Eliassen-Palm Flux Research Articles

Observational perspective on sudden stratospheric warmings and blocking from Eliassen–Palm fluxes

Observational perspective on sudden stratospheric warmings and blocking from Eliassen–Palm fluxes

Impact of Polar Vortex Modes on Winter Weather Patterns in the Northern Hemisphere

This study is an additional investigation of stratosphere–troposphere coupling based on the recent stratospheric winter descriptions in five distinct modes: January, February, Double, Dynamical, and Dadiative. These modes, established in a previous study, categorize the main stratospheric winter typologies modulated by the timing of important sudden stratospheric warmings (SSWs) and final stratospheric warmings (FSWs). The novelty of this research is to investigate the Northern Annular Mode, mean sea level pressure (MSLP) anomalies in the Ural and Aleutian regions, and the decomposition of Eliassen–Palm flux into wavenumbers 1 and 2 within each mode. The results show that the January and Double modes exhibit similar pre-warming surface signals, characterized by Ural blocking and Aleutian trough events preceding weak polar vortex events. The January mode displays a positive MSLP anomaly of +395 hPa (−191 hPa) in the Ural (Aleutian) region in December, while the Double mode shows +311 hPa (−89 hPa) in November. These modes are primarily wave-1 driven, generating tropospheric responses via negative Arctic Oscillation patterns. Conversely, the February and Dynamical modes show opposite signals, with Aleutian blocking and Ural trough events preceding strong polar vortex events. In December, the February mode exhibits MSLP anomalies of +119 hPa (Aleutian) and −180 hPa (Ural), while the Dynamical mode shows +77 hPa and −184 hPa, respectively. These modes, along with important SSWs in February and dynamical FSWs, are driven by both wave-1 and wave-2 and do not significantly impact the troposphere. The Radiative mode’s occurrence is strongly related to the Aleutian blocking presence. These findings confirm that SSW timing is influenced by specific dynamical forcing related to surface precursors and underscore its importance in subsequent tropospheric responses. This study establishes a connection between early winter tropospheric conditions and upcoming stratospheric states, potentially improving seasonal forecasts in the northern hemisphere.

In Situ Generation of Planetary Waves in the Mesosphere by Zonally Asymmetric Gravity Wave Drag: A Revisit

Abstract This study investigates the in situ generation of planetary waves (PWs) by zonally asymmetric gravity wave drag (GWD) in the mesosphere using a fully nonlinear general circulation model extending to the lower thermosphere. To isolate the effects of GWD, we establish a highly idealized but efficient framework that excludes stationary PWs propagating from the troposphere and in situ PWs generated by barotropic and baroclinic instabilities. The GWD is prescribed in a zonally sinusoidal form with a zonal wavenumber (ZWN) of either 1 or 2 in the lower mesosphere of the Northern Hemisphere midlatitude. Our idealized simulations clearly show that zonally asymmetric GWD generates PWs by serving as a nonconservative source Z′ of linearized disturbance quasigeostrophic potential vorticity q′. While Z′ initially amplifies PWs through enhancing q′ tendency, the subsequent zonal advection of q′ gradually balances with Z′, thereby attaining steady-state PWs. The GWD-induced PWs predominantly have the same ZWN as the applied GWD with minor contributions from higher ZWN components attributed to nonlinear processes. The amplitude of the induced PWs increases in proportion with the magnitude of the peak GWD, while it decreases in proportion to the square of ZWN. Moreover, the amplitude of PWs increases as the meridional range of GWD expands and as GWD shifts toward lower latitudes. These PWs deposit substantial positive Eliassen–Palm flux divergence (EPFD) of ∼30 m s−1 day−1 at their origin and negative EPFD of 5–10 m s−1 day−1 during propagation. In addition, the in situ PWs exhibit interhemispheric propagation following westerlies that extend into the Southern Hemisphere.

