The impact of a major sudden stratospheric warming (SSW) in the
Arctic in February 2018 on the midlatitude mesosphere is investigated by
performing the microwave radiometer measurements of carbon monoxide (CO) and
zonal wind above Kharkiv, Ukraine (50.0
Major sudden stratospheric warming (SSW) events, which happen roughly every
other year in the North Pole region, are produced by strong planetary wave
activity according to the model developed by Matsuno (1971) which is
supported by numerous observations (Alexander and Shepherd, 2010;
Kuttippurath and Nikulin, 2012; Tao et al., 2015). A major SSW event is
accompanied by a sharp increase in the stratosphere temperature up to 50 K
and the reversal of the zonal wind from climatological westerlies to
easterlies over a period of several days (Charlton and Polvani, 2007;
Chandran and Collins, 2014; Hu et al., 2014; Tripathi et al., 2016; Butler
et al., 2017; Karpechko et al., 2018; Taguchi, 2018; Rao et al., 2018). The
primary definition of a SSW event provided by the World Meteorological
Organization requires a stratosphere temperature increase and an
accompanying zonal wind reversal to easterlies at the 10 hPa pressure level
(approximately 30 km altitude) and 60
The source of the SSW is planetary wave activity born in the troposphere that propagates upward through the tropopause to the stratosphere (Matsuno, 1971; Alexander and Shepherd, 2010; Butler et al., 2015). The enhanced wave activity results in the rapid warming of the polar stratosphere and the breakdown of the stratospheric polar vortex (Matsuno, 1971; de la Torre et al., 2012; Chandran and Collins, 2014; Pedatella et al., 2018). The important feature of a SSW event is its impact on lower altitudes, when temperature and wind anomalies descend downward into the high- and midlatitude troposphere during the following weeks to month and influence the surface weather (Baldwin and Dunkerton, 2001; Zhou et al., 2002; Butler et al., 2015; Yu et al., 2018). The major SSW events may also impact the atmospheric composition of the whole Northern Hemisphere (NH) stratosphere including midlatitudes (Solomon et al., 1985; Allen et al., 1999; Tao et al., 2015).
During the SSW, vertical coupling covers not only the troposphere but extends upward to the mesosphere. Mesospheric responses to the SSW are observed as enhancement in planetary wave amplitude, zonal wind reversal, and significant air cooling (Shepherd et al., 2014; Zülicke and Becker, 2013; Stray et al., 2015; Zülicke et al., 2018); substantial depletion of the metal layers (Feng et al., 2017; Gardner, 2018); and mesosphere-to-stratosphere descent of trace species (Manney et al., 2009; Salmi et al., 2011). The SSW events are also accompanied by the rapid descent of the stratopause into the stratosphere at the SSW onset, followed by formation of the elevated stratopause in the lower mesosphere and gradual stratopause lowering toward its typical position in the SSW recovery phase (Manney et al., 2009; Chandran et al., 2011; Salmi et al., 2011; Tomikawa et al., 2012; Limpasuvan et al., 2016; Orsolini et al., 2010, 2017). The elevated stratopause events provide evidence for the coupling between the stratosphere and the mesosphere.
Among the trace gases, the CO molecule is a good tracer of winter polar vortex dynamics in the upper stratosphere and mesosphere due to its long photochemical lifetime (Solomon et al., 1985; Allen et al., 1999; Rinsland et al., 1999; Shepherd et al., 2014). The CO mixing ratio generally increases with height in the upper stratosphere and mesosphere and increases with latitude toward the winter pole. This is due to the mean meridional circulation which transports CO from the source region in the summer hemisphere and the tropics to the extratropical winter mesosphere and stratosphere (Shepherd et al., 2014). Therefore, large abundances of CO appear in the winter polar regions under conditions of large-scale planetary wave activity. Downward meridional transport causes descent of CO between the mesosphere and stratosphere and this process is sensitive to planetary wave amplitudes, and particularly the wave amplitude changes that occur during SSWs (Rinsland et al., 1999; Manney et al., 2009; Kvissel et al., 2012). Due to the large-scale descent, high CO values of mesospheric origin are observed at stratospheric altitudes down to 25–30 km (Engel et al., 2006; Huret et al., 2006; Funke et al., 2009). At NH midlatitudes, CO also exhibits significant variability during periods of planetary wave activity associated with SSWs, when the polar vortex splits and displaces off the pole (Solomon et al., 1985; Allen et al., 1999; Funke et al., 2009).
