Zonal Wind Jets Research Articles

On the Downward Progression of Stratospheric Temperature Anomalies Using Long‐Term SABER Observations

AbstractIt is well known that stratospheric sudden warmings (SSWs) are a result of the interaction between planetary waves (PWs) and the stratospheric polar vortex. SSWs occur when breaking PWs slow down or even reverse this zonal wind jet and induce a sinking motion that adiabatically warms the stratosphere and lowers the stratopause. In this paper we characterize this downward progression of stratospheric temperature anomalies using 18 years (2003–2020) of Sounding of the Atmosphere using Broadband Radiometry (SABER) observations. SABER temperatures, derived zonal winds, PW activity and gravity wave (GW) activity during January and February of each year indicate a high‐degree of year‐to‐year variability. From 11 stratospheric warming events (9 major and 2 minor events), the descent rate of the stratopause altitude varies from 0.5 to 2 km/day and the lowest altitude the stratopause descends to varies from <20 to ∼50 km (i.e., no descent). A composite analysis of temperature and squared GW amplitude anomalies indicate that the downward descent of temperature anomalies from 50 to ∼25 km lags the downward progression of increased GW activity. This increased GW activity coincides with the weakening and reversal of the westward zonal winds in agreement with previous studies. Our study suggests that although PWs drive the onset of SSWs at 30 km, GWs also play a role in contributing to the descent of temperature anomalies from the stratopause to the middle and lower stratosphere.

Equatorial Nighttime Thermospheric Zonal Wind Jet Response to the Temporal Oscillation of Solar Wind

AbstractThis work investigates the response of the equatorial thermospheric wind jet to the temporal oscillation of the Bz component of the interplanetary magnetic field using Thermosphere Ionosphere Electrodynamics General Circulation Model simulations and CHAllenging Minisatellite Payload observations. The thermospheric wind jet is formed in the magnetic equatorial region with the strongest zonal winds. Both the morning westward jet and the nighttime eastward wind jet at the dip equator decrease during the oscillation periods of IMF Bz. Ion drag dominates the deceleration of the equatorial eastward wind jet in the nighttime, with a minor contribution from the viscous force. The relative motion between the ions and neutrals has a dominant influence on the ion drag at the 60–180°W and 107–180°E geographic longitude; at other longitudes, both relative velocity (∆velocity) and electron density changes (∆Ne) are dominant. Moreover, the decrease in the equatorial eastward wind jet at 20 magnetic local time, which is also driven by ion drag, is significant above 270 km. The generation of the equatorial westward wind jet disturbance is significantly influenced by ∆velocity at all altitudes, whereas ∆Ne becomes important below 450 km.

The role of shallow convection in the momentum budget of the trades from large‐eddy‐simulation hindcasts

AbstractMotivated by the abundance of low clouds in the subtropics, where the easterly trade winds prevail, we study the role of shallow convection in the momentum budget of the trades. To this end, we use ICON‐LEM hindcasts run over the North Atlantic for 12 days corresponding to the NARVAL1 (winter) and NARVAL2 (summer) flight campaigns. The simulation protocol consists of several nested domains, and we focus on the inner domains (≈100 × 100 km2) which have been run at resolutions of 150–600 m and are forced by analysis data, thus exhibiting realistic conditions. Combined, the resolved advection and the subgrid stresses decelerate the easterly flow over a frictional layer that balances the prevailing geostrophic wind forcing. Irrespective of the horizontal resolution, this layer is about 2 km deep in the strong winter trades and 1 km in summer, as winds and geostrophic forcing weaken and cloudiness reduces. The unresolved processes are strongest near the surface and are well captured by traditional K‐diffusion theory, but convective‐scale motions which are not considered in K‐diffusion theory contribute the most in the upper part of the mixed layer and are strongest just below cloud base. The results point out that convection in the mixed layer – the roots of trade‐wind cumuli and subcloud‐layer circulations – play an important role in slowing down easterly flow below cloud base (but little in the cloud layer itself), which helps make the zonal wind jet more distinct. Most of the friction within the clouds and near the wind jet stems from smaller‐scale turbulence stresses.

