agency's Swarm satellite constellation mission show prominent 6‐day variations in the dayside low‐latitude region at this time, which can be attributed to forcing from the middle atmosphere by the Rossby normal mode “quasi‐6‐day wave” (Q6DW). Geopotential height measurements by the Microwave Limb Sounder aboard National Aeronautics and Space Administration's Aura satellite reveal a burst of global Q6DW activity in the mesosphere and lower thermosphere during the SSW, which is one of the strongest in the record. The Q6DW is apparently generated in the polar stratosphere at 30–40 km, where the atmosphere is unstable due to strong vertical wind shear connected with planetary wave breaking. These results suggest that an Antarctic SSW can lead to ionospheric variability through wave forcing from the middle atmosphere.


Introduction
A sudden stratospheric warming (SSW) is a large-scale meteorological phenomenon in the winter stratosphere, which involves a rapid rise in the polar temperature by a few tens of kelvins in several days (Andrews et al., 1987;Labitzke & Van Loon, 1999). An SSW is triggered by an injection of stationary planetary waves (PWs) from the troposphere, which are driven by topography and land-sea temperature contrasts. PW breaking in the middle atmosphere leads to an acceleration of the zonal mean flow and changes the mean meridional circulation (Matsuno, 1971). Dynamical effects of PW breaking during SSWs are not limited in the stratosphere but are also well extended into the mesosphere and lower thermosphere (Chandran et al., 2014).
According to the definition by the World Meteorological Organization (McInturff, 1978), a "minor" SSW occurs when a large temperature increase is observed in the winter polar stratosphere, at least by 25 K in a week or less. The event is called "major" if the reversal of the zonal mean flow from eastward to westward occurs poleward of 60 • latitude at 10 hPa (32 km) or below, along with the reversal of the meridional temperature gradient. The average number of major SSWs is ∼0.6 per winter in the Northern Hemisphere (NH) (Butler et al., 2015;Charlton & Polvani, 2007). In the Southern Hemisphere (SH), the occurrence of an SSW, In the last decade, the aeronomy community has come to the realization that SSWs can be a significant source of ionospheric variability (Chau et al., 2012;Pedatella et al., 2018). In particular, the January 2009 major Arctic SSW, which took place under extremely quiet solar and geomagnetic activity conditions, enabled many studies to attribute observed ionospheric perturbations to the SSW (e.g., Chau et al., 2010;Fejer et al., 2010;Liu et al., 2011;Lin et al., 2019;Nayak & Yigit, 2019;Oyama et al., 2014;Pedatella & Forbes, 2010;Patra et al., 2014;Pancheva & Mukhtarov, 2011;Rodrigues et al., 2011;Yue et al., 2010;Yadav et al., 2017). Most studies concentrated on the dayside low-latitude region, where the ionospheric response to the SSW was most pronounced. Modeling studies have suggested that atmospheric tides played an important role in driving ionospheric variability during the January 2009 SSW (Fang et al., 2012;Fuller-Rowell et al., 2011;Jin et al., 2012;Pedatella et al., 2014;Pedatella & Maute, 2015;Sassi et al., 2013;Wang et al., 2014). Tidal waves at altitudes of the ionospheric E region (95-150 km) are, in large part, from the middle atmosphere, and their amplitudes and phases can change in response to an SSW (Stening et al., 1997). Among different tidal modes, the semidiurnal lunar tide shows a particularly strong and consistent response to SSWs (Pedatella & Maute, 2015;Zhang & Forbes, 2014). Forbes and Zhang (2012) argued that the large semidiurnal lunar tide observed during the January 2009 SSW can arise from resonant amplification associated with the atmospheric Pekeris mode. Enhanced lunar tidal perturbations in the equatorial ionosphere have been reported for a number of SSW events (Fejer et al., 2011;Liu et al., 2019;Siddiqui et al., 2015Siddiqui et al., , 2018Park et al., 2012).
