AS1.9

Rossby waves are able to communicate weather anomalies in one region to other regions. There anomalous weather events can follow if the wave is persistent and large amplitude. They can also be caused by breaking of the wave leading to blocking. The impact on the middle latitudes via stationary Rossby wave trains triggered by tropical convection anomalies has been of interest for many years. However, tropical convective events can also interact with higher latitude jet streams and the weather systems on them through a very different mechanism. In this talk, some examples will be given that indicate the flaring of tropical convection can lead to strong upper tropospheric outflows in which filaments of air with near equatorial values of PV interact with higher latitude jet streams and the weather systems on them.

How to cite: Hoskins, B.: Some aspects of Rossby waves and non-linear dynamics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16482, https://doi.org/10.5194/egusphere-egu21-16482, 2021.

In the summer of 2020, extreme fires have raged in northeastern Siberia, many of them within the Arctic Circle burning in ecotonal larch forest and tundra ecosystems. This unprecedented increase in fire activity within the Arctic Circle has been linked to record-high temperatures measured in the region, as well as to high lightning activity.

In mid-latitudes, the pronounced and long-lasting heatwaves of the last decade have been linked to amplified Rossby waves connected with weak atmospheric circulation. These amplified waves tend to phase-lock in preferred positions and thereby lead to more persistent summer weather. Linkages between atmospheric teleconnections and boreal wildfires exist for some regions, yet the influence of wave dynamics on arctic-boreal wildfires is unknown. We explored relationships between wave dynamics, heatwaves, and the unprecedented fire activity in Siberia in 2020 to assess whether the recent surge in arctic-boreal fires in Siberia is driven by large-scale atmospheric dynamics.

We determined wave amplitudes as phase positions by applying fast Fourier transformation on weekly averaged mid- to high-latitudinal mean meridional wind velocities at the 250 mb level from ERA5 reanalysis data. Gridded percentage area burned between 2001 and 2020 was derived from the Moderate Resolution Imaging Spectrometer (MODIS) Burned Area product (MCD64A1). We then quantified the importance of Rossby wave patterns on fire activity clustered by latitude in eastern Siberia.

How to cite: Scholten, R., Coumou, D., Luo, F., and Veraverbeke, S.: The role of atmospheric dynamics in extreme wildfire activity in northeastern Siberia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2006, https://doi.org/10.5194/egusphere-egu21-2006, 2021.

Several studies in recent years have highlighted the role of quasi stationary planetary and synoptic scale waves in the occurrence of extreme events such as heatwaves and cold spells. The advancement of our understanding of the dynamical properties and predictability of these events is however hindered by the poor statistics of extreme events in observations and numerical simulations. Recently we have shown how the problem of sampling extreme events in climate models can be tackled using rare event algorithms, numerical tools developed in statistical physics to reduce the computational effort required to sample rare events in dynamical systems. Here we study extreme warm summers and heatwaves over France and Scandinavia in present-day climate conditions, applying a rare event algorithm to ensemble simulations with the CESM1.2 general circulation model. The application of the rare event algorithm concentrates the ensemble members on dynamical trajectories leading to extreme seasonal and subseasonal temperatures for the target regions. In this way we generate samples of extreme heatwaves orders of magnitude larger than what is feasible with direct sampling, and we perform with high degree of precision composite and spectral analysis of dynamical quantities conditional on the occurrence of the extremes. We show how extreme warm summers and heatwaves are associated to low wavenumber hemispheric teleconnection patterns, and how the most extreme summers are related to the succession of rare subseasonal heatwaves. We then discuss the application of these methods to the detection, prediction and analysis of the dynamics of atmospheric Rossby waves related to the formation of extreme heatwaves and other time persistent extreme events.

How to cite: Ragone, F. and Bouchet, F.: Analysis of teleconnection patterns during extreme warm summers and heatwaves over Europe with a rare event algorithm, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14710, https://doi.org/10.5194/egusphere-egu21-14710, 2021.

Here, the linear relationship between European air stagnation and the large-scale circulation is explored across all seasons and during the 1979--2018 period. Dynamical based indices identifying atmospheric blocking, Rossby wave breaking, subtropical ridges, and the North Atlantic eddy-driven and subtropical jets are used to describe the large-scale circulation as predictors in a statistical model of air stagnation variability. It is found that the large-scale circulation can explain approximately 60% of the variance in monthly air stagnation in five distinct regions within Europe. The variance explained by the model does not vary strongly across regions and seasons. However, the dynamical indices most related to air stagnation do depend on region and season. The blocking and Rossby wave breaking predictors tend to be the most important for describing air stagnation variability in northern regions whereas ridges and the subtropical jet are more important to the south. The demonstrated correspondence between air stagnation and the large-scale circulation can be used to assess the representation of air stagnation in climate models, which is key for understanding how air quality and its associated health risks may change in the future.

How to cite: Maddison, J., Abalos, M., Barriopedro, D., Garcia Herrera, R., Garrido Pérez, J. M., and Ordóñez, C.: Linking air stagnation in Europe with the large-scale atmospheric circulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9465, https://doi.org/10.5194/egusphere-egu21-9465, 2021.

