Wind and temperature response of midlatitude mesopause region to the 2009 Sudden Stratospheric Warming



[1] In this paper we report winds and temperature in the mesopause region (80–102 km) over full diurnal cycles during the 2009 major Sudden Stratospheric Warming (SSW) at Fort Collins (41°N, 105°W). The measurements were made with the Colorado State University (CSU) sodium Doppler wind-temperature lidar. We deduce the diurnal mean wind and temperature profiles by removing the tidal components from the 24-h continuous observations and present their anomalous behaviors in connection with this event. These mean wind and temperature measurements reveal significant anomalies in the mesopause region: the mean temperature at 80 km was approximately 30 K lower than the climatological mean; the mean zonal wind ranged between ∼ −10 to 0 m/s from 80 to 97 km and then turned eastward in lower thermosphere in a reversal of the climatological mean wind profile. We further use observations from the TIMED/SABER satellite observations and simulations from the WACCM model to investigate the global structure of this dynamical anomaly at Fort Collins. The satellite observations and model reveal that the anomaly is part of a disturbance that extended from the polar region to Fort Collins. These simultaneous wind- and temperature-lidar observations document the direct impact of a major SSW on the dynamic and thermal circulation of the midlatitude mesopause region.

1. Introduction

[2] There has been intense interest in sudden stratospheric warming (SSW) because of their significant global impact on the whole atmosphere and the manifestation of an atmospheric coupling from the troposphere to the thermosphere [Limpasuvan et al., 2004; Liu and Roble, 2002]. A SSW begins with a large transient amplification of Rossby planetary waves [Andrews et al., 1987] in the lower and mid-stratosphere at the high latitudes of the winter hemisphere. The amplified Rossby wave is, then, dissipated through wave-mean flow interaction [Matsuno, 1971], resulting in a westward forcing in the stratosphere that leads to a deceleration of the eastward zonal wind. This anomalous zonal circulation, which may persist from just few days to a week or longer, changes the gravity wave filtering properties of the stratosphere and lower mesosphere, allowing more eastward propagating gravity waves into the mesopause region [Yamashita et al., 2010]. These eastward-propagating gravity waves can break and deposit their momentum in the mesopause region, leading to zonal wind acceleration, upward and equatorial flow, and adiabatic cooling in the mesopause region. The anomalous circulation during an SSW also results in transport of atmospheric constituents [Siskind et al., 2005, Manney et al., 2009; Smith et al., 2009]. While the effects of SSW are most pronounced at high latitudes [Labitzke, 1972, 1981; Jacobi et al., 2003], recent studies have explored impacts on the mesosphere, thermosphere and ionosphere at midlatitudes [Hoffmann et al., 2007; Mukhtarov et al., 2007; Goncharenko and Zhang, 2008; Cevolani, 1991; Vergasova and Kazimirovsky, 2010, and references therein] and equatorial latitudes [Goncharenko et al., 2010; Chau et al., 2009; Shepherd et al., 2007].

[3] In January 2009 there was a major disturbance of the Arctic middle atmosphere, and on 23 January there was a reversal of the zonal mean zonal wind at 60°N and 10 hPa indicating a major SSW event [Labitzke, 1981]. The 2009 SSW has been extensively reported and classified as one of the strongest and most pronounced SSWs ever recorded [Manney et al., 2009; Thurairajah et al., 2010a; Coy et al., 2011]. During this event the stratospheric vortex split and the SSW had a more profound impact on the lower stratosphere than previously recorded SSWs. However, despite this interest in the 2009 SSW, to our knowledge there has been no report of direct and collocated measurements of both temperature and horizontal wind during a major SSW event that show the direct impact (as opposed to secondary impacts through mean flow-planetary wave-tidal interactions) of the SSW on the midlatitude mesopause region.

[4] In this paper we present sodium (Na) Doppler wind-temperature lidar observations of temperature and wind made over a three-day period (19 January–21 January) at a midlatitude site (Colorado State University (CSU), Fort Collins, (41°N, 105°W) during the 2009 SSW. We document the wind temperature anomalies and quantify them in terms of the multiyear measurements from the site. We then draw on observations from the Sounding of the Atmosphere through Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and simulations from the Whole Atmosphere Community Climate Model (WACCM) to show that the anomalies observed at Fort Collins are a direct manifestation of the 2009 SSW.

