Variability in the ionosphere during the 2020–2021 sudden stratospheric warming (SSW) is investigated using a combination of Constellation Observing System for Meteorology, Ionosphere, and Climate-2 ...(COSMIC-2) observations and the Whole Atmosphere Community Climate Model with thermosphere–ionosphere eXtension (WACCM-X) simulations. The unprecedented spatial–temporal sampling of the low latitude ionosphere afforded by COSMIC-2 enables investigating the short-term (<5 days) variability in the ionosphere during the SSW event. The COSMIC-2 observations reveal a reduction in the diurnal and zonal mean ionosphere total electron content (ITEC) and reduced amplitude of the diurnal variation in the ionosphere during the SSW. Enhanced ITEC amplitudes of the semidiurnal solar and lunar migrating tides and the westward propagating semidiurnal tide with zonal wavenumber 3 are also observed. The WACCM-X simulations demonstrate that these variations are driven by variability in the stratosphere–mesosphere during the 2020–2021 SSW event. The results show the impact of the 2020–2021 SSW on the mean state, diurnal, and semidiurnal variations in the ionosphere, as well as the capabilities of the COSMIC-2 mission to observe short-term variability in the ionosphere that is driven by meteorological variability in the lower atmosphere.
An interhemispheric asymmetry of thermospheric Oxygen (O) to Nitrogen (N2) column density ratio (∑O/N2) variations was observed by the Global‐scale Observations of the Limb and Disk (GOLD) during the ...3–5 November 2021 geomagnetic storm. ∑O/N2 depletion is more equatorward in the local morning than in the local afternoon in the northern hemisphere (NH), while more equatorward in the local afternoon than the local morning in the southern hemisphere (SH). The Thermosphere Ionosphere Electrodynamics General Circulation Model simulations reproduced the observed ∑O/N2 asymmetry. The impacts of interplanetary magnetic field (IMF) east‐west component (By) on the interhemispheric asymmetry of ∑O/N2 depletion was investigated. Comparisons of simulation results with and without real By show that the dominant positive IMF By results in the Joule heating maximum occurring in the local afternoon of NH and local morning in the SH during the storm initial phase, which generates the corresponding ∑O/N2 changes. Additionally, the IMF By during this storm are more crucial in the SH than in the NH. Without By, equatorward penetration of the ∑O/N2 depletion in GOLD Field of View are almost same in the SH. However, in the NH, there is still more equatorward depletion in the local morning than the local afternoon without By.
Key Points
Global‐scale Observations of the Limb and Disk (GOLD) observed an interhemispheric asymmetry of ∑O/N2 depletion during the storm on 4 November 2021, and was reproduced by model
Dominant positive interplanetary magnetic field By enhances magnitude of depletions in both hemispheres compared with the ones without By
Without By, equatorward penetration of the ∑O/N2 depletion in GOLD Field of View are almost same in the southern hemisphere
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
3.
Sudden Stratospheric Warmings Baldwin, Mark P.; Ayarzagüena, Blanca; Birner, Thomas ...
Reviews of geophysics,
March 2021, Volume:
59, Issue:
1
Journal Article
Peer reviewed
Open access
Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (∼10–50 km) are associated with a complete ...reversal of the climatological wintertime westerly winds. SSWs are caused by the breaking of planetary‐scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid descent and warming of air in polar latitudes, mirrored by ascent and cooling above the warming. The rapid warming and descent of the polar air column affect tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode, making cold air outbreaks over North America and Eurasia more likely. SSWs affect the atmosphere above the stratosphere, producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (nonionized) particles and electron densities, and electric fields. These effects span both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal prediction. This work reviews the current knowledge on the most important aspects of SSWs, from the historical background to dynamical processes, modeling, chemistry, and impact on other atmospheric layers.
Plain Language Summary
The stratosphere is the layer of the atmosphere from ∼10 to 50 km, with pressures decreasing to ∼1 hPa (0.1% of surface pressure) at the top. The polar stratosphere during winter is normally very cold, with strong westerly winds. Roughly every 2 years in the Northern Hemisphere, the quiescent vortex suddenly warms over a week or two, and the winds slow dramatically, resulting in easterly winds that are more similar to the summer. These events, known as sudden stratospheric warmings (SSWs), were discovered in the early 1950s, and today, they are observed in detail by satellites. After several decades researching SSWs, considerable progress has been made in dynamical aspects of SSWs, but our understanding of how they affect both surface weather and the upper atmosphere is incomplete. We observe that variability of the stratospheric circulation (SSWs being an extreme event) is associated with shifts in the jet stream and the paths of storms, with associated effects on rainfall and temperatures. The likelihood of cold weather spells and damaging wind storms is also affected. Almost all SSWs have occurred in the Northern Hemisphere, but there was one spectacular major SSW in 2002 in the Southern Hemisphere.
