September 2019 Antarctic sudden stratospheric warming: quasi-6-day wave burst and ionospheric effects

ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters September 2019 Antarctic sudden stratospheric warming: quasi-6-day wave burst and ionospheric effects 1 2 3 4 Y. Yamazaki1 , V. Matthias2 , Y. Miyoshi3 , C. Stolle1,4 , T. Siddiqui1 , G. 5 Kervalishvili1, J. Laštovička5 , M. Kozubek5 , W. Ward6 , D. R. Themens6 , S. 6 Kristoffersen6, P. Alken7,8 1 GFZ 7 German Research Centre for Geosciences, Potsdam, Germany 2 Potsdam 8 3 Department 9 Institute for Climate Impact Research, Potsdam, Germany of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan 10 4 Faculty 11 5 Institute 6 Department 12 13 7 Cooperative of Science, University of Potsdam, Potsdam, Germany of Atmospheric Physics CAS, Prague, Czech Republic of Physics, University of New Brunswick, Fredericton, New Brunswick, Canada Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA 14 8 National 15 Centers for Environmental Information, NOAA, Boulder, CO, USA Key Points: 16 17 • An Antarctic sudden stratospheric warming (SSW) occurred in September 2019 18 • Swarm observations reveal prominent 6-day variations in the dayside low-latitude ionosphere 19 20 21 • A burst of quasi-6-day wave activity is observed in the middle atmosphere during the SSW Corresponding author: Yosuke Yamazaki, yamazaki@gfz-potsdam.de –1– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 22 23 Abstract An exceptionally strong stationary planetary wave with Zonal Wavenumber 1 led 24 to a sudden stratospheric warming (SSW) in the Southern Hemisphere in September 2019. 25 Ionospheric data from ESA’s Swarm satellite constellation mission reveal prominent 6- 26 day variations in the dayside low-latitude region at this time, which can be attributed 27 to forcing from the middle atmosphere by the Rossby normal mode “quasi-6-day wave” 28 (Q6DW). Geopotential height measurements by the Microwave Limb Sounder aboard 29 NASA’s Aura satellite show a burst of Q6DW activity in the mesosphere and lower ther- 30 mosphere during the SSW, which is one of the strongest in the record. The Q6DW is 31 apparently generated in the polar stratosphere at 30–40 km, where the atmosphere is 32 unstable due to strong vertical wind shear connected with planetary-wave breaking. These 33 results suggest that an Antarctic SSW can lead to ionospheric variability through wave 34 forcing from the middle atmosphere. –2– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 35 36 1 Introduction A sudden stratospheric warming (SSW) is a large-scale meteorological phenomenon 37 in the winter stratosphere, which involves a rapid rise in the polar temperature by a few 38 tens of K in several days (Andrews, Leovy, & Holton, 1987; Labitzke & Van Loon, 1999). 39 An SSW is triggered by an injection of stationary planetary waves (PWs) from the tro- 40 posphere, which are driven by topography and land-sea temperature contrasts. PW break- 41 ing in the middle atmosphere leads to an acceleration of the zonal mean flow and changes 42 the mean meridional circulation (Matsuno, 1971). Dynamical effects of PW breaking dur- 43 ing SSWs are not limited in the stratosphere but are also well extended into the meso- 44 sphere and lower thermosphere (Chandran, Collins, & Harvey, 2014). 45 According to the definition by the World Meteorological Organization (McInturff, 46 1978), a “minor” SSW occurs when a large temperature increase is observed in the win- 47 ter polar stratosphere, at least by 25 K in a week or less. The event is called “major” 48 if the reversal of the zonal mean flow from eastward to westward occurs poleward of 60◦ 49 latitude at 10 hPa (32 km) or below, along with the reversal of the meridional temper- 50 ature gradient. The average number of major SSWs is ∼0.6 per winter in the Northern 51 Hemisphere (NH) (Butler et al., 2015; Charlton & Polvani, 2007). In the Southern Hemi- 52 sphere (SH), the occurrence of an SSW, whether major or minor, is not as frequent as 53 in the NH because of weaker PW forcing due to smaller topographical differences and 54 land-sea contrasts. In fact, the September 2002 event (Baldwin, Hirooka, O’Neill, & Yo- 55 den, 2003; Krüger, Naujokat, & Labitzke, 2005) is the only major SSW observed in the 56 Antarctic. 