[5] This paper is laid out as follows. In section 2 we describe the methods and techniques associated with the CSU Na Doppler wind-temperature lidar, the SABER instrument, and the WACCM model that we employ in this study. In section 3 we present the lidar observations during the SSW of January 2009 and compare the atmospheric mean fields (temperature, zonal and meridional winds) anomalies in 2009 with January observations in other years (2003–2007). Also, in section 3, we present the SABER observations and WACCM simulations and analyses. In section 4 we present our summary and conclusions.

2. Methods and Techniques

2.1. Lidar

[6] The CSU Na Doppler wind-temperature lidar measures Na density, temperature, and zonal and meridional winds in the mesopause region (80–105 km) over full diurnal cycles under clear skies. During the 2009 SSW, the lidar operated for 59 h over a three-day period from UT time 04:16 of 19 January to 07:48 of 21 January and from 14:05 on 21 January to 21:04 the same day. These full-diurnal cycle observations permit the estimation of mean temperature and winds in which the influence of the tidal components (at 24-, 12-, 8- and 6-h) can be identified and removed [She, 2004]. By the end of 2008 nearly 5000 h of such diurnal observations had been acquired and allowed researchers establish the mean fields and tidal climatologies in the mesopause region at this midlatitude site [Yuan et al., 2006, 2008a, 2008b, 2010]. The mean wind and temperature profiles are deduced by applying a least square-fitting technique that includes tidal modulations to the hourly mean temperatures and wind profiles obtained from continuous observations longer than 24 h (thus with short period gravity wave (GW) perturbations removed). The tidal period perturbations are then removed and the mean winds and temperatures, as well as the amplitudes and phases of tidal components, are obtained in the 80–105 km altitude region. The data analysis algorithm and system description have been presented previously [She, 2004; Arnold and She, 2003]. Here, we compare the CSU lidar's 2009 measurements with the climatological mean of the measurements taken in January from 2003 to 2007 as listed in Table 1. We exclude the lidar observations in January 2008 from this mean to avoid possible influence by the minor SSW in 2008 [Thurairajah et al., 2010a].

Table 1. CSU Na Lidar Observations in January From 2003 to 2009
YearDate (UT Day)Hours of Observations

2.2. SABER

[7] Following the approach of Thurairajah et al. [2010a, 2010b] we use SABER measurements of temperature, pressure, and geopotential to determine gradient winds, Eliassen-Palm (EP) flux and EP flux divergence (EPD). We calculate the zonal mean gradient winds from the SABER data by applying appropriate approximation to the zonal averaged meridional momentum equation [Hitchman and Leovy, 1986; Randel, 1987; Lieberman, 1999]. Changes in EP flux represent the relative importance of wave's eddy heat flux and momentum flux in the meridional-vertical plane, while the EPD is proportional to the eddy potential vorticity flux. We calculate the EPD for quasi-geostrophic eddies using well-established techniques [e.g., Andrews et al., 1987]. The EPD is scaled by the atmospheric density to give the zonal force per unit mass exerted by eddies. SABER makes measurements at two local times per day at any given location and may contain bias from tidal modulations. However, we expect that the daily zonally averaged temperature reveal the evolution of the atmosphere's mean state during the SSW. In this study, the daily SABER data (SABER Level 2A version 1.07) was grid into 5° latitude × 30° longitude bins and averaged over a latitude circle to determine the zonal mean data.

2.3. WACCM

[8] The Whole Atmosphere Community Climate Model (WACCM), developed at the National Center for Atmospheric Research (NCAR), is derived from the Community Atmosphere Model (CAM3) and is a fully coupled chemistry-climate model [Garcia et al., 2007, and references therein]. The model domain extends from the Earth's surface to ∼145 km (4.5 × 10−6 hPa. The standard horizontal resolution is 1.9° × 2.5° (latitude × longitude). While WACCM resolves planetary waves, both orographic and non-orographic gravity waves are parameterized in the model [Garcia et al., 2007; Richter et al., 2008, 2010]. The simulations used in this study have a time step of one day with the model output at midnight UT every day. WACCM can be run in a specified dynamics (SD) mode by incorporating reanalysis data from observations in the troposphere and stratosphere. This is implemented in SD WACCM by relaxing the horizontal winds and temperatures to reanalysis data from GEOS-5.2 up to 40 km. The amount of relaxation between 40–50 km is linearly reduced so that the model becomes free running at 50 km. SD WACCM has 88 vertical levels, with a resolution of approximately two grid points per scale height in the mesosphere and lower thermosphere [Marsh, 2011]. The resolution in the horizontal is the same as in free running WACCM at 1.9° × 2.5° (latitude × longitude). In this mode it is therefore possible to use WACCM to stimulate and study a particular event. For example, Marsh [2011] has used SD WACCM to study the 2006 major SSW.