Key Points
Sudden stratospheric warmings are dramatic events of the polar stratosphere that affect the atmosphere from the surface to the thermosphere
Our understanding of sudden stratospheric warmings has accelerated recently, particularly the predictability of surface weather effects
More observations, improved climate models, and big data methods will address uncertainties in key aspects of sudden stratospheric warmings
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Thermospheric gravity waves (GWs) in the bottomside F region have been proposed to play a key role in the generation of equatorial plasma bubbles (EPBs). However, direct observations of such waves ...are scarce. This study provides a systematic survey of medium‐scale (<620 km) neutral atmosphere perturbations at this critical altitude in the tropics, using 4 years of in situ Gravity Field and Steady‐State Ocean Circulation Explorer satellite measurements of thermospheric density and zonal wind. The analysis reveals pronounced features on their global distribution and seasonal variability: (1) A prominent three‐peak longitudinal structure exists in all seasons, with stronger perturbations over continents than over oceans. (2) Their seasonal variation consists of a primary semiannual oscillations (SAO) and a secondary annual oscillation (AO). The SAO component maximizes around solstices and minimizes around equinoxes, while the AO component maximizes around June solstice. These GW features resemble those of EPBs in spatial distribution but show opposite trend in climatological variations. This may imply that stronger medium‐scale GW activity does not always lead to more EPBs. Possible origins of the bottomside GWs are discussed, among which tropical deep convection appears to be most plausible.
Key Points
Medium‐scale thermospheric GWs in bottomside F region show similar longitudinal distribution as equatorial plasma bubbles
Thermosphere GWs show opposite seasonal variation to that of equatorial plasma bubbles
Thermospheric GWs in the tropics are likely secondary waves generated by deep convection
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This study investigates the day-to-day variability of equatorial plasma bubbles (EPBs) over the Atlantic–American region and their connections to atmospheric planetary waves during the sudden ...stratospheric warming (SSW) event of 2021. The investigation is conducted on the basis of the GOLD (Global Observations of the Limb and Disk) observations, the ICON (Ionospheric Connection Explorer) neutral wind dataset, ionosonde measurements, and simulations from the WACCM-X (Whole Atmosphere Community Climate Model with thermosphere–ionosphere eXtension). We found that the intensity of EPBs was notably reduced by 35% during the SSW compared with the non-SSW period. Furthermore, GOLD observations and ionosonde data show that significant quasi-6-day oscillation (Q6DO) was observed in both the intensity of EPBs and the localized growth rate of Rayleigh–Taylor (R-T) instability during the 2021 SSW event. The analysis of WACCM-X simulations and ICON neutral winds reveals that the Q6DO pattern coincided with an amplification of the quasi-6-day wave (Q6DW) in WACCM-X simulations and noticeable ∼6-day periodicity in ICON zonal winds. The combination of these multi-instrument observations and numerical simulations demonstrates that certain planetary waves like the Q6DW can significantly influence the day-to-day variability of EPBs, especially during the SSW period, through modulating the strength of prereversal enhancement and the growth rate of R-T instability via the wind-driven dynamo. These findings provide novel insights into the connection between atmospheric planetary waves and ionospheric EPBs.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The OI 630.0 nm airglow emission variability provides salient information on the dynamical changes taking place in the upper atmosphere at around 250 km. The emission rates vary with the changes in ...the ambient electron densities and the neutral constituents that are associated with these emissions. On several occasions, enhancements in these emissions are observed during post‐sunset hours, around 21 local time (LT), as measured from Mt. Abu (24.6°N, 72.7°E, 19°N Mag), a low‐latitude location at Indian longitudes. These enhancements occur following the typical monotonic decrease in emission intensity after sunset. The presence of poleward meridional wind preceded by cessation and reversal of equatorward wind at the post‐sunset hours was shown to be the cause for such observed emission enhancements in an earlier study. In this study, the cause of such reversal in meridional winds during post‐sunset hours has been investigated using the variation in electron densities and meridional winds simulated by the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X), which also shows enhancements in electron densities similar to those observed in the post‐sunset OI 630.0 nm nightglow emissions, and simultaneous reversal in meridional winds as well. The amplitudes and phases of different components of tides obtained from WACCM‐X meridional winds reveal a significant contribution of higher‐order tides, especially, quarter‐diurnal tides, to the observed reversal in the meridional winds during post‐sunset hours.
