Propagation Of Planetary Waves Research Articles

Roles of Gravity Waves in Preconditioning of a Stratospheric Sudden Warming

AbstractAs well as strong upward propagation of planetary waves from the troposphere, the state of the stratospheric mean flow has been recognized as a key factor for the occurrence of stratospheric sudden warmings (SSWs). The modification of the mean flow to a suitable state for an SSW occurrence is called “vortex preconditioning.” Recently, increasing attention has been paid to the role of gravity waves (GWs) in the preconditioning mechanism. However, because of the limited availability of data sets covering the whole neutral atmosphere, much uncertainty still exists in the role of GWs in the preconditioning. The aim of this study is to investigate the mechanism of modification of the mean flow in the stratosphere and mesosphere before SSWs from a climatological viewpoint and elucidate the role of GWs in it. We use two state‐of‐the‐art data sets covering the whole neutral atmosphere: a 17‐year medium‐resolution reanalysis data set and the output data from hindcast simulations performed with a GW‐permitting general circulation model. It is shown that the second principal component of the zonal‐mean zonal wind in the stratosphere and mesosphere tends to show a maximum prior to an SSW, characterizing preconditioning. GW forcing alters the structure of the upper part of the jet and contributes to the preconditioning along with planetary waves. Comparison of GW forcing between the reanalysis and GW‐permitting model suggests that the magnitude of parameterized GW forcing is approximately half that of the GW forcing in the polar upper stratosphere where the forcing is responsible for the preconditioning.

Influence of autumn Kara Sea ice on the subsequent winter minimum temperature over the Northeast China

AbstractMany previous studies have examined the influence of Arctic sea ice on the weather/climate of the Northern Hemisphere. However, the precursor signals of Arctic sea ice regarding East Asian winter low temperatures, which are important for climate prediction, have received little attention. This study identified an out‐of‐phase relationship between the autumn sea ice area of the Kara Sea (SICK) and subsequent winter minimum temperature in Northeast China (NEC) during 1979–2019, that is, diminished SICK facilitates cooling anomalies at NEC. Further results showed that the Arctic Oscillation (AO) acts as a bridge in the linkage between the autumn SICK and minimum temperature in NEC. Diminished autumn SICK leads to a weakened polar vortex in both the troposphere and the stratosphere in the subsequent winter through vertical propagation of planetary waves, contributing to a negative phase of the AO. Accordingly, the SICK is followed by anomalous Rossby wave train that originates from Mediterranean Sea and propagates eastward to Northeast Asia. Thus, the SICK has substantially influences on the Mongolia cyclone and minimum temperature in NEC by modulation of the AO. Moreover, the shrinking SICK could lead to the upper‐level decelerated westerly anomalies at East Asia through altering the equator‐to‐pole temperature gradient and induce negative minimum temperature anomalies at NEC. The results of this study have importance regarding the prediction of low temperature anomalies over NEC.

Satellite Observations of the Influence of Energetic Electron Precipitation on the Mesosphere and Stratosphere in the Northern Hemisphere

AbstractThe Earth’s atmosphere is influenced by energetic electrons coming from the magnetosphere. This energetic electron precipitation (EEP) is energized by the solar wind and directly affects in the high‐latitude mesosphere and lower thermosphere (MLT). EEP forms odd nitrogen (NOx) and hydrogen oxides (HOx) which destroy ozone. During winter EEP‐NOx descends to the stratosphere, establishing the indirect EEP effect. Several studies have found that EEP is related to changes in temperature and winds in the northern winter stratosphere. One of the most prominent effects of EEP is the influence on the northern polar vortex, a westerly wind system surrounding the winter pole in the middle atmosphere. Most studies of the EEP effect on dynamical features of the middle atmosphere have relied on either model simulations or reanalysis datasets which are mainly limited to stratospheric heights. We study here EEP effects on chemical and dynamical properties of the stratosphere and mesosphere in the northern hemisphere by using EOS Aura satellite’s measurements of atmospheric properties and POES satellites' measurements of precipitating electrons. We confirm earlier results showing that EEP decreases ozone and affects the temperature in the polar middle atmosphere and strengthens the stratospheric polar vortex. We show that EEP weakens the mesospheric polar vortex in late winter. This effect on polar vortex is partly due to changes in propagation and convergence of planetary waves. Accordingly, the EEP effect on the northern polar vortex depends on planetary waves not only in the stratosphere, as found in earlier studies, but also in the mesosphere.

