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.     

Midlatitude mesopause region winds and waves and comparison with stratospheric variability

We present time series of January–May mean mesosphere/lower thermosphere (MLT) mean winds and planetary wave (PW) proxies over Europe together with stratospheric stationary planetary waves (SPW) at 50°N and time series of European ozone laminae occurrence. The MLT winds are connected with stratospheric PW and laminae at time scales of several years to decades. There is a tendency for increased wave activity after 1990, together with more ozone laminae and stronger MLT zonal winds. However, possible coupling processes are not straightforward. While mean MLT winds before the 1990s show similar interannual variations than stratospheric PW at 100 hPa, later a tendency towards a connection of the MLT with the middle stratosphere SPW is registered. There is also a tendency for a change in the correlation between lower and middle stratosphere SPW, indicating that coupling processes involving the European middle atmosphere from the lower stratosphere to the mesopause region have changed.     

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.     

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.     

External impact on wind in the mesosphere/lower thermosphere region

Regular measurements of the velocity and direction of the horizontal wind in the mesosphere/lower thermosphere (MLT) region at a height of ∼95 km have been conducted since 1975 over Eastern Siberia (Badary observatory near Irkutsk), using the spaced-diversity reception method in the LF range. The accumulated database of measurement results (for more than 20 years, from 1974 to 1996) makes it possible to get information on the impact on wind in the MLT region from both below (stratospheric warmings) and above (geomagnetic storms as a consequence of magnetospheric disturbances) with sufficient statistical reliability. Effects of stratospheric warmings and strong geomagnetic storms in the prevailing wind and amplitude of the semidiurnal tide are evaluated by the superposed epoch method. It is shown that the effects of stratospheric warmings depend on the type (intensity) of stratospheric warming and on the phase of quasi-biennial oscillations of the wind in the equatorial stratosphere at the 30 hPa level. The response of MLT winds to external impacts is different for the 21st and 22nd cycles of solar activity. Effects of geomagnetic storms (A _p > 100) are manifested in the decrease in the eastward prevailing wind and increase in the semidiurnal tide amplitude.     

Long-term changes of (polar) mesosphere summer echoes

Strong VHF radar echoes have been observed not only during summer months at polar latitudes (polar mesosphere summer echoes, PMSE) but also at middle latitudes (mesosphere summer echoes, MSE). These echoes are closely connected with small ice particles, thus containing information about mesospheric temperature and water vapour content. But the (P)MSE also depend on the ionisation due to solar wave radiation and precipitating high energetic particles. Observations with VHF radars at Andenes (69.3°N; 16.0°E) since 1994 and at Kühlungsborn (54.6°N; 11.8°E) since 1998 are used for investigations of the solar and geomagnetic control of the (P)MSE as well as of possible long-term changes. The (P)MSE are positively correlated with the solar Lyman α radiation and the geomagnetic activity and have slightly positive trends. Due to the limited measuring period, the significance levels of the detected (P)MSE trends are small. Positive trends in noctilucent clouds (NLC) and polar mesospheric clouds (PMC) are in general agreement with (P)MSE trends.     

First results of warm mesospheric temperature over Gadanki (13.5°N, 79.2°E) during the sudden stratospheric warming of 2009

Rayleigh lidar observations at Gadanki (13.5°N, 79.2°E) show an enhancement of the nightly mean temperature by 10–15 K at altitudes 70–80 km and of gravity wave potential energy at 60–70 km during the 2009 major stratospheric warming event. An enhanced quasi-16-day wave activity is observed at 50–70 km in the wavelet spectrum of TIMED–SABER temperatures, possibly due to the absence of a critical level in the low-latitude stratosphere because of less westward winds caused by this warming event. The observed low-latitude mesospheric warming could be due to wave breaking, as waves are damped at 80 km.     

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.     

Regional response of the mesosphere–lower thermosphere dynamics over Scandinavia to solar proton events and geomagnetic storms in late October 2003

The short-term regional responses of the mesosphere–lower thermosphere (MLT) dynamics over Scandinavia to the exceptionally strong solar storms with their accompanying solar proton fluxes on the Earth in late October 2003 have been investigated using radar measurements at Andenes (69°N, 16°E) and Esrange (68°N, 21°E). Several solar activity storms resulted in solar proton events (SPEs) at this time, but a particularly active period of high proton fluxes occurred between 28 and 31 October 2003. The significant temperature drop (∼25 K), detected by the meteor radar at Andenes at altitude ∼90 km, was in line with the enhancement of the proton fluxes and was caused by the dramatic reduction of the ozone in the high-latitude middle atmosphere monitored by satellite measurements. This exceptionally strong phenomenon in late October 2003 was composed of three geomagnetic storms, with the first one occurring in the daytime of 29 October and the other two storms in the nighttime of 29 and 30 October, respectively. The responses of the prevailing wind and the main tides (24- and 12-h tides) were studied in detail. It was found that the response of the MLT dynamics to the first geomagnetic storm occurring in the daytime and accompanied by solar proton fluxes is very different from those to the second and third geomagnetic storms with onsets during the nighttime. Some physical mechanisms have been suggested in order to explain the observed short-term variability of the MLT dynamics. This case study revealed the impact of the SPEs observed in late October 2003 and the timing of the geomagnetic storms on the MLT neutral wind responses observed over Scandinavia.     