As mentioned earlier, SSWs rarely occur in the SH, and the ionospheric response to Antarctic SSWs has been largely unexplored. The only exception is the study by Olson et al. (2013), which examined ionospheric variability during the September 2002 major Antarctic SSW. Although Olson et al. (2013) observed multiday variations in the equatorial ionosphere, their association with the SSW remained somewhat uncertain because of high geomagnetic activity during the event. The main objective of this study is to present observations from the ionosphere and middle atmosphere during the recent Antarctic SSW event in September 2019 and note the presence of unusually strong traveling PW activity throughout the atmosphere and ionosphere at this time. Figure 1 gives an overview of the September 2019 SSW. The polar temperature at 10 hPa, obtained from the MERRA-2 reanalysis (Gelaro et al., 2017), shows a rapid increase from 207.7 K on 5 September to 258.5 K on 11 September 2019 (ΔT = 50.8 K/week) ( Figure 1a). This is the largest increase in the Antarctic polar temperature per week in the entire MERRA-2 data set starting from January 1980. The maximum temperature rise during the September 2002 major SSW was ΔT = 38.5 K/week. Figure 1b presents the vertical structure of the zonal mean zonal wind at 60 • S, as derived from geopotential height measurements by the Aura Microwave Limb Sounder (MLS) (Schwartz et al., 2008;Waters et al., 2006). It can be seen that the eastward zonal mean wind first reversed in the upper mesosphere on 2 September 2019, and in the subsequent days, the region of the wind reversal descended to lower layers, reaching 40 km on 18 September 2019. Since the wind reversal did not reach the 10-hPa level (∼32 km), the event is categorized as a minor warming. Figure 1c shows that there was an enhancement in the amplitude of the stationary PW with Zonal Wavenumber (ZW) 1 during 14-20 August 2019 and during 28 August to 5 September 2019. In both cases, the amplitude attained the largest recorded by Aura/MLS since August 2004. The former event can contribute to the SSW by weakening the zonal mean flow, which is often referred to as preconditioning (e.g., Cámara et al., 2017;Limpasuvan et al., 2004;McIntyre, 1982). Forcing due to PW breaking during the latter event is the likely cause of the zonal wind reversal in the middle atmosphere and hence the SSW. No similar enhancement is found in the amplitude of the stationary PW with ZW2.

September 2019 SSW
As a brief summary, the September 2019 Antarctic SSW was a minor warming but it involved an exceptionally strong stationary PW with ZW1 and a large temperature rise. Furthermore, the event took place during the minimum phase of the solar cycle, similar to the January 2009 SSW, and as will be shown later,  overall solar and geomagnetic activities were low, which helps identify SSW effects on the ionosphere. Therefore, the September 2019 event provides an excellent (and rare) opportunity to investigate the ionospheric response to an Antarctic SSW, which is not well understood from previous studies.

Ionospheric Observations by Swarm
European Space Agency's Earth observation mission Swarm (Friis-Christensen et al., 2006) involves three identical satellites (A, B, and C), equipped with scientific instruments that are suitable for investigating Earth's magnetic field and its source currents (Friis-Christensen et al., 2008). The three spacecraft were launched into polar orbits on 22 November 2013, and since 17 April 2014, Swarm A and C fly side by side at an altitude of ∼460 km, while Swarm B flies at ∼510 km. Figures 2a-2c show the temporal variability of the equatorial electrojet (EEJ) intensity (e.g., Alken et al., 2015), electron density (e.g., Buchert et al., 2015), and total electron content (TEC) (e.g., Park et al., 2017) as observed by Swarm B during 5 September to 5 October 2019. The data used here were collected from the descending parts of the orbit in 11:00-14:00 magnetic local time (MLT) (see also Figure 2g). Figures 2h and 2i show that overall solar and geomagnetic activity levels were low during this time interval, which is typical for solar minimum conditions. Moderately high geomagnetic activity was observed during 27 September to 1 October 2019, which needs to be taken into account when the ionospheric data are interpreted. Unlike the September 2002 Antarctic SSW, which was examined by Olson et al. (2013), severe geomagnetic activity with Kp > 6 was not observed. The low F 10.7 conditions are preferable for the study of SSW effects on the ionosphere. Modeling studies have shown that the ionospheric response to lower atmospheric forcing would be more pronounced under lower solar flux conditions Liu & Richmond, 2013).
