Search

We analyzed the impact of minor Arctic sudden stratospheric warming (SSW) event of 2017 on the ionospheric F2 region, using Digisonde data from a low-mid latitude Indian station, Delhi (28.6°N, 77.2°E, 19.2°N geomagnetic latitude, 42.4°N dip). Our study revealed significant ionospheric changes, with electron densities exhibiting variations of more than 200% during this warming event. To further investigate, we examined the deviation in critical frequency ΔfoF2 from median values in the first six months of 2017 and found that the F2 layer critical frequency experiences maximum and minimum variations during the SSW period. Additionally, we observed periodicities of 7, 11, 14 and 30 days in characteristic F2 layer frequency during this event.

Using the ERA5 reanalysis, sea surface temperature, sea ice observations, and the real-time multivariate Madden-Julian Oscillation (MJO) index, the evolution of the stratospheric extreme circulation in the winter of 2022/2023 is explored. The stratospheric polar vortex was disturbed three times in the 2022/23 winter, contrasted with only one disturbance during the other three recent winters with an SSW. Possible favorable conditions for the strong stratospheric disturbances and their effects on stratospheric ozone, water vapor distribution, and near-surface temperature were examined. Around 7 December 2022 when a short but strong pulse of planetary wavenumber 2 appeared from the troposphere to stratosphere, a weakened and elongated stratospheric polar vortex formed at 10 hPa. This pulse is related to the intensifying Ural ridge and the deepening East Asian trough. After the first stratospheric disturbance, a large fraction of cold anomalies occurred in the Eurasian continent. A lagged impact after these stratospheric disturbances was observed as strong cold anomalies formed in North America from 13 to 23 December. On 28 January 2023, a minor SSW event occurred due to a displacement of the stratospheric polar vortex. A strong pulse of eddy heat flux contributed alternately by planetary wavenumber 1 and 2 showed a large accumulative effect on the stratospheric disturbance. However, the downward impact of this second disturbance was weak, and cold surges were not noticeable after this minor SSW. The third stratospheric disturbance this winter is a major displace-type SSW that occurred on 16 February 2023, and the total eddy heat flux primarily contributed by planetary wavenumber 1 increased rapidly. Following the major SSW, the North American continent was covered by large patches of strong cold anomalies until the end of March. During the three disturbances, the residual circulation correspondingly strengthened. The water vapor and ozone in the middle and lower layers of the polar stratosphere showed positive anomaly disturbances, especially after the major SSW onset. The unprecedented frequent stratospheric disturbances in winter 2022/23 were accompanied by severe loss of Barents-Laptev Sea ice and anomalously cold tropical Pacific sea surface temperatures (La Niña), which have been reported to be conducive to the enhancement of planetary waves 1 and 2 respectively. Further, two weeks before the major SSW, existing MJO developed into phases 4–6, also contributing to the occurrence of major SSW.

Using a network of meteor radar observations, observational evidence of polar-to-tropical mesospheric coupling during the 2018 major sudden stratosphere warming (SSW) event in the northern hemisphere is presented. In the tropical lower mesosphere, a maximum zonal wind reversal (−24 m/s) is noted and compared with that identified in the extra-tropical regions. Moreover, a time delay in the wind reversal between the tropical/polar stations and the mid-latitudes is detected. A wide spectrum of waves with periods of 2 to 16 days and 30–60 days were observed. The wind reversal in the mesosphere is due to the propagation of dominant intra-seasonal oscillations (ISOs) of 30–60 days and the presence and superposition of 8-day period planetary waves (PWs). The ISO phase propagation is observed from high to low latitudes (60° N to 20° N) in contrast to the 8-day PW phase propagation, indicating the change in the meridional propagation of winds during SSW, hence the change in the meridional circulation. The superposition of dominant ISOs and weak 8-day PWs could be responsible for the delay of the wind reversal in the tropical mesosphere. Therefore, this study has strong implications for understanding the reversed (polar to tropical) mesospheric meridional circulation by considering the ISOs during SSW. [ABSTRACT FROM AUTHOR]

In this study, the neutral density and horizontal wind observed by the four meteor radars, as well as the temperature measured by the Microwave Limb Sounder (MLS) onboard the Aura satellite are used to examine the response of neutral density, wind, and temperature in the MLT region to the stratospheric sudden warmings (SSWs) during 2005 to 2021 in the Northern Hemisphere. The four meteor radars include the Svalbard (78.3°N, 16°E) and Tromsø (69.6°N, 19.2°E) meteor radars at high latitudes and the Mohe (53.5°N, 122.3°E) and Beijing (40.3°N, 116.2°E) meteor radars at middle latitudes. The superposed epoch analysis results indicate that: 1) the neutral density over Svalbard and Tromsø at high latitude increased at the beginning of SSWs and decreased after the zonal mean stratospheric temperature reached the maximum. However, the neutral density over Mohe at midlatitudes decreased in neutral density at the beginning of SSW and increase after the zonal mean stratospheric temperature reached the maximum. 2) The zonal wind at high latitudes show a westward enhancement at the beginning of SSWs and then shows an eastward enhancement after the stratospheric temperature reaches maximum. However, the zonal wind at midlatitudes shows an opposite variation to at high latitudes, with an eastward enhancement at the onset and changing to westward enhancements after the stratospheric temperature maximum. The meridional winds at high and midlatitudes show a southward enhancement after the onset of SSW and then show a northward enhancement after the stratospheric temperature maximum. 3) In general, the temperature in the MLT region decreased throughout SSWs. However, as the latitudes decrease, the temperature cooling appears to lag a few days to the higher latitudes, and the degree of cooling will decrease relatively.