On the influence of zonal gravity wave distributions on the Southern Hemisphere winter circulation

Abstract. A mechanistic global circulation model is used to simulate the Southern Hemisphere stratospheric, mesospheric, and lower thermospheric circulation during austral winter. The model includes a gravity wave (GW) parameterization that is initiated by prescribed 2-D fields of GW parameters in the troposphere. These are based on observations of GW potential energy calculated using GPS radio occultations and show enhanced GW activity east of the Andes and around the Antarctic. In order to detect the influence of an observation-based and thus realistic 2-D GW distribution on the middle atmosphere circulation, we perform model experiments with zonal mean and 2-D GW initialization, and additionally with and without forcing of stationary planetary waves (SPWs) at the lower boundary of the model. As a result, we find additional forcing of SPWs in the stratosphere, a weaker zonal wind jet in the mesosphere, cooling of the mesosphere and warming near the mesopause above the jet. SPW wavenumber 1 (SPW1) amplitudes are generally increased by about 10 % when GWs are introduced being longitudinally dependent. However, at the upper part of the zonal wind jet, SPW1 in zonal wind and GW acceleration are out of phase, which reduces the amplitudes there.

On the possible role of zonal dynamics in the formation and evolution of F3 layers over equator

In the present study, occurrences of F3 layer over Thiruvananthapuram (8.5°N; 77°E; dip latitude ~0.5°N), a dip equatorial station in India have been investigated using ionosonde data for the years 2004–2007. The F3 layers appearing in the ionograms during the pre noon hours only have been included in the analysis. The result indicates that a weak EIA resulting in low ionospheric height and high ionization density prevails before the occurrence of F3 layer and serves as an essential condition for its occurrence. The relative Slant Total Electron Content (rSTEC) measured using collocated ground based coherent low earth orbiting (LEO) radio beacon receiver has also been used along with electron densities measured by CHAMP satellite for the year 2006 and 2007 to illustrate this difference in the evolution of Equatorial Ionization Anomaly (EIA) on F3 and non F3 days. A new mechanism for F3 generation has been proposed. It has been shown that the coupling of the thermospheric zonal wind jet over equator and enhanced ionospheric density at lower heights over Indian longitude can account for the generation of F3 layer through ion-drag. The vertical wind associated with the thermospheric heating resulting from ion-drag causes the generation of an additional eastward field which, along with the prevailing F-region electric field, leads to the upward excursion of the F3 layer.

Gravity wave variations in the polar stratosphere and mesosphere from SOFIE/AIM temperature observations

A 6 year (2007-2013) temperature data set from the Solar Occultation for Ice Experiment (SOFIE) onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite is used to extract gravity waves (GWs) in the polar stratosphere and mesosphere of both hemispheres. These data are continuous in the polar regions. The monthly mean GW potential energy (PE) increases exponentially with a scale height of similar to 13 km in the upper stratosphere and mesosphere. GWs are stronger in the winter than in the summer and exhibit strong annual variation. GWs are stronger in the southern polar region (SPR) than in the northern polar region (NPR) except in the summer months. This is likely because there are stronger and longer-lasting zonal wind jets in the SPR stratosphere, as revealed from Modern-Era Retrospective analysis for Research and Applications (MERRA) wind data. The longitudinal variations of PE in the winter polar stratosphere are consistent with the elevated regions. In the mesosphere, the longitudinal variations of PE do not vary with height significantly. The correlations between GW PE and the column ice water content (IWC, an indicator of the polar mesosphere cloud) exhibit longitudinal and annual variations.

Bidirectional infrasonic ducts associated with sudden stratospheric warming events

In January 2011, the state of the polar vortex in the midlatitudes changed significantly due to a minor sudden stratospheric warming event. As a result, a bidirectional duct for infrasound propagation developed in the middle atmosphere that persisted for 2 weeks. The ducts were due to two zonal wind jets, one between 30 and 50 km and the other around 70 km altitude. In this paper, using microbarom source modeling, a previously unidentified source region in the eastern Mediterranean is identified, besides the more well known microbarom source regions in the Atlantic Ocean. Infrasound data are then presented in which the above mentioned bidirectional duct is observed in microbarom signals recorded at the International Monitoring System station I48TN in Tunisia, from the Mediterranean region to the east and from the Atlantic Ocean to the west. While the frequency bands of the two sources overlap, the Mediterranean signal is coherent up to about 0.6 Hz. This observation is consistent with the microbarom source modeling; the discrepancy in the frequency band is related to differences in the ocean wave spectra for the two basins considered. This work demonstrates the sensitivity of infrasound to stratospheric dynamics and illustrates that the classic paradigm of a unidirectional stratospheric duct for infrasound propagation can be broken during a sudden stratospheric warming event.