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.

Solar influences on the Earth’s atmosphere: solved and unsolved questions

Solar influences on the Earth’s atmosphere: solved and unsolved questions

Interplay between Boreal Summer Intraseasonal Oscillation and Southern Hemisphere Stratospheric Polar Vortex Warming

Abstract The boreal summer intraseasonal oscillation (BSISO) features more distinctive and complex propagation characteristics than its wintertime counterpart, the Madden–Julian oscillation (MJO). While the relationship between the MJO and the Arctic stratosphere during boreal winter has been widely documented, the linkage between the BSISO and the Antarctic stratosphere during austral winter has not been extensively discussed. Here, after identifying the Southern Hemisphere (SH) stratospheric polar vortex warming (SPVW) events, we reveal the bidirectional connection between BSISO and SPVW. Before onset of the SPVW events, the occurrence frequency and amplitude of BSISO phase 5 (P5) shows a significant increase. The most significant responses of the SH polar stratospheric temperature to the BSISO are found about 10 days after BSISO P5. Thus, to some extent, BSISO P5 can be regarded as a precursor to the SH SPVW event, which is attributed to the enhanced upward propagation and dissipation of planetary waves in the SH stratosphere induced by the BSISO P5. After onset of the SPVW events, a significant increase in the occurrence and amplitude of BSISO P6 is observed, corresponding to the enhanced convection over the South China Sea and southern Philippine Sea. Tropical upwelling associated with the strengthened Brewer–Dobson circulation (BDC) induced by the SPVW tends to result in unstable circumstances in the tropical upper troposphere. Then the high correlation between static stability at 150 hPa and outgoing longwave radiation (OLR) anomalies over the South China Sea and southern Philippine Sea provides robust evidence that the intensity of convective activity in the tropics can indeed be modulated by the variability in SH stratospheric polar vortex. Significance Statement The relationship between the boreal summer intraseasonal oscillation (BSISO) and the Southern Hemisphere (SH) polar stratosphere remains unclear. After selecting the stratospheric polar vortex warming (SPVW) events, we reveal the two-way connection between BSISO and SPVW. Before the SPVW, the significant increase in occurrence and amplitude of BSISO phase 5 (P5) suggests that BSISO P5 can be regarded as a precursor to the weakened polar vortex. After the SPVW, the significant increase in occurrence and amplitude of BSISO phase 6 (P6) establishes that environmental alteration in tropical upper troposphere induced by the SPVW provides a favorable condition for the growth of ensuing convective activity. The results here help us better understand the potential link between stratospheric polar vortex and tropical intraseasonal oscillation.

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.

Change of the CP ENSO’s role in the occurrence frequency of Arctic daily warming events triggered by Atlantic storms

Arctic daily warming is gradually garnering the attention of academics. Here we discuss an interdecadal change around the mid-1980s in the role of the Central Pacific El Niño-Southern Oscillation (CP ENSO) in the occurrence frequency of Arctic daily warming events triggered by Atlantic storms in winter (called the Atlantic pattern-Arctic rapid tropospheric daily warming (A-RTDW) event) and the possible mechanism. Before the mid-1980s, the Central Pacific El Niño/La Niña events weakened/strengthened the Iceland Low (IL); the resulting anomalous northerly/southerly at the east of the IL prevented/favored the A-RTDW event occurrence by leading Atlantic storms away from/into the Arctic. Thus, the CP ENSO could affect the occurrence frequency of A-RTDW events by the CP ENSO/IL teleconnection. In contrast, this role hardly exists after the mid-1980s. Before the mid-1980s, the CP ENSO could affect the polar vortex by planetary wave propagation upwards into the stratosphere to create the CP ENSO/IL teleconnection; thereby, the CP ENSO and A-RTDW could establish a connection. However, after the mid-1980s, the planetary wave associated with CP ENSO could not propagate upwards into the stratosphere; thus, the ENSO/IL teleconnection disappears, resulting in CP ENSO having no effect on the occurrence frequency of A-RTDW events.