Near‐space balloon networks have the potential to improve numerical weather prediction (NWP) through data assimilation (DA) by providing in situ observations in an otherwise data‐sparse stratosphere. This study investigates the prospective value of stratospheric balloon winds to NOAA NWP by examining Loon data quality and conducting a 2‐month observing system experiment (OSE) using NOAA's Finite‐Volume Cubed‐Sphere Global Forecast System. During the study period (December 2018–January 2019), Loon winds show good correspondence to collocated rawinsonde (RAOBS) winds, while a considerable difference in tropical stratospheric winds is observed between NOAA and ECMWF operational analyses. The OSE, one run with Loon winds assimilated and another without, suggests that additional stratospheric wind observations can improve NWP. Loon wind assimilation acts to decelerate the strong tropical easterly stratospheric jet west of where Loon winds are observed while accelerating less intense subtropical easterly motions. Differences in observation‐minus‐background statistics between the two runs reveal that the backgrounds for Loon winds and RAOBS winds and temperatures improve when Loon winds are assimilated. This positively impacts short‐term forecasts of the lower stratospheric circulation during the study period. Additionally, Loon wind assimilation improves the tropical stratospheric response to the 2019 New Year sudden stratospheric warming (SSW) event by significantly reducing the short‐term forecast error following the SSW. The findings emphasize the value of stratospheric winds toward improving global NWP, and imply the importance of including near‐space observing systems in the Earth‐observing architecture. Plain Language Summary: Networks of large superpressure balloons have the potential to address a known data gap in the global observing system by providing in situ observations of the lower stratosphere. This study investigates the potential value of stratospheric winds observed by Loon balloons to NOAA by conducting two experiments, one run with Loon winds added and another without, using NOAA's Finite‐Volume Cubed‐Sphere Global Forecast System over a 2‐month period (December 2018–January 2019). Comparisons between the two runs reveal the impact of adding Loon winds to the system. We find that Loon winds act to weaken the strong tropical easterly stratospheric jet in the model near where Loon winds are observed. Differences between the observed and model background winds show that the addition of Loon winds helps to improve the model wind and temperature backgrounds in the stratosphere, leading to better short‐term forecasts of the stratospheric circulation during the study period. Additionally, Loon winds help improve the prediction of changes to the tropical stratosphere brought on by the 2019 New Year sudden stratospheric warming event by significantly reducing the forecast error following the event. These results emphasize the benefit of adding in situ stratospheric wind observations to global forecast models. Key Points: Stratospheric winds observed by long‐duration superpressure Loon balloons are assimilated in NOAA NWP, and the impact is assessedLoon wind assimilation leads to local improvements in short‐term forecasts of the lower stratospheric circulation during the study periodLoon wind assimilation significantly improves the tropical stratospheric response to the 2019 New Year sudden stratospheric warming event [ABSTRACT FROM AUTHOR]

Using different scenario experiments by the CESM2-WACCM model, the simulation and projection of the SSW and its impact on the near surface are explored as compared with the ERA5 and NCEP/NCAR reanalyses. The SSW frequency ranges from 4 to 7 per decade in CESM2-WACCM experiments (piControl, AMIP, historical, 1pctCO2, abrupt-4xCO2, SSP245, SSP585), comparable to ERA5 (6 per decade). Projected relative change in the displacement and split SSWs is much more uncertain due to the underestimation of the SSW frequency in the model and uncertainty in the greenhouse gas emission pathways. In all CO2 increase experiments, the downward propagation of annular stratospheric signals at short lags associated with displacement SSWs likely reinforces, whereas the downward coupling is projected to change little at long lags for displacements and at all lags for splits. CESM2-WACCM also projects a weakening of the wavenumber-2 forcing for split SSW in the future. Enhanced tropospheric negative annular mode response is projected at short lags for displacements in all future scenarios, and there is no significant enhancement of the negative NAO-like response to displacements at long lags. In contrast, the projected change in the tropospheric response to splits is zonally heterogenous at short lags, resembling a wave train pattern with the East Asian trough deepening, but the NAO-like response to splits at long lags is not projected to change significantly. The cold pattern over North Eurasia following displacement SSWs might expand further equatorward in the future projections, whereas cold anomalies over North America following splits might enhance in the future. [ABSTRACT FROM AUTHOR]

It is still not well understood if subseasonal variability of the local PM2.5 in the Beijing-Tianjin-Hebei (BTH) region is affected by the stratospheric state. Using PM2.5 observations and the ERA5 reanalysis, the evolution of the air quality in BTH during the January 2021 sudden stratospheric warming (SSW) is explored. The subseasonal variability of the PM2.5 concentration after the SSW onset is evidently enhanced. Stratospheric circumpolar easterly anomalies lasted for 53 days during the January–February 2021 SSW with two evident stratospheric pulses arriving at the ground. During the tropospheric wave weakening period and the intermittent period of dormant stratospheric pulses, the East Asian winter monsoon weakened, anomalous temperature inversion developed in the lower troposphere, anomalous surface southerlies prevailed, atmospheric moisture increased, and the boundary layer top height lowered, all of which favor the accumulation of pollutant particulates, leading to two periods of pollution processes in the BTH region. In the phase of strengthened East Asian winter monsoon around the very beginning of the SSW and another two periods when stratospheric pulses had reached the near surface, opposite-signed circulation patterns and meteorological conditions were observed, which helped to dilute and diffuse air pollutants in the BTH region. As a result, the air quality was excellent during the two periods when the stratospheric pulse had reached the near surface. The increased subseasonal variation of the regional pollutant particulates after the SSW onset highlights the important role of the stratosphere in the regional environment and provides implications for the environmental prediction. [ABSTRACT FROM AUTHOR]

Using a network of meteor radar observations, observational evidence of polar-to-tropical mesospheric coupling during the 2018 major sudden stratosphere warming (SSW) event in the northern hemisphere is presented. In the tropical lower mesosphere, a maximum zonal wind reversal (−24 m/s) is noted and compared with that identified in the extra-tropical regions. Moreover, a time delay in the wind reversal between the tropical/polar stations and the mid-latitudes is detected. A wide spectrum of waves with periods of 2 to 16 days and 30–60 days were observed. The wind reversal in the mesosphere is due to the propagation of dominant intra-seasonal oscillations (ISOs) of 30–60 days and the presence and superposition of 8-day period planetary waves (PWs). The ISO phase propagation is observed from high to low latitudes (60° N to 20° N) in contrast to the 8-day PW phase propagation, indicating the change in the meridional propagation of winds during SSW, hence the change in the meridional circulation. The superposition of dominant ISOs and weak 8-day PWs could be responsible for the delay of the wind reversal in the tropical mesosphere. Therefore, this study has strong implications for understanding the reversed (polar to tropical) mesospheric meridional circulation by considering the ISOs during SSW.