Climatology of the terms and variables of transformed Eulerian-mean (TEM) equations from multiple reanalyses: MERRA-2, JRA-55, ERA-Interim, and CFSR

Abstract. A 30-year (1980–2010) climatology of the major variables and terms of the transformed Eulerian-mean (TEM) momentum and thermodynamic equations is constructed by using four global atmospheric reanalyses: the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2); the Japanese 55-year Reanalysis (JRA-55); the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim); and the Climate Forecast System Reanalysis (CFSR). Both the reanalysis ensemble mean (REM) and the differences in each reanalysis from the REM are investigated in the latitude–pressure domain for December–January–February and for June–July–August. For the REM investigation, two residual vertical velocities (the original one and one evaluated from residual meridional velocity) and two mass streamfunctions (from meridional and vertical velocities) are compared. Longwave (LW) radiative heating and shortwave (SW) radiative heating are also shown and discussed. For the TEM equations, the residual terms are also calculated and investigated for their potential usefulness, as the residual term for the momentum equation should include the effects of parameterized processes such as gravity waves, while that for the thermodynamic equation should indicate the analysis increment. Inter-reanalysis differences are investigated for the mass streamfunction, LW and SW heating, the two major terms of the TEM momentum equation (the Coriolis term and the Eliassen–Palm flux divergence term), and the two major terms of the TEM thermodynamic equation (the vertical temperature advection term and the total diabatic heating term). The spread among reanalysis TEM momentum balance terms is around 10 % in Northern Hemisphere winter and up to 50 % in Southern Hemisphere winter. The largest uncertainties in the thermodynamic equation (about 50 %) are found in the vertical advection, for which the structure is inconsistent with the differences in heating. The results shown in this paper provide basic information on the degree of agreement among recent reanalyses in the stratosphere and upper troposphere in the TEM framework.

Analysis of Planetary Scale Waves Using Idealized Sudden Stratospheric Warming Simulations in Different Dynamical Cores

AbstractPlanetary waves from the troposphere are known to play an important role in sudden stratospheric warming (SSW). To evaluate the representation of large‐scale waves in idealized SSW simulations, three dynamical cores of the National Center for Atmospheric Research Community Atmosphere Model were tested in this study: the Eulerian (EUL) spectral‐transform, finite‐volume (FV), and spectral element (SE) models. Notable differences were observed among the dynamical cores, with FV being unable to generate major SSWs and simulating minor SSWs less effectively than the other models. Wave decomposition analysis revealed distinct planetary wave propagation patterns during the development of each event. Zonal Wave Number 2 wave activities primarily determined SSW types, and ZWN1 led SSWs to recovery during the ensuing periods. However, FV failed to adequately propagate large‐scale waves in minor SSW events, preventing the reversal of stratospheric zonal‐mean zonal winds. Interestingly, FV showed decreased upward Eliassen–Palm flux compared to that of other dynamical cores, even during climatology. Additional tests were performed to examine the reasons for FV's atypical results, but the dynamic impacts on planetary waves remain poorly understood.

The Variation Characteristics of Stratospheric Circulation under the Interdecadal Variability of Antarctic Total Column Ozone in Early Austral Spring

Antarctic Total Column Ozone (TCO) gradually began to recover around 2000, and a large number of studies have pointed out that the recovery of the Antarctic TCO is most significant in the austral early spring (September). Based on the Bodeker Scientific Filled Total Column Ozone and ERA5 reanalysis dataset covering 1979–2019, the variation characteristics of the Antarctic TCO and stratospheric circulation for the TCO ‘depletion’ period (1979–1999) and the ‘recovery’ period (2000–2019) are analyzed in September. Results show that: (1) Stratospheric elements significantly related to the TCO have corresponding changes during the two eras. (2) The interannual variability of the TCO and the above-mentioned stratospheric circulation elements in the recovery period are stronger than those in the depletion period. (3) Compared with the depletion period, due to the stronger amplitude of the planetary wave 1, stronger Eliassen–Palm (EP) flux corresponds to EP flux convergence, larger negative eddy heat flux, and positive eddy momentum flux in the stratosphere during the recovery period. The polar temperature rises in the lower and middle stratosphere and the polar vortex weakens in the middle and upper stratosphere, accompanied by the diminished area of PSC. This contributes to the understanding of Antarctic ozone recovery.

Role of Southern Hemisphere sudden stratospheric warming in altering the intensity of Brewer-Dobson circulation and its impact on stratospheric ozone and water vapor

The changes in the intensity of Brewer-Dobson circulation (BDC) in the record-breaking Southern Hemisphere sudden stratospheric warming (SH SSW) event in 2019 are investigated in the present study. Wave driving was observed to be at its highest in 2019 SH SSW, despite the event being classified as a minor warming event. The Eliassen-Palm flux divergence, indicative of wave driving, was observed to be increasing before the SSW, and subsiding afterwards. The stratospheric circulation followed the wave driving and intensified along the warming episode. The resulting stratospheric distribution from Aura Microwave Limb Sounder showed a negative ozone anomaly (∼20%) in the tropical lower stratosphere and a downward propagating positive anomaly in the Southern Hemisphere polar stratosphere around the central dates. The water vapor mixing ratio showed an increase of more than 25% in the polar lower stratosphere and 10% in the tropical upper stratosphere. A cooling of about 5 K was observed in the tropical middle stratosphere. The results indicate that SH SSW can change the intensity of the stratospheric circulation which in turn affects the ozone and water vapor transport from the tropics to the pole.