Influence of Preconditioned Stratospheric State on the Surface Response to Displacement and Split Sudden Stratospheric Warmings

AbstractSudden stratospheric warmings (SSWs) can affect tropospheric weather and climate, while the factors affecting the surface response to SSWs are not fully understood. This study shows the important effects of preconditioned stratospheric state on the surface response to displacement and split SSWs using ERA5 reanalysis and historical simulations from 12 Coupled Model Intercomparison Project 6 (CMIP6) models. Both ERA5 and CMIP6 models show that preconditioned stratospheric signals (e.g., positive meridional potential vorticity gradient anomaly in the upper stratosphere) can promote the upward propagation of stratospheric planetary waves by decreasing the negative refractive index frequency before SSWs. The modulation of strong stratospheric preconditions causes larger upper‐stratospheric anomalies for subsequent SSWs and prominent barotropic downward impacts for split SSWs. Meanwhile, the strong preconditioned signals affect the preexisting tropospheric variability, masking the downward impacts of displacement SSWs. The surface response is thus weaker than that to displacement SSWs with weak preconditions.

Impacts of Quasi-Biennial Oscillation and El Niño–Southern Oscillation on Stratospheric Isentropic Mixing Process

The present study investigates the influences of stratospheric quasi-biennial oscillation (QBO) and El Niño–Southern Oscillation (ENSO) on the intensity of stratospheric isentropic mixing based on ERA-Interim and MERRA-2 reanalysis products. It is found that isentropic mixing in the stratosphere is modulated by QBO and ENSO. An analysis of the QBO basis function in the multiple regression model reveals that isentropic mixing in the lower stratosphere is suppressed in the equatorial region in the WQBO phase, while the mixing enhances in the subtropical and mid-latitude regions. This result is not consistent with the Holton–Tan mechanism. However, isentropic mixing in the mid-latitudes becomes stronger in the middle stratosphere in the EQBO phase, which agrees well with the Holton–Tan effect. Composite analysis indicates that QBO-induced changes in the direction and speed of the stratospheric zonal wind can affect wave propagation and wave breaking. In the WQBO phase, zonal wind weakens, and a planetary wave is anomalously converging near 30°N, which leads to an increase in isentropic mixing; on the contrary, wind speed becomes large, and the upward propagation of planetary wave divergence, which lead to the isentropic mixing, becomes weak near 60°N. In the EQBO phase, the wind is relatively weak around 60°N, and the isentropic mixing is strong. Multiple regression analysis reveals the ENSO impact on the intensity of isentropic mixing, which shows weak mixing in the middle and high latitudes and strong mixing in the low latitudes of the lower stratosphere in the El Niño years. In the middle stratosphere, isentropic mixing enhances in the mid-latitude region due to intensified upward propagation of planetary waves but weakens in the polar region. Composite analysis reveals a clear relationship between the mixing strength zones of the El Niño and La Niña years with the position of the polar jet and changes in zonal wind speed.