57 In the last decade, the aeronomy community has come to the realization that SSWs 58 can be a significant source of ionospheric variability (Chau, Goncharenko, Fejer, & Liu, 59 2012; Pedatella et al., 2018). In particular, the January 2009 major Arctic SSW, which 60 took place under extremely quiet solar- and geomagnetic-activity conditions, enabled many 61 studies to attribute observed ionospheric perturbations to the SSW (e.g., Chau et al., 62 2010; Fejer et al., 2010; Goncharenko, Chau, Liu, & Coster, 2010; Goncharenko, Coster, 63 Chau, & Valladares, 2010; Lin et al., 2019; H. Liu et al., 2011; Nayak & Yiğit, 2019; Oyama 64 et al., 2014; Pancheva & Mukhtarov, 2011; Patra, Pavan Chaitanya, Sripathi, & Alex, 65 2014; Pedatella & Forbes, 2010; Rodrigues, Crowley, Azeem, & Heelis, 2011; Yadav et 66 al., 2017; Yue et al., 2010). Most studies concentrated on the dayside low-latitude re- –3– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 67 gion, where the ionospheric response to the SSW was most pronounced. Modeling stud- 68 ies have suggested that atmospheric tides played an important role in driving ionospheric 69 variability during the January 2009 SSW (Fang et al., 2012; Fuller-Rowell et al., 2011; 70 Jin et al., 2012; Pedatella et al., 2014; Pedatella & Maute, 2015; Sassi, Liu, Ma, & Gar- 71 cia, 2013; Wang et al., 2014). Tidal waves at altitudes of the ionospheric E region (95– 72 150 km) are, in large part, from the middle atmosphere, and their amplitudes and phases 73 can change in response to an SSW (Stening, Forbes, Hagan, & Richmond, 1997). Among 74 different tidal modes, the semidiurnal lunar tide shows a particularly strong and con- 75 sistent response to SSWs (Chau, Hoffmann, Pedatella, Matthias, & Stober, 2015; Zhang 76 & Forbes, 2014). Forbes and Zhang (2012) argued that the large semidiurnal lunar tide 77 observed during the January 2009 SSW can arise from resonant amplification associated 78 with the atmospheric Pekeris mode. Enhanced lunar tidal perturbations in the equato- 79 rial ionosphere have been reported for a number of SSW events (Fejer, Tracy, Olson, & 80 Chau, 2011; J. Liu, Zhang, Hao, & Xiao, 2019; Park, Lühr, Kunze, Fejer, & Min, 2012; 81 Siddiqui et al., 2018; Siddiqui, Stolle, Lühr, & Matzka, 2015; Yamazaki, Richmond, & 82 Yumoto, 2012). 83 As mentioned earlier, SSWs rarely occur in the SH, and the ionospheric response 84 to Antarctic SSWs has been largely unexplored. The only exception is the study by Ol- 85 son, Fejer, Stolle, Lühr, and Chau (2013), which examined ionospheric variability dur- 86 ing the September 2002 major Antarctic SSW. Although Olson et al. (2013) observed 87 multi-day variations in the equatorial ionosphere, their association with the SSW remained 88 somewhat uncertain because of high geomagnetic activity during the event. The main 89 objective of this study is to present observations from the ionosphere and middle atmo- 90 sphere during the recent Antarctic SSW event in September 2019 and note the presence 91 of unusually strong traveling PW activity throughout the atmosphere and ionosphere 92 at this time. 93 2 Results and Discussion 94 2.1 September 2019 sudden stratospheric warming 95 Figure 1 gives an overview of the September 2019 SSW. The polar temperature at 96 10 hPa, obtained from the MERRA-2 reanalysis (Gelaro et al., 2017), shows a rapid in- 97 crease from 207.7 K on 5 September to 258.5 K on 11 September 2019 (∆T =50.8 K/week) –4– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 98 (Figure 1a). This is the largest increase in the Antarctic polar temperature per week in 99 the entire MERRA-2 data set starting from January 1980. The maximum temperature 100 rise during the September 2002 major SSW was ∆T =38.5 K/week. Figure 1b presents 101 the vertical structure of the zonal mean zonal wind at 60◦ S, as derived from geopoten- 102 tial height (GPH) measurements by the Aura Microwave Limb Sounder (MLS) (Schwartz 103 et al., 2008; Waters et al., 2006). It can be seen that the eastward zonal mean wind first 104 reversed in the upper mesosphere on 2 September 2019, and in the subsequent days, the 105 region of the wind reversal descended to lower layers, reaching 40 km on 18 September 106 2019. Since the wind reversal did not reach the 10 hPa level (∼32 km), the event is cat- 107 egorized as a minor warming. Figure 1c shows that there was an enhancement in the am- 108 plitude of the stationary PW with Zonal Wavenumber (ZW) 1 during 14–20 August 2019 109 and during 28 August–5 September 2019. In both cases, the amplitude attained the largest 110 recorded by Aura/MLS since August 2004. The