Zonal Wave Research Articles

Trends in summer heatwaves in central Asia from 1917 to 2016: Association with large‐scale atmospheric circulation patterns

AbstractThe changes in the frequency, duration, and intensity of summer heatwaves over central Asia during the period 1917–2016 were studied. On average, the frequency, duration, and intensity of heatwaves showed significant positive trends during the period 1917–2016, with enhanced rates during the last 50 years. During 1967–2016, the heatwave indices increased significantly in most of central Asia, especially in the western part. The number of heatwaves has increased by 1.3 times since the 1960s. Remarkable changes in the frequency and duration of heatwaves occurred during the 1990s in association with the inter‐decadal shift in the Silk Road pattern of atmospheric circulation around 1997. The results based on the ERA‐Interim reanalysis data set were well‐matched with the station observations during the period 1979–2016, whereas those based on the NCEP‐NCAR data set were less well matched. Heatwaves in central Asia were closely related to a zonal wave circulation pattern at 500 hPa with a centre of positive geopotential height anomalies over central Asia. This anomalous circulation pattern was rapidly enhanced during the 1990s, suggesting that large‐scale patterns of atmospheric circulation had a role in modulating the occurrence of heatwaves in central Asia.

Global analysis for periodic variations in gravity wave squared amplitudes and momentum fluxes in the middle atmosphere

Abstract. Atmospheric gravity waves (GWs) are an important coupling mechanism in the middle atmosphere. For instance, they provide a large part of the driving of long-period atmospheric oscillations such as the Quasi-Biennial Oscillation (QBO) and the semiannual oscillation (SAO) and are in turn modulated. They also induce the wind reversal in the mesosphere–lower thermosphere region (MLT) and the residual mean circulation at these altitudes. In this study, the variations in monthly zonal mean gravity wave square temperature amplitudes (GWSTAs) and, for the first time, absolute gravity wave momentum flux (GWMF) on different timescales such as the annual, semiannual, terannual and quasi-biennial variations are investigated by spectrally analyzing SABER observations from 2002 to 2015. Latitude–altitude cross sections of spectral amplitudes and phases of GWSTA and absolute GWMF in the stratosphere and mesosphere are presented and physically interpreted. It is shown that the time series of GWSTA and GWMF at a certain altitude and latitude results from the complex interplay of GW sources, propagation through and filtering in lower altitudes, oblique propagation superposing GWs from different source locations, and, finally, the modulation of the GW spectrum by the winds at a considered altitude and latitude. The strongest component is the annual variation, dominated in the summer hemisphere by subtropical convective sources and in the winter hemisphere by polar vortex dynamics. At heights of the wind reversal, a 180∘ phase shift also occurs, which is at different altitudes for GWSTA and GWMF. In the intermediate latitudes a semiannual variation (SAV) is found. Dedicated GW modeling is used to investigate the nature of this SAV, which is a different phenomenon from the tropical SAO also seen in the data. In the tropics a stratospheric and a mesospheric QBO are found, which are, as expected, in antiphase. Indication for a QBO influence is also found at higher latitudes. In previous studies a terannual variation (TAV) was identified. In the current study we explain its origin. In particular the observed patterns for the shorter periods, SAV and TAV, can only be explained by poleward propagation of GWs from the lower-stratosphere subtropics into the midlatitude and high-latitude mesosphere. In this way, critical wind filtering in the lowermost stratosphere is avoided and this oblique propagation is hence likely an important factor for MLT dynamics.