Using the ERA5 and MERRA2 reanalysis, and surface meteorological observation data, this study explores the possible impact of the sudden stratospheric warming (SSW) events on air quality in the Beijing-Tianjin-Hebei (BTH) region. As the duration of split SSW events is longer and the stratospheric signal pulses propagate further downward than displacement SSWs, subseasonal variability of the atmospheric particulates in the BTH is larger during split SSWs. The air particulate concentration is light before the SSW onset due to the enhanced perturbation in the troposphere associated with strengthened planetary waves. The air particulate concentration around the SSW onset dates begins to rise due to weakening of the tropospheric disturbance as the enhanced planetary waves enter the stratosphere. In the decaying period of the SSW, the air particulate concentration decreases as the stratospheric negative Northern Annular Mode (NAM) signal propagates downward. Specifically, in the pre-SSW period of displacement (split) SSW events, a wavenumber-1-like (wavenumber-2-like) anomaly pattern is strengthened. The East Asian winter monsoon intensifies as the east Asian trough is deepened especially before the split SSW event onset, leading to a cleaning period. Around the SSW onset period as the tropospheric perturbation diminishes and the East Asian winter monsoon weakens, a surge of air particulate concentration is observed. After the SSW onset, due to the downward propagation of the stratospheric negative NAM signal, cold anomalies form in the northeastern East Asia especially for split SSWs, corresponding to a cleaning period in the BHT region. The local meteorological conditions during the SSWs are also discussed. [ABSTRACT FROM AUTHOR]

Using reanalysis, observations, and subseasonal to seasonal (S2S) forecasts, the most recent sudden stratospheric warming (SSW) in January 2021 and its predictability are explored. Previous work has shown that SSWs can be forecasted at relatively long lead time if favorable conditions are present. However, favorable conditions were not present for this most recent SSW, which occurred under the tropical westerly quasi‐biennial oscillation (QBO) and weak convection over tropical Pacific. In mid‐December 2020 and early January 2021, the Ural ridge and East Asian trough were anomalously strong, corresponding to enhanced climatological wavenumber 2. In late December 2020 and mid‐January 2021, negative (positive) height anomalies over the North Pacific (Atlantic) enhanced the climatological wavenumber 1. Alternate wave pulses by the wavenumber 1 and 2 finally led to the SSW onset and the long lifetime of the January 2021 SSW. As the composite for QBO westerlies displays a strengthened stratospheric polar vortex, the weak vortex in January 2020 was not attributed to this tropical forcing. Therefore, the predictability for the occurrence of the January 2021 SSW is not beyond two weeks with the required hit ratio >50%. Splitting of the vortex adds further difficulty to the prediction of this event. The cold anomalies over North Eurasia initiated one to two weeks before the SSW onset were likely due to the SSW precursors, which then persisted until mid‐January 2021 as the SSW signal began to propagate downward. However, the persistent cold anomalies can only be forecasted in SSW‐hit members and SSW‐hit S2S models. The observed Arctic sea ice loss is unlikely to extend the predictability of this event in S2S models. Plain Language Summary: The wintertime Arctic stratosphere is typically characterized by strong westerly winds, but on occasion the stratospheric polar vortex deforms and even splits into two smaller vortices as the Arctic stratosphere strongly warms. Such an abrupt warming phenomenon, also known as a sudden stratospheric warming (SSW) occurred on January 5, 2021. This most recent SSW was forced by alternate wave pulses from the troposphere as a blocking high developed over the Urals in mid‐December 2020 and early January 2021, and the East Asian trough deepened in late December 2020 and mid‐January 2021. The amplifying wave amplitudes led to the elongation and splitting of stratospheric vortex days before the SSW onset and displacement from the Arctic after the SSW onset. As the tropospheric precursors in the extratropics preceded the SSW onset by two weeks, the maximum predictive limit of this SSW is not beyond two weeks. Arctic ice loss does not extend the predictability of this event. A cold air outbreak happened in Asia before the SSW onset, so cold anomalies over North Eurasia in the post‐SSW period are not solely caused by the downward impact of this SSW. Cold anomalies persisted in North Eurasia after the SSW and models only succeed in capturing this extended cold if they capture the SSW. Key Points: A sudden stratospheric warming (SSW) event occurred on January 5, 2021, despite unfavorable tropical forcings (westerly quasi‐biennial oscillation and weak Madden‐Julian Oscillation convection)Without favorable tropical conditions, the maximum predictive limit to this SSW is not beyond the average predictability, two weeksThe persistence of the cold anomalies over Eurasia is better captured in SSW‐hit members than in SSW‐missed members [ABSTRACT FROM AUTHOR]

Abstract The impact of the El Niño Southern Oscillation (ENSO) on the Northern Hemisphere mid-winter zonal wind, temperature, and stationary planetary waves (SPWs) is evaluated using the Middle and Upper Atmosphere Model and Modern-Era Retrospective Analysis for Research and Applications (MERRA). The composites determined using simulated ensembles and MERRA winters with different ENSO phases show that the mean zonal wind in the stratosphere at higher-middle latitudes is weaker and polar region is warmer, and the activity of SPW1 is higher during El Niño events. The simulated and observed SPW2 amplitude behaves in the opposite way, and it is stronger in the stratosphere during La Niña. The observed changes of SPW1 and SPW2 amplitudes under La Niña and El Niño events should affect an efficiency of the stratosphere–troposphere coupling in different longitudinal sectors through the changes in horizontal distributions of the downward wave activity flux.