3. Observations and Model Results

3.1. Lidar Measurements of Local Wind and Temperature at Fort Collins

[9] Figure 1a shows the mean temperature climatology profile based on lidar observations in January from 2003 to 2007. In comparison, we see a dramatic cooling in Jan 2009 temperature profile below 90 km. The error bars on the 2009 profile represent the uncertainty of the harmonic fitting when weighting every lidar data equally. The standard deviations of the diurnal means for climatology profiles are shown as natural variability, while the fitting errors for climatology profiles are comparable with those of the 2009 profiles. As the figure shows, the largest temperature drop was observed near 80 km, where lidar detected a 30 K cooling. Above 90 km, the temperature follows the climatological profile quite well up to 102 km. The simultaneous lidar zonal wind observations also indicated an intriguing zonal wind behavior in the mesopause region during the 2009 warming event. In Figure 1b, a zonal wind reversal to the small westward zonal wind (around −5 m/s from 82 to 95 km) in 2009 is clearly seen. In fact, the zonal wind around the mesopause during this SSW is weak or slightly westward with a return to eastward above 98 km. Though the standard deviation is larger than the fitting uncertainties of the climatology, Figure 1b clearly shows that the 2009 zonal wind is exactly opposite to the climatological zonal wind, which is eastward in January below 95 km and changes to westward above this altitude [Yuan et al., 2008b]. In addition, a significant drop of Na abundance below 90 km (almost 90% decreasing in Na density near 80 km compared to the climatological value) was observed during the same time frame whereas there is little change above. The disappearance of Na below 90 km is consistent with the temperature change in this region as early model suggested [Plane et al., 1999]. In addition, Collins and Smith [2004] have shown that a temperature change of 30 K would lead to a 0.6 relative change in density at 80 km in the steady state Na layer.

Figure 1.

The lidar observed full-diurnal mean (a) temperature, (b) zonal wind and (c) Na density profiles, based on lidar measurements between Jan 19 and 21, approaching the peak of 2009 SSW. The standard deviations are shown as the error bars for climatology profiles (black) and fitting errors for 2009 profiles (Red). The fitting errors for climatology profiles are comparable to those of the 2009.

[10] To study the evolution of mesopause region wind and temperature during this period, we apply a 24-h running average through the lidar temperature and horizontal winds data to document the variations of the mean states during this period of time (Figure 2). We apply the technique to the 52 h continuous observations between UT time 04:16 of 19 January and 07:48 of 21 January. As shown in Figure 2a, throughout the observation, the mesopause cooled from typical winter values (greater than 210 K below 85 km in the climatology) to significantly lower values (190 K). At altitudes higher than 90 km the isotherms show steady cooling (∼6 K/day). For example, at 90 km (95 km), the temperature dropped from 200 K (188 K) on UT day 19.7 to 191 K (183 K) on UT day 20.8. In the vertical direction, the temperatures were slowly but consistently getting warmer toward lower altitudes at a rate commensurate with climatology. However, below ∼90 km, the warming rate toward lower altitude is much reduced in response to the forcing that leads to mesopause cooling associated with SSW. The mean zonal wind (Figure 2b) shows relatively weak westward winds with small variations (from −10 m/s to +10 m/s) throughout most of the mesopause region. Above 95 km, the mean zonal wind returns from westward to eastward near the end 19 January. The simultaneous mean meridional wind measurement (Figure 2c) indicates a ∼30 m/s change in wind speed change throughout the whole mesopause region. The meridional wind reversed direction from mostly northward at the beginning of the observation to completely southward at the end of lidar run. It is worth noticing that the region where mean meridional wind was southward corresponds to the region where the mean zonal wind became eastward.

Figure 2.

The time series of lidar observed full diurnal mean (a) temperature, (b) zonal wind and (c) meridional wind during the 2009 SSW. Tidal modulations are removed by running a 24-h window through the course of the lidar data at one-hour step.

[11] In summary, the lidar observations of lidar mean temperature and wind profiles show significant cooling and wind reversal. The observations reveal the upward extent of the anomaly to 90 km in the mesopause region. These single site measurements are consistent with the response of the mesosphere to a major SSW, and suggest that we have documented the direct response of the middle atmosphere at 41°N to the SSW. To gain a global perspective of the local lidar observations and to understand the related dynamical behavior, we employ satellite-based SABER observations and the simulations of SD-WACCM to reveal the global structure of this 2009 SSW.