Extreme precipitation events (EPEs) frequently cause flooding with dramatic socioeconomic impacts in many parts of the world. Previous studies considered two synoptic-scale processes, Rossby wave breaking and intense moisture transport, typically in isolation, and their linkage to such EPEs in several regions. This study presents for the first time a global and systematic climatological analysis of these two synoptic-scale processes, in tandem and in isolation, for the occurrence of EPEs. To this end, we use 40-year ERA-Interim reanalysis data (1979-2018) and apply object-based identification methods for (i) daily EPEs, (ii) stratospheric potential vorticity (PV) streamers as indicators of Rossby wave breaking, and (iii) structures of high vertically integrated horizontal water vapor transport (IVT). First, the importance of these two synoptic-scale processes is demonstrated by case studies of previously documented flood events that inflicted catastrophic impacts in different parts of the world. Next, a climatological quantification shows that Rossby wave breaking is associated with > 90 % of EPEs near high topography and over the Mediterranean, whereas intense moisture transport is linked to > 95 % of EPEs over many coastal zones, consistent with findings of atmospheric river-related studies. Combined Rossby wave breaking and intense moisture transport contributes up to 70 % of EPEs in several subtropical and extratropical regions, including (semi)arid desert regions where tropical-extratropical interactions are of key importance for (heavy) rainfall. A detailed analysis shows that five categories with different combinations of wave breaking and intense moisture transport can reflect a large range of EPE-related weather systems across various climate zones. Odds ratios of EPEs linked to the two synoptic-scale processes suggest that intense moisture transport is stronger associated with the occurrence of EPEs than wave breaking. Furthermore, the relationship between the PV and IVT characteristics and the precipitation volumes shows that the depth of the wave breaking and moisture transport intensity are intimately connected with the extreme precipitation severity. Finally, composites reveal that subtropical and extratropical EPEs, linked to Rossby wave breaking, go along with the formation of upper-level troughs and cyclogenetic processes near the surface downstream, reduced static stability beneath the upper-level forcing (only over water), and dynamical lifting ahead (over water and land). This study concludes with a concept that reconciles well-established meteorological principles with the importance of Rossby wave breaking and intense moisture transport for extreme precipitation events. The findings of this study may contribute to an improved understanding of the atmospheric processes that lead to EPEs, and may find application in climatic studies on extreme precipitation changes in a warming climate.

How to cite: De Vries, A. J.: A global climatological perspective on the importance of Rossby wave breaking and intense moisture transport for extreme precipitation events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-288, https://doi.org/10.5194/egusphere-egu21-288, 2021.

This study highlights the relevance of North Atlantic SSTs and certain jet stream properties for the onset of high European temperatures by using the ERA-5/ERA20c reanalysis product and a targeted experiment with the OpenIFS model. We found that certain European heat wave events could be related to the simultaneous appearance of cold North Atlantic SST events, specific jet stream wave numbers and further to transient and recurrent Rossby wave activity.  

The coexistence of cold North Atlantic sea surface temperature (SST) and positive European surface temperature anomalies during several summer seasons, like in 1994, 2015 and 2018 motivated us to evaluate whether and how widespread and significant North Atlantic SST anomalies could be associated with European heat waves.Therefore we investigated the role of the jet stream in serving as a medium for a downstream signal propagation.  

A composite study reveals that cold North Atlantic SST anomalies in summer are accompanied by a more undulating jet stream and a preferred trough-ridge pattern in the North Atlantic-European sector. A  wave analysis covering two-dimensional probability density functions of phase speed and amplitude after compositing cold SSTs show that cold North Atlantic SST events reveal a preference for a dominance of transient waves. In the presence of a trough during cold North Atlantic events, we obtain a slow-down of the transient waves, but not necessarily an amplification or stationarity. The deceleration of the transient waves result in a longer duration of a trough over the North Atlantic accompanied by a ridge downstream over Europe, favouring the conditions for the onset of European heat episodes.

A study of the jet stream energetics via a kinetic energy power spectrum of meridional wind anomalies reveals that generally a trend shows up towards wave numbers 4 to 6. This is supported by an enhanced activity of specific wave numbers within this increased range during summer seasons of European heat wave events happening in the last two decades. An arising question poses whether the increased energy for a certain wave number originates from an SST forcing or different drivers. We investigate this by performing targeted OpenIFS model runs forced by different SST conditions.

How to cite: Krüger, J., Kjellsson, J., Pilch-Kedzierski, R., Bumke, K., and Matthes, K.: North Atlantic SST and jet stream anomalies related to European heat waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2054, https://doi.org/10.5194/egusphere-egu21-2054, 2021.

Rossby waves, found in the westerly flow at the upper troposphere, transfer energy, moisture, and momentum across large distances, being responsible for atmospheric teleconnections. Large-amplitude waves may contribute to rapid changes in wind and temperature, making them import for creating local temperature or precipitation extremes. Wirth et al (2018) separated Rossby waves into a low-frequency type, referred to as Rossby wave trains, and high-frequency, or synoptic, waves. In this work we explore a relative role of these two types in creating seasonal and synoptic temperature extremes in the midlatitudes.