General circulation modeling of the Martian upper atmosphere during global dust storms

Simulations with a general circulation model (GCM) have been performed to study the Martian upper atmosphere during two major dust storms. The GCM extending from the surface to about 160 km included a spectral parameterization of subgrid‐scale gravity waves suitable for planetary thermospheres, and prescribed four‐dimensional dust distributions corresponding to storm events near the equinox and solstice (Martian years 25 and 28, respectively). The results show that the wind and temperature fields in the upper atmosphere above ∼100 km respond to such storms as intensively as in the lower atmosphere. During the equinoctial storm, the temperature above the mesopause dropped by up to 30 K everywhere except in the Northern Hemisphere high latitudes, where it rose by up to 15 K. At the solstitial storm, the temperature above the mesopause decreased by up to 40 K in the winter hemisphere, by 15 K in the summer hemisphere, and increased by 30–40 K in tropics and in the summer hemisphere above 130 km. Prograde and retrograde zonal wind jets intensified throughout the atmosphere at all heights and during both dust scenarios. These changes are the result of the altered meridional overturning circulation induced by resolved and unresolved waves. Atmospheric density during dust storms enhanced in average by a factor of 2 to 3 in the mesosphere and lower thermosphere, which agrees well with observations.

General circulation modeling of the Martian upper atmosphere during global dust storms

[1] Simulations with a general circulation model (GCM) have been performed to study the Martian upper atmosphere during two major dust storms. The GCM extending from the surface to about 160 km included a spectral parameterization of subgrid-scale gravity waves suitable for planetary thermospheres, and prescribed four-dimensional dust distributions corresponding to storm events near the equinox and solstice (Martian years 25 and 28, respectively). The results show that the wind and temperature fields in the upper atmosphere above ∼100 km respond to such storms as intensively as in the lower atmosphere. During the equinoctial storm, the temperature above the mesopause dropped by up to 30 K everywhere except in the Northern Hemisphere high latitudes, where it rose by up to 15 K. At the solstitial storm, the temperature above the mesopause decreased by up to 40 K in the winter hemisphere, by 15 K in the summer hemisphere, and increased by 30–40 K in tropics and in the summer hemisphere above 130 km. Prograde and retrograde zonal wind jets intensified throughout the atmosphere at all heights and during both dust scenarios. These changes are the result of the altered meridional overturning circulation induced by resolved and unresolved waves. Atmospheric density during dust storms enhanced in average by a factor of 2 to 3 in the mesosphere and lower thermosphere, which agrees well with observations.

Long-term trends and decadal variability of upper mesosphere/lower thermosphere gravity waves at midlatitudes

Mesosphere/lower thermosphere (MLT) winds over Germany as measured with a low-frequency spaced receiver system at Collm 1984–2007 have been analysed with respect to variations at the time scales of gravity waves. Background winds are also registered to analyse possible gravity wave-mean flow interactions at decadal and interdecadal time scales. In both winter and summer an increasing mesospheric zonal wind jet with time is registered, which is accompanied with increasing gravity wave variances. At greater altitudes in summer, the mean wind jet trend reverses, and negative trends of gravity wave variances are found. This connection between gravity waves and mean wind is also observed on a quasi-decadal scale: during solar maximum stronger mesospheric zonal wind jets as well as larger gravity wave amplitudes are observed. This results in a solar cycle modulation of gravity waves with larger amplitudes during solar maximum. The observed positive correlation between gravity wave amplitudes and the mean zonal wind may follow the theory of saturated waves in the atmosphere, such that stronger mesospheric zonal winds are connected with larger gravity wave amplitudes.

