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manuscript submitted to Geophysical Research Letters
Variations of Low-latitude Thermospheric Winds and1
Temperature during the 2020/2021 Major Sudden2
Stratospheric Warming as Observed by ICON and3
GOLD Satellites4
Erdal Yi˘git1, Ayden L. S. Gann 1, Alexander S. Medvedev2, Federico5
Gasperini3, Md Nazmus Sakib1, Qian Wu4,5
6
1George Mason University, Department of Physics and Astronomy, Space Weather Lab, Fairfax, VA,7
USA.8
2Max Planck Institute for Solar System Research, G¨ottingen, Germany.9
3Orion Space Solutions, Louisville, CO, USA10
4High Altitude Observatory, NCAR, Boulder, CO, USA11
5COSMIC Program UCAR/UCP, Boulder, CO, USA12
Key Points:13
•Effects of the 2020/2021 SSW on thermospheric winds and temperature are stud-14
ied using ICON and GOLD satellites15
•Thermospheric mean winds undergo substantial changes during the SSW, some16
changes occurring before the warming onset.17
•The low-latitude thermosphere cools around 150 km during the SSW, with a cool-18
ing trend starting before the warming onset.19
Corresponding author: Erdal Yi˘git, eyigit@gmu.edu, May 22, 2023
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manuscript submitted to Geophysical Research Letters
Abstract20
Using ICON and GOLD satellite observations, the response of the thermospheric day-21
time horizontal winds and neutral temperature to the 2020/2021 major sudden strato-22
spheric warming (SSW) is studied at low- to middle latitudes (0◦- 40◦N). Comparison23
with observations during the non-SSW winter of 2019/2020 and the pre-SSW period (De-24
cember 2020) clearly demonstrates the SSW-induced changes. The northward and west-25
ward thermospheric winds are enhanced during the warming event, while temperature26
around 150 km drops by up about 50 K compared to the pre-SSW phase. Changes in27
the horizontal circulation during the SSW can generate upwelling at low-latitudes, which28
can contribute to the adiabatic cooling of the low-latitude thermosphere. The observed29
changes during the major SSW are a manifestation of long-range vertical coupling in the30
atmosphere.31
Plain Language Summary32
NASA’s ICON and GOLD satellites are used to determine to what extent the 2020/202133
major sudden stratospheric warming (SSW) influenced the thermosphere above 90 km.34
Observations show that the horizontal circulation becomes more westward and poleward35
and the temperature cools by up to 50 K during the warming event. Changes in the strato-36
spheric circulation during the major SSW modulate the upward propagation of atmo-37
spheric waves of various scales. These altered waves can reach the thermosphere, inter-38
act with the background atmosphere and induce upward motions at low-latitudes, thus39
explaining, to some degree, the significant cooling observed by GOLD. Our observations40
provide evidence for SSW-induced long-range vertical coupling in the atmosphere.41
1 Introduction42
Sudden stratospheric warmings (SSWs) are remarkable phenomena that occur in43
the polar lower stratosphere (mostly in the Northern hemisphere) during winters and last44
for several days. Although five types of warmings are currently distinguished – major,45
midwinter, minor, final, and Canadian (Butler et al., 2015), – they often are categorized46
as either major or minor events. In a major warming, the zonal mean winds ¯uat 60◦N47
reverse their direction from eastward to westward at or below 10 hPa (∼30 km) and the48
zonal mean temperature ¯
Tincreases poleward of 60◦N. During a minor warming, the zonal49
mean temperature increases poleward of 60◦N, while the eastward zonal mean winds weaken50
but do not fully reverse. SSWs are caused by large-scale planetary waves propagating51
upward from the troposphere and interacting with the stratospheric mean flow (Holton,52
1976; Matsuno, 1971).53
The dynamical and thermodynamical effects of SSWs are wide-reaching and include54
not only the troposphere-stratosphere coupling, but extend across all layers from the tro-55
posphere to the thermosphere and ionosphere (Yi˘git & Medvedev, 2015; Miyoshi et al.,56
2015; Goncharenko et al., 2021). While the peak of temperature increase occurs over the57