3.2. SABER Observations of Zonally Averaged Structure and Planetary Wave Activity

[12] Figure 3 shows zonal mean temperature, zonal mean gradient wind and EPD near 40°N observed by or deduced from TIMED/SABER observations. The SABER zonal mean temperature (Figure 3a) indicates that the upper stratosphere at 40°N latitude was indeed experiencing warming starting near 17 January, although not as dramatic as that observed near high latitude [Thurairajah et al., 2010b]. The deduced zonal gradient wind (Figure 3b), clearly indicates wind reversal (from eastward to westward) in the upper mesosphere from 19 to 22 of January, which is in good agreement with the Na lidar observations of mean zonal (Figure 1b). In fact, Figure 3b shows the strong zonal wind deceleration started in the upper stratosphere and mesosphere on 12 January and the wind reversal occurred on 18 January in the upper mesosphere near 85 km. The SABER observations show that during the 2009 SSW the zonal wind responds to the anomalous forcing more readily and is a more sensitive indicator of SSW signals, especially at the edge of a disturbed region at midlatitudes.

Figure 3.

Temporal evolution of TIMED/SABER (a) zonal mean temperature, (b) zonal gradient wind and (c) EPD in January 2009 at 40°N. The units for temperature, wind and EPD (per unit mass) are respectively K, m/s and m/s/day. The data near the blank area between 21 and 24 Jan above 90 km in Figure 3c may be questionable.

[13] Figure 2b also shows that the zero wind line propagated down to lower mesosphere and stratosphere on 19–20 of January. Indeed, the EPD (Figure 2c) in the mesosphere (∼50–80 km) started to turn negative in the pre-deceleration phase and was clearly negative by 14 January and remained negative until 24 January. The EPD was mostly zero outside this time frame. Since the sampling time of the TIMED/SABER observation are not fine enough to capture gravity wave perturbations, the observed negative EPD indicates westward forcing resulting from transient planetary wave breaking in the upper stratosphere and mesosphere. The negative EPD at 40°N occurred at the same time as at 65°N [Thurairajah et al., 2010a, Figure 10], though the westward forcing in the midlatitudes was located at slightly higher altitude than in the higher latitudes. Therefore, SABER EPD shows that, near midlatitude, the upward propagating planetary waves break at higher altitude during the warming event than they do at high-latitudes.

[14] The geopotential height amplitudes of planetary wave number 1 (PW1, Figure 4a) and wave number 2 (PW2, Figure 4b) are also deduced from SABER data; we plot these amplitudes as a function of latitude and altitude for 16, 18, 20, and 24 January. Clearly, the amplitude of PW2 amplitude was much greater than the amplitude of PW1 during this period. The PW2 peak amplitude occurred at 40 km near 60°N during 18–20 January (Figure 4b, top) corresponding to the main warming phase, and also had significant amplitude at the latitude of the Fort Collins (41°N). The PW2 amplitude in upper stratosphere reached its maximum between 18 and 20 January, corresponding to the middle of lidar observation, and decayed to similar magnitude as PW1 on 24 January. Comparing Figures 4a and 4b, it appears that the changing of planetary wave amplitude in PW1 and PW2 in stratosphere is anti-correlated during the 2009 SSW. For example, the amplitude of PW1 amplitude on 18 January was quite small but increased on 24 January (Figure 4a). On the other hand, the PW2 amplitude in the mesosphere reached a maximum before 16 January and then steadily decreased from 16 January through 24 January 16 (Figure 4b). The increase in both PW1 and PW2 amplitudes at high latitudes above 90 km after the warming (on 24 January) may reflect momentum forcing associated with the breaking of gravity waves and generation of secondary stationary planetary waves in the upper mesosphere and lower thermosphere that have been observed by satellite [Smith, 2003; Garcia et al., 2005].

Figure 4a.

Latitudinal distributions of geopotential amplitudes of planetary wave number 1 for Jan. 16, 18, 20 and 24, 2009. Contour level values are 300 m (dotted line), 600 m (dashed line), 900 m (dash-dotted line) and 1200 m (solid line).

Figure 4b.

Same as Figure 4a but for planetary wave number 2.

3.3. SD-WACCM Simulation of Thermal Structure of the Northern Hemisphere

[15] The SD-WACCM simulation incorporates reanalysis data up to 40 km. The temperature map at 40 km reveals that the disturbed stratospheric vortex extended equatorward to 41°N (Figure 5). The vortex at these altitudes is evident as shown by the warmer temperatures, while the colder temperatures are associated with stratospheric antic-cyclones. The splitting of the vortex is clearly evident on 20, 21 and 22 January corresponding to the days of the lidar observations, when the vortex passes over Fort Collins (indicated by a red star). The model simulation, which highlights the splitting of the polar vortex, reflects the strength and latitudinal extent of the PW2 activity associated with the 2009 SSW.