former event can contribute to the SSW 111 by weakening the zonal mean flow, which is often referred to as preconditioning (e.g., 112 Cámara et al., 2017; Limpasuvan, Thompson, & Hartmann, 2004; McIntyre, 1982). Forc- 113 ing due to PW breaking during the latter event is the likely cause of the zonal wind re- 114 versal in the middle atmosphere, and hence the SSW. No similar enhancement is found 115 in the amplitude of the stationary PW with ZW2. As a brief summary, the September 2019 Antarctic SSW was a minor warming but 116 117 it involved an exceptionally strong stationary PW with ZW1 and a large temperature 118 rise. Furthermore, the event took place during the minimum phase of the solar cycle, sim- 119 ilar to the January 2009 SSW, and as will be shown later, overall solar and geomagnetic 120 activities were low, which helps identify SSW effects on the ionosphere. Therefore, the 121 September 2019 event provides an excellent (and rare) opportunity to investigate the iono- 122 spheric response to an Antarctic SSW, which is not well understood from previous stud- 123 ies. 124 2.2 Ionospheric observations by Swarm 125 ESA’s Earth observation mission Swarm (Friis-Christensen, Lühr, & Hulot, 2006) 126 involves three identical satellites (A, B and C), equipped with scientific instruments that 127 are suitable for investigating Earth’s magnetic field and its source currents (Friis-Christensen, 128 Lühr, Knudsen, & Haagmans, 2008). The three spacecraft were launched into polar or- 129 bits on 22 November 2013, and since 17 April 2014, Swarm A and C fly side-by-side at –5– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 130 an altitude of ∼460 km while Swarm B flies at ∼510 km. Figures 2a–2c show the tem- 131 poral variability of the equatorial electrojet (EEJ) intensity (e.g., Alken et al., 2015), elec- 132 tron density (e.g., Buchert et al., 2015), and total electron content (TEC) (e.g., Park et 133 al., 2017) as observed by Swarm B during 5 September–5 October 2019. The data used 134 here were collected from the descending parts of the orbit in 11:00–14:00 magnetic lo- 135 cal time (MLT) (see also Figure 2g). Figures 2h and 2i show that overall solar and ge- 136 omagnetic activity levels were low during this time interval, which is typical for solar min- 137 imum conditions. Moderately high geomagnetic activity was observed during 27 September– 138 1 October 2019, which needs to be taken into account when the ionospheric data are in- 139 terpreted. Unlike the September 2002 Antarctic SSW, which was examined by Olson et 140 al. (2013), severe geomagnetic activity with Kp>6 was not observed. The low F10.7 con- 141 ditions are preferable for the study of SSW effects on the ionosphere. Modeling studies 142 have shown that the ionospheric response to lower atmospheric forcing would be more 143 pronounced under lower solar flux conditions (Fang, Fuller-Rowell, Wang, Akmaev, & 144 Wu, 2014; H.-L. Liu & Richmond, 2013). 145 The EEJ is a narrow band of a zonal electric current that flows along the magnetic 146 equator in the dayside E-region ionosphere at 100-115 km altitude (e.g., Yamazaki & Maute, 147 2017). During geomagnetically quiet periods, day-to-day variations of the EEJ intensity 148 are dominated by the changes in neutral winds at E-region heights associated with at- 149 mospheric waves from the lower layers (Yamazaki et al., 2014), and thus are a good in- 150 dicator of lower-atmospheric influence on the E-region ionosphere. The methods for de- 151 riving the EEJ intensity and equatorial zonal electric field (EEF) from Swarm magnetic 152 field measurements are detailed in Alken, Maus, Vigneron, Sirol, and Hulot (2013). Fig- 153 ure 2a reveals that the EEJ variability was dominated by 6-day variations during this 154 period. The westward phase propagation of the EEJ intensity perturbations with ZW1 155 can also be seen. Similar spatial and temporal variability was found in the equatorial 156 zonal electric field. Figure 2d shows relative changes in the EEF from the time mean. 157 It can be seen that the EEF underwent 6-day variations of ±40% that are out-of-phase 158 for a 180◦ longitudinal separation. The amplitude varies in the range of 20–70% depend- 159 ing on the longitude. In a recent study, Yamazaki, Stolle, Matzka, and Alken (2018) re- 160 ported that the EEJ intensity occasionally shows ∼6-day variations that have charac- 161 teristics of a westward-propagating wave with ZW1. They attributed the EEJ variations 162 to the quasi-6-day wave (Q6DW) that was simultaneously observed in the lower ther- –6– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 163 mosphere. The behavior of the EEJ presented in Figure 2a is similar to those reported 164 by Yamazaki et al. (2018). 