The EEJ is a narrow band of a zonal electric current that flows along the magnetic equator in the dayside E region ionosphere at 100-to 115-km altitude (e.g., Yamazaki & Maute, 2017). Under solar minimum conditions with low geomagnetic activity and nearly constant solar flux, day-to-day variations of the EEJ intensity are dominated by the changes in neutral winds at E region heights associated with atmospheric waves from the lower layers (Yamazaki et al., 2014) and thus are a good indicator of lower atmospheric influence on the 10.1029/2019GL086577 E region ionosphere. The methods for deriving the EEJ intensity and equatorial zonal electric field (EEF) from Swarm magnetic field measurements are detailed in Alken et al. (2013). Figure 2a reveals that the EEJ variability was dominated by 6-day variations during this period. The westward phase propagation of the EEJ intensity perturbations with ZW1 can also be seen. The PW spectrum of the EEJ intensity shows the predominance of the westward propagating ZW1 wave with period 6.0 ± 0.2 days (see Figure S1 in the supporting information). Similar spatial and temporal variability was found in the equatorial zonal electric field. Figure 2d shows relative changes in the EEF from the 29-day running mean, calculated separately at ±90 • longitudes. It can be seen that the EEF underwent 6-day variations of ±40% that are out of phase for a 180 • longitudinal separation. The amplitude varies in the range of 20-70% depending on the longitude. In a recent study, Yamazaki et al. (2018) reported that the EEJ intensity occasionally shows ∼6-day variations that have characteristics of a westward propagating wave with ZW1. They attributed the EEJ variations to the quasi-6-day wave (Q6DW) that was simultaneously observed in the lower thermosphere. The behavior of the EEJ presented in Figure 2a is similar to those reported by Yamazaki et al. (2018). The amplitude of the 6-day variations in the EEJ, 10-30 mA/m, is also comparable with those in Yamazaki et al. (2018).
The Q6DW is a westward propagating PW with ZW1, which is occasionally observed in the middle atmosphere (e.g., Forbes & Zhang, 2017;Hirota & Hirooka, 1984;Pancheva et al., 2018;Riggin et al., 2006;Talaat et al., 2001;Wu et al., 1994). It is often regarded as the (1,1) Rossby normal mode, which is predicted by classical atmospheric wave theory (Forbes, 1995;Kasahara, 1976;Madden, 1979Madden, , 2007Salby, 1984), for its ZW, phase speed, and latitudinal structure. The Q6DW can be excited in the troposphere by heating due to moist convection (Miyoshi & Hirooka, 1999). Additionally, the wave can be excited/amplified in the middle atmosphere due to baroclinic/barotropic instability (Lieberman et al., 2003;Liu et al., 2004;Meyer & Forbes, 1997). Zonal wind perturbations of the Q6DW are largest around the equator and can be up to a few tens of meters per second at E region heights, which is sufficient to cause detectable changes in dayside ionospheric electric fields and currents (Gan et al., 2016;Miyoshi, 1999;Pedatella et al., 2012). These electric field perturbations in the E region ionosphere are transmitted to the F region along equipotential magnetic field lines and affect the distribution of low-latitude F region plasmas by modulating their E × B plasma drift motions. In this way, the Q6DW can affect the F region plasma density, as first revealed in the 1990s by ionosonde measurements (e.g., Apostolov et al., 1994;Altadill & Laštovička, 1996;Laštovička, 2006). More recent studies based on global TEC maps have established that the Q6DW effect on the plasma density is largest in the afternoon local time sector near the equatorial ionization anomaly crests (±20 • magnetic latitudes) (Gu et al., 2014Qin et al., 2019;Yamazaki, 2018).
The 6-day variations can be seen in both electron density (Figures 2b and 2e) and topside TEC (Figures 2c and  2f) at 20 • magnetic latitude. ( Figure S2 in the supporting information shows the electron density variations at various latitudes.) The variations are consistent with those in the EEJ/EEF (Figures 2a and 2d), indicating electrodynamic coupling between the E and F region ionosphere. The response time of the F region plasma density to a change in the E region electric field is 2-4 hr (e.g., Stolle et al., 2008;Venkatesh et al., 2015), which would not be visible in the figures. The relative change in the electron density is in the range of 20-40%, which is appreciably larger than that of TEC, 5-10%. This is not surprising as the amplitude of the Q6DW decreases with altitude in the topside ionosphere, as demonstrated by .
The plasma density and TEC data from the ascending parts of the Swarm B orbit (23:00-02:00 MLT) were also examined, but the 6-day variations were not as evident as the results derived from the descending orbits. Similarly, the ionospheric data (EEJ, electron density, and TEC) from Swarm A, which was flying around 02:00-05:00 MLT (descending orbits) and 14:00-17:00 MLT (ascending orbits), did not show strong 6-day variations. The electron density variations from Swarm B (ascending) and Swarm A (ascending and descending) are presented in the supporting information ( Figure S3). The different behavior of 6-day variations in different Swarm data sets reflects the fact that the ionospheric response to the Q6DW depends on MLT and height, as well as on magnetic latitude . Further studies are required to determine the three-dimensional structure of the 6-day ionospheric variations during this event, which could clarify the cause of the differences observed between Swarm A and B.