We identify wave propagation regions at 300 hPa using ERA-Interim dataset for JFM 1980 – 2017. Our analysis is based on the daily data. This time scale allows identification of waveguides at a wide range of latitudes, suggesting possibility of Rossby wave propagation between midlatitudes and polar regions, as well as tropics. We show that winter temperature extremes in the midlatitudes are associated with anomalies in both high and low latitudes, while the relative importance of these areas differs across midlatitude regions. Furthermore, we demonstrate, that warm Arctic regions can create cold outbreaks in Siberia and North America.

Analysis of the evolution of midlatitude synoptic extremes reveals the importance of a pre-existing local temperature anomaly, that triggers amplification of large-scale Rossby wave trains and creates a local anomaly in the waveguide. The latter modifies propagation of synoptic scale Rossby waves that further amplify the local temperature anomaly.

References:

Wirth, V., M. Riemer, E. K. M. Chang, and O. Martius, 2018: Rossby Wave Packets on the Midlatitude Waveguide—A Review. Mon. Wea. Rev., 146, 1965–2001. https://doi.org/10.1175/MWR-D-16-0483.1.

How to cite: Rudeva, I. and Simmonds, I.: Roles of wave trains and synoptic Rossby waves in creating midlatitude temperature extremes during winter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6955, https://doi.org/10.5194/egusphere-egu21-6955, 2021.

Sudden stratospheric warmings (SSWs) are impressive phenomena that consist of a rapid stratospheric polar vortex breakdown. SSWs can have a strong impact on the tropospheric weather and are mainly associated with the negative phases of the Arctic and North Atlantic Oscillations (AO, NAO), and with northern European cold outbreaks, thus causing high societal impact. However, the mechanisms behind the downward impact from the stratosphere are insufficiently understood, especially the role played by the troposphere. In this work, we investigate this coupling and its associated predictability limits by studying the 2018 SSW event.

By analyzing ECMWF 15-day ensemble forecasts and partitioning them into different weather regimes, we search for possible dynamical tropospheric events that may have favored the downward stratosphere-troposphere coupling during and after the SSW. It is found that two cyclogenesis events were the main drivers of the negative NAO pattern associated with a Greenland Blocking, causing a rapid change from prevailing westerlies into a blocked state in the North Atlantic region. Unless these cyclogenesis events are simulated in the forecasts, the prediction of a Greenland Blocking does not become highly prevalent. No important stratospheric differences between WRs were found. A possible oceanic contribution to this blocked state is also found. This work corroborates that individual synoptic events might constitute a “predictability barrier" for subsequent forecast lead times. It also sheds light, on the specific topic of troposphere-stratosphere coupling.

How to cite: González-Alemán, J. J., Grams, C. M., Ayarzagüena, B., Zurita-Gotor, P., Domeisen, D. I. V. D., Gómara, Í., and Rodríguez-Fonseca, B.: Understanding the role of the troposphere in the surface impact of the 2018 sudden stratospheric warming event, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8495, https://doi.org/10.5194/egusphere-egu21-8495, 2021.

Upper-level synoptic-scale Rossby wave packets are well-known to affect surface weather. When these Rossby wave packets occur repeatedly in the same phase at a specific location, they can result in persistent hot, cold, dry, and wet conditions. The repeated and in-phase occurrence of Rossby wave packets is termed as recurrent synoptic-scale Rossby wave packets (RRWPs). RRWPs result from multiple transient synoptic-scale wave packets amplifying in the same geographical region over several weeks.

Our climatological analyses using reanalysis data have shown that RRWPs can significantly modulate the persistence of hot, cold, dry, and wet spells in several regions in the Northern and the Southern Hemisphere.  RRWPs can both shorten or extend hot, cold, and dry spell durations. The spatial patterns of statistically significant links between RRWPs and spell durations are distinct for the type of the spell (hot, cold, dry, or wet) and the season (MJJASO or NDJFMA). In the Northern Hemisphere, the spatial patterns where RRWPs either extend or shorten the spell durations are wave-like. In the Southern Hemisphere, the spatial patterns are either wave-like (hot and cold spells) or latitudinally banded (dry and wet spells).

Furthermore, we explore the atmospheric drivers behind RRWP events. This includes both the background flow and potential wave-triggers such as the Madden Julian Oscillation or blocking. For 100 events of intense Rossby wave recurrence in the Atlantic, the background flow, the intensity of tropical convection, and the occurrence of blocking are studied using flow composites.

How to cite: Ali, S. M., Martius, O., and Röthlisberger, M.: Recurrent Rossby wave packets and persistent extreme weather, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1549, https://doi.org/10.5194/egusphere-egu21-1549, 2021.

In recent history, the eastern Mediterranean and Saudi Arabia have experienced extreme precipitation events involving significant financial and human losses. An important subset of these events is associated with the activation of the Red Sea trough (RST). In this study, the effect and role of Rossby wave propagation during three cases (Dec 1993, Jan 2011 and May 1982) of the active RST is investigated. Meanwhile, the synoptic and dynamic factors related to the tropical-extratropical interaction and the lower and upper levels of troposphere are discussed for each event. The data used were extracted from the Era-Interim subsection of the ECMWF database with a time step of 6 hours and a spatial step of 80 km in both latitude and longitude directions.