Recent atmospheric models are being extended up to 80–150 km and are used for the study of SSWs (de la Torre et al., 2012; Chandran and Collins, 2014; Shepherd at al., 2014; Limpasuvan et al., 2016; Newnham et al., 2016). For example, de la Torre et al. (2012) applied the Whole Atmosphere Community Climate Model (WACCM) and Shepherd at al. (2014) used the Canadian Middle Atmosphere Model (CMAM) for SSW modeling. The reference wind profiles for the models are mainly retrieved from observations of the radiation of the mesospheric ozone molecules, which allow robust measurements at altitudes up to approximately 65 km (e.g., Hagen et al., 2018). These data are generally consistent with the most commonly used reanalysis products. However, there are still insufficient observations of middle atmospheric winds at altitudes between 60 and 85 km made with a high vertical resolution to verify atmospheric models and possible long-term trends (Keuer et al., 2007; Hagen et al., 2018; Rüfenacht et al., 2018). This altitude range, where temperature generally decreases with height, which causes inherent vertical instability, is situated below the winter mesopause region at 95–100 km (e.g., Xu et al., 2009) and plays a significant role in the mass and energy exchange between the stratosphere and the mesosphere (Shepherd et al., 2014; Limpasuvan et al., 2016; Gardner, 2018).
Microwave radiometry is a ground-based technique that can provide vertical
profiles of CO,
The first ground-based microwave measurements of CO were made in the 1970s
and they confirmed theoretical estimations of the vertical CO profile
(Waters et al., 1976; Goldsmith et al., 1979). Since the 1990s,
ground-based microwave radiometers measuring CO have been installed in the
Northern Hemisphere at high and middle latitudes to provide measurements on
a regular basis. Microwave radiometers are operating in Onsala and Kiruna,
Sweden, since 2008. The results are described in Hoffmann et al. (2011) and
in Forkman et al. (2012). The microwave radiometer operated in Bern,
Switzerland, since 2010 aims to contribute to the significant gap that exists
in the middle atmosphere between 40 and 70 km altitude for wind data
(Rüfenacht et al., 2012). In the Arctic,
Since 2014, the microwave measuring system for CO observations has been
operated in Kharkiv, Ukraine (Piddyachiy et al., 2010,
2017). Microwave radiometer measurements of CO are used to retrieve
mesospheric winds near the mesopause region (70–85 km). Methods deriving
the wind speed from mesospheric CO measurements are based on the
determination of CO and
Our observations in February 2018 using the new microwave radiometer at the midlatitude Kharkiv station have recorded the mesospheric effects of a major SSW. In mid-February 2018, the stratospheric polar vortex in the Arctic split into two sister vortices (Fig. 1), the zonal wind reversed in the stratosphere–mesosphere from westerly to easterly, and warm air penetrated into the polar cap regions (Rao et al., 2018; Karpechko et al., 2018; Vargin and Kiryushov, 2019). This caused large-scale disturbances in the middle atmosphere of the polar and middle latitudes. The major SSW in 2018 is not yet widely discussed in publications (Rao et al., 2018; Karpechko et al., 2018; Vargin and Kiryushov, 2019) and in this paper we give a detailed description of the observed mesospheric CO and zonal wind variations.
The polar vortex split at the 10 hPa pressure level during the SSW event in February 2018. Geopotential heights are calculated from ERA-Interim reanalysis data.
In Sect. 2, the microwave radiometer and data processing software are briefly described. The SSW event in February 2018 is considered in Sect. 3. The effects of the SSW on midlatitude mesosphere–stratosphere conditions in the Ukrainian longitudinal sector are presented in Sect. 4. Discussion is given in Sect. 5 followed by conclusions in Sect. 6.