Previous studies found a significant contribution of the semidiurnal lunar tide to ionospheric variability during NH SSWs (e.g., Park et al., 2012), but it is not known whether the lunar tide plays an equally important role during SH SSWs. The semidiurnal lunar variations in the EEJ intensity derived from the Swarm A and B data during 5 September to 5 October 2019 are presented in the supporting information ( Figure  S4). It is found that the amplitude of the EEJ semidiurnal lunar variation is 17.7 ± 2.1 mA/m for Swarm A (14:00-17:00 MLT) and 16.6 ± 2.8 mA/m for Swarm B (11:00-14:00 MLT), which is greater than the climatological value of 9.0 ± 0.4 mA/m as reported by  for September daytime (08:00-16:00 local solar time) conditions. The phase, which is defined as the lunar time of maximum, is 10.2 ± 0.2 hr for Swarm A and 10.0 ± 0.4 hr for Swarm B, which is in good agreement with the climatological value of 10.0 ± 0.1 hr. Despite the significant enhancement, the lunar variation accounts for only a small part of the observed EEJ variability (cf. Figures 2a and S4). The relative amplitude of the semidiurnal lunar variation in the topside electron density is 9.9 ± 0.7% for Swarm A and 11.1 ± 0.1% for Swarm B (also shown in Figure S4). Again, these variations are smaller than the 6-day variations observed during the same period (Figure 2e).
It is noted that since Swarm slowly precesses in local solar time, it is not possible to resolve short-term variability of solar tides. Changes in upward-propagating solar tides can occur during SSWs due to changes in the zonal mean atmosphere (Jin et al., 2012;Pedatella & Liu, 2013), tidal sources Siddiqui et al., 2019), and tidal interaction with PWs (Chang et al., 2009;Liu et al., 2010;Maute et al., 2014). Possible changes in solar tides during the September 2019 SSW remain to be investigated.

Q6DW in the Middle Atmosphere
Traveling PWs in the middle atmosphere are examined using the geopotential height data from Aura/MLS. The analysis method was described in detail in the previous work (Yamazaki & Matthias, 2019) and thus is only briefly summarized here. The amplitude A and phase of waves with period were derived by fitting the following formula to the data at a given latitude and height: where t is the universal time, is the longitude, and s is the ZW. Eastward and westward propagating waves correspond to s < 0 and s > 0, respectively. The data were analyzed for each day using a time window that is 3 times the wave period. The 1-error in the amplitude is typically below 0.05 km. Figures 3a and 3b show the amplitudes for the westward and eastward propagating waves with ZW1 at 45 • S in the lower thermosphere at ∼97 km. Enhanced wave activity can be seen in the westward propagating component (Figure 3a) with period 4-7 days during September 2019, which can be identified as the Q6DW. It is consistent with the appearance of 6-day variations in the ionosphere (Figures 2a-2c). Such enhanced wave activity is not present in the eastward propagating ZW1 component (Figure 3b) or other components with higher ZWs (not shown here). Although studies have found that the amplitude of the Q6DW in the middle atmosphere is greatest during equinoctial months (Forbes & Zhang, 2017;Qin et al., 2019;Yamazaki, 2018), the wave enhancement in September 2019 was exceptional, with the maximum amplitude larger than 0.4 km in the lower thermosphere, which is much larger than the climatological amplitude (0.15 km) or amplitudes recorded during other individual years during 2004-2018 ( Figure 3d). Thus, the large-amplitude Q6DW observed in September 2019 cannot be explained merely as a seasonal effect.
The latitude and height structures of the 6-day wave during 10-30 September 2019 are presented in Figure 3c. The amplitude and phase were derived at wave period of exactly 6.0 days, so that the phases calculated at different heights and latitudes can be compared. In the mesosphere and lower thermosphere (above 50 km), the amplitude structure is symmetric about the equator with peaks at approximately ±45 • latitudes, and the phase tends to be horizontally uniform with downward phase progression. These features are in conformity with the theoretically expected Q6DW in the presence of the mean winds and dissipation (e.g., Salby, 1981bSalby, , 1981a. Below 50 km, the phase progression is poleward as well as downward, especially in the SH, indicating equatorward and upward energy propagation from the high-latitude region. Using reanalysis data, Gan et al. (2018) demonstrated how the Q6DW generated in the SH high latitude can propagate into the NH, growing to be a global mode in the mesosphere and lower thermosphere under September equinox conditions.
In Figure 3c, there is a region of locally enhanced amplitudes at 70-80 • S and 20-to 50-km altitude, which can be regarded as a source of the large-amplitude Q6DW observed above. The amplification of the Q6DW from the seasonal background in this region is depicted in Figure 3e. Enhanced wave activity is observed in the same region over a wide range of wavenumbers (s from −3 to 3) and periods ( = 3-20 days) (not shown here). A possible explanation for the wave amplification is baroclinic/barotropic instability 10.1029/2019GL086577 Lieberman et al., 2003;Liu et al., 2004;Meyer & Forbes, 1997), in which waves can rapidly grow by extracting energy from the unstable mean flow. Figure 3f shows that the wave amplification in the polar middle atmosphere is not uncommon around this time of year, but in 2019, it took place at lower altitudes (∼30 km) than in other years (∼50 km).
