AS1.5

AS1 EDI
Stratospheric dynamics 

The variability in the stratosphere is important for many atmospheric phenomena. Examples include the dynamical two-way coupling between the stratosphere and troposphere, the transport of trace gases through the meridional circulation of the stratosphere, or the connection between the Quasi-Biennial Oscillation of the tropical stratosphere and the Madden-Julian Oscillation. This session is interested in the causes for and consequences of stratospheric circulation variability, including the mechanisms behind the vertical coupling between the stratosphere and troposphere in tropics and extratropics, and the role of stratospheric dynamics for short-term atmospheric weather, sub-seasonal to seasonal predictability, and long-term climate variability. We welcome abstracts that study these problems from observational, modelling, or theoretical viewpoints on all temporal and spatial scales.

Convener: Thomas Reichler | Co-conveners: Blanca Ayarzagüena, Bo Christiansen, Seok-Woo Son, Zheng WuECSECS
Presentations
| Wed, 25 May, 15:55–18:29 (CEST)
 
Room 0.31/32

Presentations: Wed, 25 May | Room 0.31/32

Chairpersons: Thomas Reichler, Bo Christiansen, Zheng Wu
15:55–15:56
Waves
15:56–16:02
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EGU22-4113
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ECS
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Presentation form not yet defined
Wave geometry of the stratospheric polar vortex during extreme and moderate El Niño events
(withdrawn)
Xin Zhou, Quanliang Chen, and Zhaolu Hou
16:02–16:08
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EGU22-1022
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Virtual presentation
Chaim Garfinkel and Israel Weinberger

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 (Fz) at 100 hPa events are preceded by extreme Fz 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.

16:08–16:14
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EGU22-340
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ECS
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Virtual presentation
Daria Sobaeva, Yulia Zyulyaeva, and Sergey Gulev

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.

16:14–16:20
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EGU22-200
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ECS
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On-site presentation
Wolfgang Wicker, Inna Polichtchouk, and Daniela Domeisen

Sudden stratospheric warmings (SSW) are major weather events in the stratosphere with a long-lasting impact on tropospheric weather conditions and, thus, offer a great potential to extend the predictability of surface weather on subseasonal time scales. However, underestimating the warming signal in the stratosphere itself hinders prediction systems to exploit this source of tropospheric predictability. In this study, hindcast experiments with the ECMWF IFS model reveal sensitivity to vertical resolution both for the amplitude and the persistence of the stratospheric warming signal and the prediction skill of surface variables. A potential mechanism for the extended and strengthened warming in the stratosphere with higher vertical resolution are better resolved gravity waves that break in the proximity of the zero-wind line in the upper stratosphere. The enhanced gravity wave drag with higher vertical resolution increases positive temperature anomalies in the middle stratosphere, consistent with anomalous subsidence over the polar cap during the SSWs. Nudging experiments confirm that the enhanced gravity wave drag results directly from increased vertical resolution, as opposed to the modified background state, and that increased surface skill and longer predictable lead times are of stratospheric origin.

How to cite: Wicker, W., Polichtchouk, I., and Domeisen, D.: The influence of resolved gravity waves in the stratosphere for subseasonal hindcasts of the troposphere during SSW events, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-200, https://doi.org/10.5194/egusphere-egu22-200, 2022.

16:20–16:24
16:24–16:30
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EGU22-6370
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ECS
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Virtual presentation
Hilla Gerstman, Bernat Jimenez-Esteve, and Daniela I.V. Domeisen

Sudden stratospheric warmings (SSWs) are extreme stratospheric events which can be followed by a significant impact on surface weather. Roughly two thirds of the observed SSW events are followed by an equatorward shift of the tropospheric midlatitude jet in the North Atlantic, while a third of the events generally show a poleward jet shift. However, it is not yet resolved which factors lead to the large inter-event variability in the surface impact.

Here, the sensitivity of the North Atlantic jet response to stratospheric forcing is investigated using an intermediate complexity atmospheric model. We analyze the contribution of different stratospheric and tropospheric drivers for determining the downward response, focusing on persistent anomalies in the lower stratosphere, downstream influence from the northeastern Pacific, and local tropospheric conditions in the North Atlantic at the time of the initial response. Both the model and reanalysis show that most of the variance in the tropospheric jet response after SSW events can be explained by the lower stratospheric geopotential height anomalies. To isolate the role of the stratosphere from tropospheric variability, we use model runs where the zonal mean stratospheric winds are nudged towards climatology. When stratospheric variability is suppressed, the coupling between the North Atlantic and the northeastern Pacific is found to be weaker. 