Diagnostics of a WN2‐Type Major Sudden Stratospheric Warming Event in February 2018 Using a New Three‐Dimensional Wave Activity Flux

AbstractObservational features of atmospheric fields during a zonal wave number 2‐type major sudden stratospheric warming (MSSW) event in February 2018 (MSSW18) are analyzed using the Japanese 55‐year Reanalysis, satellite measurements by a microwave limb sounder, and a new three‐dimensional wave activity flux. MSSW18 is characterized by a clear polar vortex split, continuance of easterlies with evident double peaks, clear planetary wave propagation in the easterly region of the upper stratosphere and extraordinary upward propagation of wave number 2 planetary waves from the upper troposphere. The upward propagation peak is comparable to that during an MSSW event in January 2009 (MSSW09), although MSSW18 shows relatively modest warming, unclear disappearance of the stratopause and weak stratopause elevation compared to those during MSSW09. Further analyses reveal that wave packets propagates upward simultaneously in both the Eastern and Western Hemispheres. Those observed in the Eastern Hemisphere converge strongly at the western edge of the Aleutian High, and most of them do not propagate further into the upper stratosphere. The wave packets observed in the Western Hemisphere propagate locally into the upper stratosphere over North America despite the fact that easterlies were predominant there in the mature stage of MSSW18. Furthermore, the results of our study reveal that the regions and levels of wave packet attenuation correspond well to the areas that smaller‐scale waves have weak eastward phase tilt or are in the quasi‐barotropic condition during MSSW18, whereas during MSSW09 the westward phase tilts of smaller‐scale waves were clearly visible in the upper stratosphere.

Extratropical age of air trends and causative factors in climate projection simulations

Abstract. Climate model simulations show an acceleration of the Brewer–Dobson circulation (BDC) in response to climate change. While the general mechanisms for the BDC strengthening are widely understood, there are still open questions concerning the influence of the details of the wave driving. Mean age of stratospheric air (AoA) is a useful transport diagnostic for assessing changes in the BDC. Analyzing AoA from a subset of Chemistry–Climate Model Initiative part 1 climate projection simulations, we find a remarkable agreement between most of the models in simulating the largest negative AoA trends in the extratropical lower to middle stratosphere of both hemispheres (approximately between 20 and 25 geopotential kilometers (gpkm) and 20–50∘ N and S). We show that the occurrence of AoA trend minima in those regions is directly related to the climatological AoA distribution, which is sensitive to an upward shift of the circulation in response to climate change. Also other factors like a reduction of aging by mixing (AbM) and residual circulation transit times (RCTTs) contribute to the AoA distribution changes by widening the AoA isolines. Furthermore, we analyze the time evolution of AbM and RCTT trends in the extratropics and examine the connection to possible drivers focusing on local residual circulation strength, net tropical upwelling and wave driving. However, after the correction for a vertical shift of pressure levels, we find only seasonally significant trends of residual circulation strength and zonal mean wave forcing (resolved and unresolved) without a clear relation between the trends of the analyzed quantities. This indicates that additional causative factors may influence the AoA, RCTT and AbM trends. In this study, we postulate that the shrinkage of the stratosphere has the potential to influence the RCTT and AbM trends and thereby cause additional AoA changes over time.

Climatology of quasi-two day oscillations from GPS-derived total electron content during 1999–2015

The paper presents the climatology of eastward- and westward-propagating quasi-2-day oscillations (QTDOs) with zonal wave numbers 1–4 using the global ionospheric total electron content (TEC) derived at the International GNSS Service (IGS) from the network of ground Global Positioning System (GPS) receivers. The climatologies of space-time spectra and amplitudes of QTDOs in TEC during 16 years from December to February (DJF) (1998/99–2014/15) and 17 years from June to August (JJA) (1999–2015) were investigated. The space-time spectra (spectral signature) and the mean amplitude of all wave modes (1–4) of QTDOs exhibit strong latitudinal, hemispheric, inter-annual and solar-cycle variations. Latitudinaly, strengths of the spectral signature and amplitude values diminish gradually from maxima at equatorial ionization anomaly (EIA) crest towards high latitude regions. The spectral signature amplify (weaken) in the solar maximum (minimum) periods. Likewise, the amplitudes of both the eastward- and westward-propagating QTDOs attain peak values during the maximum solar activity phases of the solar cycle. Seasonally, the spectral signature and amplitude values reveal summer amplification in both hemispheres as opposed to winter suppression. However, the spectral signatures (amplitudes) in the Southern Hemisphere summer are stronger (larger) than that in the Northern Hemisphere summer. Moreover, the spectral signatures and amplitudes associated with the westward-propagating oscillations are dominant over those propagating eastward. The hemispheric and latitudinal mean differences in the long-term mean daily values of amplitude of E1 (W1) QTDOs exhibit high values in the southern (northern) hemisphere near the December (June) solstices that follow the solar declination. The global spatial mean amplitudes of E1 and W1 QTDOs are significantly higher during DJF than in JJA throughout the period of study with a few exceptions revealing the annual anomaly phenomenon in TEC which is more notable during solar maxima. Moreover winter anomaly is distinctly captured in the northern hemisphere as reflected by higher amplitudes of E1 and W1 QTDOs in summer solstice (JJA) than in winter solstice (DJF) consistent with the antecedent understanding.