The microwave radiometer dataset registered during the 2017/2018 winter
campaign in Kharkiv (50.0
The microwave radiometer (MWR) with high sensitivity, installed at Kharkiv, Ukraine, is designed for continuous observations of the atmospheric CO profiles and zonal wind speed in the mesosphere using emission lines at 115.3 GHz. The radiometer can continuously provide vertical profiles up to the mesopause region during day and night, even in cloudy conditions (Hagen et al., 2018). However, precipitation, such as strong rain or snow, can prevent the measurements.
The receiver of the radiometer has a double-sideband noise temperature of
250 K at an ambient temperature of 10
In this study, daily datasets from the ERA-Interim global atmospheric reanalysis
of European Centre for Medium-Range Weather Forecasts (ECMWF; ECMWF, 2019; Dee et al.,
2011) were downloaded from
(
Zonal wave amplitudes in geopotential height were analyzed using the
National Oceanic and Atmospheric Administration, National Centers for
Environmental Prediction, Global Data Assimilation System–Climate
Prediction Center (NOAA NCEP GDAS–CPC) data (Okamoto and Gerber, 2006) at
Descending air masses are observed throughout the mesosphere and stratosphere of the winter polar region (Orsolini et al., 2010; Chandran and Collins, 2014; Limpasuvan et al., 2016; Zülicke et al., 2018). From Aura MLS vertical profiles, a layered descending sequence of alternating cool and warm anomalies over the polar cap was observed in the 2017/2018 winter (Fig. 2a). The SSW event in Fig. 2a is identified by a rapid warming in the stratosphere and cooling in the mesosphere (upward arrow) starting from 10 February 2018 (left vertical line).
The development of the SSW in 2018 from the vertical profiles of
This event was preceded by progressively descending warm and cold anomalies
that formed in January (black and white dashed arrows, respectively).
Oscillations in the intensity of the anomalies indicate that they were
formed under the influence of large-amplitude planetary waves of zonal wave
numbers 1 and 2 (Fig. 2c–e). From 1 January to 10 February (during 41 d), descending warm anomalies with a velocity of
The splitting of the polar vortex (Fig. 1) and the zonal wind reversal (Fig. 2b) started at the time of the wave 2 pulse on 10 February (Fig. 2d and
dashed curve in Fig. 2e). Note that this is close to the SSW timing in Rao
et al. (2018) and Vargin and Kiryushov (2019), where the SSW onset date was
11 February. As seen from Fig. 2c and solid curve in Fig. 2e, the increasing
wave 1 amplitude contributed to the destabilization of the polar vortex
during January to early February and to temperature and zonal wind
oscillations in the mesosphere and stratosphere (Fig. 2a and b). These
oscillations are usually associated with the propagation of planetary waves
in the stratosphere and mesosphere (Limpasuvan et al., 2016; Rüfenacht
et al., 2016). As noted in an earlier study (Manney et al., 2009; Rao et
al., 2018), wave 1 amplitudes were also larger prior to the SSW in 2009,
suggesting a role of preconditioning. During 10–15 February, the easterly
zonal wind anomaly at the stratopause (about 1 hPa,
Local variability in the conditions of the atmosphere during the microwave
measurements in January–March 2018 at Kharkiv (50.0
The CO molecule volume mixing ratio (VMR) near the mesopause at 75–80 km
decreased from 10 ppmv of a background level to 4 ppmv on 19–21 February
(Fig. 3a), when the sharp vertical CO gradient at the lower edge of the CO
layer near 6 ppmv increased in height by about 8 km (between 75 and
83 km, thick part of the white curve in Fig. 3a). For comparison, the pre-
and post-SSW vertical variations in the 6 ppmv contour were observed in a
range 2–3 km (white curve in Fig. 3a). Moreover, similar variations in the
zonal mean 6 ppmv level are much weaker (yellow curve in Fig. 3e). This
indicates that the local and regional mesosphere over the MWR site was disturbed
by some source active during the SSW, which is identified below. We take here
the 6 ppmv contour as a conditional lower edge of the CO layer since the CO
gradients sharply increase from 0.2–0.3 ppmv km
The 5 d mean CO field over the NH (0–90
The local mesospheric CO variability from the MWR observations over Kharkiv
agrees with the regional one from the MLS data averaged over the adjacent area
47.5–52.5
Unlike the mesosphere, the CO descent and an increase in CO abundance is observed in the stratosphere from both regional and zonal mean MLS data shortly after the SSW start (contour 0.1 ppmv in Fig. 3d and g, respectively). The CO-rich air of 0.1–0.5 ppmv, which is typical for the lower mesosphere (Fig. 3c) descended down to about 30 km (Fig. 3d and g), far exceeding typical stratospheric CO mixing ratios on the order of about 0.01–0.02 ppmv (Engel et al., 2006; Huret et al., 2006; Funke et al., 2009). The CO-rich stratospheric anomaly is close in time to the wave 1 peak on 10–15 February (solid curve in Fig. 2e), that was observed through the stratosphere down to the 30 km altitude (Fig. 2c).