Despite differences in humidity sources and the amount of hot and humid air ascent in each event, a general pattern can be deduced in all three events. The results show that in all events from a few days before the maximum rainfall, fluxes of heat and humidity are directed to Saudi Arabia and the eastern Mediterranean and the RST is strengthened and extended to the east of the Mediterranean Sea. At the same time, a trough with varying intensity at the level of 500 hPa in the eastern Mediterranean exerts a southward influence, which is caused by the anticyclonic Rossby wave breaking. At the upper levels, associated with the wave activity flux divergence and convergence areas of the Mediterranean storm track, higher amounts of Rossby wave activity enter the northeast region of Africa. Also the meridional convergence of the wave activity flux strengthens the meridional circulation in the north of the Red Sea. Increased horizontal wave activity flux to the northeast Africa and the Red Sea is led to increased head and humidity flux to the region. On the other hand, the weakening of the extension of the Azores high pressure over Africa facilitates the tropical and extratropical interactions over the region. Also in the north or northeast of the Red Sea, a surface low pressure is formed. Having a different source in each case, the mid-level troughs exhibit a northwest-southeast title with respect to the surface lows which lead to baroclinic development and intensification of precipitation events in the eastern Mediterranean and Saudi Arabia.

Keywords: Extreme precipitation, Rossby wave activity flux, Mediterranean storm track, upper level trough, meridional circulation, baroclinic development

How to cite: Alizadeh, Z., Mohebalhojeh, A., Ahmadi-Givi, F., Mirzaei, M., and Khansalari, S.: Case studies of the relation between upper-tropospheric wave propagation and the Red Sea trough, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2134, https://doi.org/10.5194/egusphere-egu21-2134, 2021.

Heatwaves are extreme weather events characterized by extreme near-surface temperature anomalies that persist for several days, which lead to catastrophic impacts on natural ecosystems, agriculture, human health, and economies. Different physical processes can contribute to the increase in temperature associated with heatwaves. Previous studies have shown that adiabatic compression due to subsidence and local land-atmosphere coupling are important drivers of summer heatwaves. However, less is known about the respective roles of these processes for heat extremes occurring in different seasons and latitudes.

By analyzing the different terms of the temperature tendency equation, we quantify the relative importance of horizontal wind advection, adiabatic, and diabatic processes (including radiation and surface fluxes) during the lifecycle of realistic and idealized heatwaves. We identify heatwaves both in reanalysis and in simulations using the ICOsahedral Nonhydrostatic (ICON) climate model. These simulations range from a simple zonally symmetric temperature relaxation and dry dynamics to a simulation using full physics, with coupled land and sea surface temperature forcing. This step-wise inclusion of physical processes and increasing model complexity allows us to identify the key drivers of extreme warm events and the characteristics of these across the different model complexities. In the simplest model configuration, i.e. only dry dynamics and no surface coupling, extreme temperature events are generally shorter but produce more intense temperature anomalies in the midlatitudes, where the horizontal temperature gradient is strongest. These idealized heatwaves are almost entirely driven by a very strong advection of warm air from more equatorward locations and are linked to local amplification of Rossby wave packets and atmospheric blocking. In contrast, in the complex model configuration as well as in reanalysis, summer heatwaves over land areas are mainly driven by adiabatic and diabatic processes, while advection is of secondary importance. On the other hand, extreme warm periods during winter are mainly driven by advection both in the model and reanalysis. Identifying the most relevant processes driving heatwaves can potentially benefit the prediction and representation of extreme events in operational weather and climate forecasts.

How to cite: Jiménez-Esteve, B. and Domeisen, D. I. V.: The Lifecycle and Physical Drivers of Heatwaves in a Hierarchy of Model Simulations , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2349, https://doi.org/10.5194/egusphere-egu21-2349, 2021.

Rossby Wave Packets (RWPs) are key to the improvement of  long-range forecasting and for the prediction of sub-seasonal extremes. Several studies have focused on their properties, such as time duration, trajectory, areas of detection and dissipation as well as interannual variability in the northern hemisphere, but only a few of them have focused in the southern hemisphere. Here we study the influence of low-frequency climate modes on RWPs during southern hemisphere summer using NCEP DOE 2 Reanalysis data. Focusing on long-lived RWPs, which we define as RWPs with a lifespan above 8 days,  we determine how El Niño-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) modify their frequency of occurrence and their main areas of detection and dissipation. We found that during El Niño and negative SAM years, the number of long-lived RWPs is maximum. In addition, years with the highest amount of long-lived RWPs show a zonally symmetric and narrow upper level jet that is shifted northward from its climatological position. On the other hand, when the jet is shifted southward, particularly in the southeastern Pacific, during positive SAM phases, only a small number of long-lived RWPs is detected. Therefore, negative SAM conditions provide a background mean flow that favours the occurrence of long-lived RWPs while positive SAM conditions have the opposite effect. The dependence on ENSO phase is not as symmetric: while El Niño sets atmospheric conditions that favour the formation of long-lived RWPs, La Niña years present high interannual variability in the frequency of occurrence. Furthermore, in El Niño events the main formation area is between 61-120ºE and the main dissipation area between 300-359ºE. During La Niña events, the main formation area is located by 241-300ºE and no main dissipation area is identified. In the case of positive SAM two main formation areas appear at 61-120ºE and 241-300ºE and two main dissipation areas within 121-180 and 301-359ºE. Lastly in the case of negative SAM one main formation area at 241-300ºE is detected and no main dissipation area is detected. The robustness of the results was tested repeating the analysis using data from the ERA5 Reanalysis and supports the finding that the maximum number of long-lived RWPs occur during negative SAM and El Niño years