Distinct Propagation Characteristics of Intraseasonal Variability Over the Tropical West Pacific

AbstractTropical intraseasonal variability, with the Madden‐Julian oscillation (MJO) as its most prominent mode, exerts extensive influences on global weather extremes. It is found that strong interannual variability of intraseasonal convection exists in the west Pacific (WP), in the form of years with strong eastward propagation (i.e., associated with the MJO) and years with strong westward propagation. Years with strong westward propagation on intraseasonal timescales are dominated by a westward propagating intraseasonal mode (WPIM), which is the second leading intraseasonal mode after the MJO over the tropical WP. Initiated over the central Pacific, the WPIM exhibits slow equatorial westward propagation (5 m/s) with a period of 25 days and a spatial scale of zonal wave number 3–4. Unlike the MJO, the WPIM lacks a significant tilt with height in specific humidity and vertical velocity. A strong anticorrelation is found between MJO and WPIM activity on interannual timescales over the WP. Budget analyses of the moist static energy suggest that both modes are driven by horizontal moist static energy advection and that substantial differences in winter mean large‐scale moisture and zonal winds largely define their distinct propagation behaviors. The WPIM is favored, while the MJO is suppressed when mean equatorial low‐level easterlies between 150°E and 160°W are enhanced and equatorial mean low‐level moisture is reduced near the Dateline and enhanced in the off‐equatorial WP (110°–150°E). While the WPIM bears resemblance to low‐frequency equatorial Rossby waves, a more detailed analysis must be conducted to affirm if they are the same phenomenon.

A quantitative study of moisture transport variation on the interdecadal variation of the summer precipitation in South China from 1979 to 2015

The summer precipitation in South China (SC) has experienced a pronounced interdecadal variation during 1983–2013 with trend transition in the late 1990s. This study quantitatively investigates the precipitation variation and its connection to water vapor transport by combining the Lagrangian trajectory-based Dynamic Recycling Model and the clustering method of self-organizing map. The external moisture outside of SC explains most (84%) of the mean and the interdecadal variation of the summer rainfall, mainly through the southwest transport pathways. A long-distance southwest pathway related to cross-equatorial flow and eastward flow over the Northern Indian Ocean explains 31.5% of mean precipitation and 50.4% of the upward precipitation trend before 1997. The other branch of the southwest pathways has relatively shorter length over North Indian Ocean, South China Sea, and Southeast Asia, explaining 35.7% of the mean and 51.2% of the downward trend after 1997. Also, for the downward trend, the westerly-driven moisture transport over Eurasia acts as the second contributor (32.2%) to the precipitation decrease. However, the western-Pacific pathway explains the smallest portion (≤ 3%) of the trends, suggesting weak influence from the subtropical high. The large-scale circulation anomaly in the form of zonal and meridional wave trains control the interdecadal variability of the SC precipitation. It is found that the circumglobal teleconnection and Pacific–Japan teleconnection significantly correlate to the two wave trains, whose match relation strongly modulates the trend transition in the 1990s for the SC summer precipitation.

