Stationary Planetary Waves Research Articles

Simulating influences of QBO phases and orographic gravity wave forcing on planetary waves in the middle atmosphere

Recently developed parameterization of stationary orographic gravity waves (OGWs) generated by the Earth’s topography was implemented into a general circulation model of the middle and upper atmosphere. We performed numerical simulations of the zonal mean wind and amplitudes of stationary planetary waves and normal atmospheric modes with periods of 4–16 days at altitudes from the troposphere to the lower thermosphere in January for easterly and westerly phases of the quasi-biennial oscillation (QBO) including and excluding the stationary OGW parameterization. Simulations show that accounting dynamical and thermal effects of stationary OGWs can lead to substantial changes (up to 50–90 %) in the amplitudes of stationary planetary waves. Amplitudes of westward travelling normal atmospheric modes change (up to 50–90 %) at different altitudes and latitudes of the northern hemisphere due to OGW effects. Transitions from the easterly to westerly QBO phases can change planetary wave amplitudes up to ±30–90 % at middle and high latitudes. These changes in PW amplitudes are consistent with distributions of EP-flux and refractive index under different QBO phases simulated including our parameterization of stationary OGWs.

Evidence of nonlinear interaction between quasi 2 day wave and quasi‐stationary wave

AbstractThe nonlinear interaction between the westward quasi 2 day wave (QTDW) with zonal wave number s = 3 (W3) and stationary planetary wave with s = 1 (SPW1) is first investigated using both Thermosphere, Ionosphere, and Mesosphere Electric Dynamics (TIMED) satellite observations and the thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) simulations. A QTDW with westward s = 2 (W2) is identified in the mesosphere and lower thermosphere (MLT) region in TIMED/Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature and TIMED/TIMED Doppler Imager (TIDI) wind observations during 2011/2012 austral summer period, which coincides with a strong SPW1 episode at high latitude of the northern winter hemisphere. The temperature perturbation of W2 QTDW reaches a maximum amplitude of ~8 K at ~30°S and ~88 km in the Southern Hemisphere, with a smaller amplitude in the Northern Hemisphere at similar latitude and minimum amplitude at the equator. The maximum meridional wind amplitude of the W2 QTDW is observed to be ~40 m/s at 95 km in the equatorial region. The TIME‐GCM is utilized to simulate the nonlinear interactions between W3 QTDW and SPW1 by specifying both W3 QTDW and SPW1 perturbations at the lower model boundary. The model results show a clear W2 QTDW signature in the MLT region, which agrees well with the TIMED/SABER temperature and TIMED/TIDI horizontal wind observations. We conclude that the W2 QTDW during the 2011/2012 austral summer period results from the nonlinear interaction between W3 QTDW and SPW1.

Structure and origins of the Weddell Sea Anomaly from tidal and planetary wave signatures in FORMOSAT‐3/COSMIC observations and GAIA GCM simulations

AbstractThe Weddell Sea Anomaly (WSA) is a recurrent feature of the austral summer midlatitude ionosphere where electron densities are observed to maximize during the local nighttime. In this study, tidal decomposition is applied to FORMOSAT‐3 (Formosa Satellite)/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) total electron content (TEC) and electron density observations between 2007 and 2012 to quantify the components dominating local time and spatial variation in the WSA region. Our results present some of the first three‐dimensional spaceborne analyses of the WSA from a tidal perspective over multiple years. We find that the features of the WSA can be reconstructed as the result of superposition between the dominant diurnal standing (D0), eastward wave number 1 (DE1), westward wave number 2 (DW2), and stationary planetary wave 1 (SPW1) components in TECs, producing the characteristic midnight WSA peak. The D0, DE1, DW2, and SPW1 components are found to be an interannually recurring feature of the southern midlatitude to high‐latitude ionosphere during the summer, manifesting as enhancements in electron density around 300 km altitude of the summer middle to high magnetic latitudes. The phases of the aforementioned nonmigrating diurnal signatures in electron density in this region are near evanescent, suggesting in situ generation, rather than upward propagation from below. However, the SPW1 signature shows some signs of an eastward tilt with altitude, suggesting possible downward propagation. The relation of these components to possible generation via in situ photoionization or plasma transport along magnetic field lines is also discussed using results from the Ground‐to‐topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) general circulation model (GCM), connecting the tidal interpretation of the WSA to previously examined generation mechanisms.