These findings shed light on the relative contribution of the stratosphere and the troposphere to the diverse downward impacts of SSW events. The implications of these results for improved long-range prediction of tropospheric jet variability the North Atlantic will be discussed.

How to cite: Gerstman, H., Jimenez-Esteve, B., and Domeisen, D. I. V.: Evaluating the relative contribution of stratospheric and tropospheric drivers for the North Atlantic jet response after sudden stratospheric warmings, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6370, https://doi.org/10.5194/egusphere-egu22-6370, 2022.

16:30–16:36
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EGU22-11634
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Virtual presentation
Philip Rupp, Sheena Loeffel, Hella Garny, Xiaoyang Chen, Joaquim Pinto, and Thomas Birner

February-March 2020 was marked by highly anomalous large-scale circulations in the Northern extratropical troposphere and stratosphere. The Atlantic jet reached extreme strength, linked to some of the strongest and most persistent positive values of the Arctic Oscillation index on record, which provided conditions for extreme windstorms hitting Europe. Likewise, the stratospheric polar vortex reached extreme strength that persisted for an unusually long period. Past research indicated that such circulation extremes occurring throughout the troposphere-stratosphere system are dynamically coupled, although the nature of this coupling is still not fully understood and generally difficult to quantify. 

We employ sets of numerical ensemble simulations to statistically characterize the mutual coupling of the early 2020 extremes. We find the extreme vortex strength to be linked to the reflection of upward propagating planetary waves and the occurrence of this reflection to be sensitive to the details of the vortex structure. Our results show an overall robust coupling between tropospheric and stratospheric anomalies: ensemble members with polar vortex exceeding a certain strength tend to exhibit a stronger tropospheric jet and vice versa. Moreover, members exhibiting a breakdown of the stratospheric circulation (e.g. a sudden stratospheric warming) tend to lack periods of persistently enhanced tropospheric circulation. Despite indications for vertical coupling, our simulations underline the role of internal variability within each atmospheric layer. The circulation extremes during early 2020 may be viewed as resulting from a fortuitous alignment of dynamical evolutions within the troposphere and stratosphere, aided by each layer's modification of the other layer's boundary condition.

How to cite: Rupp, P., Loeffel, S., Garny, H., Chen, X., Pinto, J., and Birner, T.: Potential links between tropospheric and stratospheric circulation extremes during early 2020, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11634, https://doi.org/10.5194/egusphere-egu22-11634, 2022.

Coffee break
Chairpersons: Bo Christiansen, Zheng Wu, Thomas Reichler
17:00–17:06
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EGU22-493
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ECS
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Virtual presentation
Hyeong-Oh Cho, Min-Jee Kang, Seok-Woo Son, Dong-Chan Hong, and Joonsuk M. Kang

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 26th 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.

QBO
17:06–17:12
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EGU22-6662
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ECS
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Virtual presentation
Min-Jee Kang, Seok-Woo Son, and Hye-Yeong Chun

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.

17:12–17:18
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EGU22-3329
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ECS
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Virtual presentation
Lan Luan, Paul Staten, William Randel, and Ying-Hwa Kuo

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.

17:18–17:24
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EGU22-3314
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ECS
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Virtual presentation
Yuna Lim and Seok-Woo Son

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.

17:24–17:30
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EGU22-9785
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Virtual presentation
Lon Hood and Thomas Galarneau, Jr.

The tropical Madden-Julian oscillation (MJO) is the strongest of the intraseasonal climate oscillations.  It generates a Rossby wave train that can be associated with high-impact weather events at northern midlatitudes in winter and spring.  Here, we investigate using 41 years of ECMWF reanalysis data (1979-2019) why static stabilities in the tropical lower stratosphere are unusually low under easterly QBO and solar minimum conditions, leading to stronger MJO episodes.  Results indicate an important role for extratropical wave forcing events, including stratospheric warmings, occurring preferentially in late fall and early winter during QBOE and SMIN.  This increases the tropical upwelling rate beyond that caused by the QBO induced meridional circulation alone, further reducing lower stratospheric temperatures and static stability during northern winter.  In many but not all years, major sudden stratospheric warmings (SSWs) contribute significantly to the results obtained here.  Of the 11 clear QBOE years in the study period, six had SSWs in early winter prior to Jan. 15.  Of the 12 clear QBOW years, none had early winter SSWs while six had SSWs in late winter after Jan. 15.  There are two main implications of these results: (1) Observations of wave forcing and tropical static stabilities in late fall / early winter, combined with the known QBO and solar phases, may provide a means of projecting the likely strength of the MJO in a given winter; (2) A necessary prerequisite for a successful simulation of the QBO/solar - MJO connection in a global climate model may be the ability to simulate a preferred occurrence of extratropical wave forcing events, including SSWs, in early winter under QBOE and SMIN conditions. 