Polar Cyclones at the Origin of the Reoccurrence of the Maud Rise Polynya in Austral Winter 2017

AbstractThis study examines the role of atmospheric forcings in the occurrence of open‐ocean polynyas by investigating the case of the austral winter 2017's polynya located in the Lazarev Sea sector to the east of the Weddell Sea, known as the Maud Rise polynya or the Weddell Polynya. The ice‐free zone appeared in mid‐September 2017 and grew to as large as 80,000 km2 by the end of October 2017 before merging with the open ocean after the sea ice started to retreat at the beginning of the austral summer. Using a combination of satellite observations and reanalysis data at high spatiotemporal resolution, we found that severe cyclones, occurring over the ice pack, have a deterministic role in creating strong divergence in the sea ice field through strong cyclonic surface winds leading to the opening of the polynya. The occurrence of intense and frequent cyclones over the ice pack during austral winter 2017 was unusual, and it occurred under an enhanced strong positive meridional transport of heat flux and moisture toward Antarctica associated with an amplification of the atmospheric zonal wave 3 and a strong positive Southern Annular Mode index. We found that the opening of the polynya was not primarily due to direct ice melt by thermodynamic effects but rather to strong dynamical forcing by the winds on the sea ice, as in the case of coastal polynyas. Indeed, the meridional transport of heat toward Antarctica occurred over the Weddell Sea sector (i.e., to the east of the Lazarev Sea sector where the polynya is located) whereas the Lazarev Sea sector was under the influence of equatorward transport of cold air masses at that time. Our results show that the supply of warm and moist air coming from the west side of the South Atlantic Ocean into the Weddell Sea significantly increased the potential for cyclone formation as measured by the Eady growth rate leading to intense and frequent cyclogenesis over the ice pack, far south from the ice edge. After cyclogenesis in the Weddell Sea, these cyclones intensified as they moved eastward spinning over the Lazarev Sea with intensity comparable to category 11—violent storms—in the Beaufort scale. The cyclonic winds generated sea ice divergence by pushing the ice away from the cyclone center: To the east, north of it and to the west, south of it, which led to the reoccurrence of the Maud Rise polynya in mid‐September 2017.

Mesospheric semidiurnal tides and near-12 h waves through jointly analyzing observations of five specular meteor radars from three longitudinal sectors at boreal midlatitudes

Abstract. In the last decades, mesospheric tides have been intensively investigated with observations from both ground-based radars and satellites. Single-site radar observations provide continuous measurements at fixed locations without horizontal information, whereas single-spacecraft missions typically provide global coverage with limited temporal coverage at a given location. In this work, by combining 8 years (2009–2016) of mesospheric winds collected by five specular meteor radars from three different longitudinal sectors at boreal midlatitudes (49±8.5∘ N), we develop an approach to investigate the most intense global-scale oscillation, namely at the period T=12±0.5 h. Six waves are resolved: the semidiurnal westward-traveling tidal modes with zonal wave numbers 1, 2, and 3 (SW1, SW2, SW3), the lunar semidiurnal tide M2, and the upper and lower sidebands (USB and LSB) of the 16 d wave nonlinear modulation on SW2. The temporal variations of the waves are studied statistically with a special focus on their responses to sudden stratospheric warming events (SSWs) and on their climatological seasonal variations. In response to SSWs, USB, LSB, and M2 enhance, while SW2 decreases. However, SW1 and SW3 do not respond noticeably to SSWs, contrary to the broadly reported enhancements in the literature. The USB, LSB, and SW2 responses could be explained in terms of energy exchange through the nonlinear modulation, while LSB and USB might previously have been misinterpreted as SW1 and SW3, respectively. Besides, we find that LSB and M2 enhancements depend on the SSW classification with respect to the associated split or displacement of the polar vortex. In the case of seasonal variations, our results are qualitatively consistent with previous studies and show a moderate correlation with an empirical tidal model derived from satellite observations.

