Realistic modeling of the winds and dynamical variations in the mesosphere and lower thermosphere (MLT) at Southern Hemisphere (SH) mid‐to‐high latitudes near 60°S where dramatic motions occur has ...been a challenge. This work presents an evaluation of the MLT zonal and meridional winds from ∼80 to 98 km altitude produced by the high‐altitude version of the Navy Global Environmental Model (NAVGEM‐HA) numerical weather prediction system during the Antarctic Sudden Stratospheric Warming (SSW) in September 2019. These results are compared with the coincident measurements by five meteor radars at Tierra del Fuego (TDF; 53.7°S, 67.7°W), King Edward Point (KEP; 54.3°S, 36.5°W), King Sejong Station (KSS; 62.2°S, 58.8°W), Rothera (ROT; 67.5°S, 68.0°W), and Davis (DAV; 68.6°S, 78.0°E) across SH mid‐to‐high latitudes. We find that the day‐to‐day variations in NAVGEM‐HA winds related to tidal motions are overall consistent with variations in the radar winds, and the daily mean winds have a correlation of 0.7–0.9 between them. Three‐hourly NAVGEM‐HA winds have a correlation of ∼0.5 and mean difference <10 m/s to the radar observations at most stations, and the Root Mean Square (RMS) error ranges from ∼25 to 35 m/s. Above 90 km altitude, the correlation coefficient decreases, and the difference and RMS error increase, indicating an upper limit to the validity of the NAVGEM‐HA results. Both the analyzed and observed winds reveal an enhancement in diurnal and semidiurnal tidal amplitude during this SH SSW. NAVGEM‐HA shows some evidence that nonmigrating tidal enhancements are produced through the interaction of migrating tides with planetary waves.
Plain Language Summary
High Altitude (HA) meteorological analysis products of the Navy Global Environmental Model (NAVGEM) that assimilates various observations are believed to be able to provide a realistic description of the state of the mesosphere and lower thermosphere (MLT). Dramatic motions of the MLT region are detected during austral winter in the Southern Hemisphere (SH) over the area extending from the Southern Andes to the Drake Passage and the Antarctic Peninsula. Yet, due to lack of global wind observations, this dynamically active region has not been well explored. Sudden Stratospheric Warmings (SSWs) are manifestations of dynamic disruptions in the winter polar area, and in September 2019 an unusual SSW occurred over Antarctica. This study evaluates the evolution of MLT winds and related tidal variations during this SSW, using both the NAVGEM‐HA analysis results and the meteor radar observations at several locations within this very dynamic region. We have performed a one‐to‐one comparison and found that the analyzed daily mean winds generally agree with the observations. The NAVGEM‐HA numerical forecast system also captures the enhanced tidal motions observed by the radars during this Antarctic SSW.
Key Points
High‐altitude meteorological analysis results with data assimilation are consistent with the meteor radar measurements near 60°S latitude
Day‐to‐day variations in the zonal and meridional winds at ∼80–90 km altitude are captured by the analysis system
Both analyzed and observed winds reveal a large tidal variation at ∼90 km altitude during the 2019 Antarctic stratospheric warming
Sudden stratospheric warmings (SSWs) could act as an important mediator in the vertical coupling of atmospheric regions and dramatic variations in the mesosphere and lower thermosphere (MLT) in ...response to SSWs have been documented. However, due to rare occurrences, SSWs in the Southern Hemisphere (SH) and their impacts on the MLT dynamics are not well understood. This study presents an analysis of MLT winds at ∼80–98 km altitudes measured by meteor radars located at Tierra del Fuego (53.7°S, 67.7°W), King Edward Point (54.3°S, 36.5°W) and King Sejong Station (62.2°S, 58.8°W) near 60°S latitude during the Antarctic winter. Eastward zonal winds from these stations are observed to decrease significantly near the peak date of the 2019 Antarctic SSW, and both zonal and meridional winds in 2019 exhibit considerable differences to the mean winds averaged over other non‐SSW years. A quasi 6‐day oscillation is observed at all three radar locations, being consistent with the presence of the westward propagating zonal wave‐1 planetary wave. The vertical wavelength of this wave is estimated to be ∼55 km, and the enhancement of the wave amplitude during this SSW is noticeable. Evidence of the interaction between the 6‐day wave and the semidiurnal diurnal tide is provided, which suggests a possible mechanism for SSWs to impact the upper atmosphere. This study reports the large‐scale variations in winds in the MLT region at SH midlatitudes to high latitudes in a key dynamic but largely unexplored latitudinal band in response to the 2019 Antarctic SSW.
