Major Sudden Stratospheric Warming Research Articles

Mid-latitude ionospheric disturbances during the major Sudden Stratospheric Warming event of 2018 observed by sub-ionospheric VLF/LF signals

This study examines the impact of a major sudden stratospheric warming (SSW) event in February 2018 on the ionosphere of the mid-to–high latitudes in the Northern Hemisphere. The SSW event commenced on February 11 and persisted for about 18 days. The polar stratospheric temperature at 10 hPa increased by about 50 K during February 17 (daytime) and February 18 (nighttime). To detect lower ionospheric variations, VLF/LF signals from four transmitters received by three receivers in the European sector and one receiver in the eastern USA are analyzed along the five propagation paths. The analysis of the diurnal variations in the European sector indicates a significant fluctuation in signal amplitude exceeding ∼5 dB during nighttime on the most disturbed day (February 13). The 1-h averaged nighttime amplitude also showed a maximum deviation of more than ±2σ on February 13. Notably, this occurrence took place two days after the onset of the SSW event at the European receiver locations. No delay in the occurrence of the maximum anomaly was observed in the westward propagation path in the eastern USA. We also analyzed the critical frequency of the F2 layer (foF2) of the ionosphere during February 2018, recorded by Lowell digisonde stations located near the VLF/LF receivers. The foF2 data shows a significant maximum anomaly of ∼1 MHz during nighttime on February 13 at the European stations, coinciding with the SSW period. This anomaly is consistent with the observed VLF/LF anomaly and aligns with the observed temperature variations. However, the deviations in foF2 at the USA stations are not as prominent as those observed at the European stations.

Signature of Two‐Step Non‐Linear Interactions Associated to Zonally Symmetric Waves During Major Sudden Stratospheric Warmings

AbstractAtmospheric tides and associated dynamics during two major boreal sudden stratospheric warmings (SSWs) have been investigated. The evolutionary Lomb Scargle and wavelet spectral analysis of specular meteor radar (SMR)‐derived hourly winds reveal evidence of non‐linear interactions between the semidiurnal solar tide and the quasi‐20‐day wave (Q20dw) during SSWs. The zonal wavenumber (ZWN) diagnosis indicates possible non‐linear interaction between the dominant semidiurnal migrating tide (SW2) and zonally symmetric 20‐day wave (20dw0) component, producing the secondary waves. The non‐linear interaction between the ZWN 2 component of stationary planetary wave (SPW2) and westward propagating 20‐day wave (20dwW2) in the stratosphere seems crucial to produce the 20dw0. As observed in the SMR‐derived wind spectra, the excited 20dw0 possibly interacts non‐linearly with SW2 to generate secondary waves. Therefore, the present study provides the first observational evidence of a two‐step non‐linear interaction associated with zonally symmetric planetary waves during major SSWs.

A Case Study of the Solar and Lunar Semidiurnal Tide Response to the 2013 Sudden Stratospheric Warming

AbstractThis study investigates the response of the semidiurnal tide (SDT) to the 2013 major sudden stratospheric warming (SSW) event using meteor radar wind observations and mechanistic tidal model simulations. In the model, the background atmosphere is constrained to meteorological fields from the Navy Global Environmental Model—High Altitude analysis system. The solar (thermal) and lunar (gravitational) SDT components are forced by incorporating hourly temperature tendency fields from the ERA5 forecast model, and by specifying the M2 and N2 lunar gravitational potentials, respectively. The simulated SDT response is compared against meteor wind observations from the CMOR (43.3°N, 80.8°W), Collm (51.3°N, 13.0°E), and Kiruna (67.5°N, 20.1°E) radars, showing close agreement with the observed amplitude and phase variability. Numerical experiments investigate the individual roles of the solar and lunar SDT components in shaping the net SDT response. Further experiments isolate the impact of changing propagation conditions through the zonal mean background atmosphere, non‐linear wave‐wave interactions, and the SSW‐induced stratospheric ozone redistribution. Results indicate that between 80 and 97 km altitude in the northern hemisphere mid‐to‐high latitudes the net SDT response is driven by the solar SDT component, which itself is shaped by changing propagation conditions through the zonal mean background atmosphere and by non‐linear wave‐wave interactions. In addition, it is demonstrated that as a result of the rapidly varying solar SDT during the SSW the contribution of the lunar SDT to the total measured tidal field can be significantly overestimated.

