AS1.5

The connection between the polar stratospheric vortex and the vertical component of the Eliassen–Palm flux in the lower stratosphere and upper troposphere is examined in model level data from ERA5. The particular focus of this work is on the conditions that lead to upward wave propagation between the tropopause and the bottom of the vortex near 100 hPa. The ability of four different versions of the index of refraction to capture this wave propagation is evaluated. The original Charney and Drazin index of refraction includes terms ignored by Matsuno that are shown to be critical for understanding upward wave propagation just above the tropopause both in the climatology and during extreme heat flux events. By adding these terms to the Matsuno index of refraction, it is possible to construct a useful tool that describes wave flux immediately above the tropopause and at the same time also describes the role of meridional variations within the stratosphere. It is shown that a stronger tropopause inversion layer tends to restrict upward wave propagation. It is also shown that while only 38% of extreme wave-1 Eliassen–Palm flux vertical component (F_z) at 100 hPa events are preceded by extreme F_z at 300 hPa, there are almost no extreme events at 100 hPa in which the anomaly at 300 hPa is of opposite sign or very weak. Overall, wave propagation near the tropopause is sensitive to vertical gradients in buoyancy frequency, and these vertical gradients may not be accurately captured in models or reanalysis products with lower vertical resolutions.

To better understand the role of the TIL for transmission and reflection of waves,  an analytical quasi-geostrophic planetary scale model is used to examine the role of the tropopause inversion layer (TIL) in wave propagation and reflection. The model consists of three different layers: troposphere, TIL and stratosphere. It is shown that a larger buoyancy frequency in the TIL leads to weaker upward transmission to the stratosphere and enhanced reflection back to the troposphere, and thus reflection of wave packets is sensitive not just to the zonal wind but also to the TIL’s buoyancy frequency. The vertical-zonal cross section of a wavepacket for a more prominent TIL in the analytical model is similar to the corresponding wavepacket for observational events in which the wave amplitude decays rapidly just above the tropopause. Similarly, a less prominent TIL both in the model and in reanalysis data is associated with enhanced wave transmission and a non-detectable change in wave phase above the tropopause. Models
with a poor representation of the TIL will necessarily miss all of these effects.

• Weinberger, I., C.I. Garfinkel, I.P White, and T. Birner (2021), The Efficiency of Upward Wave Propagation Near the Tropopause: importance of the form of the refractive index, JAS, https://doi.org/10.1175/JAS-D-20-0267.1.
• Weinberger, I., C.I. Garfinkel, N. Harnik, N. Paldor (under review)  Transmission and reflection of upward propagating Rossby waves in the lowermost stratosphere: Importance of the Tropopause Inversion Layer, JAS

How to cite: Garfinkel, C. and Weinberger, I.: The Efficiency of Upward Wave Propagation Near the Tropopause and Reflection from the TIL: importance of the form of the refractive index, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1022, https://doi.org/10.5194/egusphere-egu22-1022, 2022.

Stratospheric dynamics have predictive skills on a subseasonal timescale for troposphere synoptic processes, which plays a crucial role in the “seamless” forecasting approach. Therefore, predicting the state of the stratospheric polar vortex (SPV) is one of the top priority tasks for modern meteorology.

Early research showed that the intensity of the vertical propagation of wave 1 over Eastern Siberia could be a predictor for an extremely strong/weak SPV in the next month during the winter season. However, this connection does not always exist. During the negative phase of the Pacific Decadal Oscillation (PDO), 70% of the variability of the SPV intensity is explained by the dynamics of wave 1 in the previous month, and during the positive phase of the PDO, there is no statistically significant connection between them. It can be concluded that the nature of the spatial propagation of planetary waves differs in different phases of the PDO.

The work aimed to confirm the effect of large-scale SST anomalies on planetary waves propagation using numerical experiments with ISCA model and to prove results of observational analysis based on JRA-55 data that showed that wave 1 is more “stationary” during the negative PDO phases than during the positive ones. Distributions of the wave 1 ridges’ location for different PDO phases are significantly different at the 8% level according to Student’s t-test.

We analyzed the differences in the vertical components of the Plumb flux for isolated large-scale SST anomalies condition corresponding to the main modes of SST variability, such as PDO, El-Nino Southern Oscillation (ENSO), and for SST anomalies in the Kara-Barents Seas region. The experiments with combined conditions were carried out as well.

How to cite: Sobaeva, D., Zyulyaeva, Y., and Gulev, S.: The effect of SST anomalies on planetary waves dynamics: numerical experiments with ISCA, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-340, https://doi.org/10.5194/egusphere-egu22-340, 2022.