Plain Language Summary
Airglow emissions occur when atoms and molecules are deexcited after being excited by the solar energetic photons. OI 630.0 nm emissions (redline), which peak around 250 km and have a width of emissions around 100 km, act as tracers of that altitudinal region. Typically, the redline emissions decrease monotonically with time after sunset in the absence of solar radiation. However, on several occasions, an enhancement in redline emissions has been measured during post‐sunset hours over a low‐latitude location in the Indian longitudes. A previous study attributed this enhancement to the increase in electron density at the emission altitudes in the presence of poleward meridional winds, despite usually being equatorward during such times. The cause of such reversal in the equatorward winds and simultaneous increase in electron density at post‐sunset hours has been examined in this study using the variation in electron densities and winds simulated by the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X). Remarkably, the simulation results also show enhancements in electron density similar to those observed in redline emissions. A strong tidal contribution, especially quarter‐diurnal component, has been found to be the cause behind the poleward reversal of meridional winds after sunset that causes an enhancement in redline emissions.
Key Points
OI 630.0 nm post‐sunset emission enhancement over low‐latitudes is consistent with the presence of poleward meridional winds
On some days, WACCM‐X simulated meridional winds show a poleward wind reversal during post‐sunset hours
Quarter‐diurnal tides seem to play a significant role in reversing the meridional winds after sunset
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Abstract
The NCAR Whole Atmosphere Community Climate Model (WACCM) is used to investigate the dynamical influence of the lower and middle atmosphere on the upper mesosphere and lower thermosphere. In ...simulations using a methodology adapted from the “specified dynamics” (nudged) version of the model, horizontal winds and temperature over part of the vertical range of the atmosphere are relaxed toward results from a previous simulation that serves as the true simulation, equivalent to meteorological analysis. In the upper mesosphere, the magnitude of the divergence of the constrained simulations from the true simulation depends on the vertical extent and frequency of the data used for nudging the model and grows with altitude. The simulations quantify the error growth of the model dynamical fields when data and forcing terms are known exactly and there are no model biases. The error growth rate and the ultimate discrepancy between the nudged and true fields depend strongly on the method used for representing gravity wave drag. The largest error growth occurs when the gravity wave parameterization uses interactive wave sources that depend on convective activity or fronts. Errors are reduced when the same parameterization is used with smoothly varying specified wave sources. The smallest errors are seen when the parameterized gravity wave drag is replaced by linear Rayleigh friction damping on the wind speed. These comparisons demonstrate the role of gravity waves in transporting the variability of the troposphere into the mesosphere and lower thermosphere.
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We provide observational evidence that the stability of the stratospheric Polar vortex (PV) is a significant driver of sub‐seasonal variability in the thermosphere during geomagnetically quiet times ...when the PV is anomalously strong or weak. We find strong positive correlations between the Northern Annular Mode (NAM) index and subseasonal (10–90 days) Global Observations of the Limb and Disk (GOLD) O/N2 perturbations at low to mid‐northern latitudes, with a largest value of +0.55 at ∼30.0°N when anomalously strong or weak (NAM >2.5 or < −2.1) vortex times are considered. Strong agreement for O/N2 variability and O/N2‐NAM correlations is found between GOLD observations and the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) simulations, which is then used to delineate the global distribution of O/N2‐NAM correlations. We find negative correlations between subseasonal variability in WACCM‐X O/N2 and NAM at high northern and southern latitudes (as large as −0.54 at ∼60.0°S during anomalous vortex times). These correlations suggest that PV driven upwelling at low latitudes is accompanied by corresponding downwelling at high latitudes in the lower thermosphere (∼80–120 km), which is confirmed using calculations of residual mean meridional circulation from WACCM‐X.
Plain Language Summary
The stratospheric Polar vortex (PV) is a large‐scale circulation pattern that forms over the poles during the winter months, characterized by strong winds circulating in a counterclockwise direction in the Northern Hemisphere. It has previously been demonstrated that the breakup of the PV during sudden stratospheric warming (SSW) events has a large effect on the composition of the thermosphere. We demonstrate that the PV also influences thermospheric composition during non‐SSW times, including time periods in which the vortex is strong and times in which the vortex is inactive, by correlating observations of thermospheric O/N2 from the Global Observations of the Limb and Disk (GOLD) instrument with the Northern Annular Mode (NAM) index, which tracks the strength of the PV. We also find strong anti‐correlations between O/N2 and tidal amplitudes from model simulations, which suggests that the PV influences thermospheric composition via changes in mean circulation induced by the dissipation of enhanced waves. We use model simulations to confirm that the NAM‐O/N2 correlations reflect PV induced changes in global circulation patterns. We find that the PV causes enhanced upwelling at low latitudes and corresponding downwelling at high latitudes in both the northern and southern hemispheres.