A Comparison of Stratospheric Gravity Waves in a High‐Resolution General Circulation Model With 3‐D Satellite Observations

AbstractAtmospheric gravity waves (GWs) play a key role in determining the thermodynamical structure of the Earth's middle atmosphere. Despite the small spatial and temporal scales of these waves, a few high‐top general circulation models (GCMs) that can resolve them explicitly have recently become available. This study compares global GW characteristics simulated in one such GCM, the Japanese Atmospheric GCM for Upper‐Atmosphere Research (JAGUAR), with those derived from three‐dimensional (3‐D) temperatures observed by the Atmospheric Infrared Sounder (AIRS) aboard NASA's Aqua satellite. The target period is from 15 December 2018 to 8 January 2019, including the onset of a major sudden stratospheric warming (SSW). The 3‐D Stockwell transform method is used for GW spectral analysis. The amplitudes and momentum fluxes of GWs in JAGUAR are generally in good quantitative agreement with those in the AIRS observations in both magnitude and distribution. As the SSW event progressed, the GW amplitudes and eastward momentum flux increased at low latitudes in the summer hemisphere in both the model and observation datasets. Case studies demonstrate that the model is able to reproduce comparable wave events to those in the AIRS observations with some differences, especially noticeable at low latitudes in the summer hemisphere. Through a comparison between the model results with and without the AIRS observational filter applied, it is suggested that the amplitudes of GWs near the entrance or exit of an eastward jet streak are underestimated in AIRS observations.

Whether Sudden Stratospheric Warming Effects Are Seen in the Midlatitude Thermosphere of the Opposite Hemisphere?

AbstractThe reaction of midlatitude daytime foF2 and thermospheric parameters in two Hemispheres has been analyzed for a minor Arctic Sudden Stratospheric Warming (SSW) in January 2008, three major Arctic SSWs in January 2006, 2009, 2013, and a minor Antarctic SSW in September 2019. Arctic SSWs always result in a simultaneous foF2 depression observed at some stations in the Northern Hemisphere but not necessary in the opposite Hemisphere, that is, not all SSWs have global appearance. Thermospheric parameters retrieved from ionospheric observations in two Hemispheres manifest a dependence on the type of SSW (major/minor) and its magnitude. Pronounced thermospheric SSW effects in the opposite (Southern) Hemisphere may be expected only for strong major Arctic SSW events. Retrieved exospheric temperature does not manifest any visible reaction to SSWs both for major and minor SSW events. The duration of foF2 and atomic oxygen decrease related to SSW is 3–5 days in the vicinity of the SSW peak. Both observed neutral gas density and retrieved thermospheric parameters do not manifest a significant difference comparing western and eastern regions of North America during the minor Antarctic SSW in September 2019. Therefore, the previously reported (80–100%) increase in TEC in the western region of North America was not related with variations of neutral composition and winds.

Direct Observational Evidence of Altered Mesosphere Lower Thermosphere Mean Circulation From a Major Sudden Stratospheric Warming

AbstractSudden stratospheric warmings (SSWs) are large‐scale phenomena characterized by dramatic dynamic disruptions in the stratospheric winter polar regions. Previous studies, especially those employing whole atmosphere models, indicate that SSWs have strong impacts on the circulation of the mesosphere lower thermosphere (MLT) and drive a reversal in the mean meridional circulation (MMC) near 90–125 km altitude. However, the robustness of these effects and the roles of SSW‐induced changes in global‐scale wave activity to drive the reversal have been difficult to observe simultaneously. This work employs horizontal lower thermospheric (∼93–106 km altitude) winds near 10°S‐40°N latitude from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer (ICON) to present observational evidence of a prominent MLT MMC reversal associated with the January 2021 major SSW event and to demonstrate connections to semidiurnal tidal activity and possible associations with a ∼3‐day ultra‐fast Kevin wave.