Using NCEP/NCAR reanalysis data, we investigate the statistical characteristics and the long-term variations of major sudden stratospheric warming (SSW) events in the Northern Hemisphere. We find that the strength and duration of major SSW events have increased from 1958 to 2019 because of the strengthening of winter planetary wave activity. The frequency of the SSW events related to displacement or split of the polar vortex differs between early, middle, and late winter. Early (middle) winter is dominated by displacement (split) SSW events, while late winter sees almost equal frequency of these two types of events. This is due to the differences in the relative strength of wavenumber-1 and wavenumber-2 planetary wave activity among the three winter periods. As a result of the increase in upward planetary wave activity and the decrease in westerly winds around the polar vortex in middle winter, more SSW events tend to occur in middle winter. In addition, we reveal the influence of the downward propagation of different types of SSW events on the surface temperature anomaly. Compared with early displacement SSW events, middle split SSW events are followed by more surface cold centers in Russia, northern China, and North America. [ABSTRACT FROM AUTHOR]

Mesospheric winds from three longitudinal sectors at 65°N and 54°N latitude are combined to diagnose the zonal wave numbers (m) of spectral wave signatures during the Southern Hemisphere sudden stratospheric warming (SSW) 2019. Diagnosed are quasi‐10‐ and 6‐day planetary waves (Q10DW and Q6DW, m = 1), solar semidiurnal tides with m = 1, 2, 3 (SW1, SW2, and SW3), lunar semidiurnal tide, and the upper and lower sidebands (USB and LSB, m = 1 and 3) of Q10DW‐SW2 nonlinear interactions. We further present 7‐year composite analyses to distinguish SSW effects from climatological features. Before (after) the SSW onset, LSB (USB) enhances, accompanied by the enhancing (fading) Q10DW, and a weakening of climatological SW2 maximum. These behaviors are explained in terms of Manley‐Rowe relation, that is, the energy goes first from SW2 to Q10DW and LSB, and then from SW2 and Q10DW to USB. Our results illustrate that the interactions can explain most wind variabilities associated with the SSW. Plain Language Summary: Sudden stratospheric warming events occur typically over the winter Arctic and are well known for being accompanied by various tides and Rossby waves. A rare SSW occurred in the Southern Hemisphere in September 2019. Here, we combine mesospheric observations from the Northern Hemisphere to study the wave activities before and during the warming event. A dual‐station approach is implemented on high‐frequency‐resolved spectral peaks to diagnose the horizontal scales of the dominant waves. Diagnosed are multiple tidal components, multiple Rossby normal modes, and two secondary waves arising from nonlinear interactions between a tide component and a Rossby wave. Most of these waves do not occur in a climatological sense and occur around the warming onset. Furthermore, the evolution of these waves can be explained using theoretical energy arguments. Key Points: Mesospheric winds from multiple longitudes in the NH are combined to diagnose zonal wave numbers of waves during the Antarctic SSW 2019Diagnosed are Q6DW, Q10DW, M2, SW1, SW2, SW3, and LSB and USB of Q10DW‐SW2 nonlinear interactionsLSB and USB are generated asynchronously, during which their parent waves evolve following the Manley‐Rowe energy relations [ABSTRACT FROM AUTHOR]

A rare sudden stratosphere warming (SSW) occurred in the Southern Hemisphere polar region in 2019. The polar stratosphere temperature and planetary wave (PW) enhancements are found to be unusual from the history for 40 years; hence, it is an "Extremely‐Rare" SSW. The distinct features of the mesosphere winds were observed during the SSW, in association with the traveling PWs in the stratosphere. The mesosphere zonal winds reversed for about 20 days before the peak SSW. Meteor radar (MR) and Modern‐Era Retrospective Analysis for Research and Applications (MERRA)‐2 observations indicate that the zonal wind reversal was descended with time, and the reversal was larger over ~72°S than the MR site (62°S). The MR detected the PWs of 14–22 days before and 8–12 days following the SSW in the mesosphere. We further noticed the enhancement of wavenumber 1 signature in the mesosphere during the peak SSW over the polar region. Thus, the polar middle‐atmosphere is greatly affected by the SSW. Key Points: Unusual increase of stratosphere temperature and amplitude of PW were observed in SH during the 2019 SSW, more severe than in the 2002 SSWA long‐lasting (~20 days) mesosphere zonal wind reversal is noted before the peak SSW, and the mesosphere was greatly affected by the SSWKSS meteor radar observations detected the signature of 14‐ to 22‐day PWs before the SSW and 8‐ to 12‐day PWs following the SSW [ABSTRACT FROM AUTHOR]

A minor sudden stratospheric warming (SSW) happened in September 2019 in the Southern Hemisphere (SH) with winds at 10 hPa, 60°S reaching their minimum value on 18 September. Using multiple data sets and real‐time predictions from 11 subseasonal to seasonal (S2S) models, the evolution, favorable conditions, and predictability for this SSW event are explored. The September 2019 SSW happened during several favorable conditions, including easterly equatorial quasi‐biennial oscillation (QBO) winds at 10 hPa, solar minimum, positive Indian Ocean Dipole (IOD) sea surface temperatures (SST), warm SST anomalies in the central Pacific, and a blocking high near the Antarctic Peninsula. With these favorable initial and boundary conditions, the predictive limit to this SSW is around 18 days in some S2S models, and more than 50% of the ensemble members forecast the zonal wind deceleration in reforecasts initialized around 29 August. A vortex slowdown is evident in some initializations from around 22 August, but with a forecast‐reanalysis pattern correlation %3C0.5, while initializations later than 29 August capture the wavelike pattern in the troposphere and the subsequent stratospheric evolution. The ensemble spread in the magnitude of the vortex deceleration during the SSW is mainly explained by the ensemble spread in the magnitude of upward propagation of waves in the troposphere and in the stratosphere, with an underestimated tropospheric wave amplitude leading to a too‐small deceleration of the vortex. The September 2019 SH SSW did not show a near‐instantaneous downward impact on the tropospheric southern annular mode (SAM) in late September and early October 2019. The Australian drought and hot weather in September possibly associated with the positive IOD might have been exacerbated by the negative SAM in October and later months due to the weak stratospheric polar vortex. However, models tend to forecast a near‐instantaneous tropospheric response to the SSW. Key Points: An SH SSW happened in September 2019, with westerly winds at 10 hPa, 50°S reversed on 16 SeptemberThis SH SSW appeared during several favorable conditions, including easterly QBO winds, solar minimum, positive IOD, and warm SST anomalies in the central PacificThe predictive limit to this SSW is ~18 days in some S2S models, but models forecast a faster tropospheric response to the SSW than observations [ABSTRACT FROM AUTHOR]