Plain Language Summary
Sudden stratospheric warmings (SSWs) manifest dynamic disruptions in the polar winter stratosphere, characterized as rapid changes in temperature and wind within a few days. Although SSWs are by definition a stratospheric phenomenon, they have significant impacts throughout the middle and upper atmosphere. Many studies of the SSW impacts on the mesosphere and lower thermosphere (MLT) have been performed, mostly for the Northern Hemisphere (NH). In the Southern Hemisphere (SH), SSW events are rare and thus the Antarctic SSWs are not well known. An unusual SSW occurred in the SH during September 2019, and the work presented here focuses on studying the MLT winds observed by three meteor radars located at Tierra del Fuego (53.7°S, 67.7°W) in Argentina, King Edward Point (54.3°S, 36.5°W) on South Georgia Island, and King Sejong Station (62.2°S, 58.8°W) in King George Island. These observations are over a key dynamic but largely unexplored region around the Drake Passage. This study presents the large‐scale variations in the MLT winds at SH midlatitudes to high latitudes which are believed to be in response to the 2019 Antarctic SSW. Possible mechanisms for SSWs to impact the upper atmosphere are discussed.
Key Points
Large wind disturbances are observed at ∼80–98 km altitude ∼60°S by three meteor radars during the 2019 Antarctic stratospheric warming
A quasi 6‐day oscillation is observed at different longitudes indicating the presence of the westward propagating zonal wave‐1 wave
This study provides observational evidence of the nonlinear interaction between the 6‐day planetary wave and the semidiurnal tide
D‐Region High‐Latitude Forcing Factors Macotela, Edith L.; Clilverd, Mark; Manninen, Jyrki ...
Journal of geophysical research. Space physics,
January 2019, 2019-01-00, 20190101, Letnik:
124, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The subionospheric very low frequency (VLF) radio wave technique provides the possibility of investigating the response of the ionospheric D‐region to a diversity of transient and long‐term physical ...phenomena originating from above (e.g., energetic particle precipitation) and from below (e.g., atmospheric waves). In this study, we identify the periodicities that appear in VLF measurements and investigate how they may be related to changes in space weather and atmospheric activity. The powerful VLF signal transmitted from NAA (24 kHz) on the east coast of the United States, and received at Sodankylä, Finland, was analyzed. Wavelet transform, wavelet power spectrum, wavelet coherence, and cross‐wavelet spectrum were computed for daily averages of selected ionospheric, space weather, and atmospheric parameters from November 2008 until June 2018. Our results show that the significant VLF periods that appear during solar cycle 24 are the annual oscillation, semiannual oscillation, 121‐day, 86‐day, 61‐day, and solar rotation oscillations. We found that the annual oscillation corresponds to variability in mesospheric temperature and solar Lyman‐α (Ly‐α) flux and the semiannual oscillation to variability in space weather‐related parameters. The solar rotation oscillation observed in the VLF variability is mainly related to the Ly‐α flux variation at solar maximum and to geomagnetic activity variation during the declining phase of the solar cycle. Our results are important since they strengthen our understanding of the Earth's D‐region response to solar and atmospheric forcing.
Key Points
D‐region periodic variations are studied using wavelet analysis of 10 years of high‐latitude subionospheric VLF waves
Mesospheric temperature is more related to daytime VLF annual oscillations but nighttime is more related to solar Lyman‐α flux
The 27‐day solar rotation period dominates solar declining phase and all periods shorter than annual are transitory
Energetic electrons are deposited into the atmosphere from Earth's inner magnetosphere, resulting in the production of odd nitrogen (NOx). During polar night, NOx can be transported to low altitudes, ...where it can destroy ozone, affecting the atmospheric radiation balance. Since the flux of energetic electrons trapped in the magnetosphere is related to solar activity, the precipitation of these electrons into Earth's atmosphere provides a link between solar variability and changes in atmospheric chemistry which may affect Earth's climate. To determine the global distribution of the precipitating flux, we have built a statistical model binned by auroral electrojet (AE) index, magnetic local time (MLT), and L shell of E > 30 keV precipitating electrons from the Medium Energy Proton and Electron Detector (MEPED) on board the NOAA Polar Orbiting Environmental Satellites (POES) low‐altitude satellites NOAA‐15, NOAA‐16, NOAA‐17, and NOAA‐18. We show that the precipitating flux increases with geomagnetic activity, suggesting that the flux is related to substorm activity. The precipitating fluxes maximize during active conditions where they are primarily seen outside of the plasmapause on the dawnside. The global distribution of the precipitating flux of E > 30 keV electrons is well‐correlated with the global distribution of lower‐band chorus waves as observed by the plasma wave experiment onboard the Combined Release and Radiation Effects Satellite (CRRES) satellite. In addition, the electron precipitation occurs where the pitch angle diffusion coefficient due to resonant interaction between electrons and whistler mode chorus waves is high, as calculated using the pitch angle and energy diffusion of ions and electrons (PADIE) code. Our results suggest that lower‐band chorus is very important for scattering >30 keV electrons from Earth's inner magnetosphere into the atmosphere.