Figures 3h-3j illustrate the development of the atmospheric instability. The areas highlighted by the light-yellow color indicate the regions where the necessary condition for barotropic/baroclinic instability is met; that is, the meridional gradient of the quasi-geostrophic potential vorticity is negative (e.g., Liu et al., 2004). It can be seen that unstable regions are formed mainly around the edge of the polar vortex due to the strong vertical and horizontal shear in the zonal wind. As the westward mean flow descends to lower layers, the unstable regions at high latitudes (70-80 • S) also move down and hence exciting/amplifying waves at lower altitudes compared to other years. As these waves propagate equatorward and upward, the amplitude at 45 • S is greater than other years above ∼40 km ( Figure 3g). As numerically demonstrated by Salby (1981b), the vertical growth of amplitude is enhanced where the zonal mean zonal wind is weak and eastward relative to the phase speed of the wave. The westward phase speed of the Q6DW is ∼55 m/s at 45 • S and ∼13 m/s at 80 • S. Thus, the reduced eastward mean flow and the weak wind reversal during the SSW (Figures 3h-3j) provide favorable conditions for the vertical propagation of the Q6DW. Interactions of the Q6DW with tides and gravity waves could also affect the vertical structure of the Q6DW (e.g., Forbes et al., 2018;Meyer, 1999). A better understanding of the Q6DW propagation in the mesosphere and lower thermopshere during the September 2019 SSW would benefit from a more comprehensive analysis of dynamic fields from an atmospheric reanalysis or general circulation model.
For the NH, possible SSW influences on the vertical propagation of traveling planetary waves in the middle atmosphere have been discussed in a number of studies (e.g., Hirooka & Hirota, 1985;Matthias et al., 2012;Pancheva et al., 2008;Sassi et al., 2012;Yamazaki & Matthias, 2019). In some cases, a strong Q6DW was observed during an SSW (e.g., Gong et al., 2018;Pancheva et al., 2018) but in general, there is no one-to-one correspondence between the occurrence of SSW and Q6DW enhancement in the NH (Yamazaki & Matthias, 2019). Modeling studies also found enhanced Q6DW activity following some SSWs, which has been attributed to barotropic/baroclinic instability in the NH high latitude (Chandran et al., 2013;Tomikawa et al., 2012). For the SH, studies are few because of infrequent occurrence of SSWs. Dowdy et al. (2004) and Espy et al. (2005) observed a westward propagating planetary wave with ZW1 and period around 14 days at 70-to 100-km altitude during the September 2002 Antarctic SSW. The present study finds a strong response of the Q6DW in the mesosphere and lower thermosphere during the September 2019 Antarctic SSW. It is possible that the response of traveling planetary waves to Antarctic SSWs varies from event to event. More studies are needed to clarify this point.

Summary and Conclusions
An SSW occurred in the SH in September 2019. Although it was a minor warming, it involved an exceptionally strong Wave 1 planetary wave and a large polar temperature enhancement by 50.8 K/week. The event also took place under solar minimum conditions, which is preferable for studying the ionospheric response. Earlier studies focused on the effect of NH SSWs on the ionosphere, and few studies investigated SH cases.
The analysis of ionospheric data from European Space Agency's Swarm mission during the September 2019 SSW reveals prominent 6-day variations in the dayside low-latitude region, including 20-70% variations in the equatorial zonal electric field, 20-40% variations in the topside electron density, and 5-10% variations in the topside TEC. These variations are attributed to the Q6DW simultaneously observed in the middle atmosphere. Evidence is also found for enhanced lunar tidal perturbations in the ionosphere, but their amplitudes are relatively small (e.g., less than 15% in the topside electron density).
The amplitude of the Q6DW in the lower thermosphere is more than 0.4 km in geopotential height, which is found to be the largest observed by Aura/MLS in the SH since August 2004 and thus cannot be explained merely as a seasonal effect. The latitudinal and vertical structures of the Q6DW suggest that the waves are excited/amplified in the polar region at 30-to 40-km altitude, where the atmosphere is unstable due to strong vertical shear in the zonal wind connected with planetary wave breaking. As the Q6DW grows in the vertical, the wave attains large amplitudes in the lower thermosphere, which drives ionospheric variability.