Sudden stratospheric warming events (SSWs) are the most spectacular atmospheric vertical coupling processes, well‐known for being associated with diverse wave activities in the upper atmosphere and ionosphere. The first four solar tidal harmonics have been reported as being engaged. Here, combining mesospheric winds detected by three midlatitude radars, we demonstrate at least the first six harmonics that occurred during SSW 2018. Wave number diagnosis demonstrates that all six harmonics are dominated by migrating components. Wavelet analyses reveal that the fourth, fifth, and sixth harmonics quench after the SSW onset. The six harmonics and the quenching appear also in a statistical analysis based on near‐12‐year observations from one of the radars. We attribute the quenching to reversal of the background eastward wind. Plain Language Summary: Solar tides are the most predictably occurring waves in the upper atmosphere. Although the dynamical theory can be dated back to Laplace in the sixteenth century, upper atmospheric tides were rarely studied observationally until satellites and ground‐based radars became common. To date, observational studies have mainly dealt with low‐order solar‐day harmonics. Here, we combine mesospheric wind observations from three longitudinal sectors to investigate high‐order harmonics. Results illustrate that the first six harmonics appear in early 2018, all of which are dominated by Sun‐synchronous components. Among these harmonics, the fourth, fifth, and sixth quench at the sudden stratospheric warming onset, which we attribute to variations in the background zonal wind. Key Points: First six solar tidal harmonics occur in the mesospheric wind during SSW 2018 among which the fourth, fifth, and sixth harmonics quench at the SSW onsetWave number diagnosis using multistation techniques suggests that all six harmonics are dominated by migrating tidesIn a near‐12‐year statistics, the six harmonics and quenching also occur [ABSTRACT FROM AUTHOR]

We investigate the atmospheric and ionospheric response to the 2013 sudden stratospheric warming (SSW) by using multiple instruments located in Eastern Asia. Three meteor radars and five ionosondes are used to investigate the mesospheric zonal wind fields and ionospheric parameters of $F$ -layer virtual height ($h$ ’F), F2-layer peak height ($h_{m}\text{F}_{2}$), and critical frequency ($f_{o}\text{F}_{2}$) at mid- and low-latitudes (10.7°N to 40.3°N). The vertical total electron content (TEC) data derived from the ground-based global positioning system receiver network are analyzed to study the ionospheric perturbations in the equatorial ionization anomaly (EIA) region. The changes in equatorial electrojet (EEJ) are observed by using the magnetometer data from stations on and off the magnetic equator. The variations of the $h_{m}\text{F}_{2}$ at Sanya and EEJ strength presented the semidiurnal pattern with increase/decrease and eastward/westward currents in the morning/afternoon hours. In addition, the EIA crest moved poleward/equatorward in the morning/afternoon. The $f_{o}\text{F}_{2}$ showed the most significant enhancements during daytime at Wuhan and Shaoyang but the $f_{o}\text{F}_{2}$ at Sanya and Chumphon reduced mildly. Most importantly, based on the time-period wavelet analysis, the diurnal tidal components in the $f_{o}\text{F}_{2}$ over Beijing, Wuhan, and Sanya seemed similar those in zonal winds and the semidiurnal tides in the low-latitude $h_{m}\text{F}_{2}$ showed the similar temporal variations as those in EEJ strength during the later phase of SSW. Therefore, apart from the local tides propagating from lower atmosphere having influence on the mid- and low-latitude ionosphere directly during the early phase, the equatorial fountain effect modulated by the enhanced tides also disturbed the EIA region. [ABSTRACT FROM AUTHOR]

Abstract We have investigated the coupling between the stratosphere and mesosphere–lower thermosphere (MLT) in the Southern Hemisphere (SH) during 2010 minor sudden stratospheric warmings (SSWs). Three episodic SSWs were noticed in 2010. Mesospheric zonal winds between 82 and 92 km obtained from King Sejong Station (62.22°S, 58.78°W) meteor radar showed the significant difference from usual trend. The zonal wind reversal in the mesosphere is noticed a week before the associated SSW similar to 2002 major SSW. The mesosphere wind reversal is also noticed in “Specified Dynamics” version of Whole Atmosphere Community Climate Model (SD-WACCM) and Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) simulations. The similar zonal wind weakening/reversal in the lower thermosphere between 100 and 140 km is simulated by GAIA. Further, we observed the mesospheric cooling in consistency with SSWs using Microwave Limb Sounder data. However, the GAIA simulations showed warming between 130 and 140 km after few days of SSW. Thus, the observation and model simulation indicate for the first time that the 2010 minor SSW also affects dynamics of the MLT region over SH in a manner similar to 2002 major SSW. Graphical abstract .

• Advanced meteor radar is used to study SSW effects on the tropical mesosphere. • Maximum wind reversal (∼37 m/s) noticed at 80 km. • The 14–16 day waves are observed during the minor SSW like a major SSW. • Large SDT enhancement (∼30 m/s) is noticed in the zonal wind. The observations of the advanced meteor radar at the Indian tropical station Tirupati (13.63°N, 79.4°E) have been used to investigate the effects of the 2017 minor Sudden Stratospheric Warming (SSW) on the tropical mesosphere and lower thermosphere (MLT). The episodic minor warmings were observed in February 2017 with the gain of ∼40 K in the polar stratospheric temperature (PST), followed by a weakening of the eastward wind by 27 m/s. The observations show the large temporal variations in the zonal wind at 75–80 km during the SSW. We report the occurrence of 14–16 day waves in the MLT zonal wind during SSW and the secondary waves (2–7 days) in the meridional wind after SSW. A large enhancement (∼30 m/s) is observed in the amplitude of the semi-diurnal tide (SDT) during the SSW. The present results are similar to those observed during the major SSW. Therefore, the present results stress that the studies of minor SSW on the tropical MLT dynamics must be important. [ABSTRACT FROM AUTHOR]