Long‐term variabilities of monthly zonal (U) and meridional winds (V) in northern polar mesosphere and lower thermosphere (MLT, ∼80–100 km) are investigated using meteor radar observations during ...1999–2022 over Esrange (67.9°N, 21.1°E). The summer (June‐August) mean zonal winds are characterized by westward flow up to ∼88–90 km and eastward flow above this height. The summer mean meridional winds are equatorward with strong jet at ∼85–90 km and it weakens above this height. The U and V exhibit strong interannual variability that varies with altitude and month or season. The responses of U and V anomalies (from 1999 to 2003) to solar cycle (SC), Quasi Biennial Oscillation at 10 and 30 hPa, El Niño‐Southern Oscillation, North Atlantic Oscillation, ozone (O3) and carbon dioxide (CO2) are analyzed using multiple linear regression. From analysis, significant regions of correlations between MLT winds and above potential drivers vary with altitude and month. The positive responses of U and V to SC (up to 15 m/s/100 sfu) indicates the strengthening of eastward winds in mid‐late winter, and poleward winds in late autumn and early winter. The O3 likely intensifies the eastward and poleward winds (∼100 m/s/ppmv) in winter and early spring. The CO2 significantly influence the eastward flow in late winter and summer (above ∼90–95 km) and strengthen the meridional circulation. The significant positive trend in U peaks in summer, late autumn and early winter (∼0.6 m/s/year), the negative trend in V is more prominent in summer above ∼90–95 km.
Plain Language Summary
The transition region between middle atmosphere and thermosphere is known as the mesosphere and lower thermosphere (MLT). The dynamics and circulation in this region are significant for global transport of important trace chemical species. Further the MLT winds play crucial role for the dynamical coupling of the middle and upper atmosphere. In the present study, the long‐term variability and tendencies in monthly mean zonal and meridional winds are investigated in the Arctic MLT between ∼80 and ∼100 km from meteor radar observations during 1999–2022 over Esrange (67.9°N, 21.1°E) in Sweden. The ability of radar provided the unique, consistent, and long‐term data set of polar MLT winds for duration of about two solar cycles. The MLT winds show important characteristic features and significant interannual variability that vary with altitude and month or season. In addition, the possible influence of climate forcings viz., solar activity, Quasi Biennial Oscillation (at 10 and 30 hPa), El Niño‐Southern Oscillation, North Atlantic Oscillation, ozone, and carbon dioxide on the variabilities of polar MLT winds has been analyzed using multiple linear regression. The significant interannual variabilities and tendencies in northern polar MLT zonal and meridional winds can be attributed to the above potential drivers.
Key Points
The long‐term variabilities in arctic mesosphere and lower thermosphere winds in response to potential climate forcings have been investigated for 1999–2022 over Esrange
The variability in U and V significantly correlated with O3 in winter and early spring, and with CO2 in summer based on altitude
The interannual variability in U and V is found to vary with altitude and month or season
The mesosphere and lower thermosphere (MLT) plays a critical role in linking the middle and upper atmosphere. However, many General Circulation Models do not model the MLT and those that do remain ...poorly constrained. We use long‐term meteor radar observations (2005–2021) from Rothera (67°S, 68°W) on the Antarctic Peninsula to evaluate the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) and investigate interannual variability. We find some significant differences between WACCM‐X and observations. In particular, at upper heights, observations reveal eastwards wintertime (April–September) winds, whereas the model predicts westwards winds. In summer (October–March), the observed winds are northwards but predictions are southwards. Both the model and observations reveal significant interannual variability. We characterize the trend and the correlation between the winds and key phenomena: (a) the 11‐year solar cycle, (b) El Niño Southern Oscillation, (c) Quasi‐Biennial Oscillation and (d) Southern Annular Mode using a linear regression method. Observations of the zonal wind show significant changes with time. The summertime westwards wind near 80 km is weakening by up to 4–5 ms−1 per decade, whilst the eastward wintertime winds around 85–95 km are strengthening at by around 7 ms−1 per decade. We find that at some times of year there are significant correlations between the phenomena and the observed/modeled winds. The significance of this work lies in quantifying the biases in a leading General Circulation Model and demonstrating notable interannual variability in both modeled and observed winds.