How to cite: Perez, I., Barreiro, M., and Masoller, C.: ENSO and SAM influence on the generation of long episodes of Rossby Wave Packets during Southern Hemisphere Summer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-877, https://doi.org/10.5194/egusphere-egu21-877, 2021.

The northern midlatitude summer has experienced rapid warming since the 1990s, especially in Europe, Central Siberia-Mongolia, the West Coast of North America as well as several continental Arctic regions. These “hot spots” are connected by a chain of high-pressure ridges from an anomalous wavenumber-5 Rossby wave train in the upper troposphere.  Here by cross-examining reanalysis datasets and a suite of Coupled Model Intercomparison Project Phase 6 (CMIP6) baseline experiments, we demonstrate that the anthropogenically forced response may be intertwined with internal multidecadal variability, making it difficult to partition the 1979-2020 trend with state-of-the-art climate models. Instead, we take a “storyline” approach with a planetary wave model and sensitivity experiments with an Earth system model to explore key underlying driving factors. Our results highlight the importance of multiscale interaction with synoptic eddy via atmosphere-ocean and atmosphere-land coupling in shaping the multidecadal regional warming trend which has enormous socioeconomic implications. 

How to cite: Teng, H., Leung, R., Branstator, G., Lu, J., and Ding, Q.: Accelerated warming in the northern midlatitude summer since the 1990s, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6506, https://doi.org/10.5194/egusphere-egu21-6506, 2021.

Weather over the Euro-Atlantic region during winter is highly variable, with rich and chaotic internal atmospheric dynamics. In particular, the non-linear breaking of Rossby waves irreversibly mixes potential vorticity contours and so triggers shifts in the latitude of the eddy driven jet and establishes persistent anticyclonic blocking events. The concept of atmospheric regimes captures the tendency for blocks – and for the jet – to persist in a small number of preferred locations. Regimes then provide a non-linear basis through which model deficiencies, interdecadal variability and forced trends in the Euro-Atlantic circulation can be studied.

A drawback of past regime approaches is that they were unable to easily capture both the dynamics of the jet and of blocking anticyclones simultaneously. In this work we apply a recently developed regime framework, which is able to capture both these important aspects while reducing sampling variability, to the CMIP6 climate model ensemble. We analyse both the historical variability and biases of blocking and jet structure in this latest generation of climate models, and make new estimates of the anthropogenic forced trend over the coming century.

How to cite: Dorrington, J.: A regime perspective on jet and blocking dynamics in CMIP6, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-919, https://doi.org/10.5194/egusphere-egu21-919, 2021.

The large-scale extratropical upper-tropospheric flow tends to organize itself into eastward-propagating Rossby wave packets (RWPs). Investigating the spatiotemporal evolution of RWPs and the underlying physical processes has been beneficial in showcasing the role of the upper-tropospheric flow in temperature and precipitation extremes. The use of recently developed diagnostics of local in space and time wave properties has provided further insight in this regard. Motivated by the above, these diagnostic methods are now being employed to investigate the intraseasonal to decadal variability of key RWP properties such as their amplitude, phase speed, and group velocity in reanalysis datasets. It is shown that these properties exhibit a distinct seasonal and interregional variability, while interesting patterns thereof emerge. Moreover, the interannual and long-term variability in these RWP properties is explored and significant decadal trends for specific regions and seasons are highlighted. Ongoing work aims at further utilizing the presented diagnostics and analyses toward an improved understanding of the extratropical large-scale flow variability from weather to climate time scales.

How to cite: Fragkoulidis, G. and Wirth, V.: Intraseasonal to decadal variability of Rossby wave packet properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8828, https://doi.org/10.5194/egusphere-egu21-8828, 2021.

The east coast of North America experienced a record-breaking jet stream event on 20 Feb 2019, with peak wind speeds exceeding 110 m/s observed by weather balloons over Nova Scotia. At the time this was the strongest wind speed ever recorded over North America. The extreme `jet streak' propagated out over the North Atlantic where it played a key role in the subsequent development of a large and rapidly deepening cyclone on 22 Feb 2019. The cyclone had little societal impact because it did not make landfall. It did however act to amplify a large scale Rossby wave, producing a strong poleward advection of warm air towards western Europe, and leading to record-breaking February warmth in several European countries on 27 Feb 2019. The whole sequence of events took just over a week to complete.