Using state-of-the-art models from the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5/6), future changes of sudden stratospheric warming (SSW) events under a moderate emission scenario (RCP45/SSP245) and a strong emissions scenario (RCP85/SSP585) are evaluated with respect to the historical simulations. Changes in four characteristics of SSWs are examined in 54 models: the SSW frequency, the seasonal distribution, stratosphere–troposphere coupling, and the persistency of the distorted or displaced polar vortex. The composite results show that none of these four aspects will change robustly. An insignificant (though positive) change in the SSW frequency from historical simulations to RCP45/SSP245 and then to RCP85/SSP585 is consistently projected by CMIP5 and CMIP6 multimodel ensembles in most wintertime months (December–March). This increase in the SSW frequency is most pronounced in mid- (late-) winter in CMIP6 (CMIP5). No shift in the seasonality of SSWs is simulated especially in the CMIP6 future scenarios. Both the reanalysis and CMIP5/6 historical simulations exhibit strong stratosphere–troposphere coupling during SSWs, and the coupling strength is nearly unchanged in the future scenario simulations. The near surface responds immediately after the onset of SSWs in both historical and future scenarios experiments, denoted by the deep downward propagation of zonal-mean easterly anomalies from the stratosphere to the troposphere. On average, the composite circumpolar easterly winds persist for 8 d in the reanalysis and CMIP5/6 historical experiments, which are projected to remain unchanged in both the moderate and strong emissions scenarios, implying the lifecycle of SSWs will not change.

Using the ERA5 reanalysis, sea surface temperature and sea ice observations, and the real-time multivariate Madden–Julian index, this study explores a sudden stratospheric warming (SSW) in January 2021, its favorable conditions, and the near surface impact. Wavenumbers 1 and 2 alternately contributed to the total eddy heat flux from mid-December 2020 to late January 2021, and the wavenumber 2 during the onset period nearly split the stratospheric polar vortex. In mid-December 2020 and during the 2021 New Year period (1–5 January 2021), a blocking developed over the Urals, which enhanced the local ridge and the climatological wavenumber 2. Composite results confirm that the Arctic sea ice loss in autumn and La Niña favor the deepening of the high latitude North Pacific low and the increase of the Urals height ridge, which together enhance the planetary waves and hence disturb the stratospheric polar vortex. However, the Madden–Julian oscillation (MJO) in the tropics was dormant in mid-to-late December 2020 and early January 2021, and the well-established statistical relationship between the MJO convection over the western Pacific and the SSW is not applicable to this special case. The cold air outbreak in China during the 2021 New Year period before the January 2021 SSW onset is not explained by the SSW signal which developed in the stratosphere. In contrast, the downward-propagating signal reached the near surface in mid-February 2021, which may contribute to the cold air outbreak in US and may help to explain the extreme coldness of Texas in middle February.

Abstract: The winter upper atmosphere is associated with semidiurnal tidal variants, referring collectively to enhancements of near‐12 h periodicities, including the lunar tide‐like (M2) periodicity, solar semidiurnal (S2) spectral sidebands, and the quasi‐semidiurnal westward propagating modes with zonal wavenumbers m = 1 and 3 (qSW1 and qSW3). Here we formulate a multipoint technique and implement the technique for a configuration of two midlatitude meteor radars, from Germany and China, to investigate the tidal variants. Statistical results illustrate that the 12 h periodicity is dominated consistently by the expected migrating mode ( m = 2) between 2012 and 2016, consistent with the tidal climatology and in turn validating the technique. Our case study of 2013 sudden stratospheric warming reveals that the 11.6 h periodicity is characterized by m = 3, whereas the 12.4 h periodicity is dominated by m = 2 mode with a maximum amplitude 7.5 m/s and also comprises an additional mode m = 1 with a maximum amplitude 3.3 m/s. These observational evidences demonstrate, explicitly and for the first time, that (1) two independently reported categories of the variants, namely, the sidebands and the qSW1/qSW3 enhancements, are two different perspectives of identical phenomena, namely, the secondary waves of nonlinear interactions between SW2 and planetary waves, and (2) while M2 and the qSW1‐associated secondary wave are entangled in the 12.4 h periodicity, M2 is superior to the sideband. [ABSTRACT FROM AUTHOR]

Using the historical simulation from the CESM1-WACCM coupled model and based on the JRA55 and NCEP/NCAR reanalyses, the general statistical characteristics of the major sudden stratospheric warmings (SSWs) in this stratosphere-resolving model are assessed. The statistical and diagnostic results show that CESM1-WACCM can successfully reproduce the frequency of SSW events. As in the JRA55 and NCEP/NCAR reanalyses, five or six SSW events, on average, occur in a model decade. The seasonal distribution of SSWs is also well simulated with the highest frequency in January (35%). The unprecedented low SSW frequency observed in 1990s from the two reanalyses is also identified in a model decade (1930s). In addition, the overestimated duration of SSW events in the earlier WACCM version is not identified in CESM1-WACCM when compared with the two reanalyses. The model can well reproduce the downward propagation of the stratospheric anomaly signals (i.e., zonal wind, height, temperature) following SSWs. Both the modelling and observational evidences indicate that SSWs are proceeded by the positive Pacific−North America (PNA) and negative Western Pacific (WP) pattern. The negative North Atlantic Oscillation (NAO) develops throughout the SSW life cycle, which is successfully modeled. A cold Eurasian continent−warm North American continent pattern is observed before SSWs at 850 h Pa, while the two continents are anomalously cold after SSWs in both the reanalyses and the model.