165 The Q6DW is a westward-propagating planetary wave with ZW1, which is occa- 166 sionally observed in the middle atmosphere (e.g. Forbes & Zhang, 2017; Hirota & Hi- 167 rooka, 1984; Pancheva, Mukhtarov, & Siskind, 2018; Riggin et al., 2006; Talaat, Yee, & 168 Zhu, 2001, 2002; Wu, Hays, & Skinner, 1994). It is often regarded as the (1,1) Rossby 169 normal mode, which is predicted by classical atmospheric wave theory (Forbes, 1995; Kasa- 170 hara, 1976; Madden, 1979, 2007; Salby, 1984), for its zonal wavenumber, phase speed, 171 and latitudinal structure. The Q6DW can be excited in the troposphere by heating due 172 to moist convection (Miyoshi & Hirooka, 1999). Additionally, the wave can be excited/amplified 173 in the middle atmosphere due to baroclinic/barotropic instability (Lieberman et al., 2003; 174 H.-L. Liu et al., 2004; Meyer & Forbes, 1997). Zonal wind perturbations of the Q6DW 175 are largest around the equator and can be up to a few tens of m/s at E-region heights, 176 which is sufficient to cause detectable changes in dayside ionospheric electric fields and 177 currents (Gan et al., 2016; Miyoshi, 1999; Pedatella, Liu, & Hagan, 2012). These elec- 178 tric field perturbations in the E-region ionosphere are transmitted to the F region along 179 equipotential magnetic field lines, and affect the distribution of low-latitude F-region plas- 180 mas by modulating their E×B plasma drift motions. In this way, the Q6DW can affect 181 the F-region plasma density, as first revealed in the 1990s by ionosonde measurements 182 (e.g., Altadill & Laštovička, 1996; Apostolov, Alberca, & Altadill, 1994; Laštovička, 2006). 183 More recent studies based on global TEC maps have established that the Q6DW effect 184 on the plasma density is largest in the afternoon local time sector near the equatorial 185 ionization anomaly crests (±20◦ magnetic latitudes) (Gu et al., 2014; Gu, Ruan, et al., 186 2018; Qin et al., 2019; Yamazaki, 2018). 187 The 6-day variations can be seen in both electron density (Figures 2b and 2e) and 188 top-side TEC (Figures 2c and 2f) at 20◦ magnetic latitude. (Figure S1 in Supporting 189 Information shows the electron density variations at various latitudes.) The variations 190 are consistent with those in the EEJ/EEF (Figures 2a and 2d), indicating electrodynamic 191 coupling between the E- and F-region ionosphere. The response time of the F-region plasma 192 density to a change in the E-region electric field is 2–4 hours (e.g., Stolle, Manoj, Lühr, 193 Maus, & Alken, 2008; Venkatesh et al., 2015), which would not be visible in the figures. 194 The relative change in the electron density is in the range of 20–40%, which is appre- 195 ciably larger than that of TEC, 5–10%. This is not surprising as the amplitude of the –7– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 196 Q6DW decreases with altitude in the top-side ionosphere, as demonstrated by Gu, Ruan, 197 et al. (2018). 198 The plasma density and TEC data from the ascending parts of the Swarm B or- 199 bit (02:00–23:00 MLT) were also examined, but the 6-day variations were not as evident 200 as the results derived from the descending orbits. Similarly, the ionospheric data (EEJ, 201 electron density, and TEC) from Swarm A, which was flying around 02:00–05:00 MLT 202 (descending orbits) and 14:00–17:00 MLT (ascending orbits), did not show strong 6-day 203 variations. The electron density variations from Swarm B (ascending) and Swarm A (as- 204 cending and descending) are presented in Supporting Information (Figure S2). The dif- 205 ferent behavior of 6-day variations in different Swarm datasets reflects the fact that the 206 ionospheric response to the Q6DW depends on MLT and height, as well as on magnetic 207 latitude (Gu, Ruan, et al., 2018). Further studies are required to determine the three 208 dimensional structure of the 6-day ionospheric variations during this event. 