Zonal Wave Research Articles

Global observations and modeling of nonmigrating diurnal tides generated by tide‐planetary wave interactions

AbstractAdvective processes that couple planetary waves with tides have long been proposed as sources of nonmigrating diurnal tides. This paper reports observations of short‐term variability in global observations of nonmigrating tides predicted to arise from the interaction of the migrating diurnal tide (DW1) with a quasi‐stationary planetary wave number one (PW1). PW1 and tidal definitions are extracted from satellite temperatures and high‐altitude meteorological analyses. During winter months, the evolution of westward traveling diurnal tides with zonal wave number 2 (DW2) generally tracks that of strong‐amplitude stratospheric PW1. DW1 and PW1 spectra are used to compute nonlinear tidal forcing terms arising from advection. We then examine the response of a primitive equation model to the observation‐based nonlinear forcing. The model experiments indicate that meridional advection of PW1 zonal momentum by DW1 is a significant source of lower thermospheric DW2. Modeled DW2 amplitudes are very consistent with observed DW2 amplitudes when stratospheric PW1 penetrates to equatorial latitudes. The model experiments also indicate that the interaction can imprint short‐term variability associated with wintertime PW1 upon DW2 in the summer hemisphere and the lower thermosphere.

Quasi‐10‐day wave in the atmosphere

AbstractIn the classical theory of oscillations on a spherical‐rotating Earth, the quasi‐10‐day wave (Q10DW) exists as a westward propagating “free” or “unforced” normal mode oscillation with zonal wave number s = 1. In the present study, we employ Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry temperature measurements between 20 and 100 km and ±50° latitude, and extending from 2002 to 2013, to provide a comprehensive perspective on the Q10DW as it actually exists in the atmosphere. Climatological seasonal‐latitudinal structures are presented which demonstrate that the Q10DW is weakest during summer months and equatorward of ±50° latitude but otherwise has amplitudes ranging from 1.0 K at ∼45 km to ∼5 K at 100 km. Seasonal asymmetries and significant interannual variability also exist. The mean period of the Q10DW is 9.8 days with a standard deviation of about 0.4 day. On average the Q10DW conforms reasonably well with theoretical expectations for a normal mode subject to the effects of dissipation and mean winds, at least below 80 km. Above 80 km this conformity often breaks down. Several factors potentially contributing to this nonconformity are discussed.

Role of the strengthened El Niño teleconnection in the May 2015 floods over the southern Great Plains

AbstractThe climate anomalies leading to the May 2015 floods in Texas and Oklahoma were analyzed in the context of El Niño teleconnection in a warmer climate. A developing El Niño tends to increase late‐spring precipitation in the southern Great Plains, and this effect has intensified since 1980. Anthropogenic global warming contributed to the physical processes that caused the persistent precipitation in May 2015: Warming in the tropical Pacific acted to strengthen the teleconnection toward North America, modification of zonal wave 5 circulation that deepened the stationary trough west of Texas, and enhanced Great Plains low‐level southerlies increasing moisture supply from the Gulf of Mexico. Attribution analysis using the Coupled Model Intercomparison Project Phase 5 single‐forcing experiments and the Community Earth System Model Large Ensemble Project indicated a significant increase in the El Niño‐induced precipitation anomalies over Texas and Oklahoma when increases in the anthropogenic greenhouse gases were taken into account.

Solar cycle influence on troposphere and middle atmosphere via ozone layer in the presence of planetary waves: Simulation with ARM

AbstractGlobal circulation model of the Troposphere‐Middle Atmosphere‐Lower Thermosphere ARM (Atmospheric Research Model) is used to simulate the thermal and wind response to solar cycle‐induced UV variations. ARM covers altitudes from 1 to 135 km and has corresponding spatial resolution: 1 km in altitude; 11.25° in longitude; 5° in latitude. Internal Gravity Waves parameterization and planetary waves (PWs) structure on the basis of observations are determined at the lower boundary of the model. Changes in UV radiation, which is absorbed by ozone and molecular oxygen, are introduced into the model to find the corresponding global wind and temperature response. Stationary PWs with zonal wave numbers 1–3 are included at lower boundary in model runs. The simulations show that atmospheric response to solar cycle has a visible nonzonal character with the amplitude of about 5 K in the troposphere for the winter season. The effect is rather smaller for summer due to the trapping PWs at lower altitudes. So, in accordance with the results of simulations, the link between the solar UV variability and the middle and low atmosphere strongly depends on the ozone and PWs activity.