pole (usually at North), these events produce changes across the hemisphere that last58
for several weeks. The lower and middle atmospheric effects of SSWs have been exten-59
sively studied (Siskind et al., 2010; Gu et al., 2020; Roy & Kuttippurath, 2022), how-60
ever the response of the upper atmosphere to SSWs is understood to a lesser degree. Nev-61
ertheless, an increasing amount of modeling efforts and observations provided a solid frame-62
work for characterizing the SSW effects in the upper atmosphere (Goncharenko et al.,63
2021; Kouck´a Kn´ıˇzov´a et al., 2021).64
A variety of observational and modeling techniques have been used to quantify the65
response of the thermosphere-ionosphere to SSWs. Sudden warmings affect both the mean66
state and variability of thermospheric temperatures and horizontal winds at various scales,67
as simulated by general circulation models (GCMs) (Miyoshi et al., 2015; Liu et al., 2013).68
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Observations demonstrated a persistent connection between the 2009 major SSW and69
the ionospheric variations at low-latitudes (Goncharenko, Chau, et al., 2010). They re-70
vealed that the SSW-induced changes in the ionosphere increase the latitudinal asym-71
metry of the equatorial ionization anomaly (Azeem et al., 2015). Studies of ionospheric72
variations with ground-based measurements by digisondes at midlatitudes showed that73
the peak electron density around the F2region and TEC increased during an SSW (Moˇsna74
et al., 2021).75
Gravity (buoyancy) waves (GW) and solar tides of various scales propagate directly76
from the lower atmosphere to the thermosphere producing multi-scale coupling and in-77
fluence the general circulation and temperature structure of the upper atmosphere (Yi˘git78
& Medvedev, 2009; Miyoshi & Fujiwara, 2008; Heale et al., 2014; Gavrilov & Kshevet-79
skii, 2015; Gasperini et al., 2022; Pancheva et al., 2009). SSWs alter the propagation and80
dissipation of atmospheric waves in the whole atmosphere system (Yi˘git & Medvedev,81
2016). While during minor warmings GW activity increases in the thermosphere (Yi˘git82
& Medvedev, 2012; Yi˘git et al., 2014), during a major warming, GW activity in the iono-83
sphere can slightly increase in the early phase, but ultimately decreases in the main phase84
of the warming, as demonstrated by GPS-TEC analysis (Nayak & Yi˘git, 2019). Also,85
high resolution GCMs show that the total GW energy and the associated drag decrease86
in the thermosphere above 110 km (Miyoshi et al., 2015), while observations show that87
nonmigrating tides amplify in the middle atmosphere (Pancheva et al., 2009). More re-88
cently, analysis of the ICON observations between 93–106 km indicated that the semid-89
iurnal tidal and 3-day ultra-fast Kelvin wave activity contribute to the structure of the90
mean meridional circulation in the upper mesosphere and lower thermosphere (MLT)91
(Gasperini et al., 2023).92
Planetary wave amplification with subsequent breaking and changes in GW dynam-93
ics can significantly modify the stratospheric and mesospheric circulation and temper-94
ature during major SSWs (Siskind et al., 2010, 2005; Gavrilov et al., 2018; Gu et al., 2020;95
Koval et al., 2021). The impact of SSWs on the thermospheric winds, circulation, and96
temperature has been insufficiently explored, due primarily to limited coverage in ob-97
servations. In this paper, we use ICON and GOLD horizontal wind and temperature mea-98
surements for characterizing the impact of the major 2020/2021 SSW on the low-latitude99
thermosphere. This is the first observational study that reports on coincident measure-100
ments of wind and temperature above 120 km during the major warming event, which101
commenced on 1st January 2021, peaked on 5th January 2021 and lasted for a few weeks.102
2 Observations and Data Analysis103
We employ the measurements of horizontal winds by ICON (Immel et al., 2018)104
and of temperature by GOLD satellites (Eastes et al., 2017). Specifically, we consider105
the ICON/MIGHTI version 5 zonal and meridional winds based on green line measure-106
ments along with the GOLD neutral temperatures obtained from Level 2 (L2) Tdisk ver-107