Plain Language Summary
The mesosphere and lower thermosphere (MLT), at heights of 80–100 km is an important region for the coupling of the middle and upper atmosphere. We carry out a study of the winds above Rothera (Antarctic Peninsula) for the years 2005–2021. We use observations from a meteor radar which measures winds at heights of 80–100 km and compare with the eXtended version of the Whole Atmosphere Community Climate Model (WACCM‐X), a leading general circulation model. We find that although most of the seasonal cycle in the winds is captured well, WACCM‐X exhibits biases in the winds at upper heights. In wintertime, the zonal winds are westwards whereas in observations they are eastward. In summertime WACCM‐X model meridional winds at 90–100 km are southwards but observations northwards. The observed and modeled winds also display significant interannual variability. We characterize the trends of the winds and the correlation with various drivers (the 11‐year solar cycle, El Niño Southern Oscillation, the Quasi‐Biennial Oscillation and the Southern Annular Mode), using a multi‐linear regression method. The study uses a uniquely long data set of Antarctic MLT winds to test and further develop general circulation models and quantifies the relationship between these winds and drivers such as the solar cycle.
Key Points
We characterize the variability of monthly mean winds in the mesosphere and lower thermosphere (MLT) over 17 years at Rothera using meteor radar observations and the eXtended version of the Whole Atmosphere Community Climate Model (WACCM‐X)
WACCM‐X displays biases in the wintertime winds in the upper MLT. Observed winds are eastwards whilst WACCM‐X winds are westwards
Significant variability, trends and intermittent correlations with the solar cycle, Quasi‐Biennial Oscillation and Southern Annular Mode are found in the observed and modeled winds
The solar tides of the mesosphere and lower thermosphere (MLT) show great variability on time scales of days to years, with significant variability at interannual time scales. However, the nature and ...causes of this variability remain poorly understood. Here, we present measurements made over the interval 2005–2020 of the interannual variability of the 12‐hr tide as measured at heights of 80–100 km by a meteor radar over Rothera (68°S, 68°W). We use a linear regression analysis to investigate correlations between the 12‐hr tidal amplitudes and several climate indices, specifically the solar cycle (as measured by F10.7 solar flux), El Niño Southern Oscillation (ENSO), the Quasi‐Biennial Oscillation (QBO) at 10 and 30 hPa and the Southern Annular Mode (SAM). Our observations reveal that the 12‐hr tide has a large amplitude and a clearly defined seasonal cycle with monthly mean values as large as 35 m s−1. We observe substantial interannual variability, with monthly mean 12‐hr tidal amplitudes at 95 km exhibiting a two standard‐deviation range (2σ) in spring of 13.4 m s−1, 11.2 m s−1 in summer, 18.6 m s−1 in autumn, and 7.0 m s−1 in winter. We find that F10.7, QBO10, QBO30, and SAM all have significant correlations to the 12‐hr tidal amplitudes at the 95% level, with a linear trend also present. Whereas we detect very minimal correlation with ENSO. These results suggest that variations in F10.7, the QBO and SAM may contribute significantly to the interannual variability of 12‐hr tidal amplitudes in the Antarctic MLT.
Key Points
Substantial interannual variability in monthly mean amplitudes of the 12‐hr tide above Rothera is characterized
The climate indices F10.7, QBO10, QBO30, and SAM display significant correlations with interannual variability of 12‐hr tidal amplitudes
El Niño Southern Oscillation does not show a significant correlation to the 12‐hr tidal amplitudes over Rothera, suggesting no linear link
In this study we explore the seasonal variability of the mean winds and diurnal and semidiurnal tidal amplitude and phases, as well as the Reynolds stress components during 2019, utilizing meteor ...radars at six Southern Hemisphere locations ranging from midlatitudes to polar latitudes. These include Tierra del Fuego, King Edward Point on South Georgia island, King Sejong Station, Rothera, Davis, and McMurdo stations. The year 2019 was exceptional in the Southern Hemisphere, due to the occurrence of a rare minor stratospheric warming in September. Our results show a substantial longitudinal and latitudinal seasonal variability of mean winds and tides, pointing towards a wobbling and asymmetric polar vortex. Furthermore, the derived momentum fluxes and wind variances, utilizing a recently developed algorithm, reveal a characteristic seasonal pattern at each location included in this study. The longitudinal and latitudinal variability of vertical flux of zonal and meridional momentum is discussed in the context of polar vortex asymmetry, spatial and temporal variability, and the longitude and latitude dependence of the vertical propagation conditions of gravity waves. The horizontal momentum fluxes exhibit a rather consistent seasonal structure between the stations, while the wind variances indicate a clear seasonal behavior and altitude dependence, showing the largest values at higher altitudes during the hemispheric winter and two variance minima during the equinoxes. Also the hemispheric summer mesopause and the zonal wind reversal can be identified in the wind variances.