Teleconnection between the central tropical Pacific and the Antarctic stratosphere: spatial patterns and time lags

Coupling between the central tropical Pacific (CTP) sea surface temperature (SST) variability and the Antarctic stratosphere known from previous studies is analysed using a lagged correlation method. Monthly mean SST, stratospheric temperatures and zonal winds from the NCEP–NCAR reanalysis and total ozone from the Merged Ozone Data Set over the 1979–2011 period are used. Indices for interannual variability were created and sequential lagged correlations were used to determine the spatial patterns and time lags of the strongest teleconnection. It has been revealed that interannual SST variations in a westward-oriented V-shaped pattern in the CTP region (160–220°E, 30°N–20°S) are associated with statistically significant positive signals in zonally asymmetric region of the Antarctic stratosphere temperature field. Stratospheric pathway dominates in the poleward and downward propagation of the CTP related anomaly in the Southern Hemisphere zonal wind variability. Abrupt modification of the teleconnection patterns in October is accompanied by occurrence of the intense meridional wave train in the lower stratosphere and troposphere. The strongest lagged correlations between the CTP SST index in June and the Southern Hemisphere atmosphere variables in October (r = ±0.7) could be related to the corresponding seasonal peaks in the stationary planetary wave activity. Possible causes of the V-pattern formation and the lagged teleconnection mechanisms are briefly discussed.

Moderate geomagnetic storms of January 22–25, 2012 and their influences on the wave components in ionosphere and upper stratosphere-mesosphere regions

Moderate geomagnetic storms occurred during January 22–25, 2012 period. The geomagnetic storms are characterized by different indices and parameters. The SYM-H value on January 22 increased abruptly to 67nT at sudden storm commencement (SSC), followed by a sharp decrease to −87nT. A second SSC on January 24 followed by a shock on January 25 was also observed. These SSCs before the main storms and the short recovery periods imply the geomagnetic storms are CME-driven. The sudden jump of solar wind dynamic pressure and IMF Bz are also consistent with occurrence of CMEs. This is also reflected in the change in total electron content (TEC) during the storm relative to quiet days globally. The response of the ionospheric to geomagnetic storms can also be detected from wave components that account for the majority of TEC variance during the period. The dominant coherent modes of TEC variability are diurnal and semidiurnal signals which account upto 83% and 30% of the total TEC variance over fairly exclusive ionospheric regions respectively. Comparison of TEC anomalies attributed to diurnal (DW1) and semidiurnal (SW2) tides, as well as stationary planetary waves (SPW1) at 12 UTC shows enhancement in the positive anomalies following the storm. Moreover, the impact of the geomagnetic storms are distinctly marked in the daily time series of amplitudes of DW1, SW2 and SPW1. The abrupt changes in amplitudes of DW1 (5 TECU) and SW2 (2 TECU) are observed within 20°S–20°N latitude band and along 20°N respectively while that of SPW1 is about 3 TECU. Coherent oscillation with a period of 2.4days between interplanetary magnetic field and TEC was detected during the storm. This oscillation is also detected in the amplitudes of DW1 over EIA regions in both hemispheres. Eventhough upward coupling of quasi two day wave (QTDWs) of the same periodicity, known to have caused such oscillation, are detected in both ionosphere and upper stratosphere, this one can likely be attributed to the geomagnetic storm as it happens after the storm commencement. Moreover, further analysis has indicated that QTDWs in the ionosphere are strengthened as a result of coherent oscillation of interplanetary magnetic field with the same frequency as QTDWs. On the otherhand, occurrences of minor SSW and geomagnetic storms in quick succession complicated clear demarcation of attribution of the respective events to variability of QTDWs amplitudes over upper stratosphere.