sion 4 data. ICON observes the thermosphere at low- to midlatitudes (∼10◦S - 40◦N).108
Characterization of the mean horizontal winds and the associated circulation by ICON/MIGHTI109
for the Northern Hemisphere summer solstice has recently been performed in the work110
by Yi˘git et al. (2022). GOLD measures the Far Ultraviolet (FUV) spectrum of Earth’s111
atmosphere at geostationary orbit, from 0610 to 0040 Universal Time (UT) every day,112
providing, among others, daytime thermospheric temperatures near 150 km at low- and113
midlatitudes (0◦to ±60◦), depending on the solar zenith angle (see Section 1 in SI for114
further information).115
We first characterize the SSW in the stratosphere based on the MERRA-2 reanal-116
ysis data output every three hours and compare with a non-SSW winter. Figure 1 shows117
the evolution of the December 2020–January 2021 major SSW at 10 hPa (30 km) in terms118
of the zonal mean temperature ¯
Tand zonal wind ¯u(red lines). They are compared with119
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those for the non-SSW winter of December 2019–January 2020 (black lines). The tem-120
perature is plotted at 60◦N and 90◦N and the zonal wind at 35◦N and 60◦N. Figure 1121
demonstrates that, after the onset of the warming on 1 January 2021, the Northern po-122
lar temperature increases by 50 K – from about 200 K to 250 K, peaking on 5 January123
2021. During the ascending phase of the warming, ¯uat 60◦N gradually changes to west-124
ward – from ∼30 m s−1at the onset of the SSW to about –10 m s−1at the peak phase,125
demonstrating a reversal of the mean flow direction. The recovery phase of the SSW is126
relatively long, during which the temperatures remain elevated and ¯uis westward com-127
pared to the pre-SSW period in December 2020 and during the non-SSW season in Jan-128
uary 2021. It is noticeable that the December 2019 (non-SSW winter) and December 2020129
exhibit some minor differences in mean winds, owing, partially, to interannual variations130
in planetary wave activity and behavior of large-scale internal waves.131
For the thermospheric data from both instruments, we selected a period centered132
around the onset of the SSW, i.e., from 6 December 2020 to 26 January 2021 (hereafter133
called “SSW winter”). The results are compared to those for the non-SSW winter (6 De-134
cember 2019 to 26 January 2020). While between ∼90-109 km both daytime and night-135
time data are available, only daytime winds are available above ∼109 km up to about136
210 km. Therefore, we use only daytime winds from 90-200 km to produce a uniform anal-137
ysis of the mean wind variations.138
The solar and geomagnetic activity were relatively low during the studied periods,139
with somewhat higher solar activity during the SSW winter (F10.7∼75−90 W m−2Hz−1
140
vs F10.7∼70 −75 W m−2Hz−1for the non-SSW period). The magnetic activity, al-141
though generally low, exhibits some degree of day-to-day variability, reaching occasion-142
ally Ap∼12 (Kp∼3−) (see Section 2 and Figure S3 in Supporting Information for143
details). In order to reduce the impact of these elevated space weather conditions on our144
analysis of temperature variations, we have excluded geomagnetically disturbed days with145
Ap>7 (Kp>2o) in temperature plots.146
3 Results and Discussion147
3.1 Observations of Thermospheric Horizontal Winds148
In order to assess changes in the thermospheric winds induced by the major SSW,149
we consider ICON/MIGHTI measurements for two periods with a common spatiotem-150
poral coverage. Figure 2 presents the evolution of the daytime zonal mean horizontal151
winds during the SSW (December 2020 – January 2021) and non-SSW winters (Decem-152
ber 2019 – January 2020) at two representative latitude bands: at low-latitude (0 - 20◦N)153
and low- to midlatitude (20◦- 40◦N) regions. Altitudes and days, for which observations154
are not available, are shown in gray shading.155
Even without an SSW, the observed thermospheric horizontal winds exhibit a sig-156
nificant degree of day-to-day variability. This could be related to a combination of phys-157
ical processes, such as a) changes in the dynamics of internal atmospheric waves, b) vari-158