Atmospheric 8‐ and 6‐hr tides are observed for the first time in the zonal and meridional winds at ~82–97 km altitudes simultaneously at Tierra del Fuego (TDF; 53.7°S, 67.7°W), King Edward Point ...(KEP; 54.3°S, 36.5°W), and Rothera (ROT; 67.5°S, 68.0°W) at Southern Hemisphere (SH) middle‐to‐high latitudes during long time spans, allowing to reveal climatology and migrating nature. The monthly averaged amplitudes vary between ~1 and 8 m/s for the 8‐hr tides while the amplitudes of 6‐hr tides are smaller ~0.5–4 m/s. Both tides exhibit an annual pattern having the amplitude maxima during SH winter and minima in SH summer. The tidal phases are smaller (earlier) in the zonal wind than in the meridional wind by about 90°. The phase differences observed between TDF and KEP, which are located at similar latitudes but different longitudes suggest the propagation of migrating tides. The study finds that 8‐ and 6‐hr tides are correlated.
Plain Language Summary
Atmospheric oscillations with 8‐ and 6‐hr periods at middle‐to‐high latitudes in the Southern Hemisphere are poorly understood due to the lack of measurements. In this study, we report the climatology of these oscillations in the altitude range from ~82–97 km in the mesosphere 13 and lower thermosphere through the analysis of the coordinated observations by three meteor radars located at Rio Grande, Tierra del Fuego (TDF; 53.7°S, 67.7°W) in Argentina, King Edward Point station (KEP; 54.3°S, 36.5°W) on South Georgia Island, and Rothera station (ROT; 67.5°S, 68.0°W) on Adelaide Island west of the Antarctic Peninsula. The oscillations are observed in both zonal and meridional winds with the zonal component leading the meridional component corresponding to the counterclockwise rotation. The observations from TDF and KEP at similar latitudes suggest that the oscillations are propagating westward in the phase speed of ~360° longitude/day following the apparent motion of the Sun. Approximately 11 years of continuous observations at TDF and ROT show that the long‐term changes between 8‐ and 6‐hr oscillations are correlated with each other. These short‐period tides should have significant impacts on the variability of the thermosphere and the ionosphere.
Key Points
First study of 8‐ and 6‐hr tides from coordinated observations of three meteor radars at Southern Hemisphere middle‐to‐high latitudes is presented
First time‐measured longitudinal phase differences between two sites suggest the propagation of migrating 8‐ and 6‐hr tides
Correlation between 8‐ and 6‐hr tides is identified for the first time, suggesting that these tides are generated by the same source
Geosynchronous Los Alamos National Laboratory (LANL‐97A) satellite particle data, riometer data, and radio wave data recorded at high geomagnetic latitudes in the region south of Australia and New ...Zealand are used to perform the first complete modeling study of the effect of substorm electron precipitation fluxes on low‐frequency radio wave propagation conditions associated with dispersionless substorm injection events. We find that the precipitated electron energy spectrum is consistent with an e‐folding energy of 50 keV for energies <400 keV but also contains higher fluxes of electrons from 400 to 2000 keV. To reproduce the peak subionospheric radio wave absorption signatures seen at Casey (Australian Antarctic Division), and the peak riometer absorption observed at Macquarie Island, requires the precipitation of 50–90% of the peak fluxes observed by LANL‐97A. Additionally, there is a concurrent and previously unreported substorm signature at L < 2.8, observed as a substorm‐associated phase advance on radio waves propagating between Australia and New Zealand. Two mechanisms are discussed to explain the phase advances. We find that the most likely mechanism is the triggering of wave‐induced electron precipitation caused by waves enhanced in the plasmasphere during the substorm and that either plasmaspheric hiss waves or electromagnetic ion cyclotron waves are a potential source capable of precipitating the type of high‐energy electron spectrum required. However, the presence of these waves at such low L shells has not been confirmed in this study.
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