The Climatology of Elevated Stratopause Events in the UA‐ICON Model and the Contribution of Gravity Waves

AbstractThe climatologies of the stratopause height and temperature in the UA‐ICON model are examined by comparing them to 17‐years (2005–2021) of Microwave Limb Sounder (MLS) observations. In addition, the elevated stratopause (ES) event occurrence, their main characteristics, and driving mechanisms in the UA‐ICON model are examined using three 30‐year time‐slice experiments. While UA‐ICON reasonably simulates the large‐scale stratopause properties similar to MLS observations, at polar latitudes in the Southern Hemisphere the stratopause is ∼8 K warmer and ∼3 km higher than observed. A time lag of about two months also exists in the occurrence of the tropical semiannual oscillation of the stratopause compared to the observations. ES events occur in ∼20% of the boreal winters, after major sudden stratospheric warmings (SSWs). Compared to the SSWs not followed by ES events (SSW‐only), the ES events are associated with the persistent tropospheric forcing and prolonged anomalies of the stratospheric jet. Our modeling results suggest that the contributions of both gravity waves (GW)s and resolved waves are important in explaining the enhanced residual circulation following ES events compared to the SSW‐only events but their contributions vary through the lifetime of ES events. We emphasize the role of the resolved wave drag in the ES formation as in the sensitivity test when the non‐orographic GW drag is absent, the anomalously enhanced resolved wave forcing in the mesosphere gives rise to the formation of the elevated stratopause at about 85 km.

On the Different Quasi-2-Day Wave Behaviors during Sudden Stratospheric Warming Periods

The temporal variations in the sudden stratospheric warming (SSW) events in the winter stratosphere always coincide with the quasi-2-day wave (Q2DW) in the summer mesosphere, and the impact of SSW on Q2DW is interesting but still a mystery. Major SSWs occurred in both 2006 and 2009, while the Q2DW activity was quite different. The Second Modern Era-Retrospective Analysis for Research and Applications (MERRA-2) reanalysis dataset was used to comparatively analyze these two major SSW events and elucidate the reasons for the different Q2DW behaviors. We noticed that the summer easterly jet shows a large interannual variability. We conclude that the summer mesospheric Q2DWs are modulated by the winter SSW, whereas the modulation process is also affected by the interannual variability of the summer easterly flow itself. The effects of the SSW on the Q2DWs may differ from year to year due to the variability of the summer easterly flow itself, resulting in different anomalous Q2DW behavior. This conclusion may also be true for the interannual variability of other phenomena during the SSW period.

Response of Total Column Ozone at High Latitudes to Sudden Stratospheric Warmings

The total column ozone (TCO) at northern high latitudes is increased over a course of 1–2 months after a major sudden stratospheric warming as a consequence of enhanced ozone eddy transport and diffusive ozone fluxes. We analyzed ground-based measurements of TCO from Oslo, Andøya and Ny Ålesund from 2000 to 2020. During this time interval, 15 major sudden stratospheric warmings (SSWs) occurred. The observed TCO variations are in a good agreement with those of ECMWF Reanalysis v5 (ERA5), showing that TCO from ERA5 is reliable, even during dynamically active periods. ERA5 has the advantage that it has no data gaps during the polar night. We found that TCO was increased by up to 190 DU after the SSW of February 2010, over one month. The composite analysis of the 15 SSWs provided the result that TCO is increased on average by about 50 DU over one month after the central date of the SSW.

Life Cycle of Major Sudden Stratospheric Warmings in the Southern Hemisphere from a Multimillennial GCM Simulation

Abstract Sudden stratospheric warmings (SSWs) are rare in the Southern Hemisphere (SH), making it difficult to study possible precursors or subsequent impacts. Using a multimillennial coupled climate model simulation producing 161 SSWs in the SH, we present a detailed study of their life cycle. We show that SH SSWs are predominantly displacement events forced by wave-1 planetary waves, and that a surface signature similar to the negative phase of the Southern Annular Mode (SAM) is detectable up to two months before the onset date, but there is a tendency for a transition from wave 1 before to zonally symmetric anomalies after onset. We identify a strong weakening of the Amundsen Sea low as one of the most prominent precursors, which weakens the climatological wave-2 and wave-3 stationary waves and strengthens wave-1 forcing. Compared to their northern counterparts, SH SSWs generally have a longer time scale, and while there is evidence of pre-onset forcing related to tropical sea surface temperatures, the Indian Ocean dipole is more important than El Niño–Southern Oscillation. Significance Statement Sudden stratospheric warmings (SSWs) are extreme events where the winter polar stratosphere warms within a few days to temperatures usually only experienced in summer. These events are rare in the Southern Hemisphere. Therefore, both the observational record and standard climate model simulations are not enough to understand how SSWs develop, or how they might change surface weather. Here we use very long global climate simulations that produce a large number of SSWs in the Southern Hemisphere to study the development and impact of these events. This includes possible precursors as well as the influence they have on surface weather after they occur.