Traveling planetary wave activity from mesopause region airglow temperatures determined by the Network for the Detection of Mesospheric Change (NDMC)

The global distribution of traveling planetary wave (PW) activity in the mesopause region is estimated for the first time from ground-based airglow measurements. Monthly and total mean climatologies of PW power are determined from rotational temperatures measured at 19 sites from 78° N to 76° S which contribute to the Network for the Detection of Mesospheric Change (NDMC). Wave power is expressed as the standard deviation of nocturnal mean temperature around the seasonal temperature variation. The results from 20° N confirm the SABER traveling PW proxy by Offermann et al. (2009, J. Geophys. Res. 114, D06110) at two altitudes. Most sites between 69° S and 69° N show total mean traveling PW activity of about 6K, and only some high latitude sites have considerably higher activity levels. At the two tropical sites, there is practically no seasonal variation of PW activity. At 70% of the midlatitude sites, the seasonal variation is moderate for most of the year, but it is quite appreciable at all high latitude sites. Results about traveling PW activity at 87km and 95km available from several sites signal similar behavior at both altitudes. The total mean climatological results here obtained have further been used to separate the traveling PW contribution from the superposition of wave types contained in OH rotational temperature fluctuations measured by the SCIAMACHY instrument on Envisat. A narrow equatorial wave activity maximum is probably caused by gravity waves, while a tendency towards greater activity at higher northern latitudes may be due to stationary planetary waves.

Impact of Mongolian Plateau versus Tibetan Plateau on the westerly jet over North Pacific Ocean

Mountains have long been considered to play an important role in the formation of modern climate. Particularly in the Asia-Pacific sector, the existence of Tibetan Plateau (TP) is believed to maintain the stationary planetary wave patterns and to intensify the westerly jet over North Pacific. However, the potential role of Mongolian Plateau (MP) has often been neglected in these studies. With an atmospheric general circulation model, we show that the MP, despite its smaller size, exerts a great influence on the planetary-scale circulation and the subtropical westerly jet. The MP amplifies the meridional thermal gradient at the mid-upper troposphere and thus strengthens the jet, which is primarily facilitated by its high-latitude location. By blocking westerly winds, the MP forces their northern branch further northward, which allows the southward penetration of cold air at the lee side, and intensifies the East Asian trough. In contrast, the impact of the TP itself is not as large as expected. Hence, one should not simply ascribe all the mountain-induced climate change to the TP uplift, because other smaller topography might be also very important.

The character of polar tidal signatures in the extended Canadian Middle Atmosphere Model

AbstractThe characteristics of the diurnal, semidiurnal, and terdiurnal tides (zonal wave numbers −5 to +5 in temperature and zonal wind) in the polar mesosphere and lower thermosphere region as simulated by the extended Canadian Middle Atmosphere Model are examined. The most significant diurnal, semidiurnal, and terdiurnal tides in the polar regions are Ds0, Dw1, and De1; Sw3, Sw2, Sw1, Ss0, Se1, and Se2; and Tw3, Ts0, and Tw1, respectively, and their latitudinal structures, seasonal variations, and hemispheric asymmetries noted. Of these components, Ds0, Tw1, Ts0, and Tw3 exhibit a seasonally symmetric variation with both hemispheres strengthening simultaneously. On the other hand, Dw1 strengthens asymmetrically so that when one hemisphere is strong, the other is weak. The remainder show no seasonal tendency but vacillate on shorter than seasonal time scales in a symmetric or antisymmetric manner at different times of the year. Global‐scale correlations of the amplitudes of the migrating tides Dw1/Sw2 and the stationary planetary wave 1 and the assumed “child” nonmigrating tides are also examined. The results indicate that the correlations are highly time scale‐dependent and the significant correlations seen with the original time series are mainly due to longer‐term variations (>18 days). There are no consistent global correlations associated with the short‐term variations (<18 days) among these waves.

TIME‐GCM study of the ionospheric equatorial vertical drift changes during the 2006 stratospheric sudden warming