ENSO teleconnections in terms of non-NAO and NAO atmospheric variability

The validity of the long-held understanding or assumption that El Niño-Southern Oscillation (ENSO) has a remote influence on the North Atlantic Oscillation (NAO) in the January–February–March (JFM) months has been questioned recently. We examine this claim further using atmospheric data filtered to separate the variability orthogonal and parallel to NAO. This decomposition of the atmospheric fields is based on the Principal Component/Empirical Orthogonal Function method whereby the leading mode of the sea-level pressure in the North Atlantic sector is recognised as the NAO, while the remaining variability is orthogonal (unrelated) to NAO. Composite analyses indicate that ENSO has statistically significant links with both the non-NAO and NAO variability at various atmospheric levels. Additional bootstrap tests carried out to quantify the uncertainty and statistical significance confirm these relationships. Consistent with previous studies, we find that an ENSO teleconnection in the NAO-related variability is characterised by lower-stratospheric eddy heat flux anomalies (related to the vertical propagation of planetary waves) which appear in November–December and strengthen through JFM. Under El Niño (La Niña), there is constructive (destructive) interference of anomalous eddy heat flux with the climatological pattern, enhancing (reducing) fluxes over the northern Pacific and Barents Sea areas. We further show that the teleconnection of extreme El Niño is essentially a non-NAO phenomenon. Some non-linearity of the teleconnections is suggested, with El Niño including more NAO-related variability than La Niña, but the statistical significance is degraded due to weaker signals and smaller sample sizes after the partitioning. Our findings have implications for the general understanding of the nature of ENSO teleconnections over the North Atlantic, as well as for refining methods to characterise and evaluate them in models.

Волновая активность мезосферы в диапазоне планетарных волн по наблюдениям эмиссии ОН (3-1) на станциях Маймага и Тикси за период 2015–2020 гг.

The article compares the interannual variability of the atmosphere at the OH glow height, which can be associated with planetary wave propagation, at stations spaced in latitude. As a characteristic reflecting planetary wave activity we consider standard deviations of the average overnight temperature σpw from its monthly average after taking into account the seasonal variation. Joint mesopause temperature measurements at high latitudes at two optical stations Maimaga (63.04° N, 129.51° E) and Tiksi (71.58° N, 128.77° E) began in 2015. The stations are equipped with identical Shamrock (Andor) high image quality infrared spectrographs for registration of OH (3-1) in the near infrared region (~1.5 μm). The main result of studying the planetary wave activity during the 5-year period of simultaneous observations is that at Tiksi station it slightly (by about 1–2 K) exceeds that at Maimaga station. In average annual activity fluctuations, the presence of quasi-biennial oscillations is traced.

Wave activity of the mesosphere in the planetary wave range according to OH (3-1) emission observations at Maimaga and Tiksi stations for 2015–2020

The article compares the interannual variability of the atmosphere at the OH glow height, which can be associated with planetary wave propagation, at stations spaced in latitude. As a characteristic reflecting planetary wave activity we consider standard deviations of the average overnight temperature σpw from its monthly average after taking into account the seasonal variation. Joint mesopause temperature measurements at high latitudes at two optical stations Maimaga (63.04° N, 129.51° E) and Tiksi (71.58° N, 128.77° E) began in 2015. The stations are equipped with identical Shamrock (Andor) high image quality infrared spectrographs for registration of OH (3-1) in the near infrared region (~1.5 μm). The main result of studying the planetary wave activity during the 5-year period of simultaneous observations is that at Tiksi station it slightly (by about 1–2 K) exceeds that at Maimaga station. In average annual activity fluctuations, the presence of quasi-biennial oscillations is traced.

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.

The Delayed Response of the Troposphere‐Stratosphere‐Mesosphere Coupling to the 2019 Southern SSW

AbstractA strong Southern Hemisphere (SH) sudden stratospheric warming event occurred in September 2019 and significantly weakened the stratospheric polar vortex. Due to the positive zonal wind anomalies in the troposphere, the barotropic/baroclinic instability, primarily controlled by the horizontal/vertical wind shear, weakened in the upper troposphere at midlatitudes in late September and early October. As a result, planetary waves (PWs) were deflected equatorward near the tropopause rather than upward into the stratosphere, resulting in less perturbation to the stratospheric polar vortex. After October 15, the westward zonal wind anomalies propagate downward and reach the troposphere, increasing the tropospheric barotropic/baroclinic instability. This benefits the propagation of PWs into the stratosphere, leading to the early breaking of the stratospheric polar vortex. In turn, the SH mesosphere becomes anomalously cold due to the stratospheric wind filtering on the gravity waves, leading to the much earlier onset of SH polar mesospheric clouds.