Horizontal distributions of the CO VMR in the Northern Hemisphere at the stratospheric and mesospheric altitudes in Fig. 4 suggest causes for the different CO variability in the stratosphere and mesosphere in Fig. 3. The dynamical deformation, elongation, and displacements of the polar vortex relative to the pole lead to temporal shifts between the low and high CO amounts over the MWR site at Kharkiv (white circle in Fig. 4). The tendency of the planetary wave westward tilt with altitude (dashed lines in Fig. 4, see also Figs. S1 and S2 in the Supplement for more details) also contributes to a relative zonal shift between the stratosphere and the mesosphere of the low and high CO amounts over Kharkiv.
The observed decrease in the local CO in the mesosphere during the SSW (white curve in Fig. 3a) is consistent with regional data from the satellite observations (white curve in Fig. 3b). The decrease is due to the displacement of the CO-rich air to the west relative to Kharkiv (white circle and contours outlined the CO-rich area in Fig. 4a–c and e–g). This is a result of the dominance of easterlies during the SSW that led to placing of the CO-poor air over Kharkiv with the lowest CO levels on 19–23 February (Fig. 4c and g) in correspondence with the MWR (Fig. 3a) and MLS (Fig. 3b) measurements. Recovery to the westerly regime in early March reversed the rotation of the vortex (2–6 March in Fig. 4d and h) and caused recovery of high CO levels over Kharkiv (since about 1 March in Fig. 3a and b).
The polar vortex split influenced the local CO change in the middle
stratosphere (Fig. 4m–o). The low CO level at
It should be noted that the lower edge of the midlatitude CO-rich air
descended in January to mid-February (dashed lines in Fig. 3d and g)
similarly to the temperature anomaly in the polar region (Fig. 2a). Descent
velocity was about
Note also that the vortex split in the CO distribution can be identified only in the middle and upper stratosphere (Figs. 4n and o and S1j and S1k), but not at the stratopause level (Fig. 4j and k) or in the mesosphere (Fig. S2, second and third columns for 9–13 and 19–23 February 2018, respectively).
The reversal of the local zonal wind estimated from the CO measurements at
the Kharkiv MWR site near the mesopause region was observed. The averaged
wind velocity in the altitude range 70–85 km changed between 10
and
During the SSW event, local zonal wind over the station became easterly
between the lower stratosphere and lower mesosphere (
The recovery of the local westerly wind in the upper mesosphere began in late February (Fig. 5a) and later, in early March, in the lower mesosphere–stratosphere (Fig. 5b). The longer persistence of the westerly anomaly in the stratosphere than at the stratopause level is also seen in the polar region (Fig. 2b). This is a manifestation of the downward migration of the circulation anomalies in the SSW recovery phase, while a near-instantaneous vertical coupling is observed at the SSW start on 10 February (Figs. 2a–d and 5).
The MLS temperature profiles show that high temperature variability over the
Kharkiv region concentrated at the stratopause level, particularly before
and during the SSW of 2018 (Fig. 6). As known, the SSW events are accompanied
by polar stratopause descent to 30–40 km, by stratopause breakdown, and
subsequent reformation at very high altitudes of about 70–80 km (Manney et
al., 2009; Chandran et al., 2011; Limpasuvan et al., 2016; Orsolini et al.,
2017). The midlatitude stratopause exhibits less sharp, but significant,
oscillations between 40 and 50 km in January to the first half of February 2018
(dotted curve in Fig. 6) and the highest temperature near
MLS temperature profiles
Similarly to the CO profile in Fig. 3, the zonal mean temperature variability is much lower above the stratopause than the regional one (Fig. 6b and a, respectively). The stratosphere is equally disturbed in both regional and zonal mean characteristics (Figs. 3d and g and 6a and b). This difference may be associated with the influence of the split (non-split) polar vortex in the stratosphere (mesosphere). The vortex fragments introduce higher local, regional, and zonal mean variability in the stratosphere, whereas the vortex region is more uniform in the mesosphere (Fig. 4). That results in the weaker zonal mean variability.