How to cite: Hood, L. and Galarneau, Jr., T.: QBO/Solar Modulation of the Boreal Winter Madden-Julian Oscillation: The Role of Extratropical Wave Forcing in Late Fall / Early Winter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9785, https://doi.org/10.5194/egusphere-egu22-9785, 2022.

17:30–17:36
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EGU22-236
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ECS
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Virtual presentation
Chang-Hyun Park, Seok-Woo Son, Yuna Lim, and Jung Choi

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.

17:36–17:41
17:41–17:47
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EGU22-8995
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ECS
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Virtual presentation
Jiyoung Oh, Seok-Woo Son, Jung Choi, Eun-Pa Lim, Chaim Carfinkel, Harry Hendon, Yoonjae Kim, and Hyun-Suk Kang

Antarctic ozone has been regarded as a major driver of the Southern Hemisphere (SH) circulation change in the recent past. Here, we show that Antarctic ozone can also affect the subseasonal-to-seasonal (S2S) prediction during the SH spring. Its impact is quantified by conducting two reforecast experiments with the Global Seasonal Forecasting System 5 (GloSea5). Both reforecasts are initialized on September 1st of each year from 2004 to 2020 but with different stratospheric ozone: one with climatological ozone and the other with year to-year varying ozone. The reforecast with climatological ozone, which is common in the operational S2S prediction, shows the skill re-emergence in October after a couple of weeks of no prediction skill in the troposphere. This skill re-emergence, mostly due to the stratosphere-troposphere dynamical coupling, becomes stronger in the reforecast with year to-year varying ozone. The surface prediction skill also increases over Australia. This result  suggests that a more realistic stratospheric ozone could lead to improved S2S prediction in  the SH spring.

How to cite: Oh, J., Son, S.-W., Choi, J., Lim, E.-P., Carfinkel, C., Hendon, H., Kim, Y., and Kang, H.-S.: Impact of Stratospheric Ozone on the Subseasonal Prediction in the Southern Hemisphere Spring, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8995, https://doi.org/10.5194/egusphere-egu22-8995, 2022.

17:47–17:53
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EGU22-253
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ECS
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On-site presentation
Rachel Wai-Ying Wu, Zheng Wu, and Daniela I.V. Domeisen

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.

17:53–17:55
17:55–18:01
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EGU22-9823
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Virtual presentation
Florian Ladstädter, Andrea K. Steiner, and Hans Gleisner

Historically, retrieving the detailed structure of atmospheric temperature trends from observations has been demanding. For decades, observations of upper-air temperature have either lacked the necessary vertical resolution, or the horizontal coverage. This has resulted in limited knowledge about the important transition zone around the tropopause. Recent advances in satellite measurement techniques provide new insight into the thermal structure of the upper troposphere/lower stratosphere region. This is a prerequisite for understanding the complex processes of this part of the atmosphere. With unprecedented resolution, latest climate observations from GPS Radio Occultation satellites reveal a significant warming of the atmosphere. The tropical upper troposphere has already warmed about 1 K in the 21st century alone, and the stratospheric trend structure indicates a possible change in stratospheric circulation.

How to cite: Ladstädter, F., Steiner, A. K., and Gleisner, H.: Observational evidence of large changes of Earth's atmospheric thermal structure in the 21st century, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9823, https://doi.org/10.5194/egusphere-egu22-9823, 2022.

18:01–18:07
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EGU22-874
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ECS
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On-site presentation
Radek Zajíček, Michal Kozubek, and Jan Laštovička

This study analyses long-term trends in temperature and wind climatology based on ERA5 data. We study climatology and trends separately for every decade from 1980 to 2020 and their changes during this period for winter (DJF for the NH and JJA for the SH) for 40–90°N/S . This study is focused on the pressure levels between 100–1 hPa, which essentially covers the whole stratosphere. We also analyze the impact of the sudden stratospheric warmings (SSW), North Atlantic Oscillation (NAO), El Nino Southern Oscillation (ENSO) and Quasi-biennial oscillation (QBO). This helps us to find details of climatology and trend behavior in the stratosphere in connection to these phenomena. ERA5 is one of the newest reanalysis, which is widely used for the middle atmosphere. We identify the largest differences which occur between 1990–2000 and 2000–2010 in both temperature climatology and trends. We suggest that these differences could relate to the different occurrence frequency of SSWs in 1990–2000 versus 2000–2010.