Summer climate of Madagascar and monsoon pulsing of its vortex

This study analyzes the climate of Madagascar (12°–26°S, 43°–50°E) and its relation to the Indian Ocean during austral summer (Dec–Mar). Moisture converges onto a standing easterly wave and floods are prevalent in late summer. All-island daytime land temperatures exceed 38 °C in October and are ~4 °C above sea temperatures during summer. Analysis of thermally induced diurnal convection and circulation revealed inflow during the afternoon recirculated from the southeastern mountains and the warm Mozambique Channel. Summer rainfall follows latent and sensible heat flux during the first half of the day, and gains a surplus by evening via thunderstorms over the western plains. At the inter-annual time-scale, 2.3 years oscillations in all-island rainfall appear linked with the stratospheric quasi-biennial oscillation and corresponding 80 Dobson Unit ozone fluctuations during flood events. Wet spells at frequencies from 11–27 days derive from locally-formed tropical cyclones and NW-cloud bands. Flood case studies exhibit moisture recycling in the confluence zone between the sub-tropical anticyclone and the lee-side vortex. Hovmoller analysis of daily rainfall reinforces the concept of local generation and pulsing by cross-equatorial (Indian winter) monsoon flow rather than zonal atmospheric waves. Since the surface water budget is critical to agriculture in Madagascar, this study represents a further step to understand its meso-scale summer climate.

Longitudinal and seasonal variation of the equatorial flux tube integrated Rayleigh‐Taylor instability growth rate

AbstractUsing the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM), the ionospheric Rayleigh‐Taylor instability growth rate is calculated. The seasonal and longitudinal variations of the growth rate from the TIEGCM appear to match that of the spread F observed by various satellite missions. The growth rate is strongly dependent on the angle between the sunset terminator and the geomagnetic field line near the magnetic equator. The TIEGCM simulations with nonmigrating tides show the zonal wave number 4 structure in the Rayleigh‐Taylor instability due to the inclusion of the nonmigrating diurnal eastward zonal wave number 3 and semidiurnal eastward zonal wave number 2 tides.

On the significance of the interannual relationship between the Asian‐Pacific Oscillation and the North Atlantic Oscillation

AbstractUsing monthly data from the European Centre for Medium‐Range Weather Forecasts 40 year Re‐Analysis, this study investigated the interannual relationship between the Asian‐Pacific Oscillation (APO) and the North Atlantic Oscillation (NAO) for the period of May to August. The NAO is represented by the leading Empirical Orthogonal Function of sea level pressure anomalies over the Atlantic sector. The difference of upper tropospheric eddy air temperature (T′) between Asia and the North Pacific is used to measure the APO. Results clearly show that the APO and the NAO correlated very well on the interannual time scale, with the positive (negative) phase of the APO corresponding to the strong (weak) NAO. Such a relationship is well supported by the atmospheric dynamic process. When the APO is in the positive phase (characterized by the positive T′ anomaly over Asia and the negative T′ anomaly over the North Pacific in the upper troposphere), the upper tropospheric geopotential height increases over Asia and decreases over the North Pacific. Accordingly, anticyclonic circulation and cyclonic circulation anomalies are introduced, respectively, over Asia and south of the North Pacific, and a zonal teleconnection wave train is excited from Asia to the North Atlantic. As a consequence, an anomalous anticyclonic circulation prevails in the low level of the North Atlantic and an anomalous cyclonic circulation appears to its north, which corresponds to the atmospheric pattern of strong NAO. Further study proposes that the quasi‐stationary wave pattern emanating from the western North Pacific region may serve as a bridge for this linkage.