This case provides an illustration of how climate extremes (here the record warmth in western Europe) are often the result of complex and chaotic nonlinear interactions of the atmosphere on weather timescales. The particular sequence of events is not uncommon, but both the strength of the initial jet streak over North America and the resulting temperatures in Europe were. Given the observed trend in surface temperatures, it seems likely that the temperatures were at least partly enhanced in a passive way by the warming climate. A more difficult question to answer is whether climate change is also impacting the frequency or amplitude of the preceding sequence of weather events. As a first step to answering this question, this study asks the question: do we expect extreme jet streak events to intensify in future?

Based on an analysis of CMIP simulations over the North Atlantic, we find a robust intensification of wintertime jet extremes in future climates, with the strongest instantaneous wind speeds increasing in every model. This contrasts with the strength of the time mean jet streams, which do not exhibit a robust change across the ensemble. Possible reasons for the differing behaviour of the mean winds and the extreme winds are discussed and a hypothesis is suggested to explain the robust increase in the latter.

How to cite: Harvey, B.: A Robust Intensification of Wintertime Jet Stream Extremes in Future Climates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14521, https://doi.org/10.5194/egusphere-egu21-14521, 2021.

Variability of the East Asian summer jet stream (EAJ) has a significant impact on the climate of East Asia, primarily through its modulation of East Asian precipitation. In recent decades the impact of sea surface temperatures (SSTs) in the tropical Indian and Pacific oceans on the EAJ have been studied in considerable detail, however much less is known about the drivers of EAJ variability on decadal or multi-decadal timescales. Investigating this problem is made more challenging by the temporal limitations of reanalysis datasets.

In order to establish whether SSTs can provide a source of skill in predicting decadal variations of the EAJ, we analyse long pre-industrial control runs of the CMIP6 models. One issue with studying coupled model runs is that it is often unclear whether particular SST anomalies are forcing the atmosphere, and thus can provide a meaningful source of skill, or whether they are merely responding to local atmospheric anomalies. We address this issue by combining SST and turbulent heat flux information to indicate the direction of the forcing.

How to cite: Patterson, M., Woollings, T., O'Reilly, C., and Weisheimer, A.: Decadal variability of the East Asian summer jet and its relationship to sea surface temperatures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2140, https://doi.org/10.5194/egusphere-egu21-2140, 2021.

While the existence of regional weather regimes (e.g., over the North Atlantic) is a known result, the presence of preferred circulation patterns at the hemispheric scale is still disputed. Space-time spectral analysis can offer a different perspective to tackle this problem, as it provides a compact representation of the large-scale flow evolution. It can objectively extract the most relevant harmonics, in terms of spatial wavenumbers and temporal frequencies, that dominate the hemispheric Rossby wave pattern at a given time and is easily applicable to gridded data sets as Reanalysis or the output of general circulation models.

With the aim to highlight the existence of clusters in the spectral space, we build a data set of spectra of upper-level meridional wind over midlatitudes (35°N-75°N) in the wavenumber/phase-speed domain for the 1979-2019 Reanalysis period. A spectrum is assigned to each day being located in the center of a sliding 61-days time window. This data set contains interesting information about the stationarity and the persistence of the hemispheric Rossby wave pattern. The most persistent harmonics are the ones related to quasi-stationary or westward propagating waves, as confirmed by an analysis of the dominant harmonics during atmospheric blocking events.

Cluster analysis is performed using self-organizing maps (SOMs) on this data set. To assess its significance, the same procedure is applied to an artificially generated red noise with the same mean, variance and lag-1 covariance as the real data. This cross-check does not highlight a preferred number of circulation regimes in the spectral space. However, a subjective classification of the spectral patterns highlighted by the SOM analysis in four different groups can be attempted: 1) a ground state, with no particular deviation from climatology; 2) a state characterized by rapidly propagating, high wavenumber waves; 3) a state characterized by slowly propagating, low wavenumber waves; and 4) a state with a clear, dominant wavenumber. Spectral patterns corresponding to each of these groups are present regardless of the chosen number of SOMs.

How to cite: Riboldi, J., Rousi, E., D'Andrea, F., Rivière, G., and Lott, F.: A spectral perspective on the existence of hemispheric circulation regimes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13579, https://doi.org/10.5194/egusphere-egu21-13579, 2021.

Quasi-stationary, persistent, and recurrent states of the large-scale extratropical circulation, so-called weather regimes, characterize the atmospheric variability on sub-seasonal timescales of several days to a few weeks. Weather regimes featuring a blocking anticyclone are of particular interest due to their long lifetime and potential for high-impact weather. However, state-of-the-art numerical weather prediction and climate models struggle to correctly represent blocking life cycles, which results in large forecast errors at the medium-range to sub-seasonal timescale. Despite progress in recent years, we are still lacking a process-based conceptual understanding of blocked regime dynamics, which hinders a better representation of blocks in numerical models. In particular the relative contributions of dry and moist processes in the onset and maintenance of a block remain unclear.

Here we aim to revisit the dynamics of blocking in the Euro-Atlantic region. To this end we investigate the life cycles of blocked weather regimes from a potential vorticity (PV) perspective in ERA5 reanalysis data (from 1979 to present) from the European Centre for Medium-Range Weather Forecasts. We develop a diagnostic PV framework that allows the tracking of negative PV anomalies associated with blocked weather regimes. Complemented by piecewise PV-tendencies - separated into advective and diabatic PV tendencies - we are able to disentangle different physical processes affecting the amplitude evolution of negative PV anomalies associated with blocked regimes. Most importantly, this approach newly enables us to distinguish between the roles of dry and moist dynamics in the initiation and maintenance of blocked weather regimes in a common framework. A first application demonstrates the functionality of the developed PV framework and corroborates the importance of moist-diabatic processes in the initiation and maintenance of a block in a regime life cycle. 