During the SSW, the regional stratospheric temperature in Fig. 6a was warmer
by 10–15
As is known, upward propagation of the tropospheric planetary waves into the stratosphere is limited in the easterly zonal wind (Charney and Drazin, 1961). In the changed state of a zonal flow, the critical line for planetary waves (zero wind line) in the polar region descents in a few days that looks like downward propagation of an anomaly from above (Matsuno, 1971; Zhou et al., 2002). Possibly, this process may be delayed in the midlatitudes, as seen from Fig. 6.
The observations of the major SSW effects in February 2018 in the NH
midlatitude mesosphere by microwave radiometer at the Kharkiv site, northern
Ukraine (50.0
As noted in Sect. 1, CO abundance in the extratropical mesosphere increases with latitude toward the winter pole due to meridional transport. CO accumulation results in the formation of the CO layer with a sharp vertical gradient at its lower edge (Solomon et al., 1985; Shepherd et al., 2014). Because of the horizontal CO gradient at the polar vortex edge, its split and displacement during the SSW cause a significant CO variability at the NH midlatitudes (Solomon et al., 1985; Allen et al., 1999; Funke et al., 2009; Shepherd et al., 2014).
In Sect. 4a, based on the MWR observations, we have defined the lower CO edge at 6 ppmv and this edge uplifted during the SSW by about 8 km (between 75 and 83 km, thick part of the white curve in Fig. 3a). This uplifting noticeably stands out against the pre- and post-SSW variations of the 6 ppmv level occurring within 2–3 km (Fig. 4a). The MLS CO measurements show similar variations in the 6 ppmv level over the Kharkiv region (white curve in Fig. 3b) and their absence in the corresponding zonal mean (yellow curve in Fig. 3a, b, and e).
Mesospheric CO profile uplifting is usually associated with the stratopause elevation during the SSW, when air, poor in CO, enters the mesospheric CO layer from below (Kvissel et al., 2012; Shepherd et al., 2014). Similar ascending motions in the stratopause and mesopause regions were observed in the 2013 SSW from nitric oxide (NO) and showed that the NO contours deflected upwards throughout the mesosphere (Orsolini et al., 2017). Our analysis reveals that the local CO profile variations during the SSW of 2018 were closely associated with the changes in the planetary wave patterns in the mesosphere.
The MLS CO distribution demonstrates how deformation, elongation (wave 2
effect), and rotation of the CO-rich polar area influence the local CO level
over Kharkiv (white circle with respect to the CO contours in Figs. 4a–h
and S1). The highest elevation of the 6 ppmv CO level in Fig. 3a and b
corresponds to the lowest CO level over Kharkiv on 19–23 February, when the
most distant displacement of the CO contours 16 and 6 ppmv off the
Kharkiv location was observed (Fig. 4c and g, respectively; see also the
third column in Fig. S1). As known, the strong vertical CO gradient in the
winter mesosphere is found at the higher altitudes in the tropics than in
the extratropics (Solomon et al., 1985; Allen et al., 1999; Garcia et al.,
2014). Then, poleward displacement of the low-latitude air masses is
accompanied by the CO abundance decrease and vertical CO gradient elevation
at the middle latitudes, as is observed in Fig. 3a and b. A similar
effect related to the wave 1 influence was observed during the 2003–2004
Arctic warming (Funke et al., 2009): the vortex has shifted from the pole
toward the western sector and midlatitude air poor in CO filled the eastern
sector (0–90
The results of Figs. 4 and S1 show that meridional displacements of the
low-latitude CO-poor mesospheric air to the Kharkiv region occurred under
the planetary wave influence and caused the local CO profile variations in
the SSW of 2018 (Fig. 3a and b). These results, thus, confirm that latitudinal
displacements due to wave effects may dramatically affect the local
densities of the atmospheric species (Solomon et al., 1985). Figure 6a
demonstrates that the local stratopause elevation in February 2018 to about
60 km was relatively small in comparison with the elevation that is
characteristic for the polar region, up to 70–80 km (Chandran et al., 2011;
Tomikawa et al., 2012; Limpasuvan et al., 2016; Orsolini et al., 2010,
2017). No significant stratopause elevation was observed in the zonal mean
for 47.5–52.5
In March 2018, after the SSW, the vertical CO profile was re-established (Fig. 3a and b) according to the recovery phase following the SSW (Shepherd et al., 2014; Limpasuvan et al., 2016). In the MWR data, the SSW recovery phase in the mesosphere in early March started with the short-term but anomalously high peaks in the local CO (Fig. 3a) and westerly wind (Fig. 5a). These peaks reached the highest values in daily variations in CO and zonal wind over the 3 months of the observations (January–March). By analogy with the low-CO episode in February discussed above, the high-CO peak in early March 2018 caused a change in the vortex shape and the return of the CO-rich vortex edge region to the Kharkiv location (compare 2–6 March in Fig. 4d and h with 19–23 February in Fig. 4c and g; see also the same dates in Fig. S2).
