Dynamical response of low-latitude middle atmosphere to major sudden stratospheric warming events

The zonally averaged UK Meteorological Office (UKMO) zonal mean temperature and zonal winds for the latitudes 8.75°N and 60°N are used to investigate the low-latitude dynamical response to the high latitude sudden stratospheric warming (SSW) events that occurred during winter of the years 1998–1999, 2003–2004 and 2005–2006. The UKMO zonal mean zonal winds at 60°N show a short-term reversal to westward winds in the entire upper stratosphere and lower mesosphere and the low-latitude winds (8.75°N) show enhanced eastward flow in the upper stratosphere and strong westward flow in the lower mesosphere during the major SSW events at high latitudes. The mesosphere and lower thermosphere (MLT) zonal winds acquired by medium frequency (MF) radar at Tirunelveli (8.7°N, 77.8°E) show a change of wind direction from eastward to westward several days before the onset of SSW events and these winds decelerate and weak positive (eastward) winds prevail during the SSW events. The time variation of zonal winds over Tirunelveli is nearly similar to the one reported from high latitudes, except that the latter shows intense eastward winds during the SSW events. Besides, the comparison of daily mean meridional winds over Tirunelveli with those over Collm (52°N, 15°E) show that large equatorial winds are observed over Tirunelveli during the 2005–2006 event and over Collm during the 1998–1999 events. The variable response of MLT dynamics to different SSW events may be explained by the variability of gravity waves.     

MST radar investigation on inertia-gravity waves associated with tropical depression in the upper troposphere and lower stratosphere over Gadanki (13.5°N, 79.2°E)

Continuous measurements of 3-dimensional winds, spectral parameters, and tropopause height for ～114 h during the passage of a tropical depression using mesosphere–stratosphere–troposphere (MST) radar at Gadanki (13.5°N, 79.2°E) are discussed. The spectral analysis of zonal and meridional winds shows the presence of inertia-gravity wave (IGW) with the dominant periodicity of 56 h and intrinsic period of 27 h in the upper troposphere and lower stratosphere (UTLS). The strengthening of easterly jet and associated wind shears during the passage of the depression is one of the causative mechanisms for exciting the IGW. A well-established radar method is used to identify the tropopause and to study its response to the propagating atmospheric disturbances. The significance of the present study lies in showing the response of tropopause height to the IGW during tropical depression for the first time, which will have implications in stratosphere–troposphere exchange processes.     

Interannual variability of mesospheric mean winds observed with the MU radar

In an effort to study the interannual variation of mesospheric (65–90 km altitude) mean winds, 10 years (1986–1995) of wind data collected with the MU radar at Shigaraki, Japan (34.9°N, 136.1°E) have been analysed. The analysis reveals that the mean zonal wind circulation in the mesosphere is dominated by an annual variation. The summer westward flow in the mesosphere shows a smooth variation with a peak value in the range 40–60 m/s in June/July. In contrast to the summer westward winds, the winter eastward winds exhibit much more variability. In some years it is found that the winds exceed even 60 m/s and the peak value may occur in any one of the winter months. Scrutiny of the duration of the summer westward winds reveals a two-year periodicity, which has been compared with the quasi-biennial oscillation (QBO) phases at the equator. The search for a dependence of the mean wind on solar activity does not reveal any indications of it. Ten-year averaged winds are compared with the model atmosphere, CIRA-86, values and certain agreements and disagreements are pointed out.     

Variability of the mesospheric wind field at middle and Arctic latitudes in winter and its relation to stratospheric circulation disturbances

Continuous wind observations allow detailed investigations of the upper mesosphere circulation in winter and its coupling with the lower atmosphere. During winter the mesospheric/lower thermospheric wind field is characterized by a strong variability. Causes of this behaviour are planetary wave activity and related stratospheric warming events. Reversals of the dominating eastward directed mean zonal winds in winter to summerly westward directed winds are often observed in connection with stratospheric warmings. In particular, the amplitude and duration of these wind reversals are closely related to disturbances of the dynamical regime of the upper stratosphere.The occurrence of long-period wind oscillations and wind reversals in the mesosphere and lower thermosphere in relation to planetary wave activity and circulation disturbances in the stratosphere has been studied for 12 winters covering the years 1989–2000 on the basis of MF radar wind observations at Juliusruh (55°N, since 1989) and Andenes (69°N, since 1998). Mesospheric wind oscillations with long-periods between 10 and 18 days are observed during the presence of enhanced planetary wave activity in the stratosphere and are combined with a reversal of the meridional temperature gradient of the stratosphere or with upper stratospheric warmings.     