The role of the midlatitude cyclone on the onset of January 2021 sudden stratospheric warming (SSW) is examined by conducting a set of numerical model experiments. The control simulation initialized on 26^th December 2020, 10 days before the SSW onset, successfully reproduces the spatio-temporal evolution of SSW. Since this event is preceded by the developing cyclone over the North Pacific, its impact is tested by initializing the model without cyclonic anomaly, over the North Pacific (20°–80°N, 110°E–160°W) from 1000 hPa to 150 hPa. The potential vorticity inversion technique is used to modify the initial condition. This perturbed simulation shows much weaker polar-vortex deceleration than the control simulation resulting in no distinct SSW onset. Such a difference is attributable to the fact that constructive linear interference between the climatological wave and the North Pacific cyclone is reduced in the perturbed simulation. It weakens the upward propagation of wavenumber one into the stratosphere, thereby reducing the breaking of the planetary-scale waves in the polar stratosphere.

How to cite: Cho, H.-O., Kang, M.-J., Son, S.-W., Hong, D.-C., and Kang, J. M.: Impact of the extratropical cyclone over the North Pacific on the onset of Sudden Stratospheric Warming: A case study of 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-493, https://doi.org/10.5194/egusphere-egu22-493, 2022.

The quasi-biennial oscillation (QBO), describing alternate easterly and westerly winds in the tropical stratosphere, originally shows downward phase propagation with time. However, in February 2016 and January 2020, downward-propagating westerly winds were split into two with one propagating upward and the other propagating downward, so-called a QBO disruption. Previous studies have mainly focused on the cause of the localized negative momentum forcing initiating the QBO disruption. However, the upward displacement of the westerly QBO followed by the negative momentum forcing, clearly seen in 2015/16 but not in 2019/20, has not been investigated in detail. Here, we show that the distinct upward propagation of the westerly winds in 2015/16 can be explained by the stronger Brewer-Dobson circulation (BDC) using MERRA-2 global reanalysis data. We found that strong Rossby waves with wavenumbers 1 and 2 propagating from the troposphere mainly induce the strong BDC in 2015/16. Potential contributions of El Niño and Barents–Kara sea ice reduction to wavenumber 1–2 Rossby waves are also discussed.

How to cite: Kang, M.-J., Son, S.-W., and Chun, H.-Y.: Distinct Upward Propagation of the Westerly QBO in Winter 2015/16 Compared to 2019/20 and its Relationship with Brewer-Dobson Circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6662, https://doi.org/10.5194/egusphere-egu22-6662, 2022.

The tropical tropopause layer (TTL) is an important region where air enters from the tropical troposphere to the stratosphere. The cold point tropopause (CPT) within the TTL determines how much water vapor can enter the tropical stratosphere. The water vapor will then be transported to higher latitudes via the Brewer-Dobson circulation and further influences the stratospheric chemistry and the radiation budget around the globe. 
A dominant mode of variability in the tropical stratosphere – the quasi-biennial oscillation (QBO) – can influence the TTL and related processes through thermal wind balance and secondary circulation. The QBO consists of downward propagating easterlies and westerlies, alternating with a period of about 27–28 months. But twice since its discovery – first in 2015/16 and then again in 2019/20 – the QBO was disrupted — both in the past decade. During these anomalous years, easterlies developed at around 40–50 hPa within the westerly regime, while the westerly regime ascended and halted for about 6 months. There was also stronger tropical upwelling during QBO disruptions that favored the development of anomalous easterly wind. 
Here we focus on how the QBO disruptions can influence the TTL structure and water vapor using GPS-RO data, MLS observations, and ERA-5 reanalysis. We analyze temperature, water vapor, and tropical upwelling fields between QBO disruptions and the westerly QBO composite. We find there tends to be a colder zonal mean CPT temperature but relatively more water vapor during QBO disruptions. The increased water vapor relates to the regional pattern of the CPT temperature. During QBO disruptions, CPT temperature tends to be warmer over the western Pacific and colder over the eastern Pacific where the western Pacific is usually called the “cold trap” region and the air gets final dehydrated. Since both tropical and extratropical waves can influence the QBO and the tropical upwelling, we also investigate the roles of waves during QBO disruptions by analyzing the EP flux and its divergence and the momentum equation. We find that tropical waves and midlatitude Rossby waves both influence the zonal wind in the tropical lower stratosphere, but the stronger tropical upwelling is mainly caused by the midlatitude Rossby waves. Studying the influences of QBO on the TTL structure and roles of waves during QBO disruptions sheds light on a better understanding of the mechanisms causing QBO disruptions and their potential influences on the climate.

How to cite: Luan, L., Staten, P., Randel, W., and Kuo, Y.-H.: Tropical tropopause layer structure during QBO disruptions and the roles of waves, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3329, https://doi.org/10.5194/egusphere-egu22-3329, 2022.

The dynamical mechanism by which the quasi-biennial oscillation (QBO) might influence the temperature anomaly, associated with the Madden-Julian oscillation (MJO), in the equatorial upper troposphere and lower stratosphere (UTLS) is examined by conducting a series of initial-value experiments using a dry primitive equation model. The observed temperature response to the MJO convection becomes colder and more in-phase with the convection during easterly QBO (EQBO) than westerly QBO (WQBO) phases. This QBO-dependent MJO temperature anomaly in the UTLS is qualitatively reproduced by model experiments in which EQBO or WQBO background state is artificially imposed above 250 hPa while leaving the troposphere unaltered. As in the observations, the cold anomaly in the UTLS becomes strengthened and steepened with EQBO-like background state than WQBO-like one. It turns out that the QBO zonal wind, instead of temperature, plays a major role in determining the UTLS temperature anomaly by modulating wave energy dispersion.