Wave coupling between the lower and middle thermosphere as viewed from TIMED and GOCE

AbstractVertical coupling between the lower and middle thermosphere due to the eastward propagating diurnal tide with zonal wave number 3 (DE3) and the 3.5 day ultra‐fast Kelvin Wave (UFKW) is investigated using Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics‐Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED‐SABER) temperatures near 100 km and Gravity field and steady‐state Ocean Circulation Explorer (GOCE) neutral densities and zonal winds near 260 km. The analysis is performed between ±45∘ latitude during 2011, when reliable and continuous measurements are available. With geomagnetic and solar effects removed, DE3 and the UFKW are identified as dominant sources of day‐to‐day variability at both heights. Evidence is found for the vertical propagation of DE3 and the UFKW from the lower to middle thermosphere over a range of time scales. Over 60% of the variance due to DE3 and the UFKW at 260 km is traceable to variability occurring at 100 km. The not exact agreement is thought to be due to the influences of wave‐wave interactions, zonal mean winds, dissipation, and inherent transience that interfere with one‐to‐one mapping of structures between 100 and 260 km. Spectral and temporal analyses of the SABER and GOCE data also reveal the presence of sidebands due to the modulation of DE3 by the UFKW. These secondary waves are responsible for up to 10% to 20% of the longitudinal and day‐to‐day variability. Overall, vertical propagating waves together with sidebands from DE3‐UFKW nonlinear interactions are responsible for 60% to 80% of the total variability, while geomagnetic and solar effects correlated with ap and F10.7 account for less than 20% of the variance.

Madden Julian Oscillation impacts on global ocean surface waves

We assess the impact of the tropical Madden Julian Oscillation (MJO) on global ocean wind waves using 30 years of wave data from a wave model hindcast that is forced with high resolution surface winds from the NCEP–CFSR reanalysis. We concentrate on the boreal winter season when the MJO has its greatest amplitude and is potentially a source of predictable wave impacts at intra-seasonal lead times. Statistically significant anomalies in significant wave height (Hs), peak wave period (Tp) and zonal wave energy flux (CgE) are found to covary with the intra-seasonal variation of surface zonal wind induced by the MJO as it traverses eastward from the western tropical Indian Ocean to the eastern tropical Pacific. Tp varies generally out of phase with Hs over the life cycle of the MJO, indicating that these MJO-wave anomalies are locally wind-generated rather than remotely generated by ocean swell.Pronounced Hs anomalies develop on the northwest shelf of Australia, where the MJO is known to influence sea level and surface temperatures, and in the western Caribbean Sea and Guatemalan–Panama Seas with enhanced wave anomalies apparent in the vicinity of the Tehuantepec and Papagayo gaps. Significant wave anomalies are also detected in the North Pacific and North Atlantic oceans in connection with the MJO teleconnection to the extratropics via atmospheric wave propagation. The impact in the north Atlantic stems from induction of the high phase of the North Atlantic Oscillation (NAO) about 1 week after MJO convection traverses the Indian Ocean, and the low phase of the NAO about one week after suppressed convection traverses the Indian Ocean. Strong positive Hs anomalies maximize on the Northern European coast in the positive NAO phase and vice versa for the negative NAO phase. The MJO also influences the occurrence of daily low (below the 5th percentile) and high (above the 95th percentile) wave conditions across the tropics and in the North Pacific and North Atlantic, emphasizing that the MJO may be a valuable source of intra-seasonal predictability of surface wave variability.

