Much has been learned about the response of the mean circulation under climate change. In particular, the subtropical jet will accelerate, the eddy-driven jet will shift poleward, storminess in the Southern Hemisphere will increase whereas storminess in the Northern Hemisphere will be impacted by a tug of war between different factors. However, very little is known about how circulation extremes will respond to climate change beyond blocking. This is in stark contrast to our understanding of the response of extreme temperatures, which follow the mean via an additive increase, and the response of extreme precipitation, which increase faster than the mean because of a multiplicative increase connected to the non-linear Clausius-Clapeyron relation. Here as a starting point, we investigate changes in upper-level circulation extremes defined using a daily distribution. We show fast upper-level jet stream (zonal) winds get faster under climate change. We also show extreme jet stream meandering or waviness (meridional wind) increases under climate change. These responses are geostrophic, robust across a climate model hierarchy (CMIP/AMIP/AQUA), and not connected to sea ice loss. The increase in upper-level circulation extremes is shown via moist thermal wind to be related to a multiplicative response connected to the non-linear Clausius-Clapeyron relation. Thus, upper-level circulation extremes exhibit a multiplicative increase similar to precipitation extremes. The results can be used to explain projected changes in commercial flight times, record-breaking winds, clear-air turbulence and a potential increase in severe weather occurrence under climate change.

How to cite: Shaw, T., Miyawaki, O., and Chou, H.-H.: Moving beyond the mean to understand circulation extremes under climate change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3243, https://doi.org/10.5194/egusphere-egu24-3243, 2024.

The subtropical jet is characterized by a strong vertical shear and is usually located far equatorward of the maximum surface westerlies. In some cases, strong westerlies develop below the subtropical jet, indicating that momentum flux convergence by baroclinic eddies contributes to the jet driving. In this work we examine this variability of the subtropical jet in light of baroclinic instability theory. According to linear baroclinic instability theory, the vertical scale of eddy fluxes decreases toward the equator, making the eddies effectively stable at low latitudes. This stabilizing effect enables the subtropical jet to be maintained by angular momentum advection from the tropics, without the development of baroclinic eddies and surface westerlies below the jet. The dimensionless Charney number is used as an indication for the degree of baroclinic stability at low latitudes. This number incorporates the stabilizing effect at low latitudes, in contrast with the commonly used measure for baroclinicity – the Eady growth rate. It is found that the Charney number performs better than the Eady growth rate in estimating the lowest latitude of baroclinic growth and explaining subtropical jet variability. 

How to cite: Lachmy, O. and Peles, O.: Baroclinic stability at low latitudes: Explaining subtropical jet variability in observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9823, https://doi.org/10.5194/egusphere-egu24-9823, 2024.

Atmospheric blocking is known to be one of the most important drivers of large-scale atmospheric variability at mid-high latitudes. Blocking events consist of a disruption and/or deceleration of the mean westerly circumpolar flow, and are generally associated with large-scale high-pressure patterns, which may be connected with the occurrence of climate extremes, such as heat waves and cold spells. Atmospheric dynamics in the Arctic region may be very important in shaping the spatial and temporal patterns of blocking at mid-high latitudes in the Northern Hemisphere, and consequently the occurrence of associated climate extremes. In particular, the difference between Arctic and mid-latitude temperatures is tightly associated with the Ural blocking (UB) activity. A causation relationship has been identified, with the UB triggering Arctic warming and Eurasian cold spells, and in turn leading to a weaker Arctic-midlatitudes thermal gradient (AMG). 
The objective of this study is to investigate the physical mechanisms underlying the AMG-UB relationship. In particular, the circulation patterns associated with the nonlinear part of the UB-Arctic interannual relationship are analysed in winter (December-to-February) from 1940 to 2023. To this aim, atmospheric variables are extracted from the ERA5 reanalysis datasets.

Results show that when high UB activity and strong AMG are observed, atmospheric blocking develops also over multiple areas relevant for milder and more humid air transport from mid latitudes into the Arctic region. Conversely, when low UB activity and weak AMG – hence associated with Arctic warmer than average – are observed, UB moves northwards, over Barents-Kara seas, and remote areas in the mid-latitudes and the subtropics, such as northwestern Africa and northwestern Atlantic Ocean, are teleconnected with the Arctic region.

By highlighting the complex nature of the atmospheric blocking modulation of the AMG, these findings are relevant to the comprehension of the leading factors of Arctic Amplification, and to the understanding of the role of atmospheric blocking in determining winter cold spells and extreme temperature events over mid-latitude regions.

How to cite: Cadau, M., Messori, G., Gaetani, M., Fosser, G., Bordoni, S., Buizza, R., and Sannino, G.: The relationship between atmospheric blocking and Arctic-midlatitude thermal gradient, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8358, https://doi.org/10.5194/egusphere-egu24-8358, 2024.

Various algorithms developed to track the synoptic evolution of the subtropical jets have proved useful in diagnosing variability and trends. However, consensus about trends remains low, and issues in jet detection persist. Notably, algorithms:

• Frequently employ a variety of climatological parameters, making them unsuitable for long-trend analyses;
• Rely on instantaneous meteorological fields (or Eulerian-averaged fields), thereby overlooking the temporal coherence of jet features. This results in the inability to systematically separate the true axis of the jets from underlying waves, affecting the characterization of variability known to affect the mean position of the jets —and long-term trends.