Role of gravity waves in the forcing of quasi two‐day waves in the mesosphere: An observational study

AbstractAmplitudes of quasi two‐day waves (QTDWs) are derived from temperature observations of the High Resolution Dynamics Limb Sounder and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instruments. In particular, a global climatology of QTDW amplitudes is derived from 10 years of SABER data, covering the mesosphere and lower thermosphere. This climatology is compared with geostrophic winds and climatologies of gravity wave (GW) momentum flux and GW drag absolute values derived from the same data set. We find that QTDWs are forced shortly after the maximum of the mesospheric summertime zonal wind jet in regions of jet instability where the meridional gradient of quasi‐geostrophic zonal mean potential vorticity is strongly negative. The jet instability regions are closely linked to enhanced GW drag that likely seeds those instabilities by decelerating the jet and causing the jet curvature responsible for the negative potential vorticity gradient. The vertical phase structure and the Eliassen‐Palm flux of the QTDWs are derived from SABER data and investigated. It is shown that QTDWs propagate upward starting from the jet instability regions. They exert eastward drag in the jet core, and strong westward drag at higher altitudes. Strikingly, the QTDWs are forced in regions where the global distribution of GWs exhibits a characteristic longitudinal structure caused by the GW source patterns in the summer hemisphere. This longitudinal structure might play an important role in the forcing of QTDWs; however, no clear link has been found to the observed QTDW zonal wavenumbers.

General aspects of a T213L256 middle atmosphere general circulation model

A high‐resolution middle atmosphere general circulation model (GCM) developed for studying small‐scale atmospheric processes is presented, and the general features of the model are discussed. The GCM has T213 spectral horizontal resolution and 256 vertical levels extending from the surface to a height of 85 km with a uniform vertical spacing of 300 m. Gravity waves (GWs) are spontaneously generated by convection, topography, instability, and adjustment processes in the model, and the GCM reproduces realistic general circulation in the extratropical stratosphere and mesosphere. The oscillations similar to the stratopause semiannual oscillation and the quasi‐biennial oscillation (QBO) in the equatorial lower stratosphere are also spontaneously generated in the GCM, although the period of the QBO‐like oscillation is short (15 months). The relative roles of planetary waves, large‐scale GWs, and small‐scale GWs in maintenance of the meridional structures of the zonal wind jets in the middle atmosphere are evaluated by calculating Eliassen‐Palm diagnostics separately for each of these three groups of waves. Small‐scale GWs are found to cause deceleration of the wintertime polar night jet and the summertime easterly jet in the mesosphere, while extratropical planetary waves primarily cause deceleration of the polar night jet below a height of approximately 60 km. The meridional distribution and propagation of small‐scale GWs are shown to affect the shape of the upper part of mesospheric jets. The phase structures of orographic GWs over the South Andes and GWs emitted from the tropospheric jet stream are discussed as examples of realistic GWs reproduced by the T213L256 GCM.

Constraints on a Non-orographic Gravity Wave Drag Parameterization Using a Gravity Wave Resolving General Circulation Model

The results of a T213L250 gravity wave (GW) resolving general circulation model (GWR-GCM) are used to constrain the GW source spectra of a non-orographic GW drag parameterization (GWDP) proposed by Hines. In this study, the following two constraints were placed on Hines’s GWDP: 1) the launch level at which GW source spectra are specified, and 2) the GW-source spectra, that is, the seasonally varying geographical and azimuthal distribution of GW momentum flux and horizontal wind amplitude. Considering the importance of the lateral propagation of GWs, which is ignored in Hines’s GWDP, the GW source spectra are prescribed using information from the GWR-GCM at 70 hPa, where the GWs have already propagated laterally some distance from their source regions. The GW-source spectra have significant geographical variations and anisotropy, reflecting source distribution and the effects of critical level filtering due to the background flows. Although the effects of the lateral propagation and intermittency of GWs are ignored, a T42L80 chemistry coupled climate model using the GWDP with the constraints developed in this study realistically reproduced the meridional structures of the zonal wind jets in the stratosphere and mesosphere.

Wind variations in Jupiter's equatorial atmosphere: A QQO counterpart?