Causes of the longitudinal differences in the equatorial vertical E × B drift during the 2013 SSW period as simulated by the TIME‐GCM

AbstractDuring stratospheric sudden warming (SSW) periods large changes in the low‐latitude vertical drift have been observed at Jicamarca as well as in other longitudinal sectors. In general, a strengthening of the daytime maximum vertical drift with a shift from prenoon to the afternoon is observed. During the January 2013 stratospheric warming significant longitudinal differences in the equatorial vertical drift were observed. At Jicamarca the previously reported SSW behavior prevails; however, no shift of the daytime maximum drift was exhibited in the African sector. Using the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) the possible causes for the longitudinal difference are examined. The timing of the strong SSW effect in the vertical drift (15–20 January) coincides with moderate geomagnetic activity. The simulation indicates that approximately half of the daytime vertical drift increase in the American sector may be related to the moderate geophysical conditions (Kp = 4) with the effect being negligible in the African sector. The simulation suggests that the wind dynamo accounts for approximately 50% of the daytime vertical drift in the American sector and almost 100% in the African sector. The simulation agrees with previous findings that the migrating solar tides and the semidiurnal westward propagating tide with zonal wave number 1 (SW1) mainly contribute to the daytime wind dynamo and vertical drift. Numerical experiments suggest that the neutral wind and the geomagnetic main field contribute to the presence (absence) of a local time shift in the daytime maximum drift in the American (African) sector.

Role of water vapor and convection‐circulation decoupling in MJO simulations by a tropical channel model

AbstractA tropical channel model failed to simulate an observed MJO event. As a means to find reasons for the failure, model errors in water vapor (qv) and other fields were corrected to different extents by nudging towards a global reanalysis product. An MJO metric was developed to quantify the realism of a simulated MJO event in the amplitude and zonal propagation speed of its precipitation and zonal wind at 850 hPa (U850). Results show that correcting errors in qv alone is necessary and sufficient for a realistic simulation of the MJO event. Correcting errors in other fields is insufficient by itself and unnecessary with water vapor correction. For a realistic MJO simulation, error corrections are needed for qv perturbations of the planetary scales (zonal wave number 1–3) together with its zonal mean in both the lower and middle troposphere. Correcting errors in qv at upper levels, planetary‐scale perturbations of any single zonal wave number, synoptic‐scale perturbations, or temporal/zonal means alone is not sufficient for a realistic MJO simulation. Amplitudes of MJO precipitation and U850 are significantly correlated, but their propagation speeds are not. MJO signals in U850 are easier than those in precipitation to improve by correcting errors in qv. In the absence of MJO intrinsic dynamics, inserted MJO signals in qv in a simulation may create a phantom MJO event in precipitation but not in the circulation. This study confirms the existing knowledge on the importance of qv to MJO simulations, sheds new lights on the role of the qv spatial structure, and presents evidence for possible precipitation‐circulation decoupling in MJO simulations.

Variability of the quasi-2-day wave and interaction with longer period planetary waves in the MLT at Cachoeira Paulista (22.7°S, 45°W)

An exclusive study has been carried out with long term meteor wind data (2000–2014) to characterize the quasi-2-day wave (QTDW) in the mesosphere and lower thermosphere (MLT) and its interactions with the longer period planetary waves at Cachoeira Paulista (22.7°S, 45°W). The QTDW is observed to be dominant during late summer (January–February) all the years under consideration except 2013. All the wave parameters exhibit significant interannual variability. The maximum wave amplitude comes out to be 39m/s, which is significantly higher than the reported northern hemispheric findings. The mean MLT period exhibits a wide range of variability (36–70h) indicating the presence of multiple Rossby normal modes with varying zonal wave numbers. Modulations of the QTDW amplitude by the planetary waves with longer periodicities (>9 days) are evident, especially during summer. The nonlinear interactions between the 2-day wave and longer period waves are believed to give rise to a host of secondary waves with frequencies lying close to 2-day. The strong QTDW activity, as observed at this location, has potential to cause significant effect on the overlying ionosphere and hence the atmosphere-ionosphere dynamical coupling.