209 Previous studies found a significant contribution of the semidiurnal lunar tide to 210 ionospheric variability during NH SSWs (e.g., Park et al., 2012), but it is not known whether 211 the lunar tide plays an equally important role during SH SSWs. The semidiurnal lunar 212 variations in the EEJ intensity derived from the Swarm A and B data during 5 September– 213 5 October 2019 are presented in Supporting Information (Figure S3). It is found that 214 the amplitude of the EEJ semidiurnal lunar variation is 17.7±2.1 mA/m for Swarm A 215 (14:00–17:00 MLT) and 16.6±2.8 mA/m for Swarm B (11:00–14:00 MLT), which is greater 216 than the climatological value of 9.0±0.4 mA/m as reported by Yamazaki et al. (2017) 217 for September daytime (08:00–16:00 local solar time) conditions. The phase, which is de- 218 fined as the lunar time of maximum, is 10.2±0.2 h for Swarm A and 10.0±0.4 h for Swarm 219 B, which is in good agreement with the climatological value of 10.0±0.1 h. Despite the 220 significant enhancement, the lunar variation accounts for only a small part of the observed 221 EEJ variability (compare Figures 2a and S3). The relative amplitude of the semidiur- 222 nal lunar variation in the top-side electron density is 9.9±0.7% for Swarm A and 11.1±0.1% 223 for Swarm B (also shown in Figure S3). Again, these variations are smaller than the 6- 224 day variations observed during the same period (Figure 2e). 225 It is noted that since Swarm slowly precesses in local solar time, it is not possible 226 to resolve short-term variability of solar tides. Changes in upward-propagating solar tides 227 can occur during SSWs due to changes in the zonal mean atmosphere (Jin et al., 2012; –8– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 228 Pedatella & Liu, 2013), tidal sources (Goncharenko, Coster, Plumb, & Domeisen, 2012), 229 and tidal interaction with PWs (H.-L. Liu, Wang, Richmond, & Roble, 2010; Maute, Ha- 230 gan, Richmond, & Roble, 2014). Possible changes in solar tides during the September 231 2019 SSW remain to be investigated. 232 2.3 Q6DW in the middle atmosphere 233 Traveling PWs in the middle atmosphere are examined using the GPH data from 234 Aura/MLS. The analysis method was described in detail in the previous work (Yamazaki 235 & Matthias, 2019), and thus is only briefly summarized here. The amplitude A and phase 236 φ of waves with period τ were derived by fitting the following formula to the data at a 237 given latitude and height: 238 4 X  As cos 2π s=−4    t + sλ − φs , τ (1) 239 where t is the universal time, λ is the longitude, and s is the zonal wavenumber. Eastward- 240 and westward-propagating waves correspond to s<0 and s>0, respectively. The data were 241 analyzed for each day using a time window that is 3 times the wave period. The 1-σ er- 242 ror in the amplitude is typically below 0.05 km. 243 Figures 3a and 3b show the amplitudes for the westward- and eastward-propagating 244 waves with ZW1 at 45◦ S in the lower thermosphere at ∼97 km. Enhanced wave activ- 245 ity can be seen in the westward-propagating component (Figure 3a) with period 4–7d 246 during September 2019, which can be identified as the Q6DW. It is consistent with the 247 appearance of 6-day variations in the ionosphere (Figures 2a–2c). Such enhanced wave 248 activity is not present in the eastward-propagating ZW1 component (Figure 3b), or other 249 components with higher zonal wavenumbers (not shown here). Although studies have 250 found that the amplitude of the Q6DW in the middle atmosphere is greatest during equinoc- 251 tial months (Forbes & Zhang, 2017; Qin et al., 2019; Yamazaki, 2018), the wave enhance- 252 ment in September 2019 was exceptional, with the maximum amplitude larger than 0.4 253 km in the lower thermosphere, which is much larger than the climatological amplitude 254 (0.15 km) or amplitudes recorded during other individual years during 2004–2018 (Fig- 255 ure 3d). Thus, the large-amplitude Q6DW observed in September 2019 cannot be ex- 256 plained merely as a seasonal effect. 257 The latitude and height structures of the 6-day wave during 10–30 September 2019 258 are presented in Figure 3c. The amplitude and phase were derived at wave period of ex- –9– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 259 actly 6.0 days, so that the phases calculated at different heights and latitudes can be com- 260 pared. In the mesosphere and lower thermosphere (above 50 km), the amplitude struc- 261 ture is symmetric about the equator with peaks at approximately ±45◦ latitudes, and 262 the phase tends to be horizontally uniform with downward phase progression. These fea- 263 tures are in conformity with the theoretically expected Q6DW in the presence of the mean 264 winds and dissipation (e.g., Salby, 1981a, 1981b). Below 50 km, the phase progression 265 is poleward as well as downward, especially in the SH, indicating equatorward and up- 266 ward energy propagation from the high latitude region. Using reanalysis data, Gan, Ober- 267 heide, and Pedatella (2018) demonstrated how the Q6DW generated in the SH high lat- 268 itude can propagate into the NH, growing to be a global mode in the mesosphere and 269 lower thermosphere under September equinox conditions. 