Possible mechanism of the mid-high-latitude synoptic-scale disturbances impact on Meiyu precipitation anomalies

Precipitation during the Meiyu period in East Asia demonstrated a tendency towards increased frequency of rainstorms and escalating disaster-causing potential, exhibiting significant inter-annual variations in recent years. The relationship between Meiyu precipitation (MP) anomalies, related circulation anomalies and mid-high-latitude synoptic-scale disturbances was investigated by observation and reanalysis data from 1979 to 2018 and the regional climate model (RegCM4.6). Based on the observation results, the physical mechanism of disturbance affecting MP was proposed and verified by numerical simulation experiments. The results show that, during the Meiyu periods, the variation of synoptic-scale disturbances over Lake Baikal correlates negatively with MP anomalies, where the abnormal zonal winds and the disturbance anomalies show a strong spatially consistent coherence in the upper troposphere. Compared to drought years, the weaker conversion of disturbance energy to mean flow and the convergence of anomalous Eliassen-Palm (E-P) flux induce mid-high-latitude anomalous easterly flow during the Meiyu period in flood years. The occurrence of such conditions leads to the southward shift of the East-Asian subtropical jet (STJ). Then, anomalous stronger ascending flow over the Meiyu region finally results in stronger MP. In addition, warm temperature anomaly and warm advection anomaly exist at lower latitudes in flood years, while mid-high latitudes exhibit cold temperature anomaly and cold advection anomaly, which is conducive to the southward shift of the East-Asian STJ and the increase of MP. Based on the above diagnosis, sensitivity experiments were conducted using RegCM4.6. When the mid-high-latitude synoptic-scale disturbances are amplified during flood years or flood decades, the MP correspondingly decreases, and vice versa. The simulation results also show that the convergence (divergence) of mid-high-latitude E-P flux results in a weakening (strengthening) of the westerly flow, which leads to the southward (northward) shift of the East-Asian STJ. The alteration of the STJ's position in East Asia alters the position of the ascending branch of the vertical meridional circulation corresponding to the jet stream, subsequently impacting MP. Therefore, both diagnosis and numerical simulation reveal the possible mechanism of synoptic disturbances in mid-high latitude affecting MP in Yangtze River Basin, thus enriching the study of MP anomaly mechanism during Meiyu period.

Interdecadal change in the response of winter North Atlantic Oscillation to the preceding autumn sea ice in the Barents-Kara Seas around the early 1990s

Based on the reanalysis data for the period 1980–2019, this study reveals that the linkage of autumn sea ice concentration in the Barents-Kara Seas (BKSIC) to the subsequent winter North Atlantic Oscillation (NAO) underwent an interdecadal weakening in the early 1990s. Such a weakening is attributed to the discrepancy in the tropospheric warming resulting from the decrease in the interannual variability of the BKSIC from the former period to the latter period. During the former period (1980–1993), the decrease in autumn BKSIC can induce strong warming in the troposphere through enhancing the in-situ diabatic heating, significantly weakening the circumpolar westerly. The weakening of autumn circumpolar westerly then promotes the upward propagation of the Eliassen-Palm (EP) flux from the troposphere to the stratosphere. Till winter, the EP flux propagates downward from the stratosphere to the troposphere. It leads to significant positive stream function anomalies over the Greenland-Barents Seas region, accompanied with negative stream function anomalies over Western Europe. They are linked by a strong southeastward-propagating wave activity flux and result in a negative NAO phase with its southern center located eastward. During the latter period (1994–2019), due to smaller interannual variability of the BKSIC, its resultant warming of the tropospheric temperature is much weaker, not conducive to the aforementioned stratosphere-troposphere interaction and thus diluting the relationship between autumn BKSIC and the subsequent winter NAO.