Five‐day waves in polar stratosphere and mesosphere temperature and mesospheric ice water measured by SOFIE/AIM

AbstractThe temperature and column ice water content (IWC) of polar mesospheric clouds (PMCs) have been simultaneously measured by the Solar Occultation for Ice Experiment (SOFIE) on board NASA's Aeronomy of Ice in the Mesosphere satellite since April 2007. The 8 year (2007–2014) data of the temperature and IWC are used to extract the 5 day planetary waves (PWs) with zonal wave numbers ranging from −1 to −3 (eastward propagating mode, E1–E3), 0 (stationary mode, W0), and 1 to 3 (westward propagating mode, W1–W3) in the polar stratosphere and mesosphere. The 5 day PWs in temperature are stronger in the polar winter stratosphere and mesosphere and exhibit substantial interhemispheric asymmetry. The date‐height distributions of the 5 day waves coincide with those of the eastward jet in each hemisphere. This indicates that the 5 day PWs might be generated from barotropic/baroclinic instability in the polar stratosphere. The relative strengths of 5 day PWs decrease with increasing wave numbers. The E1 (W1) 5 day PW is stronger than any other mode in the winter stratosphere and lower mesosphere (summer upper mesosphere). SOFIE temperature and IWC data are derived from simultaneous measurements in the same air column and thus provide a good opportunity to study the phase relationship between the 5 day PWs in temperature and IWC. Our analyses show that the phase shifts of W1 5 day PW in temperature relative to that in IWC have a mean of −2.0 h (0.3 h) with a standard deviation of 3.8 h (4.2 h) in the northern (southern) polar region. This indicates that the formation of the W1 5 day PW in PMCs is controlled mainly by the W1 5 day PW in temperature and influenced by other factors and is consistent with previous studies.

Observations of planetary waves in the mesosphere-lower thermosphere during stratospheric warming events

Abstract. This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW) activity in the mesosphere–lower thermosphere (MLT). PW activity near 95 km is derived from meteor wind data using a chain of eight SuperDARN radars at high northern latitudes that span longitudes from 150° W to 25° E and latitudes from 51 to 66° N. Zonal wave number 1 and 2 components were extracted from the meridional wind for the years 2000–2008. The observed wintertime PW activity shows common features associated with the stratospheric wind reversals and the accompanying stratospheric warming events. Onset dates for seven SSW events accompanied by an elevated stratopause (ES) were identified during this time period using the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM). For the seven events, a significant enhancement in wave number 1 and 2 PW amplitudes near 95 km was found to occur after the wind reversed at 50 km, with amplitudes maximizing approximately 5 days after the onset of the wind reversal. This PW enhancement in the MLT after the event was confirmed using SD-WACCM. When all cases of polar cap wind reversals at 50 km were considered, a significant, albeit moderate, correlation of 0.4 was found between PW amplitudes near 95 km and westward polar-cap stratospheric winds at 50 km, with the maximum correlation occurring ∼ 3 days after the maximum westward wind. These results indicate that the enhancement of PW amplitudes near 95 km is a common feature of SSWs irrespective of the strength of the wind reversal.

Multiday thermospheric density oscillations associated with variations in solar radiation and geomagnetic activity

AbstractThermospheric densities observed by Challenging Minisatellite Payload and Gravity Recovery and Climate Experiment satellites during 2002–2010 and the globally averaged thermospheric densities from 1967 to 2007 have been used to investigate latitudinal, longitudinal, and height dependences of the multiday oscillations of thermospheric densities. The data show that the main multiday oscillations in thermospheric densities are 27, 13.5, 9, and 7 day oscillations. The high‐correlation coefficients between the density oscillations and the F10.7 or Ap index indicate that these oscillations are externally driven. The 27 day density oscillation, being the strongest, is induced by variations in solar radiation, as well as recurrent geomagnetic activity that is the result of corotating interaction regions (CIRs) and high‐speed solar wind streams of coronal hole origin. Density oscillations at periods of 13.5, 9, and 7 days at solar minimum and during the declining phase are stronger than those at solar maximum. These oscillations are mainly associated with recurrent geomagnetic activity due to coronal hole high‐speed streams and CIRs. The multiday, periodic oscillations of thermospheric density exhibit strong latitudinal and longitudinal variations in the geomagnetic coordinate and oscillate synchronously at different heights. Oscillations with zonal wave number 0 oscillate globally, whereas those with nonzero wave numbers are strong at high geomagnetic latitudes, and hemispherically asymmetric. They are stronger in the Southern Hemisphere. The spectral distributions of thermospheric densities at different heights have almost the same latitude and longitude structures, but the spectral magnitudes increase with height.