270 In Figure 3c, there is a region of locally enhanced amplitudes at 70–80◦S and 20– 271 50 km altitude, which can be regarded as a source of the large-amplitude Q6DW observed 272 above. The amplification of the Q6DW from the seasonal background in this region is 273 depicted in Figure 3e. Enhanced wave activity is observed in the same region over a wide 274 range of wavenumbers (s from -3 to 3) and periods (τ =3–20d) (not shown here). A pos- 275 sible explanation for the wave amplification is baroclinic/barotropic instability (Gan et 276 al., 2018; Lieberman et al., 2003; H.-L. Liu et al., 2004; Meyer & Forbes, 1997), in which 277 waves can rapidly grow by extracting energy from the unstable mean flow. Figure 3f shows 278 that the wave amplification in the polar middle atmosphere is not uncommon around 279 this time of year, but in 2019, it took place at lower altitudes (∼30 km) than in other 280 years (∼50 km). 281 Figures 3h–3j illustrates the development of the atmospheric instability. The ar- 282 eas highlighted by the light-yellow color indicate the regions where the necessary con- 283 dition for barotropic/baroclinic instability is met; that is, the meridional gradient of the 284 quasi-geostrophic potential vorticity is negative (e.g., H.-L. Liu et al., 2004). It can be 285 seen that unstable regions are formed mainly around the edge of the polar vortex due 286 to the strong vertical and horizontal shear in the zonal wind. As the westward mean flow 287 descends to lower layers, the unstable regions at high latitudes (70–80◦S) also move down, 288 and hence exciting/amplifying waves at lower altitudes compared to other years. As these 289 waves propagate equatorward and upward, the amplitude at 45◦ S is greater than other 290 years above ∼40 km (Figure 3g). As numerically demonstrated by Salby (1981b), the 291 vertical growth of amplitude is enhanced where the zonal mean zonal wind is weak and –10– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 292 eastward relative to the phase speed of the wave. The westward phase speed of the Q6DW 293 is ∼55 m/s at 45◦ S and ∼13 m/s at 80◦ S. Thus, the reduced eastward mean flow and 294 the weak wind reversal during the SSW (Figures 3h–3j) provide favorable conditions for 295 the vertical propagation of the Q6DW. Interactions of the Q6DW with tides and grav- 296 ity waves could also affect the vertical structure of the Q6DW (e.g., Forbes, Zhang, Maute, 297 & Hagan, 2018; Meyer, 1999). A better understanding of the Q6DW propagation in the 298 mesosphere and lower thermopshere during the September 2019 SSW would benefit from 299 a more comprehensive analysis of dynamic fields from an atmospheric reanalysis or gen- 300 eral circulation model. 301 For the NH, possible influence of SSWs on the vertical propagation of traveling plan- 302 etary waves in the middle atmosphere has been discussed in a number of studies (e.g., 303 Gu, Dou, Pancheva, Yi, & Chen, 2018; Hirooka & Hirota, 1985; Matthias, Hoffmann, Rapp, 304 & Baumgarten, 2012; Pancheva et al., 2008; Sassi, Garcia, & Hoppel, 2012; Yamazaki 305 & Matthias, 2019). In some cases, a strong Q6DW was observed during an SSW (e.g., 306 Gong et al., 2018; Pancheva et al., 2018) but in general, there is no one-to-one correspon- 307 dence between the occurrence of SSW and Q6DW enhancement in the NH (Yamazaki 308 & Matthias, 2019). Modeling studies also found enhanced Q6DW activity following some 309 SSWs, which has been attributed to barotropic/baroclinic instability in the NH high lat- 310 itude (Chandran, Garcia, Collins, & Chang, 2013; Tomikawa et al., 2012). For the SH, 311 studies are few because of infrequent occurrence of SSWs. Dowdy et al. (2004) and Espy, 312 Hibbins, Riggin, and Fritts (2005) observed a westward-propagating planetary wave with 313 ZW1 and period around 14d at 70–100 km altitude during the September 2002 Antarc- 314 tic SSW. The present study finds a strong response of the Q6DW in the mesosphere and 315 lower thermosphere during the September 2019 Antarctic SSW. It is possible that the 316 response of traveling planetary waves to Antarctic SSWs varies from event to event. More 317 studies are needed to clarify this point. 