How to cite: Hauser, S., Grams, C. M., Riemer, M., Knippertz, P., and Teubler, F.: Potential vorticity dynamics of life cycles of blocked weather regimes in the Euro-Atlantic region: A diagnostic framework, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12847, https://doi.org/10.5194/egusphere-egu21-12847, 2021.

Atmospheric blocking events are mid-latitude weather patterns, which obstruct the usual path of the polar jet stream. Several blocking indices (BIs) have been developed to study blocking patterns and their associated trends, but these show significant seasonal and regional differences. Despite being central features of mid-latitude synoptic-scale weather, there is no well-defined historical dataset of blocking events. Here, we introduce a new blocking index using self-organizing maps (SOMs), an unsupervised machine learning approach, and compare its detection skill to some of the most widely applied BIs. To enable this intercomparison, we first create a new ground truth time series classification of European blocking based on expert judgement. We then demonstrate that our method (SOM-BI) has several key advantages over previous BIs because it exploits all the spatial information provided in the input data and avoids the need for arbitrary thresholds. Using ERA5 reanalysis data (1979-2019), we find that the SOM-BI identifies blocking events with a higher precision and recall than other BIs. We present a case study of the 2003 European heat wave and highlight that well-defined groups of SOM nodes can be an effective tool to reliably and accurately diagnose such weather events. This contrasts with the way SOMs are commonly used, where an individual SOM node can be wrongly assumed to represent a weather pattern. We also evaluate the SOM-BI performance on about 100 years of climate model data from a preindustrial simulation with the new UK Earth System Model (UK-ESM1). For the model data, all blocking detection methods have lower skill than for the ERA5 reanalysis, but SOM-BI performs significantly better than the conventional indices. This shows that our method can be effectively applied to climate models to develop our understanding of how climate change will affect regional blocking characteristics. Overall, our results demonstrate the significant potential for unsupervised learning to complement the study of blocking events in both reanalysis and climate modelling contexts.

How to cite: Thomas, C., Voulgarakis, A., Lim, G., Haigh, J., and Nowack, P.: An unsupervised learning approach to identifying blocking events: the case of European summer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9517, https://doi.org/10.5194/egusphere-egu21-9517, 2021.

Changes in weather persistence are of particular concern in the context of climate change as periods of longer persistence can reinforce weather extremes. In our study we apply structural image recognition methods to global ERA5 reanalysis data to identify when, where and how long isolines of atmospheric geopotential height fields run in similar tracks. We identify regions and episodes around the world in which, retrospectively, unusually long-lasting weather patterns repeatedly occurred. Concerning the temperature and precipitation meteorological fields, we derive a connection between the occurrence of weather persistence and hydro-climatic extreme events.

Based on our new method we find that weather persistence has been particularly increasing in Northern Hemisphere mid-latitudes in summer confirming earlier studies. Here, highly populated regions like Central Europe have experienced long-term increases in persistent weather conditions of up to 4-5% between 1981 and 2019 amplifying the risk of prolonged heat waves and droughts. Further, we show that climate models tend to have difficulties in capturing the dynamics of weather persistence and thus may severely underestimate the frequency and magnitude of future extremes events in their climate projections.

How to cite: Hoffmann, P., Lehmann, J., Fallah, B., and Hattermann, F.: New method to detect and quantify weather persistence associated with hydro-climatic extremes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8044, https://doi.org/10.5194/egusphere-egu21-8044, 2021.

Atmospheric blocking is a large-scale weather phenomenon that interrupts the prevailing eastward progression of pressure systems and can result in weather extremes in the midlatitudes. Due to their devastating consequences, understanding the changes of blocking in response to climate change has been of great interest in recent years. In this study, we investigate the 3D structure of blocking events in reanalysis and two large-ensemble, fully coupled GCM simulations: NCAR’s CESM1 Large-Ensemble Project (LENS) and GFDL-CM3 large-ensemble project. Here we compare the climatology of blocks in the models with reanalysis and show that the structure of the blocks is remarkably reproduced well in the GCMs, given that these models are known to have biases in reproducing the climatological Northern Hemisphere large-scale circulation. The results of our composite analysis indicate that the blocks exhibit an equivalent-barotropic structure in both summer and winter seasons over both oceans and continents in the northern hemisphere. However, blocking events are stronger in winters compared to summers. We also notice a significant latent heating associated with ascending airstream on the east side of blocks. This warming, which is stronger in winter especially over the ocean basins, leads to a westward shift in the temperature anomaly during blocking episodes. Furthermore, we study the response of the blocks to climate change (RCP8.5) and find that blocking events will be weakened in the summer of three different northern hemisphere regions. However, wintertime blocks’ responses to climate change are more complex than those in summers and depend on the regions and atmospheric pressure levels. Finally, we examined the response of surface temperature associated with blocking events. We have found that the surface temperature response associated with blocking events will be weaker over all the regions in the winter season. However, during summer, the temperature responses will be slightly stronger over Russia and partially over the two ocean basins. Our results suggest that summertime blocking events over Russia are going to be more impactful compared to those over the ocean basins.