This modeling study quantifies the daytime low‐latitude vertical E×B drift changes in the longitudinal wave number 1 (wn1) to wn4 during the major extended January 2006 stratospheric sudden warming (SSW) period as simulated by the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM), and attributes the drift changes to specific tides and planetary waves (PWs). The largest drift amplitude change (approximately 5 m/s) is seen in wn1 with a strong temporal correlation to the SSW. The wn1 drift is primarily caused by the semidiurnal westward propagating tide with zonal wave number 1 (SW1), and secondarily by a stationary planetary wave with zonal wave number 1 (PW1). SW1 is generated by the nonlinear interaction of PW1 and the migrating semidiurnal tide (SW2) at high latitude around 90–100 km. The simulations suggest that the E region PW1 around 100–130 km at the different latitudes has different origins: at high latitudes, the PW1 is related to the original stratospheric PW1; at midlatitudes, the model indicates PW1 is due to the nonlinear interaction of SW1 and SW2 around 95–105 km; and at low latitudes, the PW1 might be caused by the nonlinear interaction between DE2 and DE3. The time evolution of the simulated wn4 in the vertical E×B drift amplitude shows no temporal correlation with the SSW. The wn4 in the low‐latitude vertical drift is attributed to the diurnal eastward propagating tide with zonal wave number 3 (DE3), and the contributions from SE2, TE1, and PW4 are negligible.

Evidence for nonmigrating tides produced by the interaction between tides and stationary planetary waves in the stratosphere and lower mesosphere

AbstractIn this work, 11 years (2002–2012) of Thermosphere, Ionosphere, Mesosphere Energetics, and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) global temperature data are used to study the nonlinear interaction between stationary planetary waves (SPWs) and tides in the stratosphere and mesosphere. The holistic behavior of the nonlinear interactions between all SPWs and tides is analyzed from the point of view of energetics. The results indicate that the intensities of nonmigrating diurnal, semidiurnal, terdiurnal, and 6 h tides are strongest during winter and almost vanish during summer, synchronous with SPW activity. Temporal correlations between the SPWs and nonmigrating tides for these four tidal components are strong in the region poleward of 20° and below about 80 km. In the tropics, where the SPWs are very weak in all seasons, the correlations are small. Bispectral analysis between triads of waves and tides shows which particular interactions are likely to be responsible for the generation of the nonmigrating tides that are largest in the midlatitude stratosphere. Based on the more limited SABER observations at high latitudes, the correlations there are similar to those in midlatitudes during spring, summer, and autumn; there are no high‐latitude observations by SABER in winter. These results show that nonlinear interactions between SPWs and tides in the stratosphere and the lower mesosphere may be an important source of the nonmigrating tides that then propagate into the upper mesosphere and lower thermosphere.

Interannual variability of the nonmigrating semidiurnal tide at high latitudes and stationary planetary wave in the opposite hemispheres

The westward propagating zonal wavenumber 1 nonmigrating semidiurnal tide (SW1) enhanced at high latitudes during summer in the mesosphere and lower thermosphere (MLT) is believed to originate from the nonlinear interaction between the migrating semidiurnal tide (SW2) and the stationary planetary wave zonal wavenumber 1 (SPW1) in the opposite winter hemispheres. This paper presents correlations of the SW1 over the Antarctic and Arctic and the SPW1 in the opposite hemispheres. The SW1 is determined from horizontal wind measurements by the TIMED Doppler Interferometer (TIDI) and the SPW1 is from temperature measurements by the Sounding the Atmosphere using Broadband Emission Radiometry (SABER), both aboard the NASA׳s Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. We focus on the SW1 over the Antarctic and the SPW1 in the northern hemisphere during an interval from mid-September to mid-November, and the SW1 over the Arctic and the SPW1 in the southern hemisphere during an interval from mid-March to mid-May. Large interannual variations of the SW1 and SPW1 are exhibited in both northern and southern hemispheres. For amplitudes of the SW1 at 90km and 82.5°S, positive correlations are exhibited with SPW1 amplitudes at ~55km in the equatorial region, and ~25km and 55°N. Although zonal SW1 amplitude at 95km and 86.5°N is positively correlated with SPW1 amplitudes at ~30°S above 35km, meridional SW1 amplitude is negatively correlated with SPW1 amplitudes equatorward of 30°S. We also present results of a correlation analysis for SW3 amplitudes during an interval from mid-January to mid-March over the Antarctic and from mid-July to mid-September over the Arctic with SPW1 amplitudes.