318 3 Summary and Conclusions 319 An SSW occurred in the Southern Hemisphere in September 2019. Although it was 320 a minor warming, it involved an exceptionally strong wave-1 planetary wave and a large 321 polar temperature enhancement by 50.8 K/week. The event also took place under so- 322 lar minimum conditions, which is preferable for studying the ionospheric response. Ear- –11– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 323 lier studies focused on the effect of Northern-Hemisphere SSWs on the ionosphere, and 324 few studies investigated Southern-Hemisphere cases. 325 The analysis of ionospheric data from ESA’s Swarm mission during the Septem- 326 ber 2019 SSW reveals prominent 6-day variations in the dayside low-latitude region, in- 327 cluding 20–70% variations in the equatorial zonal electric field, 20–40% variations in the 328 top-side electron density, and 5–10% variations in the top-side total electron content. These 329 variations are attributed to the Q6DW simultaneously observed in the middle atmosphere. 330 Evidence is also found for enhanced lunar tidal perturbations in the ionosphere, but their 331 amplitudes are relatively small (e.g., less than 15% in the top-side electron density). 332 The amplitude of the Q6DW in the lower thermosphere is more than 0.4 km in geopo- 333 tential height, which is found to be the largest observed by Aura/MLS in the Southern 334 Hemisphere since August 2004, and thus cannot be explained merely as a seasonal ef- 335 fect. The latitudinal and vertical structures of the Q6DW suggest that the waves are ex- 336 cited/amplified in the polar region at 30–40 km altitude, where the atmosphere is un- 337 stable due to strong vertical shear in the zonal wind connected with planetary-wave break- 338 ing. As the Q6DW grows in the vertical, the wave attains large amplitudes in the lower 339 thermosphere, which drives ionospheric variability. 340 These results suggest that a Southern-Hemisphere SSW can lead to ionospheric vari- 341 ability by altering middle atmosphere dynamics and propagation characteristics of large- 342 scale waves from the middle atmosphere to the upper atmosphere. 343 Acknowledgments 344 We thank the NASA Goddard Earth Sciences (GES) Data and Information Services Cen- 345 ter (DISC) (https://disc.gsfc.nasa.gov/) for making the Aura/MLS geopotential height 346 data and MERRA-2 data available. We also thank the European Space Agency (ESA) 347 for providing the Swarm data (http://earth.esa.int/swarm). The geomagnetic activity 348 index Kp was provided by the GFZ German Research Centre for Geosciences (https://www.gfz- 349 potsdam.de/en/kp-index/). The solar activity index F10.7 was downloaded from the SPDF 350 OMNIWeb database (https://omniweb.gsfc.nasa.gov). 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Quasi-6-day wave effects on the equatorial ionization anomaly –22– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters 683 over a solar cycle. 684 9881–9892. 685 Journal of Geophysical Research: Space Physics, 123 (11), Yamazaki, Y., & Matthias, V. (2019). Large-amplitude quasi-10-day waves in 686 the middle atmosphere during final warmings. 687 search: Atmospheres, 124 (17-18), 9874-9892. 688 agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019JD030634 689 10.1029/2019JD030634 690 Yamazaki, Y., & Maute, A. (2017). Journal of Geophysical ReRetrieved from https:// doi: Sq and eej—a review on the daily variation 691 of the geomagnetic field caused by ionospheric dynamo currents. Space Science 692 Reviews, 206 (1-4), 299–405. 693 Yamazaki, Y., Richmond, A., Maute, A., Liu, H.-L., Pedatella, N., & Sassi, F. 694 (2014). 695 periods. Journal of Geophysical Research: Space Physics, 119 (8), 6966–6980. 696 On the day-to-day variation of the equatorial electrojet during quiet Yamazaki, Y., Richmond, A., & Yumoto, K. (2012). Stratospheric warmings and the 697 geomagnetic lunar tide: 1958–2007. 698 Physics, 117 (A4). 699 Yamazaki, Y., Stolle, C., Matzka, J., & Alken, P. 700 ulation of the equatorial electrojet. 701 Physics, 123 (5), 4094–4109. 702 Journal of Geophysical Research: Space (2018). Quasi-6-day wave mod- Journal of Geophysical Research: Space Yamazaki, Y., Stolle, C., Matzka, J., Siddiqui, T. A., Lühr, H., & Alken, P. (2017). 