Upper mesospheric winds observed by the Svalbard specular meteor radar (16.01°E,78.16°N) are analyzed to study the tidal variabilities during the 2009 sudden stratospheric warming (SSW). We report a textbook case of nonlinear interactions between planetary waves (PWs) and the SW2 tide (SWm denotes semidiurnal westward propagating tidal mode with zonal wave number m). The Lomb-Scargle algorithm, bispectrum, wavelet spectra, and Manley-Rowe relations are combined to explore the frequency match, phase coherence, energy budget, and wave number relations among the interacting waves and their temporal evolution. Our results suggest that (1) 5, 10, 16 day PW normal modes interact with SW2 generating significant sidebands (S2Ss) at frequencies lower and higher than SW2, known as SW1 and SW3 enhancements, respectively; (2) SW2 is the main energy supplier for both SW1 and SW3, hence shrinks in the interactions; (3) whereas the PWs export relatively negligible energy to SW3 but accept energy from SW2 in generating SW1, therefore, the PWs is not subject to the interactions but controlled by external dynamics, which might in turn act as a key in switching on/off the SW1 and SW3 interactions independently; (4) the SW1 enhancement could be explained as a byproduct of the planetary wave amplification by stimulated tidal decay (PASTIDE); (5) PASTIDE contributes energy to the secondary PW in the late SSW stage reported in previous studies; and (6) one SW1 component associated with the 16 day PW is very close to the semidiurnal lunar mode in frequency, which might contaminate the estimation of the lunar tidal amplification in previous studies. [ABSTRACT FROM AUTHOR]

We have investigated the coupling between the stratosphere and mesosphere-lower thermosphere (MLT) in the Southern Hemisphere (SH) during 2010 minor sudden stratospheric warmings (SSWs). Three episodic SSWs were noticed in 2010. Mesospheric zonal winds between 82 and 92 km obtained from King Sejong Station (62.22°S, 58.78°W) meteor radar showed the significant difference from usual trend. The zonal wind reversal in the mesosphere is noticed a week before the associated SSW similar to 2002 major SSW. The mesosphere wind reversal is also noticed in 'Specified Dynamics' version of Whole Atmosphere Community Climate Model (SD-WACCM) and Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) simulations. The similar zonal wind weakening/reversal in the lower thermosphere between 100 and 140 km is simulated by GAIA. Further, we observed the mesospheric cooling in consistency with SSWs using Microwave Limb Sounder data. However, the GAIA simulations showed warming between 130 and 140 km after few days of SSW. Thus, the observation and model simulation indicate for the first time that the 2010 minor SSW also affects dynamics of the MLT region over SH in a manner similar to 2002 major SSW. [ABSTRACT FROM AUTHOR]

A minor stratospheric sudden warming (SSW) event was noticed in the southern hemisphere (SH) during September (day 259) 2010 along with two episodic warmings in early August (day 212) and late October (day 300) 2010. Among the three warming events, the signature of mesosphere response was detected only for the September event in the mesospheric wind dataset from both meteor radar and MF radar located at King Sejong Station (62°S, 59°W) and Rothera (68°S, 68°W), Antarctica, respectively. The zonal winds in the mesosphere reversed approximately a week before the September SSW event, as has been observed in the 2002 major SSW. Signatures of mesospheric cooling (MC) in association with stratospheric warmings are found in temperatures measured by the Microwave Limb Sounder (MLS). Simulations of specified dynamics version of Whole Atmosphere Community Climate Model (SD-WACCM) are able to reproduce these observed features. The mesospheric wind field was found to differ significantly from that of normal years probably due to enhanced planetary wave (PW) activity before the SSW. From the wavelet analysis of wind data of both stations, we find that strong 14–16 day PWs prevailed prior to the SSW and disappeared suddenly after the SSW in the mesosphere. Our study provides evidence that minor SSWs in SH can result in significant effects on the mesospheric dynamics as in the northern hemisphere. [ABSTRACT FROM AUTHOR]

Based on the reanalysis data and PM 2.5 concentration in the BTH (Beijing-Tianjin-Hebei) region, the two most recent sudden stratospheric warmings (SSWs, February 11, 2018 and January 2, 2019) in the Northern Hemisphere and their potential impact on the environmental condition are investigated. The stratospheric polar vortex changes more radically from a strong to a weak state during the 2018 SSW than the 2019 SSW, and the stratospheric signal during the 2018 SSW propagates downward deeper than that during the 2019 SSW. In the pre-SSW period, enhanced climatological wave activities lead to more exchange between midlatitudes and high latitudes, and cold air outbreak in this period corresponds to a low BTH PM 2.5 concentration. In the tropospheric wave weakening period due to its upward propagation to disturb the stratosphere (days 5–20), the BTH PM 2.5 concentration increase rapidly, reaching the light pollution level. The February 2018 SSW shows a stronger downward propagating signals in the post-SSW period, which favors weakening of the pressure contrast between Arctic and midlatitudes and strengthening of the East Asian winter monsoon systems in the middle troposphere, resulting in diffusion and dilution of the pollutants. In contrast, the PM 2.5 concentration is still high in the post-SSW period during the 2019 event, consistent with the non-downward propagation of the stratospheric signal. The lower tropospheric systems do not necessarily cooperate with the stratospheric variability to impact the regional pollutions. • The PM 2.5 concentration in the Beijing-Tianjin-Hebei (BTH) region is relatively low before the SSW onset. • BTH PM 2.5 concentration is relatively high in the tropospheric wave weakening period (days 5–20 relative to the SSW onset). • BTH PM 2.5 concentration decreases (changes little) after the SSW with (without) downward-propagating stratospheric signal. [ABSTRACT FROM AUTHOR]

The signature of the low-latitude mesosphere response to the February 2017 minor sudden stratosphere warming (SSW) was detected using the observations of a meteor radar at Fuke (19.5°N, 109.1°E) a low-latitude station in China and ERA-5 data. Radar detected the zonal wind reversal in the mesosphere three days before the SSW event, whereas the ERA-5 does not reproduce the mesospheric wind response at low-latitudes during the 2017 SSW and large discrepancies are found between the mesospheric zonal winds derived from the Fuke radar and ERA-5. The quasi oscillatory pattern was noted in the meridional winds before the SSW event, afterward, the northward winds are significant. We observed the existence of 20–28-day planetary waves (PWs) in the Fuke meteor radar measured mesospheric zonal winds before the SSW and the 2-12-days period PWs before and after the SSW in the meridional winds. The zonal structure and periodicity of the PWs have been verified with the other meteor radar at the Indian tropical station Tirupati (13.63°N, 79.4°E). However, ERA-5 does not capture the PW activity in the mesosphere as radar. The similarities and dissimilarities among the different techniques have been discussed. Therefore, the present study re-emphasizes the strong response of the low-latitude mesosphere to the 2017 minor SSW in a manner similar to the major SSW event. • The Fuke meteor radar along with ERA-5 were used to study the low-latitude mesospheric signatures during the 2017 SSW. • Large discrepancies are found between the mesospheric winds derived from the Fuke radar and ERA-5 during the 2017 SSW. • Mesospheric wind measurements from Fuke and Tirupati radars reveal PW oscillations at low-latitudes during the SSW. • ERA-5 does not capture the PW activity in the mesosphere as a meteor radar. [ABSTRACT FROM AUTHOR]