The Influence of Southeastern South American Rainfall on Weather Patterns Over the Tropical Atlantic, Northwestern Africa and Western Europe

AbstractDuring boreal winter (December–February), the South American monsoon system (SAMS) reaches its annual maximum when upper‐tropospheric westerly winds prevail over the equatorial Atlantic. Atmospheric dynamic model simulations suggest that Rossby waves generated over South America can propagate to and potentially influence weather patterns in the Northern Hemisphere (NH). However, observational evidence has been absent previously. Here we focus on southeastern South American (SESA) precipitation anomalies, which can characterize intraseasonal rainfall variability of the SAMS. Since tropical “westerly duct” and convective heating are important factors for cross‐equatorial propagation of Rossby wave (CEPRW), we identify two groups of events based on the two factors. By comparing the events associated with both SESA rainfall and tropical westerlies to the events associated with tropical westerlies only, we find that an anomalous Rossby wave train is triggered by precipitation anomalies over SESA, propagates in the southwest–northeast direction, and subsequently crosses the equator. Over a period of 4 days, near‐surface temperature over northwestern Africa and western Europe becomes warmer, accompanied by increased surface downward longwave radiation and precipitable water. The equatorward propagating Eliassen–Palm flux anomalies originated from SESA support the evidence of CEPRW. Simulations using a time‐dependent linear barotropic model forced by prescribed divergence anomalies over SESA further confirm that SESA rainfall can influence the NH weather patterns through CEPRW. Knowledge of this study will help us better understand and model interhemispheric teleconnections over the American–Atlantic–African/European sector.

Quantifying stratospheric ozone trends over 1984–2020: a comparison of ordinary and regularized multivariate regression models

Abstract. Accurate quantification of long-term trends in stratospheric ozone can be challenging due to their sensitivity to natural variability, the quality of the observational datasets, and non-linear changes in forcing processes as well as the statistical methodologies. Multivariate linear regression (MLR) is the most commonly used tool for ozone trend analysis; however, the complex coupling in many atmospheric processes can make it prone to the issue of over-fitting when using the conventional ordinary-least-squares (OLS) approach. To overcome this issue, here we adopt a regularized (ridge) regression method to estimate ozone trends and quantify the influence of individual processes. We use the Stratospheric Water and OzOne Satellite Homogenized (SWOOSH) merged dataset (v2.7) to derive stratospheric ozone profile trends for the period 1984–2020. Besides SWOOSH, we also analyse a machine-learning-based satellite-corrected gap-free global stratospheric ozone profile dataset from a chemical transport model (ML-TOMCAT) and output from a chemical transport model (TOMCAT) simulation forced with European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis. For 1984–1997, we observe smaller negative trends in the SWOOSH stratospheric ozone profile using ridge regression compared to OLS. Except for the tropical lower stratosphere, the largest differences arise in the mid-latitude lowermost stratosphere (>4 % per decade difference at 100 hPa). From 1998 and the onset of ozone recovery in the upper stratosphere, the positive trends estimated using the ridge regression model (∼1 % per decade near 2 hPa) are smaller than those using OLS (∼2 % per decade). In the lower stratosphere, post-1998 negative trends with large uncertainties are observed and ridge-based trend estimates are somewhat smaller and less variable in magnitude compared to the OLS regression. Aside from the tropical lower stratosphere, the largest difference is around 2 % per decade at 100 hPa (with ∼3 % per decade uncertainties for individual trends) in northern mid-latitudes. For both time periods the SWOOSH data produce large negative trends in the tropical lower stratosphere with a correspondingly large difference between the two trend methods. In both cases the ridge method produces a smaller trend. The regression coefficients from both OLS and ridge models, which represent ozone variations associated with natural processes (e.g. the quasi-biennial oscillation, solar variability, El Niño–Southern Oscillation, Arctic Oscillation, Antarctic Oscillation, and Eliassen–Palm flux), highlight the dominance of dynamical processes in controlling lower-stratospheric ozone concentrations. Ridge regression generally yields smaller regression coefficients due to correlated explanatory variables, and care must be exercised when comparing fit coefficients and their statistical significance across different regression methods. Comparing the ML-TOMCAT-based trend estimates with the ERA5-forced model simulation, we find ML-TOMCAT shows significant improvements with much better consistency with the SWOOSH dataset, despite the ML-TOMCAT training period overlapping with SWOOSH only for the Microwave Limb Sounder (MLS) measurement period. The largest inconsistencies with respect to SWOOSH-based trends post-1998 appear in the lower stratosphere where the ERA5-forced model simulation shows positive trends for both the tropics and the mid-latitudes. The large differences between satellite-based data and the ERA5-forced model simulation confirm significant uncertainties in ozone trend estimates, especially in the lower stratosphere, underscoring the need for caution when interpreting results obtained with different regression methods and datasets.