A comprehensive investigation of Sudden Stratospheric Warming (SSW) events and upper atmospheric signatures associated with them

We present a study of the major and minor SSWs that have occurred in the Northern Hemisphere (NH) over the past four decades (1981–2020) from a statistical perspective. This study investigates the frequency and timing of occurrence of major and minor SSWs during the specified period. A correlation is observed between the timing of the earliest minor SSWs and final events, having different correlation coefficients in all four decades, with minimum correlation during the decade 2001–2010, including the extended solar minima. In the second part of our work, we examined SSW events of the last 18 winters based on ERA5 and MERRA2 reanalysis data. Surprisingly, the stratospheric parameters have been found to show different behaviors depending on the number of SSW events in a winter. The temperature and zonal mean zonal wind variations are stronger during winters with one major SSW, rather than winters having major SSW accompanied by minor SSWs. Finally, we conducted four case studies to investigate the atmospheric and ionospheric response to major and minor SSWs over three stations, namely Okinawa (26.21°N,127.68°E), Yamagawa (35.45°N,133.62°E), and Wakkanai (45.16°N,141.75°E). Like the stratospheric parameters, ionospheric parameters are not found to be much depended on the number of SSWs in a winter, but they are found to vary from event to event. Significant latitudinal variations in ionospheric parameters were observed in all the events, with more pronounced effects toward lower latitudinal station Okinawa. Although ionospheric disturbances are more pronounced during major SSWs, some severe minor events also have notable ionospheric impacts just as the major SSW.

A new methodology for measuring traveling quasi-5-day oscillations during sudden stratospheric warming events based on satellite observations

Abstract. Enhancements of stationary planetary waves (SPWs) and traveling planetary waves (TPWs) are commonly observed in the middle atmosphere during sudden stratospheric warming (SSW) events. Based on the least squares fitting method (Wu et al., 1995), numerous studies have used satellite measurements to investigate the characteristics of TPWs during SSWs, but they have ignored the effect of the SPWs. However, a rapid and large change in the SPWs during SSWs may lead to significant disturbances in the amplitude of derived TPWs. In this study, we present a new methodology for obtaining the amplitudes and wave numbers of traveling quasi-5-day oscillations (Q5DOs) in the middle atmosphere during major SSWs. Our new fitting method is developed by inhibiting the effect of a rapid and large change in SPWs during SSWs. We demonstrate the effectiveness of the new method using both synthetic data and satellite observations. The results of the simulations indicate that the new method can suppress the aliasing from SPWs and capture the real variations in TPWs during SSWs. Based on the geopotential height data measured by the Aura satellite from 2004 to 2021, the variations in traveling Q5DOs during eight midwinter major SSWs are reevaluated using the new method. The differences in the fitted amplitudes between the least squares fitting method and the new method are usually over 100 m during the SSW onsets. Our analysis indicates that previously reported Q5DOs during SSWs might be contaminated by SPWs, leading to both overestimation and underestimation of the amplitudes of the traveling Q5DOs.

Possible influence of sudden stratospheric warmings on the atmospheric environment in the Beijing–Tianjin–Hebei region

Abstract. Using European Centre for Medium-Range Weather Forecasts fifth-generation (ERA5) and second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) reanalysis and surface meteorological observation data, this study explores the possible impact of sudden stratospheric warming (SSW) events on air quality in the Beijing–Tianjin–Hebei (BTH) region. Major SSW events are divided into polar vortex displacement SSW and polar vortex split SSW. 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 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.

Arctic Stratosphere Dynamical Processes in the Winter 2021–2022

The Arctic stratosphere winter season of 2021–2022 was characterized by a stable, cold stratospheric polar vortex with a volume of polar stratospheric clouds (PSC) close to the maximum values since 1980, before the beginning of minor sudden stratospheric warming (SSW) events in the late February and early March and major SSW on 20 March. Analysis of dynamical processes of the Arctic stratosphere using reanalysis data indicates that the main reasons for the strengthening of the stratospheric polar vortex in January–February are the minimum propagation of planetary wave activity from the troposphere to the stratosphere over the past 40 years and its reflection in the upper stratosphere–lower mesosphere in the second half of January. The first minor SSW was limited to the upper polar stratosphere, whereas the second one propagated to the middle and lower stratosphere and led to the disappearance of the PSC, which prevented significant ozone depletion. Both minor and major SSW events led to a weakening of the residual meridional circulation in the upper Arctic stratosphere and its intensification in the middle and lower stratosphere, which contributed to additional warming of the subpolar region and weakening of the polar vortex.