Height-latitude structure of stationary planetary waves in the stratosphere and lower mesosphere

Daily UK Met Office stratospheric assimilated data for the Northern and Southern Hemispheres, accumulated for the period from 2004 to 2012 and pressure range of 1000–0.1hPa, are used in this paper. The paper presents and thoroughly discusses spatial–temporal distributions of stationary planetary wave (SPW) amplitudes and phases, calculated on the basis of geopotential height, temperature, zonal and meridional wind data for zonal wave numbers 1 and 2 (SPW1 and SPW2). The climatological planetary wave amplitudes and phases are calculated by extracting waves from three types of data: daily, monthly mean and climatological monthly mean. It has been established that magnitude of amplitudes and height-latitude distribution of amplitudes of SPW1 and SPW2 depend on data processing method for all parameters. It has also been established that height-latitude distribution amplitudes and phases significantly differ for geopotential height, temperature, zonal and meridional wind and depend on wave number and hemisphere. However, height-latitude distributions of phases are little different from each other for the used methods of data processing.

Stationary planetary wave and nonmigrating tidal signatures in ionospheric wave 3 and wave 4 variations in 2007–2011 FORMOSAT‐3/COSMIC observations

The wave 3 and wave 4 modulations of the Equatorial Ionization Anomalies are a robust feature of the low‐latitude ionosphere, when viewed at constant local time. Although initially associated, respectively, with DE2 and DE3, nonmigrating diurnal tides in the mesosphere and lower thermosphere region, recent results have suggested that the wave 3 and wave 4 may also have significant contributions from other tidal and stationary planetary wave (SPW) signatures. We present observations of total electron content (TEC) variations associated with tidal and SPW signatures comprising the ionospheric wave 3 and wave 4 structures from FORMOSAT‐3/COSMIC from 2007 to 2011. We find that the wave 3 (wave 4) feature is comprised predominately by DE2 (DE3) and SPW3 (SPW4) signatures in TEC throughout all 5 years, with contributions from SE1 (SE2) being less significant. The wave 3 component also has recurring contributions from DW4 during December/January. The absolute amplitudes of all the aforementioned tidal and SPW signatures are directly related to the level of solar activity and the semiannual variation in zonal mean TEC. After normalizing by the zonal mean, the relative amplitudes of the wave 4 signatures are inversely related to solar activity through 2010, which is not seen with the wave 3‐related signatures. The seasonal variation and phases of the main constituents of wave 3 and wave 4 are consistent from year to year, as evidenced by the interannual recurrence in the peak and trough locations of wave 3 and wave 4.

The complete spectrum of the equatorial electrojet related to solar tides: CHAMP observations

Abstract. Based on 10 yr of magnetic field measurements by the CHAMP satellite we draw a detailed picture of the equatorial electrojet (EEJ) tidal variations. For the first time the complete EEJ spectrum related to average solar tides has been compiled. A large fraction of the resulting spectrum is related to the switch on/off of the EEJ between day and night. This effect has carefully been considered when interpreting the results. As expected, largest amplitudes are caused by the migrating tides representing the mean diurnal variation. Higher harmonics of the daily variations show a 1/f fall-off in amplitude. Such a spectrum is required to represent the vanishing of the EEJ current at night. The migrating tidal signal exhibits a distinct annual variation with large amplitudes during December solstice and equinox seasons but a depression by a factor of 1.7 around June–July. A rich spectrum of non-migrating tidal effects is deduced. Most prominent is the four-peaked longitudinal pattern around August. Almost 90% of the structure can be attributed to the diurnal eastward-propagating tide DE3. In addition the westward-propagating DW5 is contributing to wave-4. The second-largest non-migrating tide is the semi-diurnal SW4 around December solstice. It causes a wave-2 feature in satellite observations. The three-peaked longitudinal pattern, often quoted as typical for the December season, is significantly weaker. During the months around May–June a prominent wave-1 feature appears. To first order it represents a stationary planetary wave SPW1 which causes an intensification of the EEJ at western longitudes beyond 60° W and a weakening over Africa/India. In addition, a prominent ter-diurnal non-migrating tide TW4 causes the EEJ to peak later, at hours past 14:00 local time in the western sector. A particularly interesting non-migrating tide is the semi-diurnal SW3. It causes largest EEJ amplitudes from October through December. This tidal component shows a strong dependence on solar flux level with increasing amplitudes towards solar maximum. We are not aware of any previous studies mentioning this behaviour of SW3. The main focus of this study is to present the observed EEJ spectrum and its relation to tidal driving. For several of the identified spectral components we cannot offer convincing explanations for the generation mechanisms.