Jupiter's equatorial atmosphere, much like the Earth's, is known to show quasi-periodic variations in temperature, particularly in the stratosphere, but variations in other jovian atmospheric tracers have not been studied for any correlations to these oscillations. Data taken at NASA's Infrared Telescope Facility (IRTF) from 1979 to 2000 were used to obtain temperatures at two levels in the atmosphere, corresponding to the upper troposphere (250 mbar) and to the stratosphere (20 mbar). We find that the data show periodic signals at latitudes corresponding to the troposphere zonal wind jets, with periods ranging from 4.4 (stratosphere, 95% confidence at 4° S planetographic latitude) to 7.7 years (troposphere, 97% confidence at 6° N). We also discuss evidence that at some latitudes the troposphere temperature variations are out of phase from the stratosphere variations, even where no periodicity is evident. Hubble Space Telescope images were used, in conjunction with Voyager and Cassini data, to track small changes in the troposphere zonal winds from 20° N to 20° S latitude over the 1994–2000 time period. Oscillations with a period of 4.5 years are found near 7°–8° S, with 80–85% significance. Further, the strongest evidence for a QQO-induced tropospheric wind change tied to stratospheric temperature change occurs near these latitudes, though tropospheric temperatures show little periodicity here. Comparison of thermal winds and measured zonal winds for three dates indicate that cloud features at other latitudes are likely tracked at pressures that can vary by up to a few hundred millibar, but the cloud altitude change required is too large to explain the wind changes measured at 7° S.

Similarities in middle atmosphere structures

Similarities of middle atmosphere structures are analyzed on the basis of CRISTA 1 and SABER data which extend from the upper troposphere up into the lower thermosphere. An event of planetary wave breaking is identified that leads to the formation of a surf zone the middle mesosphere (70 km); its structures are seen in temperature, potential vorticity and trace gas mixing ratios. A second analysis compares the structures of the tropopause and the mesopause. A cold point and a dynamical (potential vorticity) are suggested similarly as they are used at the tropopause. A lapse rate mesopause, however, appears to be of little use. The dynamical appears to indicate a transport barrier similar as it is the case at the low altitudes. In summer a split mesopause is observed at middle to high latitudes. There is a zonal wind jet near to this apparent break which is similar to what can be observed at the tropopause.

On the day-to-day wind and semidiurnal tide variations at heights of the mid-latitude summer mesopause: Zonal wavenumber estimations and its consequences, case-study in 1998

This work presents an analysis of spectra and estimations of zonal wavenumbers for mean wind and semidiurnal tide amplitude oscillations observed during July–September 1998 by 3 meteor radars and one MF radar. A number of mean wind and tidal amplitude oscillations are found that are possibly related to the evolution of the summer mesospheric zonal wind jet. The mean zonal wind undergoes several significant variations (“transitions”) during this time interval. These changes were accompanied by a frequency decreasing of the QTDW (quasi-2-day wave) in such a way that oscillations with a period of roughly about 36–44 h at the beginning of the measurements were replaced by strong 48-h waves, which in its turn after the end of the QTDW burst are replaced by a set of 3–6 day oscillations. Before the strong 48-h wave appearance we observe mean zonal wind variations with a temporal scale of 4 days. The set of 3–6 days includes 3–4 day waves with s ∼ 3 and 5–6 day waves with s = 1 . The last wave is absent in the upper stratosphere during the time of observations and suggests to appear in the mesosphere. The both waves could represent a signature of the simultaneous non-linear excitation of a triad of waves. The zonal wind “transitions” also reflected in the semidiurnal tide by modulations of the tidal amplitudes with periods of 3–4 days and about 5 days. Some features observed at the end of summer 1998 could be explained by interaction between a zonally averaged wind variation and the semidiurnal tide. In a spectral representation, this variation looks like an oscillation, but is not a planetary wave. The strongest long-period modulations of the semidiurnal tide were observed during strong geopotential perturbations in the stratosphere of the Southern hemisphere.