Potassium Doppler-resonance lidar for the study of the mesosphere and lower thermosphere at the Arecibo Observatory

We have developed a lidar to study the temperature structure of the nighttime mesopause region over the Arecibo Observatory (18.35°N, 66.75°W) by measuring the lineshape of the fluorescence spectrum of atomic potassium that is deposited in the mesosphere and lower thermosphere (MLT) by meteors. To demonstrate how the potassium lidar can enhance MLT studies at Arecibo, we show recent results for: (1) comparisons with airglow temperature measurements; (2) simultaneous operations with stratospheric and mesospheric temperature profiling by Rayleigh lidar; (3) simultaneous observations of K, Ca⁺, and E-region electron density profiles; and (4) occurrences of sporadic K layers, and relationships to sporadic E layers.     

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.     

First results of convectively generated long-period Kelvin waves in the low-latitude mesosphere during Indian summer monsoon

Spectral analysis of Tirunelveli (8.7°N, 77.8°E) MF radar winds for the year 2007 indicate the presence of long-period Kelvin waves with periods ～23 and ～16 days in the low-latitude mesosphere during Indian summer monsoon months. The dominant presence of these slow-phase speed waves at mesospheric altitudes motivated us to investigate their origin and vertical propagation characteristics. Space-time Fourier analysis of NCEP winds and OLR show the presence of these periodicities with zonal wavenumber 1 indicating that tropical convection is the potential source for these waves and westward phase of stratospheric QBO winds might have favoured these waves to reach the mesosphere.     

Semidiurnal tides from the extended Canadian Middle Atmosphere Model (CMAM) and comparisons with TIMED Doppler interferometer (TIDI) and meteor radar observations

The extended Canadian Middle Atmosphere Model (extended CMAM) is a general circulation model, which extends from the surface to about 210 km. Spatial complex spectral analysis is applied to horizontal winds simulated by the extended CMAM to obtain semidiurnal tidal amplitudes and phases (from e5 to w5) in the mesosphere and lower thermosphere (MLT) region. The dominant w2 migrating component and the presence of eight nonmigrating tides (w3, w4, w5, e1, e2, e3, e4 and e5) in the mid-latitudes are identified. Components w1 and s0, which tend to maximize at high latitudes, will be discussed separately in a later paper. The migrating semidiurnal tide (w2) has amplitudes reaching over 20 m s⁻¹ for both zonal and meridional winds in the mid-latitude region. Its form compares well to the published results. The amplitudes of nonmigrating semidiurnal tides are non-negligible compared with the migrating semidiurnal tides. The amplitudes for w3 and e2 exceed 12 and 8 m s⁻¹, respectively.Comparisons are made with four nonmigrating semidiurnal components (w3, w4, e1 and e2) derived from the TIMED Doppler interferometer (TIDI) wind measurements between 85 and 105 km altitude and between 45°S and 45°N latitude. Overall, the basic CMAM and TIDI latitudinal structures of the amplitudes agree well and the agreement between the annual mean amplitudes varies with component. Relative to the TIDI results, the CMAM seasonal variations of w4 are in good agreement, of e2 are in reasonable agreement, of w3 are in partial agreement and of e1 are in poor agreement.The 11 semidiurnal components from the model are superimposed to generate the total semidiurnal winds at Jakarta (6°S, 106°E) and Kototabang (0°, 100°E) and are compared with measurements from two equatorial meteor radar stations at these sites. The relative contributions of components to the reconstructed amplitude vary from month to month. The CMAM reconstructions are generally larger than the radar results by a factor varying between one and two. The phases in the radar data are typically stationary with respect to height, whereas they generally decrease with height in the CMAM reconstruction.     

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.     

Variations in zonal mean and planetary wave properties of the stratosphere and links with the troposphere

Zonal mean data and amplitudes and phases of planetary zonal waves were derived from daily hemispheric maps for tropospheric and stratospheric levels, for the four winters 1975–76 to 1978–79. Important year-to-year fluctuation in zonal means and wave activity are described, most notable of which are the changes from 1975–76 to 1976–77. Comparison of the relative strengths of the stratospheric and tropospheric jet streams shows a strong negative correlation (–0.8) between monthly mean zonal stratospheric winds (at 10 mb, 65°N) and zonal tropospheric winds (at 200 mb, 32.5°N, in the jet core) and a positive correlation (+0.7) between the stratospheric 10 mb winds and the tropospheric 200 mb winds at 65°N. Parameters correlated were the departures from the climatological mean zonal winds. The structure of correlation between wave amplitudes in the same wave number (1, 2) at different altitudes and between wave numbers 1 and 2 is investigated. We find a high correlation (+0.93) between wave 1 in the stratosphere (10 mb height) and wave 2 (height) in the troposphere at 65°N; but only a weak correlation (+0.2) between wave 1 amplitudes in the stratosphere and troposphere. These results suggest the possible importance of wave-wave interactions in processes linking the stratosphere and troposphere. The wave correlations presented here are based on comparisons of monthly means of daily amplitudes; the correlation structure in individual wave developments may differ, in view of the likelihood of altitudinal lags in wave amplification.     