Seasonal Variations of Low‐Latitude Migrating and Nonmigrating Diurnal and Semidiurnal Tides in TIMED‐SABER Temperature and Their Relationship With Source Variations

AbstractSeasonal and source variations of migrating and nonmigrating tides are studied using Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics‐Sounding of the Atmosphere using Broadband Emission Radiometry temperature data at 10°N (5–15°N) for the year 2009. The migrating DW1 shows equinoctial maximum and summer minimum at low latitudes. It shows equinoctial asymmetry with larger amplitudes during spring equinox than fall equinox. The migrating semidiurnal tidal amplitude (SW2) shows larger amplitudes (~20 K) during March–October at 30–60°S. Its seasonal variation resembles stratospheric (10 hPa) ozone variations at southern midlatitudes. During the sudden stratospheric warming of 2009, the SW1 shows larger amplitudes over the equator and it is generated due to nonlinear interaction between SW2 and planetary wave of zonal wave number 1. The eastward nonmigrating DE4 and DE3 tides enhance in summer. The DE3 and DE4 appear to be generated due to latent heat release in the troposphere, as their amplitudes in the National Center for Environmental Prediction (NCEP)'s Precipitable water vapor (proxy for latent heat release) enhance at similar times as in mesosphere. The DW2 and DW0 tides are likely to be generated due to nonlinear interaction between DW1 and planetary wave of zonal wave number 1. The SW3 enhancement during the early winter (November‐December) may be due to nonlinear interaction between DW1 and the large‐amplitude DW2. The nonlinear interactions of DW1 with planetary wave and nonmigrating tides explain the summer minimum and equinoctial asymmetry of DW1.

The Madden‐Julian Oscillation in an Aquaplanet‐Like General Circulation Model With and Without Continents

AbstractWe have conducted experiments with the Community Atmospheric Model version 5.1 configured as an aquaplanet, with the sun at March equinox, in order to study how the model Madden‐Julian oscillation (MJO) responds to (i) the introduction of continents and (ii) realistic sea surface temperature (SST) distribution. Model results are compared with ERA‐Interim reanalysis and analyzed in terms of the moist static energy (MSE) budget to study the growth and propagation of MJO. With idealized zonally symmetric SST, the aquaplanet model produces a double Intertropical Convergence Zone (ITCZ) and an MJO‐like mode with variance at intraseasonal (30‐ to 96‐day) periods and zonal wave number 1. When we introduce continents with realistic orography and interactive surface temperature, soil moisture, and albedo, the variance of MJO is mainly confined to zonal wave numbers 1 and 2 but reduced by a factor of 3 due to weaker boundary layer moisture convergence. With prescribed climatological January SST boundary condition (which includes Indo‐west Pacific warm pool) in the presence of continents, model MJO variance is enhanced and distributed across zonal wave numbers 1 to 5, in closer agreement with observation. Thus, the presence of land by itself is not enough, but realistic SST distribution is necessary for improved space‐time characteristics of model MJO. Both in simulations and ERA‐Interim data, meridional and vertical advection of MSE promotes eastward movement of MJO. In the model experiments, meridional advection of low‐level MSE anomaly is most significant in the vicinity of the ITCZ, indicating the importance of processes that determine the location of (single or double) ITCZ.

Formation of the Y Feature at the Venusian Cloud Top by Planetary‐Scale Waves and the Mean Circulation: Analysis of Venus Express VMC Images

AbstractThe relationship between the planetary‐scale ultraviolet contrast known as the Y feature and the wind field at the Venusian cloud top was investigated by using images obtained by Venus Monitoring Camera (VMC) on ESA's Venus Express. Spectral analyses of the ultraviolet reflectivity and the wind field revealed common periodicities of 4–5 Earth days, which are attributed to Kelvin and Rossby waves with a zonal wave number of unity. Combined with the morphological relationship between the dark streaks and the enhancement of poleward flow, we propose a mechanism for the formation of the Y feature: The dark materials are supplied to the cloud top in the equatorial region by a Kelvin wave, subsequently advected poleward by the mean meridional circulation and a Rossby wave, and then stretched by the midlatitude jet to the tilted band structures. A simplified transport model was developed to demonstrate the scenario.