Dual influence of Atlantic and Pacific SST anomalies on the North Atlantic/Europe winter climate

Clustering analyses are used in both NCEP Reanalyses and model simulations to identify relationships between the low frequency variability of the zonal wind jet at 200hPa (U200) over the North Atlantic‐Europe domain and the geographical distribution of the Sea Surface Temperature (SST) anomalies. For winter months over the 1959‐1998 period, the U200 signature of the two phases of the North Atlantic Oscillation (NAO) is captured from NCEP in a cluster pair associated with the North Atlantic SST tripole. The classification also extracts a “La Niña” mode related to a strong ridge off Europe and reduced jet over the Azores. Model results confirm this dual oceanic association while exaggerating the ENSO influence. The role of the Atlantic SSTs is confirmed in model experiments where climatological SSTs are prescribed in the other basins. Tropical anomalies dominate the Atlantic SST signals and are linked to fluctuations of the jet consistently with the NAO phase.

Observations and Analysis of Longitudinal Thermal Waves on Jupiter

We analyze the properties of wave-like longitudinal temperature variations in Jupiter's upper troposphere and stratosphere, as revealed using thermal infrared (IR) ground-based observations taken yearly during the 1989–1993 period. These thermal waves are apparently the same as the “slowly moving thermal features” identified in Voyager data. We conclude that they are ubiquitous at near-equatorial latitudes on Jupiter, and have a spatial scale which typically spans wavenumber ≊2 to ≊15. Their stratospheric (20 mbar) temperature amplitude, from 7.8 μm data, is approximately a factor of 3 larger than their amplitude in the upper troposphere (≊0.3 bars), as revealed using 18 μm data. These amplitudes are consistent with the ρ −1/2growth expected for a vertically propagating Rossby wave. The phase at the two levels is the same to within 5° of longitude. The temperature wave is not advected by the zonal winds, having a phase velocity of ≊5 m sec −1in system III, but the temperature amplitude can decay on time scales ≊10 5sec. Imaging of the thermal waves shows that they are extended in latitude to a greater extent than the belt/zone structure, or the zonal wind jets. An apparently related wave is seen in broad-bandwidth (7–13 μm) IR observations which are sensitive to clouds and aerosols. This structure is advected by the zonal wind, indicating that some cloud or aerosol component shares the wave-like structure and spatial scale of the temperature wave. We interpret these waves as stationary Rossby waves, represented by small latitude excursions in the streamlines of the zonal winds. The latitude excursions could be produced by either deep structure, or by the global effect of vortices such as the Great Red Spot. In either case, temperature fluctuations are produced via “vortex stretching.” We calculate the required magnitude of the latitude excursions in the zonal winds to be ≊1° in order to produce the observed temperature amplitudes. This is a subarcsec scale for Earth-based observations, but might be detectable using observations from the Hubble space telescope.

Understanding the transport of atomic oxygen within the thermosphere, using a numerical global thermospheric model

A 2-dimensional version of the UCL global thermospheric model has been adapted by including detailed oxygen and nitrogen chemistry, with appropriate completion of the energy equation, and adding the thermal infra-red cooling by [O] and [NO]. This solution includes solar and auroral production of odd nitrogen species. A series of experiments and sensitivity studies has been carried out with this model to investigate the interactions between atmospheric dynamics and minor species transport and density, which occur as a result of varying the amount of turbulent mixing in the upper mesosphere and lower thermosphere. First results, for an equinox situation, indicate that increased turbulent transport near and above the mesopause results in a strong increase in mesospheric nitric oxide, a rather smaller increase in mesospheric atomic oxygen, and a sharp reduction in mesopause/lower thermosphere temperature. The cooling results from additional radiative cooling by nitric oxide, transported downwards from the lower thermosphere. Neutral winds of the lower thermosphere are strongly modified, with zonal wind jets of up to 100 m s −1 at middle latitudes. The numerical experiments which have been conducted indicate that relatively modest variations of turbulence within the upper mesosphere and lower thermosphere have quite dramatic effects on the dynamics and on the energy budget of the lower thermosphere. The changes in wind and thermal structure also indicate that the propagation of tidal and gravity wave perturbations may be strongly modified as a result of changes of turbulence. Energy extracted from propagating tides and gravity waves has not been included in the present simulations, and may partly mitigate the strong radiative mesopause cooling. The results of the simulations stress the necessity for combined measurements of chemistry, temperature structure and dynamics if the observations of minor species concentrations are to be correctly interpreted.