How to cite: Lim, Y. and Son, S.-W.: QBO wind influence on MJO-induced temperature anomalies in the upper troposphere and lower stratosphere in an idealized model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3314, https://doi.org/10.5194/egusphere-egu22-3314, 2022.

The impact of the quasi-biennial oscillation (QBO) on the surface air temperature in the Northern Hemisphere extratropics is investigated. It is found that the QBO, defined as 70-hPa zonal wind in the deep tropics, is negatively correlated with the surface air temperature over the western North Pacific in February and March. Cold temperature anomaly appears during the QBO westerly phase. Such relationship is likely mediated by the subtropical jet. During the QBO westerly phase, a horseshoe-shaped zonal wind anomaly forms in the upper troposphere and lower stratosphere and is connected to the equatorward shift of the Asia-Pacific jet. This equatorward jet shift is accompanied by a cyclonic circulation anomaly in the subtropical North Pacific and an anticyclonic circulation anomaly over northern Eurasia in the troposphere. The resultant temperature advection brings cold air to East Asia and the western North Pacific. This regional downward coupling in February and March, which is not sensitive to El Niño-Southern Oscillation, has become statistically significant in recent decades.

How to cite: Park, C.-H., Son, S.-W., Lim, Y., and Choi, J.: QBO-related Surface Air Temperature Change over the Western North Pacific in Late Winter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-236, https://doi.org/10.5194/egusphere-egu22-236, 2022.

In subseasonal-to-seasonal (S2S) prediction systems, strong vortex events are found to be more predictable than sudden stratospheric warming (SSW) events. The reason for this difference in predictability between different types of events is however not resolved. To investigate this question using a larger sample size, we extend the definition of strong vortex and SSW events to wind acceleration and deceleration events due to their similar dynamics. Specifically, we use the zonal mean zonal wind at 60°N, 10hPa from ERA-interim reanalysis for the winters of 1998/99 to 2017/18 to identify wind acceleration and deceleration events, which are defined as a wind change over a 10-day window. We then assess the predictability of the identified events using the ECMWF S2S hindcasts. It is found that wind acceleration events are more predictable than deceleration events. However, when expressing the predictability of deceleration and acceleration events as a function of event magnitude, they qualitatively exhibit the same predictability behaviour; that is, events of stronger magnitude are less predictable. We explain the observed predictability dependence from two perspectives: 1) In a statistical sense, strong magnitude events lie within the tails of the climatological distribution and thus are penalised more heavily than weak magnitude events, and 2) from a dynamical perspective, extreme stratospheric events are associated with strong anomalies in precursors such as wave activity and vortex background state, and are  therefore often associated with large ensemble spread and large uncertainties. In particular, the magnitude of extremely strong wave activity is underestimated in the model for strong deceleration events. Therefore, we suggest the observed predictability difference between event types can to a large extent be explained by the difference in event magnitude between event types, i.e. the fact that wind deceleration events are associated with greater magnitudes than wind acceleration events, and that SSW events are stronger in magnitude than strong vortex events. We also suggest that a better representation of extremely strong wave activity in the prediction system can enhance the predictability of stratospheric extreme events, and by extension their impacts on surface weather and climate.

How to cite: Wu, R. W.-Y., Wu, Z., and Domeisen, D. I. V.: Understanding the Differences in the Sub-seasonal Predictability of Stratospheric Extreme Events, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-253, https://doi.org/10.5194/egusphere-egu22-253, 2022.

Modes of climate variability that remotely alter the northern hemisphere stratospheric polar vortex state are explored using the Hadley Centre Climate Model (HadGEM3). Experiments are performed that sample combinations of El Niño—Southern Oscillation (ENSO) and quasi-biennial oscillation (QBO) states. These modes were chosen as El Niño and QBO easterly phases are known to weaken the polar vortex.

The El Niño induced weakening of the polar vortex is found to be more pronounced during QBO easterly than QBO westerly. Likewise, the polar vortex weakening caused by QBO easterly is stronger during El Niño than during neutral ENSO conditions.

It is also found that El Niño induces a change to the QBO itself, namely an increase in the descent rate of the QBO, but this is not large enough to explain the nonlinear response of the polar vortex. Other possible mechanisms are investigated, such as whether the QBO and ENSO teleconnections to the polar vortex are sensitive to the prior state of the polar vortex. Impacts of this nonlinearity on the surface response are also explored.

How to cite: Walsh, A., Screen, J., Scaife, A., and Smith, D.: Non-linearity in the extratropical teleconnection to ENSO and the QBO, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7446, https://doi.org/10.5194/egusphere-egu22-7446, 2022.