ability of the solar and geomagnetic activity, and c) orbital effects, e.g., ICON’s orbit159
precession toward earlier local times by about 29.8 min every day (see Figure S1 and Sec-160
tion 1 in Supporting Information). Under the non-SSW conditions (during the non-SSW161
winter and before the onset of the warming), the daytime mean zonal winds exhibit an162
alternating with altitude pattern at low- and low- to midlatitudes: typically eastward163
in the upper mesosphere, westward in the lower thermosphere and eastward again above164
∼140 km. Above ∼160 km, the westward flow dominates, in general. The mean daytime165
meridional winds without an SSW are overall northward (representing the summer-to-166
winter circulation) in the upper mesosphere, southward (winter-to-summer transport)167
in the lower thermosphere, and poleward again above ∼130 km (Figure 2c,g).168
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manuscript submitted to Geophysical Research Letters
Predominantly westward GWs surviving the winter eastward stratomesospheric jets169
are responsible for shaping the circulation in the MLT (Yi˘git et al., 2009). The associ-170
ated westward GW momentum deposition reverses the meridional winds in the winter171
MLT, thereby also reversing the mean zonal winds from eastward to westward (Lilienthal172
et al., 2020; Yi˘git et al., 2021, 2022). The measured wind reversals provide an indirect173
observational evidence for the momentum transport carried by upward propagating in-174
ternal waves, in the absence of which, the MLT would remain in radiative balance (Andrews175
et al., 1987) and the eastward and summer-to-winter meridional flow would dominate176
in the Northern Hemisphere. The momentum forcing is also supplemented by upward177
propagating diurnal and semidiurnal tides at low- and middle-latitudes, respectively (Griffith178
et al., 2021; Miyoshi & Yi˘git, 2019; Jones et al., 2019).179
After the onset of the warming in January 2021 (Figures 2b,d,f,h), westward (neg-180
ative) and northward (positive) winds relatively strengthen depending on the altitude181
and day, especially above 140 km. There is an indication that the thermospheric winds182
begin to change before the start of the SSW, which can probably be related to the fact183
that the stratospheric mean zonal wind decrease precedes the polar temperature rise by184
several days (Figure 1b). This phenomenon is known to modulate upward gravity wave185
propagation (Yi˘git & Medvedev, 2012; Miyoshi et al., 2015).186
3.2 Observations of Thermospheric Temperature187
Figure 3 presents the day-to-day evolution of the daytime neutral temperatures188
near 150 km measured by GOLD and averaged zonally and over the same two represen-189
tative latitude bands discussed above. Based on GOLD’s coverage, only longitudes be-190
tween 100◦W and 10◦E contributed to the zonal mean. The upper two rows (Figures 3a,b,c,d)191
show the temperature variations as a function of solar zenith angle χ. Note that the two192
latitude bands have different χcoverage. The observations for 25◦<χ<65◦contributed193
to the low-latitude 0 - 20◦N band, with a larger portion of measurements centered around194
65◦. The low- to midlatitude (20◦- 40◦N) band includes observations for χbetween 45◦
195
and 65◦, with a larger portion taken around χ= 55◦. Rows three and four (Figures 3e,f)196
display another aspect of temperature variations: the latitude-time cross-sections at 150197
km during the non-SSW and SSW winters, respectively. It is seen that, at all latitudes198
and solar zenith angles, thermospheric temperatures drop during the SSW. The cooling199
trend begins shortly before the SSW onset and lasts for about 15 days. The thermospheric200
cooling is more clearly seen in Figure 4, which presents the day-to-day variations of the201
average temperature in the corresponding latitude bands. The error bars indicate the202
variability around a fitted linear trend (see Section 1 in SI for further information). Start-203
ing a few days before the onset of the SSW, the thermospheric temperature decreases204
by about 50 K, from ∼730 K to 680 K, after which it returns back to ∼720 K over about205
ten days. Such cooling trend is untypical in the low-latitude thermosphere in the absence206