How to cite: Zajíček, R., Kozubek, M., and Laštovička, J.: Climatology and Long-Term Trends in the Stratospheric Temperature and Wind Using ERA5, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-874, https://doi.org/10.5194/egusphere-egu22-874, 2022.

18:07–18:09
18:09–18:15
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EGU22-6381
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On-site presentation
Priyanka Yadav, Daniela Domeisen, and Chaim Garfinkel

The sporadic nature of the Madden-Julian Oscillation (MJO) can influence the extratropical circulation response. However, there are differences in the extratropical response depending on the propagation speed of the MJO in the tropics. Here, we define slow (fast) MJO events as events that take more (less) than 20 (10) days to propagate from the Indian Ocean (phase 3) to the Pacific Ocean (phase 6). The slowly propagating MJO episodes lead to a positive North Atlantic Oscillation (NAO) response at a lag of 10 days following phase 4 of the MJO, whereas fast MJO episodes lead to a development of a positive NAO response 10-15 days following phase 2-3. The slowly propagating MJO episodes can lead to a stronger positive (negative) NAO response after a lag of 10 days following phase 4 (7-8).

In addition to this tropospheric pathway, the MJO can also impact the stratospheric circulation, which in turn can impact the NAO via downward coupling. The stronger impact on the NAO during slow MJO episodes suggests that the stratosphere plays a role in the teleconnection of the MJO to the North Atlantic region. This is evident from the zonal wind response within the stratospheric polar vortex at 60oN and 10hPa and the geopotential height response at 500 hPa and 100 hPa. In this study, we discuss the stratospheric pathways during fast and slow MJO episodes using ERA-Interim reanalysis with respect to the strength of the Northern Hemisphere stratospheric polar vortex and for stratosphere-troposphere coupling.

How to cite: Yadav, P., Domeisen, D., and Garfinkel, C.: The role of the stratosphere in tropical-extratropical interactions arising from slow MJO episodes., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6381, https://doi.org/10.5194/egusphere-egu22-6381, 2022.

18:15–18:21
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EGU22-7446
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ECS
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On-site presentation
Amber Walsh, James Screen, Adam Scaife, and Doug Smith

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.

18:21–18:23
18:23–18:29
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EGU22-10569
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On-site presentation
Froila M. Palmeiro, Javier García-Serrano, Mario Rodrigo, Marta Abalos, Bo Christiansen, and Shuting Yang

The aim of this study is to comprehensively assess the boreal winter climatology of the European Consortium Earth-system model (EC-EARTH), specifically the contributing version to CMIP6, v3.3. To identify model biases, the climatological stratospheric circulation of a 100-year long simulation with prescribed climatological boundary conditions and fixed radiative forcing, representative of present-day climate, is compared to reanalysis data. An important issue is found in the vertical distribution of stratospheric temperature from the tropics to mid-latitudes in EC-EARTH, which is seemingly linked to radiative processes of ozone, leading to a biased warm middle-upper stratosphere. Consistent with this bias, the Brewer-Dobson circulation at middle/lower levels is weaker than reanalysis while the polar vortex in EC-EARTH is stronger at the upper-stratosphere. The amplitude of Planetary waves is overall underestimated, but the magnitude of the background wave injection from the troposphere into the stratosphere is overestimated in relation to a weaker polar vortex at lower-stratospheric levels and thus less effective wave filtering. The overestimation of the background wave driving is maximum in early-winter and consistent with an increase of sudden stratospheric warmings at this time, as compared to reanalysis. When the wave injection climatology is decomposed spatially, a distinctive role of the planetary waves is revealed: while large-scale waves (wavenumbers 1-2) dominate the eddy heat flux over the North Pacific, small-scale waves (wavenumbers 3-4) are responsible for the doubled-lobe structure of the eddy heat flux over Eurasia. EC-EARTH properly simulates this climatological feature, although overestimates its amplitude over central Eurasia.

How to cite: Palmeiro, F. M., García-Serrano, J., Rodrigo, M., Abalos, M., Christiansen, B., and Yang, S.: Wintertime biases in the EC-EARTH stratosphere: CMIP6 version, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10569, https://doi.org/10.5194/egusphere-egu22-10569, 2022.