Which Sudden Stratospheric Warming Events Are Most Predictable?

The predictability of Northern Hemisphere sudden stratospheric warming (SSW) events is considered in 10 subseasonal to seasonal (S2S) forecast models for 16 major SSWs that have occurred since 1998, a larger sample size than has been considered by previous works. The four factors that most succinctly distinguish those SSWs with above average predictability are a preconditioned vortex prior to the SSW, an active Madden‐Julian Oscillation with enhanced convection in the West Pacific, the Quasi‐Biennial Oscillation phase with easterlies in the lower stratosphere, and the vortex morphology (displacement more predictable). Two of these factors appear to not have been considered in previous works focusing on a large sample of events. Most of these effects are not statistically significant at the 95% level due to the still relatively small sample size, though all would exceed a 90% criteria at least marginally. Combined, however, they account for 40% of the inter‐event spread in SSW predictability, thus indicating that SSWs with favorable precursors are significantly more predictable.

Ionospheric disturbances over the American and African sectors due to the 2019 major Sudden Stratospheric Warming (SSW 2019), under low solar activity conditions

Sudden Stratospheric Warming (SSW) is one of the most spectacular atmospheric large-scale phenomena, which takes place at high latitudes during winter months and is more frequent in the Arctic region than in the Antarctic region. SSWs can change the vertical, latitudinal, and longitudinal distributions of the neutral atmosphere and its dynamics, which in turn affects the ionospheric electrodynamic processes. Simultaneous inferred VTEC from GPS networks over the American and African sectors are used to investigate the ionospheric response due to the SSW 2019 from DOY 356 to DOY 20 (December 22, 2018–January 20, 2019). This study investigates the VTEC and EIA diurnal and day-to-day responses in the American and African sectors during the SSW. It is noted that the VTEC decreased on most of the days at several latitude regions. However, it is also noted that the VTEC increased on some days and in some latitude regions, particularly during the SSW temperature peak. The EIA exhibits significant changes in its shape, intensity, and symmetry during the SSW. This study using simultaneous observations over American and African sectors covering a large geographical extent demonstrates the similarities and differences in ionospheric response to the SSW 2019 event over different regions.

The “Polar Vortex” Winter of 2013/2014

AbstractThe term “polar vortex” remained largely a technical term until early January 2014 when the United States (US) media used it to describe an historical cold air outbreak in eastern North America. Since then, “polar vortex” has been used more frequently by the media and the public, often conflating circulation features and temperatures near the surface with only partially related features at the tropopause and in the stratosphere. The polar vortex in its most common scientific usage refers to a hemispheric‐scale stratospheric circulation over the Arctic that is present during the Northern Hemisphere cold season. Reversal of the zonal mean zonal winds circumnavigating the stratospheric polar vortex (SPV), termed major sudden stratospheric warmings, can be linked to mid‐latitude cold air outbreaks. However, this mechanism does not explain the cold US winter of 2013/2014. This study revisits the winter of 2013/2014 to understand how SPV variability may still have played a role in the severe winter weather. Observations indicate that anomalously strong vertical wave propagation occurred throughout the winter and disrupted, but did not fully break, the SPV. Instead, vertically propagating waves were reflected back downward, amplifying a blocking high near Alaska and downstream troughing across central North America, a classic signature for extreme cold air outbreaks across central and eastern North America. Thus, the association of the term “polar vortex” with winter 2013/2014, while not justified by the most common usage of the term, serves as a case study of the wave‐reflection mechanism of SPV influence on mid‐latitude weather.