Nonmigrating tidal signatures in the magnitude and the inter-hemispheric asymmetry of the equatorial ionization anomaly

Abstract. Based on nine years of observations from the satellites CHAMP and GRACE the tidal signatures in the magnitude and the inter-hemisphere asymmetry of the equatorial ionization anomaly (EIA) have been investigated in this study. The EIA magnitude parameters show longitudinal wavenumber 4 and 3 (WN4/WN3) patterns during the months around August and December, respectively, while for different EIA parameters the contributions of the various tidal parameters are different. For the crest-to-trough ratio (CTR) the dominating nonmigrating tidal component contributing to WN4 is DE3 during the months around August, while during the months around December solstice the stationary planetary wave, SPW3, takes a comparable role to DE2 in contributing to WN3. For the apex height index (ApexHC) of the EIA fluxtube the stationary planetary waves, SPW4/SPW3, exceed the amplitudes of DE3/DE2 taking the leading role in causing the longitudinal WN4/WN3 patterns. During the months around December solstice the SW3 tide is prominent in both CTR and ApexHC. SW3 shows a strong dependence on the solar flux level, while it is hardly dependent on magnetic activity. For the EIA inter-hemispheric asymmetry only WN1 and WN2 longitudinal patterns can be seen. During June solstice months the pattern can be explained by stationary planetary waves SPW1 and SPW2. Conversely, around December solstice months longitudinal features exhibit some local time evolution, in particular the diurnal nonmigrating tide D0 takes the leading role.

Influence of the El Niño Southern Oscillation on the middle and upper atmosphere

Based on Whole Atmosphere Community Climate Model (WACCM) simulations, Pedatella and Liu (2012) recently demonstrated that significant interannual variability occurs in migrating and nonmigrating tides in the mesosphere and lower thermosphere (MLT) due to the El Niño Southern Oscillation (ENSO). The role of changes in tropospheric forcing, changes in the zonal mean atmosphere, and planetary wave‐tide interactions on generating the tidal variability in the MLT are investigated in the present study. The ENSO‐driven variability in the migrating diurnal tide (DW1) is found to be primarily due to changes in the tropospheric forcing of the DW1. Changes in tropospheric forcing are also the source of the changes in the eastward propagating nonmigrating diurnal tide with zonal wave number 3 (DE3). However, changes in the zonal mean atmosphere also contribute to interannual variability of the DE3 due to the ENSO. Variability in the eastward propagating nonmigrating diurnal tide with zonal wave number 2 (DE2) is largely due to changes in the background atmosphere, with a smaller additional contribution due to changes in tropospheric forcing. Variability in the westward propagating semidiurnal tide with zonal wave number 4 (SW4) is believed to be due to changes in planetary waves during the ENSO which will enhance generation of the SW4 through the nonlinear interaction of the migrating semidiurnal tide and stationary planetary waves with zonal wave number 2. The influence of the interannual tidal variability on the longitude structure of the low‐latitude ionosphere is also investigated in the present study. Comparison of El Niño and La Niña time periods reveals that the ENSO introduces changes of ~2–4 ms−1 in the daytime vertical drift velocity at certain longitudes. Simulation results further illustrate that the variability in the vertical drift velocity drives interannual variability in the low‐latitude daytime F region maximum electron density (NmF2). The results demonstrate that the ENSO introduces variability of ~10–30% in the MLT and ~10–15% in the ionosphere. The ENSO should therefore be considered as a potentially significant source of variability in the Earth's upper atmosphere.