Stratospheric warming effects on the tropical mesospheric temperature field

Temperature observations at 20–90 km height and 5–15°N during the winter of 1992–1993, 1993–1994 and 2003–2004, from the Wind Imaging Interferometer (WINDII) and Microwave Limb Sounder (MLS) experiments on the Upper Atmosphere Research Satellite (UARS) satellite and the Sounding the Atmosphere using Broadband Emission Radiometry (SABER) experiment on the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite are analyzed together with MF radar winds and UK Meteorological Office (UKMO) assimilated fields. Mesospheric cooling is observed at the time of stratospheric warming at the tropics correlative with stratospheric warming events at middle and high latitudes. Planetary waves m=1 with periods of 4–5, 6–8, 10 and 12–18 days are found to dominate the period. Westward 7- and 16-day waves at the tropics appear enhanced by stationary planetary waves during sudden stratospheric warming events.     

On the application of a proposed global system for measuring meteor winds

Summary This paper discusses the need for a global network of meteor wind stations for determining the general circulation of the upper mesosphere and lower thermosphere. Continuous observations of horizontal motions from such a network would permit resolution of planetary scale eddy winds, tides, and gravity waves, and hypotheses that such motions propagate vertically from the lower atmosphere or are generated in situ by solar activity could be examined critically with observational data. The observed mean winds from the lower stratosphere to the meteor wind level are summarized to support the hypothesis that a standing wave pattern in the winds extends into the lower thermosphere. Data on tidal meridional momentum transports from meteor wind stations suggest that tides in the lower thermosphere are important for the maintenance of mean winds. Some of the geomagnetic and photochemical processes in the lower thermosphere that could be investigated with meteor wind data are briefly reviewed.This paper is adapted from our presentation at the 1966 Fall URSI meeting at Palo Alto, California     

Meteor wind radar observations of tidal amplitudes over a low-latitude station Trivandrum (8.5°N, 77°E): Interannual variability and the effect of background wind on diurnal tidal amplitudes

Based on the horizontal winds measured using SKiYMET meteor wind radar during the period of June 2004–May 2007, the seasonal and interannual variability of the diurnal and semidiurnal amplitudes and phases in the mesospheric and lower thermospheric (MLT) region over a low-latitude station Trivandrum (8.5°N) are investigated. The monthly values of amplitudes and phases are calculated using a composite day analysis. The zonal and meridional diurnal tidal amplitudes exhibit both annual and semiannual oscillations. The zonal and meridional components of semidiurnal tide show a significant annual oscillation. The phase values of both diurnal and semidiurnal tides exhibit annual oscillation above 90 km. The effect of background wind in the lower atmosphere on the strength of diurnal tidal amplitudes in the MLT region is studied. The effect of diurnal tides on the background wind in the lower thermosphere is also discussed.     

Planetary waves observed by TIMED/SABER in coupling the stratosphere–mesosphere–lower thermosphere during the winter of 2003/2004: Part 1—Comparison with the UKMO temperature results

Part 1 of the present paper is focused on the types of planetary wave seen in the TIMED/SABER and UK Met Office (UKMO) temperature data in the Northern Hemisphere (NH) (0–50°N) stratosphere (30–60 km) during the Arctic winter of 2003/2004, as the emphasis is on their spatial structure (latitude and altitude) and temporal evolution particularly in relation to the stratospheric warmings. A new method for analysis of satellite data is presented in this study where the migrating and nonmigrating tides and planetary waves (stationary, zonally symmetric and travelling) are simultaneously extracted from the satellite data. The comparison between the altitude and latitude structure of the SABER and UKMO planetary waves in the temperature field of the NH stratosphere indicates a high degree of qualitative and quantitative resemblance and in this way the validity of the new data analysis method is verified as well.     

Resonant interaction between two planetary waves as a precursor for stratospheric warmings?

Stratospheric warmings are attributed to an enhanced planetary wave activity, occurring nearly each winter – at least in the northern hemisphere – with different strengths. The generation of stratospheric warmings is not totally understood. One of the most promising explanations is the interaction of planetary waves: in many cases, the amplitude of the quasi-stationary planetary wave 1 builds up, until it transmits its momentum and energy to the background wind field. The role of wave 2 is usually considered to be less important.Based on ERA-40 and DYANA temperature data (January–February 1990), we found evidence that a resonant wave–wave interaction between a travelling and a stationary wave 2 was responsible for a minor stratospheric warming in February 1990. The interaction being observed during four weeks can eventually be used as an indication for an upcoming stratospheric warming.