of SSWs, as a comparison with the non-SSW winter shows. It is also seen that the ther-207
mosphere is much colder during the non-SSW winter, because it coincided with the so-208
lar minimum.209
3.3 Possible Mechanisms of Thermal Changes and Connections to Winds210
in the Low-Latitude Thermosphere211
Observations presented above demonstrate a global response of the low- to middle-212
latitude thermosphere to the SSW event. Generally, winds and neutral temperature are213
affected by a number of physical processes pertaining to external (space weather, or cou-214
pling from above) and internal forcing (coupling from below) (Yi˘git et al., 2016). Orig-215
inated in the troposphere and lower stratosphere, SSWs represent remarkable disturbances216
of the latter type, which rapidly disrupt vertical propagation of atmospheric waves that217
can directly propagate to thermospheric altitudes. A number of observational and mod-218
eling studies found that the thermospheric GW activity decreases after a major warm-219
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ing is fully developed (Nayak & Yi˘git, 2019; Miyoshi et al., 2015). On the other hand,220
the amplitude of the migrating Sun-synchronous semidiurnal tide increases during SSWs221
in the low- and midlatitude lower and upper thermosphere (Goncharenko, Coster, et al.,222
2010; Liu et al., 2013; Oberheide, 2022). These two changes can be related, because GWs223
are known to attenuate the semidiurnal tide in the thermosphere (Miyoshi & Yi˘git, 2019).224
Semidiurnal tidal sources can also be modulated owing to a redistribution of the strato-225
spheric ozone. Thus, the modified wave forcing can directly affect the residual circula-226
tion in the thermosphere (Koval et al., 2021). Systematic modeling studies are required227
for isolating the effects of gravity waves and semidiurnal tides (and their possible inter-228
actions) during stratospheric warmings.229
SSW-induced thermal and dynamical changes are intimately connected. In addi-230
tion to direct wave forcing, they can be caused by modification of the large-scale flow.231
Divergence and convergence of horizontal winds are a source of vertical motions (Rishbeth232
et al., 1969) and of the associated adiabatic heating/cooling. Using simulations with a233
whole atmosphere model, Liu et al. (2013) reported a net cooling of the thermosphere234
above 100 km during the 2008/2009 major SSW, which is qualitatively in agreement with235
our observations. A net upwelling and enhanced poleward flow initiated by SSW-induced236
changes can account for the observed cooling in the low-latitude thermosphere around237
150 km.238
Finally, a subtle decrease of solar activity (from 85 to 75 ×10−22 W m−2s−1) over239
the SSW period (see Figure S3) can contribute to some extent to the observed 50 K tem-240
perature drop around 150 km. Tests with the NRLMSIS empirical model (Picone et al.,241
2002) suggest that a reduction of the solar activity by 10 F10.7radio flux units changes242
temperature by only 5–10 K around 150 km altitude (not shown). Obviously, more ac-243
curate and self-consistent estimates can be obtained using whole atmosphere general cir-244
culation modeling.245
4 Summary & Conclusions246
Combining ICON and GOLD satellite observations, we have explored the impact247
of the 2020/2021 major sudden stratospheric warming (SSW) on the thermospheric hor-248
izontal circulation between 90 and 200 km and temperatures around 150 km. Wind and249
temperature variations during the SSW have been compared to the pre- and non-SSW250
periods. The main inferences of our study are as follows:251
1. Horizontal winds exhibit a significant degree of day-to-day variability during all252
times, which are related to a combination of orbital changes (e.g., day-to-day change253
in local time coverage) and physical and dynamical processes.254
2. Low- to midlatitude zonal winds are typically eastward in the upper mesosphere;255
reverse their direction to westward in the lower thermosphere, and change again256
to eastward above ∼120 km. Above ∼160 km, the westward flow dominates, in257
general. Mean daytime meridional winds are overall northward (poleward, rep-258