Figure 5.

Temperature longitudinal distributions on Jan. 20, 21, and 22, 2009 at 40 km altitude viewing from top of the North Pole simulated by (SD) WACCM. The red star in the top left section of each plot indicates the location of Fort Collins. The temperature is the model output at midnight UT time.

4. Summary and Conclusion

[16] The CSU Na wind-temperature lidar observations during the January 2009 SSW documented the disturbance of the midlatitude mesopause region during an extreme SSW event. The comparison to observations made in January 2003–2007, show significant anomalies in the mesopause region during the 2009 SWW. Unlike other studies, the continual lidar observations of both wind and temperature allow direct characterization of the response of the midlatitude mesopause region to an SSW. Examination of SABER data and SD-WACCM simulation (based on assimilation of reanalysis data) allows us to analyze this warming event in a global context.

[17] Based on these observations and analyses we can therefore draw the following conclusions:

[18] 1. The diurnal mean fields (lidar observed temperature and zonal wind) in the midlatitude mesopause region do show expected anomalies during the 2009 major SSW that are usually seen at high latitudes. Contrary to the 2003–2007 climatology, over the January 19–21 period, the mean temperature and wind profiles show that there was cooling in the mesosphere up to 90 km by as much as 30 K near 80 km and a reversal of zonal wind toward westward below 98 km and eastward above. SABER and SD-WACCM indicate that these anomalies are directly associated with the major SSW and that the lidar has documented the direct impact of the SSW on the midlatitude mesopause region.

[19] 2. The continuous observation of these mean fields by lidar reveals that the isotherms throughout the upper mesosphere and lower thermosphere steadily progressed to lower altitudes. During the same period, the zonal wind started to become eastward near the beginning of 20 January above 95 km, while the meridional wind also began to change from northward to southward near the same region. Such correlated reversals of zonal wind and meridional wind are consistent with the changing of gravity wave filtering and the related variations in the quasi-geostrophic circulations in the mesopause region during SSW events.

[20] 3. Unlike the zonal mean zonal wind, the zonal mean temperature from TIMED/SABER at 40°N did not indicate dramatic temperature variation during the warming event, but rather weak changes in the mesosphere as compared to observations near high latitude. This suggests that zonal wind is a more sensitive indicator of the SSW, especially in the midlatitude located at the edge of the disturbance. This fact that the anomaly is more pronounced in the wind than temperature is consistent with the direct observation of the planetary wave breaking. The SD-WACCM simulation of the 2009 event further confirms that during this major SSW where the polar vortex split, the polar vortex passed over midlatitude Fort Collins, CO.

[21] 4. During the 2009 major SSW, the mesospheric Na layer becomes thinner and a significant decrease in column abundance due to the cooling in the upper mesosphere that causes atomic Na change into its reservoir NaHCO3. Lidar results indicate an increase in the centroid height due mainly to the Na density decreasing in the lower part of the mesopause region, which is associated with the mesospheric cooling during the SSW.

[22] Overall, all these observed changes in the mesopause region suggest that the anticipated mean fields anomalies during SSW resulting from planetary wave-mean state interaction were indeed observed directly in a midlatitude mesopause region (above Fort Collins) during the 2009 major event. Compared to report of midlatitude MLT responses to 2006 major SSW at 54°N [Hoffmann et al., 2007], this simultaneous and continuous observation of mean temperature and wind fields' variations by CSU Na lidar during the SSW gives a rare, yet more comprehensive and detailed picture of mesospheric mean fields responses to this global event near midlatitude. The lidar observations during 2009 major warming at 41°N show major temperature changes were located below 90 km altitude and the zonal wind reversal from climatology in the mesopause region.


[23] The authors acknowledge the SABER science and data processing teams for providing the SABER data presented in the paper. We also want to thank Dan Marsh and Doug Kinnison for SD WACCM data. The work is supported in part by grants from National Science Foundation, ATM-0545221, ATM-0804295, and ARC-0632387. The lead author wishes to acknowledge support from CASS (Center of Atmospheric and Space Sciences) at Utah State University. Yuan and She also acknowledge the useful discussions with S. Fabrizio and D. Siskind of Navy Research Lab during the early stage of this study. We thank Sean Harrell, Wentao Huang, Joe Vance and Jia Yue for their assistance with the lidar observations in January 2009.