Intraseasonal Variability of Surface Salinity in the Eastern Tropical Pacific Associated With Mesoscale Eddies

AbstractStrong variability in sea surface salinity (SSS) in the Eastern Tropical Pacific (ETPac) on intraseasonal to interannual timescales was studied using data from the Soil Moisture and Ocean Salinity, Soil Moisture Active Passive, and Aquarius satellite missions. A zonal wave number‐frequency spectral analysis of SSS reveals a dominant timescale of 50–180 days and spatial scale of 8°–20° of longitude with a distinct seasonal cycle and interannual variability. This intraseasonal SSS signal is detailed in the study of 19 individual ETPac eddies over 2010–2016 identified by their sea level anomalies, propagating westward at a speed of about 17 cm/s. ETPac eddies trap and advect water in their core westward up to 40° of longitude away from the coast. The SSS signatures of these eddies, with an average anomaly of 0.5‐pss magnitude difference from ambient values, enable the study of their dynamics and the mixing of their core waters with the surroundings. Three categories of eddies were identified according to the location where they were first tracked: (1) in the Gulf of Tehuantepec, (2) in the Gulf of Papagayo, and (3) in the open ocean near 100°W–12°N. They all traveled westward near 10°N latitude. Category 3 is of particular interest, as eddies seeded in the Gulf of Tehuantepec grew substantially in the vicinity of the Clipperton Fracture Zone rise and in a region where the mean zonal currents have anticyclonic shear. The evolution of the SSS signature associated with the eddies indicates the importance of mixing to their dissipation.

A Statistical Approach to Quantify Atmospheric Contributions to the ITEC WN4 Structure Over Low Latitudes

AbstractBased on multiyear observations of ionospheric total electron content (TEC) and atmospheric wind, we develop a new method to interpret the atmospheric contribution to the ionospheric wave number‐4 (WN4) structure according to their coherences in annual variations. First, the ionospheric WN4 is extracted from the equatorial integrated TEC (ITEC) using the global ionospheric maps. The wind observation from the TIMED Doppler Interferometer on board the Thermosphere Ionosphere Mesosphere Energetic and Dynamics (TIMED) satellite are used to deduce the latitudinal symmetric and antisymmetric components of the zonal and meridional DE3 (diurnal, eastward, and zonal wave number 3), SE2 (semidiurnal, eastward, and zonal wave number 2), and SPW4 (stationary planetary wave with zonal wave number 4). We then develop a regression model to estimate the coupling efficiency, the background diurnal influence, and the associated contribution in ITEC WN4. (1) The zonal symmetric DE3 is the most efficient tide in generating ITEC WN4, while the zonal symmetric SPW4 plays a secondary but varied role. (2) The diurnal variation of background ionosphere/thermosphere influence's amplitude is similar to that of the zonal mean ITEC during the day, with a westward phase velocity. (3) The zonal symmetric DE3 resulted WN4 can reach 7% of the zonal daily mean ITEC, while the observed total WN4 value is about 10%. The contributions by the zonal symmetric SE2, SPW4, and meridional symmetric SE2 are comparable with respective maxima ~1.5% of the zonal daily mean ITEC. The present results confirm former suggestions that symmetric zonal DE3 is the primary source for ionospheric WN4, and the contributions due to antisymmetric wind components are relatively small in ITEC WN4.