resenting the summer-to-winter transport) in the upper mesosphere, southward259
(equatorward, or winter-to-summer flow) in the lower thermosphere, and poleward260
again above ∼130 km.261
3. After the onset of the warming, westward and northward winds strengthen depend-262
ing on the altitude and day, especially above 140 km. There is an indication that263
the thermospheric winds begin to change before the start of the SSW.264
4. The low-latitude thermosphere cools down during the SSW by about 50 K. The265
cooling trend starts about 7-10 days before the onset of the warming in the strato-266
sphere and lasts for about two weeks. The recovery phase of the temperature takes267
about about ten days.268
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manuscript submitted to Geophysical Research Letters
5. SSW-induced thermal and dynamical changes are intimately connected. The ob-269
served temperature drop in the thermosphere is likely caused by adiabatic cool-270
ing associated with changes in the large-scale horizontal flow.271
Data Availability Statement272
The MIGHTI horizontal wind data (version 5) used in this study are available at273
the ICON data center (https://icon.ssl. berkeley.edu/Data). The GOLD level 2 data used274
in this study are available at the GOLD Science Data Center (https://gold.cs.ucf275
.edu/search/) and at NASA’s Space Physics Data Facility (https://spdf.gsfc.nasa276
.gov/pub/data/gold/level2/tdisk).277
Acknowledgments278
This work was supported by NASA (Grant 80NSSC22K0016). FG acknowledges sup-279
port from NASA GIGI Grant 80NSSC22K0019. ICON is supported by NASA’s Explor-280
ers Program through contracts NNG12FA45C and NNG12FA42I. MNS was supported281
by the National Science Foundation under Grant No. 1849014282
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Figure 1. Variation of the zonal mean (a) temperature and (b) zonal winds at 10 hPa (∼30
km) based on MERRA-2; during the 2019/2020 non-SSW winter (black) and 2020/2021 SSW
winter (red). The vertical green dashed lines on the day zero marks the onset of the major warm-
ing (i.e. 1 January 2021). Mean temperature is shown at the North Pole and at 60◦N; the mean
zonal winds are shown at 35◦and 60◦N for both winters.
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Figure 2. Contour plots of the daytime mean zonal winds (upper two rows) and meridional winds (lower two rows) in m/s during
the non-SSW winter (December 2019-January 2020, first and third rows) and SSW winter (December 2020-January 2021, second and
fourth rows) plotted from 6 December to 26 January at two latitude bands, 0-20◦N (left column) and 20-40◦N (right column). The same
color scales are used for both zonal and meridional winds. Red/blue shadings (positive/negative values) represent eastward/westward
winds. The vertical black dashed lines mark the onset of the warming (1 January 2021), where the warming onset is also marked in non-
SSW winter plots for comparison. Gray shading designates data gaps.
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Figure 3. Contour plots of daytime neutral temperatures in K during the non-SSW winter (December 2019-January 2020, first
and third rows) and SSW winter (December 2020-January 2021, second and fourth rows) plotted from 6 December to 26 January.
Panels a,b,c,d are plotted with respect to the solar zenith angle (SZA) for two latitude bands, 0-20◦N (left column) and 20-40◦N
(right column). Panels e,f are presented as a function of latitude. The light grey shading represents the removed days with Apin-
dex greater than 7. The white shading represents missing data.
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Figure 4. Variation of neutral temperature at different latitude bands. Both SSW and non-SSW winters’ tempera-
ture average over the respective latitude bands – blue/orange colors (0-20◦N/20-40◦N) represent the non-SSW winter,
and green/pink colors (0-20◦N/20-40◦N) represent the SSW winter. The vertical black dashed lines mark the onset of
the warming (1 January 2021). Warming onset is also marked in non-SSW winter for comparison. Error bars are ±σof
regression residuals.
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