Wind measurements using the CO layer provide a further means to evaluate the validity of the modeled winds. Furthermore, by combining the measurements with ray tracing of gravity wave propagation (e.g., Kogure et al., 2018), this type of measurement may provide specific insights into wave–mean flow interactions, particularly where local temperature inversions alter gravity wave filtering (Hocke et al., 2018; Fritts et al., 2018).
An alternating altitudinal sequence of warm and cool anomalies progressively descending through the mesosphere and stratosphere of the polar region was observed in January–March 2018 (Fig. 2a) in consistency with many observations (Zhou et al., 2002; Orsolini et al., 2010; Shepherd et al., 2014; de Wit et al., 2014; Zülicke et al., 2018). The warm anomaly sharply intensified in the stratosphere between 20 and 50 km with simultaneous strong cooling in the mesosphere in the active phase of SSW since 10 February (vertical arrow in Fig. 2a). Unlike this, the midlatitude temperature anomalies do not show a similar vertical arrangement and regular descent with respect to the same mean climatology 2005–2017 (Fig. S3).
During the SSW of 2018, the upper (lower) stratosphere over the Kharkiv
region was cooler (warmer) up to 20
The CO profiles in Fig. 3 demonstrate opposite tendencies in the vertical
shift of the CO-rich air in the NH midlatitudes. The CO descent in the
stratosphere occurred during January–February with velocities of about 270
and 220 m d
The MLS CO maps in Fig. 4 show that the high CO amount is concentrated
inside the polar vortex and its fragments after splitting. This is a result
of meridional and downward transport of CO that is strongest in the winter
polar vortex (Rinsland et al., 1999; Manney et al., 2009; Kvissel et al.,
2012; Shepherd et al., 2014). Before (4–8 February), during (19–23 February), and after (2–6 March) the SSW, Kharkiv was outside the
stratospheric vortex or subvortex edge (Fig. 4m, o, and p, respectively)
and the CO amount was at low level typical for the midlatitude stratosphere
(of about 0.01–0.02 ppmv; Engel et al., 2006; Huret et al., 2006; Funke et
al. 2009). Descent of the 0.1 ppmv contour marked by dashed lines in Fig. 3d
and g is observed due to the episodic shift of the vortex edge toward the
Kharkiv region or to the corresponding zone 47.5–52.5
Figure 4 demonstrates that the CO amount inside the polar vortex or its fragments is much higher than in the surrounding area not only in the mesosphere but also in the stratosphere. This leads to the possibility of the enhanced CO appearance even in the stratosphere at about 25–30 km (Engel et al., 2006; Huret et al., 2006; Funke et al., 2009). By analogy, the vortex edge shift beyond the Kharkiv region (Fig. 4c and g) resulted in lowering of the regional CO mixing ratios in the mesosphere consistent with both ground-based and satellite observations (Fig. 3a and b, respectively). Meridional structure of the mesospheric CO (Sect. 1) provided the uplift of the 6 ppmv level during the SSW relative to pre- and post-SSW levels (Fig. 3a and b).