Three-wave resonant interactions and zonal flows in two-dimensional Rossby-Haurwitz wave turbulence on a rotating sphere

This paper addresses three-wave interactions of Rossby-Haurwitz waves in two-dimensional turbulence on a rotating sphere. Zonal modes are often omitted from the resonant wave even when they satisfy the conditions for three-wave interactions, as they do not transfer any energy to other modes in a manner. However, the presence of zonal flows induces phase shifts in other modes, and it is not at all clear that their influence is negligible. Since it is expected that three-wave interactions govern the entire dynamics of turbulence if the rotation rate of the sphere is sufficiently high, by analogy with the theorem regarding three-wave interactions of Rossby waves on a beta plane with sufficiently large beta previously proven by Yamada and Yoneda [Physica D 245, 1 (2013)], an appropriate definition of the wave set was determined by comparing the time evolution of several wave sets on a rapidly rotating sphere. It was found that zonal waves of the form Y-l(m=0) exp(i omega t) with odd l, where Y(l)(m )are the spherical harmonics, should be considered for inclusion in the wave set to ensure that the dynamics of the wave set determine the overall dynamics of the turbulence on a rapidly rotating sphere. Consequently, it is suggested that the minimal wave set that must be considered in the discussion of the three-wave interaction of Rossby-Haurwitz waves is the set consisting of nonzonal waves and zonal waves of the form Y-l(0) exp(icot) with odd l.

Effects of solar activity variations on dynamical processes in the atmosphere: Analysis of empirical data and modeling

The effects of the 11-year, quasi-biennial, and 27-day cycles of solar activity on dynamical processes in the atmosphere are studied using empirical data and results of numerical model calculations. Estimates of changes during the 11-year solar cycle in the wind velocity, potential vorticity, geopotential and its large-scale zonal harmonics are obtained using the ERA-Interim reanalysis data. Features of the response of these atmospheric parameters to the solar cycle in some areas of the atmosphere are revealed for a whole year and depending on season. The results point to the existence of a reliable statistical relation of large-scale dynamical and thermodynamic processes in the troposphere and stratosphere to the 11-year solar cycle. Based on the results of 200-year measurements, the phase synchronization of decadal variations of the North Atlantic Oscillation index with the 11-year solar cycle have been found within time intervals alternating with 45–50-year periodicity. According to the ERA-Interim data, the quasi-biennial oscillation in the equatorial stratospheric zonal wind velocity is phase-synchronized at the stratopause level (∼50 km) with the quasi-biennial variations of UV solar radiation. The proposed reason of the synchronization may be the the inhomogeneous heating of this layer due to the absorption of UV solar radiation by ozone, the meridional gradient of which changes in harmony with the quasi-biennial solar variations. The effect of the 27-day solar cycle on the characteristics of large-scale zonal wave harmonics of the geopotential in the Northern Hemisphere is analyzed using results of calculations by a 3-dimensional chemistry-climatic model. Noticeable correlations of the amplitude of the planetary wave components in winter in the Northern Hemisphere with the 27-day solar cycle have been found. The strongest response is obtained for perturbations with wave number 1 in the middle and high latitudes of the Northern Hemisphere.

The influence of large amplitude planetary waves on the Antarctic ozone hole of austral spring 2017

Quasi-stationary planetary wave activity in the lower Antarctic stratosphere in the late austral winter was an important contributor to the preconditioning of the ozone hole in spring 2017. Observations show that the ozone hole area (OHA) in spring 2017 was at the level of 1980s, that is, almost half the maximum size in 2000s. The observed OHA was close to that forecasted based on a least-squares linear regression between wave amplitude in August and OHA in September–November. We show that the key factor which contributed to the preconditioning of the Antarctic stratosphere for a relatively small ozone hole in the spring of 2017 was the development of large-amplitude stratospheric planetary waves of zonal wave numbers 1 and 2 in late winter. The waves likely originated from tropospheric wave trains and promoted the development of strong mid-latitude anticyclones in the lower stratosphere which interacted with the stratospheric polar vortex and strongly eroded the vortex in August and September, mitigating the overall level of ozone loss.