Link between Arctic ozone and the stratospheric polar vortex

The stratospheric ozone layer protects life on earth by preventing solar ultraviolet radiation from reaching the surface. Owing to the large population in the Northern Hemisphere and extreme ozone loss in the Arctic, changes in Arctic stratospheric ozone (ASO) and their causes have attracted broad attention recently. Using monthly mean data during the period 1980–2020 from MERRA-2, the relationship between the stratospheric polar vortex (SPV) and ASO, along with the relative contributions of chemical and dynamic processes associated with the SPV to changes in ASO, were examined in this study. Results showed that the ASO in March has a strong out-of-phase link with the strength of the SPV in March, with no obvious lead–lag correlations, i.e., an increase (decrease) in ASO corresponds to a weakened (strengthened) SPV. Further analysis suggested that the strong out-of-phase link between the SPV and ASO is related to changes in Brewer–Dobson circulation (BDC). Strong SPV events, accompanied by a low temperature condition and weakened upward propagation of planetary waves over the Arctic in the stratosphere, result in weakened BDC. The weakened downwelling at high latitudes tends to transport less ozone-rich air in the upper stratosphere at lower latitudes into the lower stratosphere at high latitudes, facilitating a decrease in ASO. The BDC's vertical velocity plays the dominant role in modulating ASO.摘要利用1980–2020年MERRA-2资料, 分析了平流层极涡 (Stratospheric polar vortex, SPV) 和北极臭氧 (Arctic stratospheric ozone, ASO) 的关系, 评估了与SPV相关的化学, 动力过程在其中的相对作用. 结果表明, 3月份ASO与同期SPV强度反相关最大. SPV-ASO二者反相关与平流层剩余环流 (Brewer-Dobson circulation, BDC) 变化密切相关. 强SPV伴随的北极平流层低温条件和行星波向上传播减弱, 导致BDC减弱, 减弱的BDC下沉支将低纬度平流层上层臭氧含量较低的空气输送到北极平流层低层, 从而导致ASO减少. BDC垂直速度在其中起主导作用.

Contributions of Early‐ and Middle‐Winter Perturbations at Higher Altitudes to Late‐Winter Anomalously Strong Arctic Polar Vortex in the Lower Stratosphere

AbstractBased on the Modern‐Era Retrospective analysis for Research Applications Version 2 reanalysis, an upper stratosphere/lower mesosphere (USLM) disturbance in early January and a sudden stratospheric warming (SSW) in early February are both coincidentally captured preceding the anomalously strong polar vortex during the late boreal winters of 2010/2011 and 2019/2020. Our analysis indicates that planetary wave breaking during the USLM disturbance and SSW decelerates the westerly polar night jet (PNJ) in the upper stratosphere, which leads to a continuous downward and poleward propagation of the PNJ during the early and middle winter. As the peak height of the PNJ descends, the region with negative vertical wind shear and negative waveguides also extends downward and reaches an altitude of ∼40 km during the late winter, which significantly limits the upward propagation of planetary waves, inducing a stable polar vortex in the lower stratosphere. In addition, the transformed Eulerian mean diagnostic results based on the specified dynamics simulation of the Whole Atmosphere Community Climate Model eXtended version confirm that the planetary wave‐induced downwelling is the main cause of the stratospheric warming during the two perturbations. More importantly, the simulation results indicate that the strength of the two upper stratospheric perturbations is appropriate to maintain the reflective configuration during the late winter, which suggests that such a specific sequence of perturbations during the early and middle winter, namely a USLM disturbance followed by a minor SSW, is favorable for the development of a late‐winter unusually strong Arctic polar vortex in the lower stratosphere.