Key Points
Polar vortex (PV) activity is correlated with sub‐seasonal O/N2 variability during low Kp winter times at all latitudes
We observe enhanced O/N2 at high southern latitudes and reduced O/N2 at low latitudes in response to northern hemisphere PV activity
PV‐O/N2 correlations are shown to reflect PV induced modulations of the residual mean meridional circulation (MMC)
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The recovery of thermosphere mass density following geomagnetic storms is a result of competing heating and cooling processes. Simulations often underestimate the speed of the recovery. In this ...study, for the first time, we report that assimilating the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Sounding of the Atmosphere using Broadband Emission Radiometry nitric oxide (NO) cooling rate profiles into a coupled thermosphere‐ionosphere model via the ensemble Kalman filter improves the thermosphere mass density recovery following a geomagnetic storm. This is due to the impact of the assimilation on both the cooling processes and the thermosphere circulation. The dynamical changes due to the assimilation include stronger upwelling and equatorial transport. These lead to an effective increase in NO at all altitudes at mid‐high latitudes, resulting in the improved recovery. The improved representation of cooling processes in the storm's main phase also results in improved >24 hr forecasts of the density recovery.
Plain Language Summary
Since nitric oxide (NO) infrared emission plays the most important role in cooling down the thermosphere during storm times, it is plausible to attribute the slow neutral mass density recovery to an inaccurate representation of NO cooling in the simulations. In this study, for the first time, we assimilate the Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry NO cooling rate profiles into a coupled thermosphere‐ionosphere model. We found that the improved density recovery is obtained due to the impact of the assimilated observations on the general circulation and cooling processes. The improved representation of cooling processes in the storm's main phase further improves forecasts of the neutral density recovery.
Key Points
Assimilating nitric oxide cooling rate profiles into coupled thermosphere‐ionosphere model results in improved thermosphere density recovery
The improved density recovery is due to the impact of the assimilated observations on the general circulation and cooling processes
Improved representation of cooling processes in the storm's main phase improves forecasts of the thermosphere mass density recovery
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Key developments have been made to the NCAR Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X). Among them, the most important are the self‐consistent ...solution of global electrodynamics, and transport of O+ in the F‐region. Other ionosphere developments include time‐dependent solution of electron/ion temperatures, metastable O+ chemistry, and high‐cadence solar EUV capability. Additional developments of the thermospheric components are improvements to the momentum and energy equation solvers to account for variable mean molecular mass and specific heat, a new divergence damping scheme, and cooling by O(3P) fine structure. Simulations using this new version of WACCM‐X (2.0) have been carried out for solar maximum and minimum conditions. Thermospheric composition, density, and temperatures are in general agreement with measurements and empirical models, including the equatorial mass density anomaly and the midnight density maximum. The amplitudes and seasonal variations of atmospheric tides in the mesosphere and lower thermosphere are in good agreement with observations. Although global mean thermospheric densities are comparable with observations of the annual variation, they lack a clear semiannual variation. In the ionosphere, the low‐latitude E × B drifts agree well with observations in their magnitudes, local time dependence, seasonal, and solar activity variations. The prereversal enhancement in the equatorial region, which is associated with ionospheric irregularities, displays patterns of longitudinal and seasonal variation that are similar to observations. Ionospheric density from the model simulations reproduces the equatorial ionosphere anomaly structures and is in general agreement with observations. The model simulations also capture important ionospheric features during storms.
Plain Language Summary
A comprehensive numerical model, the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X), has been improved, in order to simulate the entire atmosphere and ionosphere, from the Earth's surface to ∼700 km altitude. This new version (v. 2.0) adds the capability to calculate the motions and temperatures of ions and electrons in the ionosphere. The model results compare well with available ground‐based and satellite observations, under both quiet and disturbed space weather conditions. Even with constant solar forcing, the model displays large day‐to‐day weather changes in the upper atmosphere and ionosphere, with basic patterns that agree with observations. This demonstrates the model ability to describe the connections between weather near the surface and weather in space.
Key Points
The Whole Atmosphere Community Climate Model has been extended to include ionospheric electrodynamics
WACCM‐X simulates the interaction of lower atmosphere and solar influences in the ionosphere
Preliminary validation demonstrates agreement with observations
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