Ionospheric Semidiurnal Lunitidal Perturbations During the 2021 Sudden Stratospheric Warming Event: Latitudinal and Inter‐Hemispheric Variations in the American, Asian‐Australian, and African‐European Sectors

AbstractThe ionosphere exhibits enhanced semidiurnal lunitidal (M2) perturbations during sudden stratospheric warming (SSW) events, of which the manifestation and mechanism are not well documented and understood. This paper studies the latitudinal and inter‐hemispheric variations of the ionospheric M2 perturbations during the 2021 major Arctic SSW with total electron content (TEC) data in the American, Asian‐Australian, and African‐European sectors. Results show that the M2 perturbations in the three sectors all enhanced during the SSW, with the strongest M2 perturbations in the Northern and Southern Hemispheres at about 10°∼15°N and 25°∼30°S geomagnetic latitudes, respectively. Also, clear M2 perturbations in the three sectors all extended to middle latitudes during the SSW only in the Southern Hemisphere. Comparatively, the inter‐hemispheric asymmetry of the magnitude of the M2 perturbations during the SSW was more complex. The inter‐hemispheric asymmetry of the crest locations of the low‐latitude M2 perturbations in the three sectors during the SSW was probably due to the F‐region meridional wind effect on the diffusion process of the low‐latitude plasma fountain. Meanwhile, the longitudinal differences were also obvious. The TEC M2 amplitude in the American sector was obviously larger than those in the Asian‐Australian and African‐European sectors during the SSW. The M2 characteristics in the southern middle latitude in the American sector were significantly different from those in the other two sectors during the SSW, which may be related to the differences of the combined effects of the local wind and geomagnetic configuration. In addition, a moderate geomagnetic activity occurred during the 2021 SSW, which brings difficulties to the analysis of the SSW‐related TEC variation.

Migrating solar diurnal tidal variability during Northern and Southern Hemisphere Sudden Stratospheric Warmings

In this study, the variability of the migrating solar diurnal (DW1) tide in the mesosphere-lower thermosphere (MLT) region during Northern and Southern Hemisphere (NH & SH) Sudden Stratospheric Warmings (SSWs) is investigated using Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature observations and reanalysis-driven Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) simulations. The periods examined include four major NH SSWs that occurred in 2006, 2009, 2010 and 2013 and two SH SSWs that were observed in 2002 and 2019. Our analysis shows that the DW1 tide in both observations and simulations displays a reduction of amplitude at low-latitudes after the onset of NH and SH SSWs. As WACCM-X simulations qualitatively reproduce this feature of DW1 tidal variability common to both NH and SH SSWs, they have been used to examine the possible mechanism that could explain these observations in the DW1 tide. It is known that changes in the latitudinal shear of zonal winds at low-latitudes strongly affect the seasonal variation of the DW1 tide in the MLT. We show that SSW-associated changes in the latitudinal shear in the MLT could explain the observed variability of the DW1 tide during NH and SH SSWs.Graphical

Features of Winter Stratosphere Small-Scale Disturbance during Sudden Stratospheric Warmings

We analyzed the characteristics of small-scale wave disturbances emerging during the evolution and transformation of the jet stream (JS) in the winter stratosphere and the lower mesosphere of the northern hemisphere, including the periods of sudden stratospheric warming (SSW) events. Continuous generation of small-scale wave disturbances is shown to occur over quiet geomagnetic winter periods in the region of a steady jet stream in the strato–mesosphere. We studied spatial spectra for the vertical velocity variations, determined by the parameters of emerging wave disturbances. The greatest intensities of disturbances are recorded in the regions corresponding to the high velocities of the JS (from 100 m/s and higher). In the northern hemisphere, those latitudes encompass ~40–60° N. When a steady jet stream forms, the horizontal length and periods of the most intensive wavelike disturbances are shown to vary within 300–1000 km and 50–150 min correspondingly (which match the characteristic scales of internal gravity waves, or IGWs). During the SSW prewarming stage, the JS transforms substantially. Over the same periods, a disturbance intensification is recorded, as well as the emergence of larger-scale disturbances with 3000–5000-km horizontal wavelengths, and even higher. After the SSW peak and during the stratosphere circulation recovery, the velocity in the JS substantially decreases and an essential reduction in wave-disturbance generation occurs. There are decreases in the average amplitude values (by factors of 1.8–6.7). The strongest amplitude drop was observed for short waves (zonal wavelength λU = 300 km). The maximum attenuation for all wavelengths was observed for the strongest 2008/2009 winter SSW. For the analyzed events, such attenuation was observed for up to about a month after the SSW peak. Thus, JS disruption during major SSWs leads to deactivating the source for generating small-scale wave disturbances in the stratosphere. This may affect disturbances in higher atmospheric layers. The results obtained are the experimental evidence that JS itself is the primary source for the generation of IGWs in the stratosphere–lower mesosphere.