703 Longitudinal variation of the lunar tide in the equatorial electrojet. Journal of 704 Geophysical Research: Space Physics, 122 (12), 12–445. 705 Yue, X., Schreiner, W. S., Lei, J., Rocken, C., Hunt, D. C., Kuo, Y.-H., & Wan, W. 706 (2010). 707 the January 2009 stratospheric sudden warming event. Journal of Geophysical 708 Research: Space Physics, 115 (A11). 709 710 Global ionospheric response observed by COSMIC satellites during Zhang, X., & Forbes, J. M. (2014). Lunar tide in the thermosphere and weakening of the northern polar vortex. Geophysical Research Letters, 41 (23), 8201–8207. –23– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters Figure 1. Overview of middle atmosphere dynamics during the September 2019 sudden stratospheric warming. (a) Stratospheric polar temperature at 10 hPa obtained from the MERRA-2 reanalysis. The thick black line represents the data for 2019, while the thin purple lines correspond to the data for other years during 1980–2018, among which the data for 2002 are highlighted by green for the occurrence of a major SSW. The red dashed line shows the climatological mean. (b) Zonal mean zonal wind at 60◦ S derived from the geopotential height (GPH) measurements by the Aura Microwave Limb Sounder (MLS) using the method described by Matthias and Ern (2018). (c) Amplitude of the planetary wave with Zonal Wavenumber (ZW) 1 and ZW2 at 65◦ S and 48 km altitude from the Aura/MLS GPH. The red and blue solid lines represent ZW1 and ZW2 waves, respectively. The climatological amplitudes of the ZW1 and ZW2 waves are indicated by the dashed lines with corresponding colors. The gray shaded area show the range between the maximum and minimum values of the amplitude of the ZW1 wave observed by Aura/MLS since August 2004. –24– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters Figure 2. Overview of ionospheric variations during 5 September–5 October 2019. (a) Lon- gitude versus time plot of the equatorial electrojet (EEJ) intensity derived from magnetic field measurements in the descending orbits of Swarm B. The data are smoothed using a 3-day and 50◦ -longitude window. (b) Same as (a) except for the electron density at 20◦ magnetic latitude. (c) Same as (a) except for the total electron content (TEC) at 20◦ magnetic latitude at the satellite altitude of ∼510 km. (d) Percent changes in the Swarm B zonal equatorial electric field (EEF) at ±90◦ longitudes with respect to the time mean at the corresponding longitudes. (e) Same as (d) except for the Swarm B electron density at 20◦ magnetic latitude. (f) Same as (d) except for the Swarm B TEC at 20◦ magnetic latitude. (g) Magnetic local time (MLT) at equatorial crossings for the descending orbits of Swarm B. (h) Geomagnetic activity index Kp. (i) Solar activity index F10.7 . –25– ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. manuscript submitted to Geophysical Research Letters Figure 3. Overview of quasi-6-day wave (Q6DW) activity during the September 2019 SSW as derived from the geopotential height (GPH) measurements by the Aura Microwave Limb Sounder (MLS). (a) Amplitude of the westward-propagating Zonal Wavenumber (ZW) 1 waves at 45◦ S and 97 km altitude. (b) Same as (a) except for the eastward-propagating ZW1 waves. (c) Latitude versus height structures of the westward-propagating ZW1 wave with period 6.0d during 10–30 September 2019. The contour lines indicate the amplitude while the color represents the phase. (d) Amplitude of the Q6DW, defined here as the maximum amplitude of the westwardpropagating ZW1 waves at periods 5–7d, at 45◦ S and 97 km altitude. (e) Same as (d) except at 80◦ S and 32 km altitude. (f) Vertical structure of the Q6DW at 80◦ S during 10–30 September 2019. (g) Same as (f) except at 45◦ S. (h–j) Latitude versus height structures of the zonal mean zonal wind. The areas highlighted by the light-yellow color indicate the regions where the –26– meridional gradient of the quasi-geostrophic potential vorticity is negative, which is the necessary condition for barotropic/baroclinic instability. ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. Figure 1. ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. Figure 2. ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed. Figure 3. ESSOAr | https://doi.org/10.1002/essoar.10501316.1 | Non-exclusive | First posted online: Thu, 12 Dec 2019 22:03:19 | This content has not been peer reviewed.