The comparative importance of DE3, SE2, and SPW4 on the generation of wavenumber‐4 longitude structures in the low‐latitude ionosphere during September equinox

Numerical simulations are performed to investigate the generation of the wave‐4 longitude variation in the low‐latitude ionosphere due to the diurnal eastward propagating nonmigrating tide with zonal wavenumber 3 (DE3), semidiurnal eastward propagating nonmigrating tide with zonal wavenumber 2 (SE2), and stationary planetary wave 4 (SPW4). From a fixed local time perspective, the DE3, SE2, and SPW4 all appear as wave‐4 structures in longitude, and thus each of these waves must be considered as a potential source of the wave‐4 variation in the ionosphere. Both theDE3 and SPW4 are found to produce significant wave‐4 variations in the equatorial verticalE × Bdrift velocity, and in the ionospheric peak density (NmF2) at 15°N magnetic latitude. The daytime wave‐4 variation in NmF2 is driven by the combination of verticalE × Bdrift variability and in‐situ effects due largely to meridional neutral winds. The simulation results indicate that theSE2 is not a contributor to the wave‐4 longitude variation. Our results further demonstrate that the actual wave‐4 longitude variation is due to a combination of theDE3 and SPW4. We therefore conclude that, in addition to the DE3, the SPW4 also needs to be considered as an important driver of the wave‐4 longitude variation in the low‐latitude ionosphere. We additionally present evidence for the generation of theSPW4 due to the nonlinear interaction between the migrating diurnal tide and the DE3, and demonstrate the impact of DE3 variability on the amplitude of the SPW4.

Impact of Tropical SST on Stratospheric Planetary Waves in the Southern Hemisphere

Abstract The impact of tropical sea surface temperature (SST) on stratospheric planetary waves in the Southern Hemisphere (SH) is investigated in austral spring using observed SST and reanalysis data for the past three decades. Maximum covariance analysis indicates that the tropical SST and the SH stratospheric planetary wave activity are primarily coupled through two modes. The leading two modes show the La Niña–like and the central-Pacific El Niño–like SST anomalies in their positive polarities, respectively, which each are related to enhanced stratospheric planetary wave activity. These two modes also introduce phase shifts to the stratospheric stationary planetary waves: a westward shift is seen for La Niña and an eastward shift for warm SST anomalies is seen in the central Pacific. The Eliassen–Palm fluxes associated with the two modes indicate that the anomalous stratospheric wave activity originates in the troposphere and propagates upward over the mid–high latitudes, so that the linkages between tropical SST and extratropical tropospheric circulation appear to play a key role. Furthermore, the observed circulation anomaly patterns for the two modes change rapidly from spring to summer, consistent with a sharp seasonal transition in the SH basic state. Similar SST and circulation anomaly patterns associated with the two modes are simulated in chemistry–climate models.

Observations of planetary waves and nonmigrating tides by the Mars Climate Sounder

Temperature profiles from the Mars Reconnaissance Orbiter's (MRO) Mars Climate Sounder (MCS) indicate the presence of several types of stationary planetary waves and nonmigrating thermal tides in the Martian atmosphere. We analyze 35 months of MCS data to identify the dominant waves. With the contribution of the zonal mean and migrating tides removed, the temperature perturbations remaining are due to planetary waves, nonmigrating tides, and topographic and surface albedo/thermal inertia effects. We characterize waves with satellite‐relative wave numbers 1 through 4 in the Martian middle atmosphere (40–80 km). In tropical latitudes, eastward propagating diurnal Kelvin waves are the dominant component observed in the satellite‐relative wave number 2 through 4 fields; we identify these as diurnal Kelvin waves 1 through 3, respectively. The Kelvin waves have very long vertical wavelengths and amplitudes of 1–3 K. This is the first unambiguous detection of diurnal Kelvin wave 3. We also identify the zonally symmetric diurnal tide and an eastward propagating semidiurnal wave 1 tide, possibly the semidiurnal Kelvin wave 1. Stationary planetary waves with wave numbers 1 and 2 produce 5–10 K amplitudes in the middle and high latitudes of both hemispheres.

Variability of the springtime transition date and planetary waves in the stratosphere

The interannual variability of the springtime transition date of the stratospheric circulation is analyzed on the basis of the data assimilated in the UK Met Office and NCEP/NCAR models. As a proxy for the zonal mean flow we use the geostrophic wind calculated at 65.7N from geopotential heights of the 10hPa pressure level. The results show that the springtime transition date depends on the planetary-wave activity in the stratosphere and there is a tendency to the later date with a rate of about 4 days per decade. The significant correlation between the interannual variability of the spring transition date and amplitudes of stationary planetary wave with zonal wave number 1 is found. A noticeable dependence of the breakup date on the NAM index and the QBO phase is observed.