Common-volume measurements of mesospheric winds using radar and optical instruments: 1. Comparison of observations

The Millstone Hill incoherent scatter radar (42.6°N, 71.5°W) and the nearby Durham meteor wind radar (43.1°N, 70.9°W) have been used to study the structure of the winds in the mesosphere and lower thermosphere and to investigate the propagation of tidal components from the mesosphere into the lower thermosphere. In general, good agreement is found between the tidal wind amplitudes and phases determined by the two radars, but occasionally, some discontinuities have been observed in the vertical structure of the tidal components in the 90–110 km region. In order to validate the accuracy of the two techniques and the methodologies used in determining neutral winds, two common-volume experiments were conducted in 1996 and 1997 in which the two radar beams were overlayed at an altitude of 100 km. The horizontal components of the measured radar line-of-sight velocities during day-time periods were then compared at the overlapping altitudes of 95–100 km. Night-time measurements were also made using a Fabry–Perot Interferometer co-located with the radar at Millstone Hill which observed the Doppler shift of the atomic oxygen green line emission in the mesosphere. Good overall agreement is found between the instruments within the statistical uncertainties of the measurement techniques, although some differences have been found that are explained by consideration of the data statistics, the exact overlap of common volume within the different beam sizes, and the presence of altitude gradients and small scale irregularities in the sampled volumes of the atmosphere.     

Inter-hemispheric asymmetry in polar mesosphere summer echoes and temperature at 69° latitude

An inter-hemispheric asymmetry is found in the characteristics of polar mesosphere summer echoes (PMSE) and upper mesosphere temperatures at conjugate latitudes (～69°) above Antarctica and the Arctic. The second complete mesosphere–stratosphere–troposphere (MST) radar summer observation season at Davis (68.6°S) revealed that PMSE occur less frequently, with lower strength and on average 1 km higher compared with their northern counterparts at Andenes (69.3°N). We consider the thermodynamic state of the mesosphere for conjoining hemispheric summers based on satellite and ground-based radar measurements, and show the mesopause region near ～80–87 km of the Southern Hemisphere (SH) to be up to 7.5 K warmer than its Northern Hemisphere (NH) counterpart. We show that this is consistent with our observation of asymmetries in the characteristics of PMSE and demonstrate how the mesosphere meridional wind field influences the existence and strength of the echoes in both hemispheres.     

Simultaneous observations of planetary waves from 30 to 220km

Planetary wave activity at quasi 16-, 10- and 5-day periods has been compared at various altitudes through the middle and upper atmosphere over Halley (76°S, 27°W), Antarctica, during the austral winters of 1997–1999. Observational data from the mesosphere, E-region ionosphere and F-region ionosphere have been combined with stratospheric data from the ECMWF assimilative operational analysis. Fourier and wavelet techniques have shown that the relationship between planetary wave activity at different altitudes is complex and during the winter eastward wind regime does not conform to a simple combination of vertical planetary wave propagation and critical filtering. Strong planetary wave activity in the stratosphere can coincide with a complete lack of wave activity at higher altitudes; conversely, there are also times when planetary wave activity in the mesosphere, E-region or F-region has no apparent link to activity in the stratosphere. The latitudinal activity pattern of stratospheric data tentatively suggests that when the stratospheric signatures are intense over a wide range of latitudes, propagation of planetary waves into the mesosphere is less likely than when the stratospheric activity is more latitudinally restricted. It is possible that, on at least one occasion, 16-day planetary wave activity in the mesosphere may have been ducted to high latitudes from the lower latitude stratosphere. The most consistent feature is that planetary wave activity in the mesosphere is almost always anti-correlated to planetary wave activity in the E-region even though the two are in close physical proximity. The oscillatory critical filtering of vertical gravity wave propagation by planetary waves and the re-generation of the planetary wave component at higher altitudes through subsequent critical filtering or breaking of the gravity waves may provide an explanation for some of these characteristics. Alternatively the nonlinear interaction between planetary waves and tides, indicated in the E-region data, may play a role.     