Climatology of the Quasi‐6‐Day Wave in the Mesopause Region and Its Modulations on Total Electron Content During 2003–2017

AbstractThe westward quasi‐6‐day planetary wave (Q6DW) with zonal wave number 1 is a prominent oscillation in the mesosphere and lower thermosphere region, which causes significant variabilities in the ionosphere through E region wind dynamo. In this paper, the neutral wind observations from the Thermosphere Ionosphere and Mesosphere Electric Dynamics Doppler Imager experiment, the kinetic temperature observations from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument, and the total electron content (TEC) maps from globally distributed Global Positioning System (GPS) receivers during 2003–2017 are utilized to statistically study the Q6DW in both the neutral atmosphere and the ionosphere. Our statistical results show that (1) the Q6DW is most likely to occur during May and September, exhibiting a quasi‐semiannual oscillation. (2) The periods of the Q6DW show two statistical crests at ~6 and ~7 days separately. (3) The ionospheric oscillations due to the Q6DW are positively related to both the 10.7‐cm solar flux index and the neutral wave amplitudes in the mesosphere and lower thermosphere region. The stronger oscillations in TEC under higher solar activity conditions are due to the larger ionization rate and thus larger background TEC. A stronger Q6DW results in stronger wind dynamos in the E region, which leads to stronger F region responses indirectly. (4) The interhemispheric asymmetries of the ionospheric responses to Q6DW show weak negative correlations with both solar activities and neutral wave perturbations, the mechanism of which needs our further investigation.

Climatology and Anomaly of the Quasi‐Two‐Day Wave Behaviors During 2003–2018 Austral Summer Periods

AbstractThe Sounding of the Atmosphere using Broadband Emission Radiometry satellite instrument has been measuring the mesospheric global temperature since February 2002, providing the longest continuous data set to statistically study the climatology and anomaly of the quasi‐two‐day waves (QTDWs), including the westward zonal wave number 2 (W2), 3 (W3), and 4 (W4) modes. It is found that the W2 QTDW generally peaks earliest, followed by the W3 mode with the W4 QTDW being amplified lastly. This temporal pattern is always disordered during the sudden stratospheric warming (SSW) periods. The temperature oscillations of the QTDW usually peak at nearly the same period at different altitudes and different latitudes for a certain case but would be different from year to year. The statistical results show that the periods of the W2 QTDWs are more frequently observed to be at 45–48 hr and the periods of the W3 mode are mostly confined between 45 and 52 hr. Nevertheless, the oscillations of the W4 QTDW peak more dispersedly between 41 and 56 hr. The W3 QTDW is usually the strongest among the three modes, whereas it is sometimes equal to or even weaker than W2 and W4 QTDWs, especially during SSW periods. Besides, we found that the vertical distribution of the W3 QTDW splits into a bimodal structure during the SSW periods of 2006 and 2011. And the solar cycle trend of the QTDWs is frequently disturbed by a 2‐ to 5‐year variability, which seems to be a common feature for all the QTDW modes.

Estimating Subseasonal Variability and Trends in Global Atmosphere Using Reanalysis Data.

A new measure of subseasonal variability is introduced that provides a scale‐dependent estimation of vertically and meridionally integrated atmospheric variability in terms of the normal modes of linearized primitive equations. Applied to the ERA‐Interim data, the new measure shows that subseasonal variability decreases for larger zonal wave numbers. Most of variability is due to balanced (Rossby mode) dynamics but the portion associated with the inertio‐gravity (IG) modes increases as the scale reduces. Time series of globally integrated variability anomalies in ERA‐Interim show an increase in variability after year 2000. In recent years the anomalies have been about 2% above the 1981–2010 average. The relative increase in variability projecting on the IG modes is larger and more persistent than for the Rossby modes. Although the IG part is a small component of the subseasonal variability, it is an important effect likely reflecting the observed increase in the tropical precipitation variability.