Global ionospheric response to the 2009 sudden stratospheric warming event using Ionospheric Data Assimilation Four‐Dimensional (IDA4D) algorithm

AbstractA data assimilation algorithm is used to delineate the time‐dependent three‐dimensional ionospheric response to the 2009 sudden stratospheric warming (SSW) event. We use the Ionospheric Data Assimilation Four‐Dimensional (IDA4D) algorithm to study the global ionospheric response to the 2009 SSW. This is the first study to utilize global ionospheric measurements in a data assimilation framework to unambiguously characterize atmosphere‐ionosphere coupling via tidal modifications during the 2009 SSW event. Model results reveal that the dominant mode of ionospheric variability during the 2009 SSW is driven by the enhancements in westward propagating semidiurnal tide with zonal wave number 1. The IDA4D results completely characterize the tidal perturbation during the 2009 SSW for the first time and show the global 3‐D structure of the tide in total electron content (TEC) and electron density. The largest ionospheric responses were seen at low latitudes, where ionospheric plasma is extremely sensitive to the zonal electric field and susceptible to modifications by tidal winds in the lower thermosphere. The ionospheric response to the warming was characterized by an increase in TEC in the morning/early afternoon sector and a decrease during the late afternoon/evening period. The effects of coupling between the stratosphere and ionosphere were strongest between 220 km and 380 km. The IDA4D results also show a reversal of asymmetry in the equatorial ionization anomaly crests occurring several days after the peak of the 2009 SSW event. We suggest that this could be a result of the equatorial fountain effect being further modified by the summer‐to‐winter meridional neutral winds.

Ionospheric vertical plasma drift perturbations due to the quasi 2 day wave

AbstractThe thermosphere‐ionosphere‐mesosphere‐electrodynamics–general circulation model is utilized to study the vertical E × B drift perturbations due to the westward quasi 2 day wave with zonal wave numbers 2 and 3 (W2 and W3). The simulations show that both wind components contribute directly and significantly to the vertical drift, which is not merely confined to low latitudes. The vertical drifts at the equator induced by the total wind perturbations of W2 are comparable with that at middle latitudes, while the vertical drifts from W3 are much stronger at middle latitudes than at the equator. The ion drift perturbations induced by the zonal and meridional wind perturbations of W2 are nearly in‐phase with each other, whereas the phase discrepancies of the ion drift induced by the individual wind component of W3 are much larger. This is because the wind perturbations of W2 and W3 have different latitudinal structures and phases, which result in different ionospheric responses through wind dynamo.

A comprehensive survey of atmospheric quasi 3 day planetary‐scale waves and their impacts on the day‐to‐day variations of the equatorial ionosphere