Observations of peculiar sporadic sodium structures and their relation with wind variations

Resonance lidar observations of sodium density in the upper mesosphere region over Gadanki (13.5°N, 79.2°E) rarely show complex structures with rapid enhancements of sodium density, completely different from normal sporadic sodium structures. The hourly averaged meteor radar zonal winds over Trivandrum (8.5°N, 76.5°E) show an eastward shear with altitude during the nights, when these events are formed. As suggested by Kane et al. [2001. Joint observations of sodium enhancements and field-aligned ionospheric irregularities. Geophysical Research Letters 28, 1375–1378], our observations show that the complex structures may be formed due to Kelvin–Helmholtz instability, which can occur in the region of strong wind shear.     

Observations of short-period waves with an all-sky camera in the infrared airglow above Yakutsk

The parameters of internal gravity waves detected based on the variations in the hydroxyl molecule emission are statistically analyzed. The wave structures were registered with an all-sky infrared camera at Maimaga optical station (? = 63° N, λ = 129.5° E). The data obtained in the winter period of 1998–2002 are analyzed. In total, 162 waves, the majority of which propagated westward, were recorded. The wavelengths vary from 15.4 to 100 km (the average value is ～31 km); the observed horizontal phase velocities change from 19 to 166 m/s (the average value is ～60 m/s), and the estimated periods are 9–90 min (the average value is ～11 min). The statistical characteristics of the waves do not differ from those of similar waves at middle and low latitudes. The azimuthal dependence of the wave propagation direction is consistent with the theory of wave filtration by a background wind in the middle atmosphere. Probable sources of the waves are mountain ranges located at a distance of 200 km east of the observation site. Somewhat greater values of the mean wavelength and wave propagation velocities than those recorded at lower latitudes may be due to the lower loss of energy and velocity of the waves during their propagation from the source to the mesosphere, although other causes are not ruled out. Ripple-type waves have the same direction of propagation as band-type waves.     

High-altitude data assimilation system experiments for the northern summer mesosphere season of 2007

A global numerical weather prediction system is extended to the mesosphere and lower thermosphere (MLT) and used to assimilate high-altitude satellite measurements of temperature, water vapor and ozone from MLS and SABER during May–July 2007. Assimilated temperature and humidity from 100 to 0.001 hPa show minimal biases compared to satellite data and existing analysis fields. Saturation ratios derived diagnostically from these assimilated temperature and water vapor fields at PMC altitudes and latitudes compare well with seasonal variations in PMC frequency measured from the aeronomy of ice in the mesosphere (AIM) satellite. Synoptic maps of these diagnostic saturation ratios correlate geographically with three independent transient mesospheric cloud events observed at midlatitudes by SHIMMER on STPSat-1 and by ground observers during June 2007. Assimilated temperatures and winds reveal broadly realistic amplitudes of the quasi 5-day wave and migrating tides as a function of latitude and height. For example, analyzed winds capture the dominant semidiurnal MLT wind patterns at 55°N in June 2007 measured independently by a meteor radar. The 5-day wave and migrating diurnal tide also modulate water vapor mixing ratios in the polar summer MLT. Possible origins of this variability are discussed.     

MF radar observations of mean winds over Yamagawa (31.2°N, 130.6°E) and Wakkanai (45.4°N, 141.7°E)

Continuous MF radar measurements of mesospheric mean winds are in progress at the observatories in Yamagawa (31.2°N, 130.6°E) and Wakkanai (45.4°N, 141.7°E). The observations at Yamagawa and Wakkanai were started in August 1994 and September 1996, respectively. The real-time wind data are used for the study of major large scale dynamic features of the middle atmosphere such as mean winds, tides, planetary waves, and gravity waves, etc. In the present study of mean winds, we have utilized the data collected until June 1999, which include the simultaneous observation period of little more than two and a half years, for the two sites. The database permits us to draw conclusions on the characteristics of mean winds and to compare the mean wind structure over these sites. The mean prevailing zonal winds at both sites are dominated by westward/eastward motions in summer/winter seasons below 90 km. Meridional circulation at meteor heights is generally southward during most times of the year and it extends to lower mesospheric heights during summer also. The summer westward jet at Wakkanai is consistently stronger than those at Yamagawa. However, the winter eastward winds have identical strength at both locations. Meridional winds also show larger values at Wakkanai. The mean wind climatology has been examined and compared with the MU radar observations over Shigaraki (34.9°N, 136.1°E). The paper also presents the results of the comparison between the MF radar winds and the latest empirical model values (HWM93 model) proposed by Hedin et al. (1996. Journal of Atmospheric and Terrestrial Physics 58, 1421–1447). Hodograph analyses of mean winds conducted for the summer and winter seasons show interesting similarities and discrepancies.     