How to cite: Nabizadeh, E., Lubis, S., and Hassanzadeh, P.: The 3D Structure of Atmospheric Blocking: Role of Moisture and Response to Climate Change, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13585, https://doi.org/10.5194/egusphere-egu21-13585, 2021.

The aim of this study is to explore the nature of potential vorticity (PV) cutoff life cycles. While climatological frequencies of such upper-level cyclonic vortices are well known, their life cycle and in particular their three-dimensional evolution is poorly understood. To address this gap, a novel method is introduced that uses isentropic air parcel trajectories to track PV cutoffs as three-dimensional objects. With this method, we can distinguish the two fundamentally different PV cutoff lysis scenarios on isentropic surfaces: complete diabatic decay vs. reabsorption by the stratospheric reservoir.

This method is applied to the ERA-interim dataset (1979-2018) and the first global climatology of PV cutoffs is presented that is independent of the selection of a vertical level and identifies and tracks PV cutoffs as three-dimensional features. More than 140’000 PV cutoff life cycles are identified and analyzed. The climatology confirms known frequency maxima of PV cutoffs and identifies additional bands in subtropical areas in the summer hemispheres and a circumpolar band around Antarctica. The first climatological analysis of diabatic decay and reabsorption shows that both scenarios occur equally frequently – in contrast to the prevailing opinion that diabatic decay dominates.

Further, PV cutoffs are classified according to their position relative to jet streams [equatorward (type I), between two jets (type II), and poleward (type III)]. A composite analysis of PV cutoff genesis shows distinct dynamical scenarios for the three types. Type I forms due to anticyclonic Rossby wave breaking above subtropical surface anticyclones and hardly results in precipitation. Type II results from anticyclonic Rossby wave breaking downstream of the storm tracks and is frequently accompanied by surface cyclogenesis and substantial precipitation. Type III cutoffs preferentially form due to wave breaking within mature extratropical cyclones in the storm track regions. We show that important track characteristics (speed, travel distance, frequency of decay and reabsorption, vertical evolution) differ between the categories, while lifetime is similar in all categories. 

Finally, twelve PV cutoff genesis regions in DJF and JJA are selected to study the regional characteristics of PV cutoff life cycles. We find that many characteristics of these PV cutoffs reflect the preferred regional occurrence of the different life cycle types. However, a few regions are characterized by substantially longer (e.g. central subtropical North Atlantic in summer) or shorter (Mediterranean in summer) lifetimes than most other regions. Furthermore, a remarkable variability in the vertical evolution of PV cutoffs is found. While in some regions, PV cutoffs rapidly disappear at lower levels by diabatic decay, they can grow downward in other regions. We also show that in many regions PV cutoffs can be involved in surface cyclogenesis even after their formation.

This study is an important step towards quantifying fundamental dynamical characteristics and the surface impacts of PV cutoffs. The proposed classification according to the jet-relative position provides a useful way to improve the conceptual understanding of PV cutoff life cycles. However, these life cycles can be substantially modified by specific regional conditions.

How to cite: Portmann, R., Sprenger, M., and Wernli, H.: The three-dimensional life cycles of potential vorticity cutoffs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2658, https://doi.org/10.5194/egusphere-egu21-2658, 2021.

The waveguidability of an upper tropospheric zonal jet quantifies its propensity to duct Rossby waves in the zonal direction. This property has played a central role in previous attempts to explain large wave amplitudes and the subsequent occurrence of extreme weather. In these studies, waveguidability was diagnosed with the help of the refractive index using the zonal average of the observed flow as the relevant background state. Here, it is argued that this method is problematic both conceptually and mathematically.

The issue is investigated in the framework of the non-divergent barotropic model. This model allows the straightforward computation of an alternative "zonalized" background state, which is obtained through conservative symmetrisation of potential vorticity contours and which is argued to be superior to the zonal average. Using an idealized prototypical flow configuration with large-amplitude eddies, it is shown that the two different choices for the background state yield very different results; in particular, the zonal-mean background state diagnoses a zonal waveguide, while the zonalized background state does not. This result suggests that the existence of a waveguide in the zonal mean background state is a consequence of, rather than a precondition for large wave amplitudes, and it would mean that the direction of causality is opposite to the usual argument.

The analysis is applied to two heatwave episodes from summer 2003 and 2010, yielding essentially the same result. It is concluded that previous arguments about the role of waveguidability for extreme weather need to be carefully re-evaluated to prevent misinterpretation in the future.

How to cite: Wirth, V. and Polster, C.: Diagnosing jet waveguidability in the presence of large-amplitude eddies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-577, https://doi.org/10.5194/egusphere-egu21-577, 2021.

How to cite: Spensberger, C. and Spengler, T.: Bedymo: a combined quasi-geostrophic and primitive equation model in sigma coordinates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4204, https://doi.org/10.5194/egusphere-egu21-4204, 2021.