The impact of a major sudden stratospheric warming (SSW) in February 2018 on the
midlatitude mesosphere was investigated using microwave radiometer
measurements in Kharkiv, Ukraine (50.0
Among the most striking SSW manifestations over the midlatitude station in February 2018, there were (i) zonal wind reversal throughout the mesosphere–stratosphere; (ii) oscillations in the vertical profiles of CO, zonal wind and temperature; (iii) descent of the stratospheric CO, and temperature anomalies on the timescale of days to months; (iv) wave 2 peak at the vortex split date; and (v) strong mesospheric CO and westerly peaks at the start of the SSW recovery phase. Generally, the midlatitude SSW effects are known from many event analyses and in most cases they are associated with zonal asymmetry and polar vortex split and displacements relative to the pole (Solomon et al., 1985; Allen et al., 1999; Yuan et al., 2012; Chandran and Collins, 2014). Our results show that the local midlatitude atmosphere variability in the SSW of 2018 includes both the large-scale changes in the zonal circulation and temperature typical for the SSWs and local evolution of the altitude-dependent planetary wave patterns in the individual vortex split event.
The observed local CO variability can be explained mainly by horizontal air mass redistribution due to planetary wave activity with the replacement of the CO-rich air by CO-poor air and vice versa, in agreement with other studies. The MLS CO fields show that the CO-rich air masses are enclosed within the polar vortex. Horizontal (meridional and zonal) displacements of the edge of the vortex or vortex fragments relative to the ground-based midlatitude station may be a dominant cause of the observed CO profile variations during the SSW of 2018. The small subvortex located over the station at the SSW start caused the appearance of the enhanced CO level not only in the mesosphere but also in the stratosphere at about 30 km. This indicates that the polar vortex contains the CO-rich air masses with much higher CO amount than in the surrounding area and this takes place over the stratosphere–mesosphere altitude range.
Microwave observations show that a sharp altitudinal CO gradient below the mesopause could be used to define the lower edge of the CO layer and to evaluate oscillation and significant elevation of the lower CO edge during the SSW and its trend on a seasonal timescale. The presented results of microwave measurements of CO and zonal wind in the midlatitude mesosphere at 70–85 km altitudes, which is still not adequately covered by ground-based observations (Hagen et al., 2018; Rüfenacht et al., 2018), are suitable for evaluating and potentially improving atmospheric models. Simulations show that planetary wave forcing by westward-propagating wave 1 dominates between 40 and 80 km in the winter polar region during the SSW (Limpasuvan et al., 2016). Our spectral analysis reveals that the westward wave 1 during the SSW of 2018 is a dominant wave component through the midlatitude upper stratosphere–mesosphere. Instability of the westward polar jet suggested in previous studies (e.g., Limpasuvan et al., 2016) should be analyzed in the context of the westward wave 1 generation in the midlatitude upper stratosphere–mesosphere.
Our observation of variability in the CO layer during the SSW deserves further study, particularly in relation to the implications for modeling of wave dynamics and vertical coupling (Ern et al., 2016; Martineau et al., 2018) and chemical processes (Garcia et al., 2014) in the mesosphere.
The majority of the data used in this study can be obtained from the ERA-Interim reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF; Dee et al., 2011;
The supplement related to this article is available online at:
GM coordinated and led the efforts for this article. VS initiated the microwave measurements during the SSW event in Kharkiv. VS, DS, VM, and AA developed equipment and provided microwave measurements with data processing by AP and DS. GM, VS, YW, OE, AK, and AG analyzed the results and provided interpretation. GM, OE, AK, YW, VS, and WH wrote the paper with input from all authors.
The authors declare that they have no conflict of interest.
We acknowledge the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine for providing operation of the MWR facility. GM, VS, and YW acknowledge the College of Physics, International Center of Future Science, Jilin University, China, for hosting them during data analysis and paper preparation. The
microwave radiometer data have been processed using ARTS and Qpack software
packages (
This research has been supported in part by the Taras Shevchenko National University of Kyiv, Ukraine (grant no. 19BF051-08).
This paper was edited by Farahnaz Khosrawi and reviewed by two anonymous referees.