Gravity waves in Fabry–Perot measurements made at Mt. John (44°S,170°E), New Zealand

Observations of winds in mesospheric airglow layers have been made at Mt. John (44°S,170°E), New Zealand for some years. We present a modelling study of airglow emissions which shows that the properties of wind detection based on airglow emission means that high-frequency gravity waves are effectively filtered from the wind spectrum observed. This filtering means that any waves with periods of the order of hours should be detectable in the record (as they will not be hidden in the noise of the higher-frequency waves ubiquitous at these heights). One example of such a wave is shown. As part of the analysis, we show that because the airglow layers differ in width, some waves might be observed in only one airglow layer, even when present in both.     

The influence of background winds and attenuation on the propagation of atmospheric gravity waves

The influence of background winds and energy attenuation on the propagation of atmospheric gravity waves is numerically analyzed. The gravity waves, both in the internal and ducted forms, are included through employing ray-tracing method and full-wave solution method. Background winds with different directions may cause ray paths of internal gravity waves to be horizontally prolonged, vertically steepened, reflected or critically coupled, all of which change the accumulation of energy attenuation along ray paths. Only the penetrating waves propagating against winds can easily reach the ionospheric height with less energy attenuation. The propagation status of gravity waves with different periods and phase speeds is classified into the cut-off region, the reflected region and the propagating region. All the three regions are influenced significantly by winds. The area of the reflected region reduces when gravity waves propagate in the same direction of winds and expands when propagating against wind. In propagating region, the horizontal attenuation distances of gravity waves increase and the arrival heights decrease when winds blow in the same direction of gravity waves, while the attenuation distances decrease and the arrival heights increase when gravity waves propagate against winds. The results for ducted gravity waves show that the influence of winds on waves of lower atmospheric modes is not noticeable for they propagate mainly under mesosphere, where the wind field is relatively weak. However, strong winds at thermospheric height lead to considerable changes of dispersion relation and attenuation distance of upper atmospheric modes. Winds against the wave propagating direction support long-distance propagation of G mode, while the attenuation distances decrease when winds blow in the same direction of the wave. The distribution of TIDs observed by HF Doppler array at Wuhan is compared with the simulation of internal gravity waves. The observation of TIDs shows agreement with our numerical calculations.     

Secondary planetary waves in the middle atmosphere: numerical simulation and analysis of the neutral wind data

A numerical simulation of secondary waves generated by nonlinear interaction has been used to interpret the behaviour of planetary waves observed by a meteor radar in the UK (53°27′N, 1°35′W) during the summer of 1992. A new explanation is proposed for the long-period variability of the (3,0) mode quasi-two-day wave in the mesosphere and lower-thermosphere, involving the (2,0) Rossby-gravity mode and pseudo-two-day secondary waves with the same zonal wavenumbers as those of the primary (2,0) and (3,0) modes. These pseudo-two-day secondary waves arise from the nonlinear interaction of the Rossby-gravity modes with long-period oscillations of the zonally averaged flow in the equatorial stratosphere, which can be generated by the interaction between the 10 and 16 day planetary waves. Other maxima existing in the neutral wind power spectra can be identified with various secondary waves originating from nonlinear interaction between the quasi-two-day and long-period planetary waves.     

Planetary Wave Periods in <Emphasis Type="Italic">foF</Emphasis>2 Time Variations Based on Winter Data from Kaliningrad Station in 2008−2010

The study presents the results of the analysis of the F2-layer critical frequency variations obtained for the winter periods of 2008–2010, during which sudden stratospheric warmings were observed. The data were obtained at Kaliningrad ionospheric station (54.6° N, 20° E) with the Parus digital ionosonde in standard sounding mode. The mean daily foF2 values were used in the analysis. The results of spectral analysis based on continuous wavelet transform showed that, during all of the warmings that occurred in 2008–2010, the foF2 time variations demonstrated the presence of wave processes with periods of approximately 5?6 days, as well as more extended processes with periods of ~10?13 and 23?30 days. These periods coincide with the characteristic periods of planetary waves observed in the mesosphere during sudden stratospheric warmings, while the 13- and 30-day periods can be conditioned by the influence of the Sun.     