A Unique Vertical Structure of Quasi‐20‐sol Waves Observed by the MCS on the Martian Atmosphere

AbstractIn this study, we report a two‐cell structure of quasi‐20‐sol waves (Q20SWs) in the Martian atmosphere and investigate its formation mechanism. The study is based on temperature data measured by the Mars Climate Sounder, and neutral wind and temperature data obtained from the Mars Global Circulation Model reanalysis data set. The two‐cell structure is observed before the winter solstice in a latitude range from 55°N to 65°N in three consecutive Mars years (MY, MY30‐32). The lower cell has a maximum amplitude of 9 K occurring at ∼25 km, while the magnitude of the upper cell is much smaller with a peak amplitude of 5 K appearing at ∼60 km. In the period from Ls180° to Ls270°, the peak amplitudes of the two cells are highly correlated with a correlation coefficient of 0.90, and the corresponding phases are shifted by ∼180°. The Eliassen‐Palm (EP) flux and the EP flux divergence of the Q20SWs, and the zonal‐wave available potential energy transfer between the zonal mean zonal flow and the Q20SWs are calculated. Our results suggest that the formation of the two‐cell structure is likely due to the combined effects of the wave sources, the upward propagation of the Q20SWs from the lower cell, and the available potential energy transfer between the zonal mean flow and the Q20SWs.

Interannual Variability of the Mass-Weighted Isentropic Zonal Mean Meridional Circulation in the Northern Hemisphere Winter

Abstract The atmospheric general circulation diagnosed by the mass-weighted isentropic zonal mean exhibits an extratropical direct (ETD) circulation driven by eddy momentum transport. We investigated regional atmospheric modulations associated with the year-to-year variability of ETD circulation in boreal winter using a reanalysis dataset. Composite analyses showed that the interannual variability of ETD circulation accompanies a seesaw-like variation between the Aleutian low (AL) and Icelandic low (IL), which coincides with the Pacific–North American teleconnection pattern and North Atlantic dipole anomaly. A diagnosis using isentropic airmass fluxes representing the geographical Lagrangian mean motion indicated that the significant intensification of upper-tropospheric poleward warm airflow over the eastern North Pacific is responsible for the enhanced ETD circulation, together with the intensified equatorward lower cold airflow over East Asia and eastern North America. The resulting enhanced upward propagation of the stationary planetary waves corresponds to the intensification of the divergence of the Eliassen–Palm flux, which explains the modulated ETD circulation in view of the balance between the Coriolis force and eddy momentum flux convergence. These results suggest that the AL–IL seesaw promotes variations in the global general circulation, and the anomalous ETD circulation interacts with several teleconnection patterns involving the modulation of planetary waves through anomalous meridional heat transport by the basinwide-scale eddies.

Kinetic Energy Spectra and Spectral Budget of Radiative–Convective Equilibrium

Abstract The atmospheric kinetic energy spectrum and energy cascade are investigated in idealized simulations of radiative–convective equilibrium (RCE). WRF is employed to perform cloud-resolving simulations of an idealized radiative–convective equilibrium with and without aggregation with Δx = 4 km. The horizontal kinetic energy (HKE) spectrum for the aggregated simulation in the upper troposphere is steeper than the nonaggregated case and closer to −5/3. The HKE spectra for the nonaggregated simulation in the upper troposphere and the lower stratosphere are much shallower than the −5/3 spectrum. In the upper troposphere, the divergent kinetic energy has a similar magnitude to the rotational kinetic energy in both the nonaggregated simulation and aggregated simulation. Energy is mainly gained from the buoyancy flux and mainly lost from the vertical energy flux for scales larger than 20 km. Downscale energy transfer is found in the upper troposphere. Numerical dissipation is the main source of energy loss at small scales. In the lower stratosphere, the divergent kinetic energy dominates the kinetic energy spectrum in both simulations. Energy is mainly gained from the vertical energy flux and is balanced by the loss from the buoyancy flux term, transfer term, and dissipation. An Eliassen–Palm flux analysis suggests that wave–mean-flow interaction may be responsible for the upscale energy transfer found in the lower stratosphere. The magnitudes of our kinetic energy spectra are similar to spectra calculated from aircraft data. Rotation is found to promote aggregation and steepen the energy spectrum.