AbstractThis study reports a comprehensive survey of quasi 3 day (2.5–4.5 day period) planetary‐scale waves in the low‐latitude mesosphere and lower thermosphere using the temperature observations from Thermosphere Ionosphere and Mesosphere Electric Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry throughout 2002–2012. Occurrences and properties of the waves, including the eastward propagating zonal wave numbers of 1–3 (E1–E3) and vertical wavelengths, are determined for each case. The impacts of these waves on the equatorial ionosphere are investigated by searching for the corresponding variations with the same periods and wave numbers in total electron content (TEC) from the concurrent observations of the ground‐based GPS network. For a threshold amplitude of 4 K in temperature, a total of 300 waves are identified, of which there are 186 E1, 63 E2, and 51 E3 events. The mean amplitudes and vertical wavelengths of these waves are calculated to be about 7.9 K and 34 km for the E1, 5.7 K and 29 km for the E2, and 5.1 K and 27 km for the E3, having the standard deviations of 1.5 K and 6.5 km, 0.6 K and 5.6 km, and 0.5 K and 6.7 km. Occurrences of the E1 cases are not observed to depend on season, but the large‐amplitude (>8 K) cases occur more often during solstices than at equinoxes. Similarly, the E2 and E3 cases are observed to occur most often in January–February and May–August. Among these waves, 199 cases (66%) are found to have the corresponding variations in the equatorial ionosphere with amplitudes ≥4.2% relative to the mean TEC values (corresponding to 90th percentile). Most of these waves have long vertical wavelengths and large amplitudes (∼3 times more than short vertical wavelength and small‐amplitude waves). Because no seasonal or solar cycle dependence on the frequency at which these waves have corresponding variations in the ionosphere at this TEC perturbation threshold is observed, we conclude that there is no seasonal and solar cycle dependence on the propagation of such waves from the mesopause region to higher altitudes. We also identify that only 28 cases (19%) of the E1 TEC variations do not correspond to any E1 waves, which is consistent with the hypothesis that E1 waves are the primary cause of E1 TEC variations. Conditions that are favorable for 3 day waves to create ionospheric variations are present approximately two thirds of the time. This study quantifies the importance and frequency of atmospheric quasi 3 day planetary‐scale waves on the day‐to‐day variations of the equatorial ionosphere using a statistical rather than case study approach.

Nonmigrating tidal signature in the distributions of equatorial plasma bubbles and prereversal enhancement

AbstractSome wave‐like features in the longitudinal distribution of equatorial plasma bubbles understood in association with diurnal eastward propagating zonal wave number 3 nonmigrating tide (DE3) in the dayside. However, whether or not the wave features are the daytime DE3 signature has not yet been rigorously investigated. This study investigates (1) the existence of the DE3 signature in the longitudinal distribution of bubbles by analyzing the first Republic of China (ROCSAT‐1) satellite data acquired in 2000–2002 and (2) the role of daytime DE3 in the creation of bubbles by examining the linear growth rate of the generalized Rayleigh‐Taylor (R‐T) instability. The linear growth rate is derived from the “Sami2 is Another Model of the Ionosphere” model simulation results. In the longitudinal distribution of bubbles derived from ROCSAT‐1 observations, the wave number 4 component, the representative characteristic of DE3, is a weak feature. In addition, the amplitude and phase of the wave number 4 component do not show a consistent behavior in comparison with those of DE3. Our numerical calculation results show that the linear growth rate of the R‐T instability is not sensitive to the variation of the daytime vertical plasma drift. These results indicate that the DE3 signature in the occurrence rate of bubbles is not obvious and the effect of daytime DE3 on the creation of bubbles is negligible.

Three‐dimensional structures of tropical nonmigrating tides in a high‐vertical‐resolution general circulation model

AbstractThis paper investigates nonmigrating tides from the ground to the lower mesosphere using data from a high‐resolution general circulation model (KANTO GCM), as well as observational data from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on board the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite and from GPS radio occultation measurements obtained with the COSMIC/FORMOSAT‐3 mission. We extract nonmigrating tides using a composite as a function of universal time in physical space, without performing a zonal wave number decomposition. The KANTO GCM clearly demonstrates that tropical nonmigrating tides are regarded as gravity waves excited by diabatic heating enhanced over two major continents, specifically Africa and South America. They propagate zonally, in a direction away from their sources; that is, west and eastward propagating waves are dominant on the western and eastern sides of the continents, respectively. These characteristics are observed in two satellite data sets as well, except that the amplitudes in the KANTO GCM are larger than those in the observations. Seasonal variations of nonmigrating tides are also investigated. It is suggested that filtering owing to the stratopause semiannual oscillation, as well as diabatic heating in the troposphere, is important for the seasonal variations of nonmigrating tides in the stratosphere and the lower mesosphere.