Using the real‐time predictions from 11 models, this study analyzes the prediction of the downward propagation and surface impact of the 2018 and 2019 sudden stratospheric warmings (SSWs). These two SSWs differed both in their morphology types (2018: split; 2019: displacement followed by split) and magnitudes (the former being stronger). With a large sample size (>2,200) of multimodel ensemble forecasts, it is revealed that the strength of the SSW is more important than the vortex morphology in determining the magnitude of its downward impact, with strong SSWs more likely to propagate downward than weak SSWs. Therefore, based on the probabilistic forecasts, the observed strong SSW in February 2018 was more likely to have a downward and surface impact than the January 2019 SSW. The relationship between the 10‐hPa dominant wave number and the 100‐hPa polar cap height (or the Northern Annular Mode) is weak, implying that the dominant wave number might not be the primary factor determining the downward propagation of SSWs in the two analyzed cases. Hence, the high polar cap height (or negative Northern Annular Mode) response in the lower stratosphere and troposphere following the February 2018 SSW is mainly attributed to its strong intensity rather than the split morphology. Further, the 2‐m temperature anomaly pattern following the January 2019 SSW is not forecasted due to its weak downward propagation, whereas the 2‐m temperature in North Eurasia, Middle East, south China, and eastern United States could be forecasted for the downward propagating February 2018 SSW. However, regional rainfall anomalies are poorly forecasted (both in a deterministic and probabilistic sense) for both SSWs. Key Points: Better prediction of an SSW does not necessarily denote better prediction of the downward coupling or the following near‐surface impactThe T2m in North Eurasia, Middle East, south China, and eastern United States is more difficult to forecast for the 2019 SSW with weak downward propagationThe rainfall anomalies in some regions are poorly forecasted both in a deterministic and probabilistic sense for (non)downward propagating SSWs [ABSTRACT FROM AUTHOR]

Using the historical simulation from the CESM1-WACCM coupled model and based on the JRA55 and NCEP/NCAR reanalyses, the general statistical characteristics of the major sudden stratospheric warmings (SSWs) in this stratosphere-resolving model are assessed. The statistical and diagnostic results show that CESM1-WACCM can successfully reproduce the frequency of SSW events. As in the JRA55 and NCEP/NCAR reanalyses, five or six SSW events, on average, occur in a model decade. The seasonal distribution of SSWs is also well simulated with the highest frequency in January (35%). The unprecedented low SSW frequency observed in 1990s from the two reanalyses is also identified in a model decade (1930s). In addition, the overestimated duration of SSW events in the earlier WACCM version is not identified in CESM1-WACCM when compared with the two reanalyses. The model can well reproduce the downward propagation of the stratospheric anomaly signals (i.e., zonal wind, height, temperature) following SSWs. Both the modelling and observational evidences indicate that SSWs are proceeded by the positive Pacific–North America (PNA) and negative Western Pacific (WP) pattern. The negative North Atlantic Oscillation (NAO) develops throughout the SSW life cycle, which is successfully modeled. A cold Eurasian continent–warm North American continent pattern is observed before SSWs at 850 h Pa, while the two continents are anomalously cold after SSWs in both the reanalyses and the model. [ABSTRACT FROM AUTHOR]

Enhanced nonmigrating tides SW1 (SWx represents semidiurnal westward mode with zonal wave number x) and SW3 during sudden stratospheric warming (SSW) were traditionally attributed to nonlinear interactions of quasi‐stationary planetary waves with the migrating tide SW2. Recent studies specified hypothetically that responsible for the interactions is the 16‐day wave, instead of the broadly accepted quasi‐stationary planetary waves. It is suspected that the 16‐day‐wave‐triggered secondary waves, at periods ∼12.4 and ∼11.6 hr, were detected at low‐frequency resolutions and misinterpreted as SW1 and SW3, respectively. While He et al. (2018, https://doi.org/10.1002/2018JD028400) associated the 11.6‐hr oscillation conclusively to the SW3‐like signature during SSW 2013 by diagnosing its wave number, the SW1‐like 12.4hr wave, however, has never been explicitly resolved, given its proximity to the period of an active lunar tide. Here, using the coherency in the mesospheric wind between two longitudinal sectors during SSW 2009, we identify a 12.4‐hr oscillation dominated by wave number 1 and therefore associate it to the SW1‐like signature. Plain Language Summary: The winter upper atmosphere is populated by global‐scale oscillations among which the most radical one appears nearby the period of 12 hr. The existing studies suggest that the near‐12‐hr oscillation consists of at least six waves, namely, the migrating lunar and solar tides, two nonmigrating tides, and two secondary waves of the nonlinear interactions between the migrating solar tide and traveling planetary waves. The current work, together with a counterpart work (He et al., 2018), disentangles the near‐12‐hr oscillation into high‐frequency‐resolved isolated spectral peaks and investigates the longitudinal variation in the phase of the isolated peaks through jointly studying the mesospheric wind collected by longitudinally separated radars in early 2009 and early 2013. Results suggest that the nonmigrating tides did not enhance in both cases and demonstrate that the occurrence of secondary waves might have been misinterpreted as the nonmigrating tides in studies at low‐frequency resolutions. Key Points: We report an upper atmospheric pseudo‐SW1, a 12.4‐hr oscillation characterized by westward traveling wave number 1 structureThe nonlinear interaction between the 16‐day wave and the semidiurnal migrating tide SW2 triggers the pseudo‐SW1The broadly reported SW1‐like enhancements during sudden stratospheric warming events are conceptually not tides [ABSTRACT FROM AUTHOR]