Theoretical Foundation of the Linear Relationship between the Midlatitude Eddy Heat Flux and Fall-to-Spring Polar Ozone Buildup

Abstract It has been demonstrated that there is a globally unified linear relationship between the interannual variations of the fall-to-spring polar ozone accumulation and the winter-mean poleward eddy heat flux on the 100 hPa pressure surface. The foundation of this relationship is investigated using time-slice experiments on a chemistry–climate model with two levels of ozone-depleting substances (ODSs). The features of the transport field are interpreted by decomposing the horizontal ozone flux caused by the residual circulation into contributing processes including the eddy heat flux with the aid of the transformed Eulerian-mean momentum equation followed by rearrangement of terms. The linear relationship between the interannual variations of the fall-to-spring ozone buildup integrated poleward and above a reference point Pref on a meridional plane and the poleward eddy heat flux during the corresponding period at Pref is realized for each hemisphere implying that the interhemispheric unification should be treated with caution. This relationship is interpreted using the fact that the interannual variation of poleward ozone transport in the upper stratosphere is captured well by the vertical convergence of the constituent-based Eliassen–Palm (EP) flux (), which is defined as the product of the constituent (ozone) mixing ratio and EP flux. The eddy momentum flux contributes to the meridional ozone transport in combination with the eddy heat flux in the form of the divergence of , although it is not responsible for realizing the linear relationship. The dependence of the linearity on the location of Pref and ODS levels is discussed.

Compensation between Resolved Wave Forcing and Parameterized Orographic Gravity Wave Drag in the Northern Hemisphere Winter Stratosphere Revealed in NCEP CFS Reanalysis Data

Abstract Compensation between the resolved wave (RW) forcing and the parameterized orographic gravity wave drag (OGWD) accompanying barotropic/baroclinic (BT/BC) instability in the realistic atmosphere is investigated using Climate Forecast System Reanalysis data in the Northern Hemisphere winter stratosphere. When sufficiently narrow and/or strong negative OGWD drives instability, RWs are generated in situ, providing positive Eliassen–Palm flux divergence that compensates for the parameterized OGWD enhancement; this is consistent with the findings of previous studies based on the idealized general circulation models. However, dependence of the compensation rate on RW forcing differs from the nearly complete compensation in the previous studies, implying that an additional mechanism operates for the compensation: the refractive-index modification by BT/BC instability. The negative meridional gradient of the quasigeostrophic potential vorticity leads to the negative refractive index squared for RWs with phase speeds less than the zonal-mean zonal wind. This prevents RWs from entering the destabilized areas, resulting in the divergence of Eliassen–Palm fluxes that cancels out the parameterized OGWD perturbation. Although both mechanisms act simultaneously, the refractive-index modification plays an important role in the compensation processes in the stratosphere where RWs are dominated by the planetary-scale waves.

Southern Hemisphere Response to the Secondary Planetary Waves Generated During the Arctic Sudden Stratospheric Final Warmings: Influence of the Quasi‐Biennial Oscillation

AbstractBased on the Modern‐Era Retrospective analysis for Research and Applications version 2 reanalysis and Aura Microwave Limb Sounder geopotential height observations, the anomalously strong secondary planetary waves (PWs) with westward zonal wavenumber 1 and periods of 10–16 days are unexpectedly captured in the Southern (summer) Hemisphere (SH) during the Arctic sudden stratospheric final warmings (SSFWs) in 2015 and 2022, while no distinct secondary PW signal occurs in the SH during the 2005, 2014, and 2016 Arctic SSFWs. The Eliassen‐Palm (EP) flux diagnostics show that the secondary PWs during the 2015 and 2022 SSFWs have strong trans‐equatorial components at ∼35–60 km, but the southward EP flux from the Northern Hemisphere during the other three SSFWs decays dramatically around the equator. Further diagnostic results on the background conditions indicate that the phase of the quasi‐biennial oscillation (QBO) in the middle stratosphere plays a crucial role in the SH response to the secondary PW during SSFW. The strong equatorial westward winds at ∼32–42 km result in the critical layers and negative waveguides for the secondary W1 PW during the 2005, 2014, and 2016 SSFWs, which significantly block the trans‐equatorial propagation of PW. Such PW suppression is not observed during the 2015 and 2022 SSFWs since the middle stratospheric QBO is in the eastward phase. Moreover, the strong secondary PW in the SH during the 2015 and 2022 SSFWs can additionally induce the poleward residual circulation to facilitate the mesospheric total residual circulation transition from summer pattern to winter pattern in the SH.