Observations of 7-d planetary waves with MLT radars and the UARS-HRDI instrument

Long period variations in the mesosphere wind have been observed for some time by ground-based radars. These planetary scale disturbances have reoccurring periods at or near 5–7, 10, and 16 days and at times dominate the wind field at mesospheric heights. Recently, due to the continuous operation of several of the MLT radars and the availability of measurements from the UARS satellite, it has been possible to compare observations during periods of large planetary wave activity. Wind measurements from four MLT radars; the meteor radars at Durham, NH (43°N,71°W) and Sheffield, UK (53°N,2°W) and MF radars at Urbana, IL (40°N,88°W) and Saskatoon, Canada (52°N,107°W) were compared with the HRDI measurements during intervals when 7-d planetary waves were present. Wind data from the HRDI instrument on UARS has been processed to show the latitudinal structure and the seasonal variation of the planetary scale wind variation. The phases and amplitudes of the waves as determined by both the satellite and the radars are in good agreement. The ground-based measurements show large modulation of tides by these long period components, and also show comparable responses of these low frequency components over thousands of kilometers. The satellite and the ground-based results both indicate a preponderance of wave occurrence during the equinoxes and at preferred latitudes.     

Long-term tendencies in the MLT prevailing winds and tides over Antarctica as observed by radars at Molodezhnaya,Mawson and Davis

Long-term tendencies in horizontal neutral wind parameters in the southern polar mesosphere/lower thermosphere are presented. The wind data analyzed were obtained from meteor and MF radars situated at Molodezhnaya (45.9°E, 67.7°S), Mawson (62.9°E, 67.6°S) and Davis (78.0°E, 68.6°S). The composite dataset covers years from 1970 to 2006. A Bayesian approach in the form proposed by Wang and Zivot [2000. A Bayesian time series model of multiple structural changes in level, trend and variance. Journal of Business and Economic Statistics 8, 374–386] is used for the trend assessment. This approach allows structural breaks to be identified in the trend parameters (slope, mean or variance of residuals) or demonstrates their absence. The results of our analysis have shown persistence through such structural breaks in trends of the winter and summer prevailing winds and in meridional tidal components. It is demonstrated that the wind parameters exhibit different stable states with transitions between the states. Correlations between the southern polar MLT wind and indices of atmospheric variability (Northern annular mode, Southern annular mode, Multivariate El-Niño/Southern Oscillation Index) were then considered. The results show that statistically significant correlations exist during some periods of observations that do not exist during others.     

Large-scale thermodynamics of the stratosphere and mesosphere during the major stratospheric warming in 2003/2004

The stratosphere–mesosphere response to the major sudden stratospheric warming (SSW) in the winter of 2003/2004 has been studied. The UKMO (UK Meteorological Office) data set was used to examine the features of the large-scale thermodynamic anomalies present in the stratosphere of the Northern Hemisphere. The vertical and latitudinal structure of the genuine anomalies, emphasized by removing the UKMO climatology, has been investigated as well. The features of the stratospheric anomalies have been related to the mesospheric ones in measured neutral winds from radars and temperatures from meteor radars (90 km). It was found that the stratospheric warming spread to the lower mesosphere, while cooling occurred in the upper mesosphere, a feature that may be related to the large vertical scales of the stationary planetary waves (SPWs). It was shown also that the beginning of the eastward wind deceleration in the stratosphere–mesosphere system coincided with the maximum amplification of the SPW1 accompanied by short-lived bursts of waves 2 and 3.     

Mesospheric bore formation from large-scale gravity wave perturbations observed by collocated all-sky OH imager and sodium lidar

On 9 October 2007, long-horizontal-wavelength gravity waves were observed for the first time to steepen and form mesospheric bores at the altitude of ～87 km, by an all-sky OH imager located at Fort Collins (41°N, 105°W), Colorado. The collocated sodium lidar simultaneously observed the presence of a temperature inversion layer as the ducting region. One mesospheric bore uniquely later evolved into a large-amplitude soliton-like perturbation. When the gravity wave and the associated soliton-like perturbation passed through the lidar beams, the lidar detected strong vertical disturbance at 90 km, indicating convective instability. A large cold front system recorded several hours before in the troposphere was aligned to phase fronts of these large gravity waves. For all of the 7 mesospheric bores observed over a 5 year period, we found a similar alignment with a cold front 1000–1500 km away as the likely source of these large-scale gravity waves.