Relations of Enhanced High‐Latitude Concurrent Blockings With Recent Warm Arctic‐Cold Continent Patterns

AbstractA warm Arctic‐cold continent (WACC) pattern has been one of the main characteristics of the Northern Hemisphere (NH) climate variability in winter during the last two decades. However, the factors contributing this pattern remain unclear. We compared the two leading modes of surface temperature in the NH winter on an interdecadal timescale and explored the role of high‐latitude concurrent blockings (HCBs) and the Arctic oscillation (AO) in these two modes. Results show that the first mode resembling the WACC pattern is more related to enhanced HCBs over the Ural Mountains and the North Pacific than the AO. The HCBs induce a warmer Arctic by simultaneously transporting large amounts of moisture and heat. The HCBs excite a tropospheric anticyclonic anomaly near the Arctic Circle, which, in turn, triggers anomalies in the propagation of planetary waves and the meridional circulation, with a subsequent redistribution of momentum and heat. This is accompanied by a weak polar night jet and a poleward shift in the subtropical westerly jet, resulting in mild cold mid‐latitude continents. The second mode represents the AO pattern, which has a different mechanism from WACC. The high‐pressure anomaly over the North Pacific has played an increasingly important part in the WACC mode in recent decades and the WACC pattern with HCBs is more easily reproduced in simulations with human activity. HCBs therefore require further consideration under the current and future conditions of global warming.

The Role of Mid‐latitude Westerly Jet in the Impacts of November Ural Blocking on Early‐Winter Warmer Arctic‐Colder Eurasia Pattern

AbstractBased on statistical analysis using observations and idealized model simulations, previous studies have revealed the potential response of early‐winter atmospheric circulation and temperature anomalies to November Ural blocking (UB) anomalies. Using a large number of coupled simulations, this study found that the response is sensitive to the intensity of November mid‐latitude westerly jet over Eurasia. Stronger‐than‐normal November UB without a significantly weakened westerly jet could not cause significant atmospheric response in early‐winter. By contrast, stronger‐than‐normal November UB with a significantly weakened jet would be followed by a warmer Arctic‐colder Eurasia (WACE) pattern in December. The significantly weakened westerly jet favors stronger upward propagation of planetary waves, which causes stronger weakening and longer persistence of the stratospheric polar vortex. This stratospheric response persists into December and propagates downward into the troposphere interfering with planetary waves (especially wavenumber‐1). The lead‐lag UB‐WACE linkage modulated by mid‐latitude jet may have implications for sub‐seasonal predictability.

Relative Effects of the Greenhouse Gases and Stratospheric Ozone Increases on Temperature and Circulation in the Stratosphere over the Arctic

Using a stratosphere-resolving general circulation model, the relative effects of stratospheric ozone and greenhouse gases (GHGs) increase on the temperature and circulation in the Arctic stratosphere are examined. Results show that stratospheric ozone or GHGs increase alone could result in a cooling and strengthening extratropical stratosphere during February, March and April. However, the contribution of stratospheric ozone increases alone on the cooling and strengthening Arctic stratosphere is approximately 2 fold that of the GHGs increase alone. Model simulations suggested that the larger responses of the Arctic stratosphere to the ozone increase alone are closely related to the wave fluxes in the stratosphere, rather than the wave activity in the stratosphere. In response to the ozone increase, the vertical propagation of planetary waves from the troposphere into the mid-latitude stratosphere weakens, mainly contributed by its wavenumber-1 component. The impeded planetary waves tend to result from the larger zonal wind shear and vertical gradient of the buoyancy frequency. The magnitudes of anomalies in the zonal wind shear and buoyancy frequency in response to GHGs increase alone are smaller than in response to the ozone increase, which is in accordance with the larger contribution of stratospheric ozone to the temperature and circulation in the Arctic stratosphere.