Wavenumbers 3 and 4 Quasi 2‐day Wave Activities Observed by Multiple Meteor Radars in the Two Hemispheres During Austral Summer

AbstractCombining observations from three meteor radars in the Northern Hemisphere (NH) and a meteor radar in the Southern Hemisphere (SH) and reanalysis data, we present successive wavenumbers 3 and 4 (W3 and W4) quasi 2 day wave activities in austral summer. In the mesosphere and lower thermosphere, the wave exhibits two burst activities. The W3 dominates the first burst, while the W4 is the predominate mode in the second burst. The mode transition arises in the late January and is completed around the end of January. The W3 and W4 have the nearly same dominate period of about 46 hr and vertical wavelengths of 110 km in the SH and about 55–65 km in the NH, and the similar latitude and altitude distributions in the zonal and meridional winds and temperature, except stronger magnitude and farther extension in the NH for the W3. The analysis indicates that the unstable regions of the summer jets in the lower mesosphere and stratopause region have the boundaries which toward lower latitudes are largely consistent with the critical layers, and Eliassen‐Palm (EP) fluxes of the W3 and W4 increase rapidly near the unstable regions. This demonstrates that both the W3 and W4 originate from the jet instabilities. Nevertheless, the EP flux of the W3 shows a stronger transfer across the hemispheres than that of the W4. The W3 and W4 have the same period and vertical scale, which is different from previous reports.

Impacts of Stratospheric Polar Vortex Shift on the East Asian Trough

Abstract Impacts of stratospheric polar vortex shift on the wintertime East Asian trough (EAT) on intraseasonal time scales are investigated using a reanalysis dataset and a climate model. The result based on composite analysis shows that the shift of the stratospheric polar vortex toward eastern Siberia (ES-shift event) is associated with higher geopotential height at 500 hPa than normal over East Asia, corresponding to the weakened EAT. Furthermore, the simulated EAT is also weakened when nudging the stratospheric state toward that during the ES-shift events. This study further found that there is no significant difference in the stratospheric polar vortex intensity between the ES-shift events and nonshift events, implying that the polar vortex strength change may have little influence on the possible connection between the polar vortex shift and the EAT change. The underlying mechanisms are listed as follows: First, the positive potential vorticity (PV) anomalies in the lower stratosphere associated with ES-shift events could explain approximately 40% of the local westerly anomalies in the upper troposphere to the north of East Asia via PV conservation, leading to the rise of the geopotential height over East Asia and the weakening of the EAT. Second, the shift of stratospheric polar vortex could modulate the synoptic-scale Rossby wave activity in the upper troposphere, favorable for the southward propagation of synoptic-scale waves and divergence of extended Eliassen-Palm flux in the upper troposphere. Finally, the transient wave feedback could enhance the tropospheric westerly anomalies in the north of East Asia and induce positive height anomalies to its south, further weakening the EAT. Our results revealed that the stratospheric polar vortex shift leads the EAT intensity variation by around 2–5 days, implying that the stratospheric polar vortex shift could be applicable to the prediction of the EAT intensity.

Sensitivity of the 4–10-Day Planetary Wave Structures in the Middle Atmosphere to the Solar Activity Effects in the Thermosphere

Numerical simulation of the general atmospheric circulation was performed to estimate changes in amplitudes of the westward-travelling planetary waves (PWs) at altitudes from the Earth’s surface up to 300 km under different solar activity (SA) levels. The three-dimensional nonlinear mechanistic model of circulation of the middle and upper atmosphere “MUAM” was used. The atmospheric general circulation and PW amplitudes were calculated based on ensembles containing 16 model runs for conditions corresponding to low and high SA. PWs having periods of 4–10 days were considered. Comparison with the data of digital ionosondes showed that the MUAM model is capable of reproducing the considered PW modes at thermospheric heights. It is shown that under high SA conditions, PW amplitudes are significantly larger in the thermosphere and smaller in the middle atmosphere. The observed PW structures are influenced not only by changes in atmospheric refractive index and Eliassen–Palm flux but also by varying PW reflection in the lower thermosphere, which can change proportions of the wave energy transferred from the lower atmosphere to the upper layers and reflected downwards.