To address these limitations, we define the jets as Lagrangian Coherent Structures; persistent features that resist synoptic variability and thereby shape the atmospheric circulation. Using this Lagrangian definition, a new algorithm named JetLag is developed and applied to the ERA5 reanalysis. JetLag employs 2 parameters –a time scale and a spatial scale, both set by the Rossby wave dispersion relation– and is virtually insensitive to changes in those parameters, within physical bounds. Compared to wind-based methods, we show that JetLag:

• Locates jet features with better temporal coherence;
• Has enhanced capabilities for detecting weak and highly variable jets;
• Produces a different seasonal cycle of mean jet position;
• Produces different decadal to multi-decadal variability, with implications for trend detection.

We also present a new jet axis dataset for use by the community.

How to cite: Rivoire, L. and Curbelo, J.: Tracking the jets as Lagrangian objects, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14165, https://doi.org/10.5194/egusphere-egu24-14165, 2024.

How to cite: Hadas, O. and Kaspi, Y.: The Response of Synoptic-Scale Weather to Jet Stream Characteristics: a Lagrangian perspective., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10685, https://doi.org/10.5194/egusphere-egu24-10685, 2024.

Summertime atmospheric teleconnection patterns over Eurasia have a significant influence on regional weather and climate. Despite extensive studies on the subtropical patterns, the high-latitude counterpart has received relatively less attention. This study proposes physical mechanisms for the formation and maintenance of the dominant high-latitude teleconnection pattern. The formation of the pattern is associated with variability in synoptic-scale eddy activity due to the meridional gradient of sea surface temperature anomalies in the vicinity of the Gulf Stream, causing a meridional shift of the central axis of storm track at the exit of Atlantic jet. The resultant convergence of transient vorticity fluxes to the west of the British Isles induces low-frequency cyclonic circulation anomalies and continued propagation of Rossby waves downstream along northern Eurasia. Once these circulation anomalies are formed, the subsequent latent heat-related diabatic anomalies over the northern Eurasian landmass act as another source of Rossby waves to maintain the teleconnection pattern. Regional temperature and precipitation variability is closely linked to the wave pattern along a route through northern Eurasia, and even precipitation over the East Asian summer monsoon region is influenced by the teleconnection pattern.

How to cite: Kim, J. and Seo, K.-H.: Physical mechanisms for the summertime high-latitude atmospheric teleconnection patterns and the related Eurasian and East Asian climates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16134, https://doi.org/10.5194/egusphere-egu24-16134, 2024.

Rossby wave breaking (RWB) can be manifested by the irreversible overturning of isentropes on constant potential vorticity (PV) surfaces. RWB events can lead to tropospheric impacts ranging from changes in intensity and position of the jet stream to extremes in precipitation resulting in significant societal impacts. Traditionally, RWB events are categorized as anticyclonic (AWB) or cyclonic (CWB) and can be identified using the orientation of streamers of high potential temperature (θ) and low θ air on a potential vorticity surface. Self-organizing maps (SOM), a machine learning method, was used to cluster RWB events into archetypal patterns, or “flavors”, for each RWB event type (i.e., AWB and CWB). This allowed for an examination of differences in RWB event flavors, and their associated tropospheric impacts, using the European Centre for Medium Range Weather Forecasts Reanalysis v5 (ERA5) dataset. AWB and CWB flavors capture variations in the θ minima/maxima of each streamer and the localized meridional θ gradient (∇θ) flanking the streamers. Variations in the magnitude and position of ∇θ between flavors correspond to a diversity of jet structures leading to differences in vertical motion patterns and troposphere-deep circulations. A subset of flavors of AWB (CWB) events are associated with the development of strong surface high (low) pressure systems and the generation of extreme poleward moisture transport. For CWB, many events occurred in similar geographical regions, but the precipitation and moisture patterns were vastly different between flavors. 

Given these impacts and their importance for regional climates, it is important to also understand how RWB events, and their associated sensible weather features, are represented in climate models. Therefore, AWB and CWB events were identified from overturning isentropes on the dynamic tropopause (DT) in the Community Earth System Large Ensemble v2 (CESM-LENS2) climate model output during December, January, and February (DJF) 1980-2014 (i.e., historical period). RWB flavors are identified in the LENS2 for comparison to the ERA5 dataset for the same time period. Composites of tropospheric dynamic and thermodynamic fields were calculated for each RWB flavor in the LENS2 which allows for an evaluation of the impact of AWB and CWB structure on sensible weather extremes. First, the frequency of occurrence of each RWB flavor between datasets was found. Second, differences in the sensible weather features associated with each flavor were quantified. This process-orientated climate model evaluation of the LENS2 as compared to the ERA5 can provide insight into the source of model errors in the LENS2 climate model.

How to cite: LaChat, G., Bowley, K. A., and Gervais, M.: Diagnosing flavors of tropospheric Rossby wave breaking and their associated dynamical and sensible weather features , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13842, https://doi.org/10.5194/egusphere-egu24-13842, 2024.

Recent studies have highlighted the link between upper-level jet dynamics, especially the persistence of certain configurations, and extreme summer weather in Europe. The weaker and more variable nature of the jets in summer makes it difficult to apply the tools developed to study them in winter, at least not without modifications. Here, in order to further investigate this link, we present two complementary approaches to characterize the jet dynamics in summer in the North Atlantic sector.

First, we apply a jet axis detection and tracking algorithm to ERA5 reanalysis data to extract individual jets and classify them in the canonical categories of polar and subtropical jets. Then, we compute a wide range of jet indices on each jet to provide easily interpretable scalar time series representing upper-tropospheric dynamics.

Second, we apply the self-organizing map (SOM) clustering algorithm to the same data to create a distance-preserving, discrete, 2D phase space. The dynamics can then be described by the time series of visited SOM nodes, in which a long stay in a given node relates to a persistent state and a rapid transition between nodes that are far apart relates to a sudden dramatic shift in the configuration of upper-level flow.

We first compare these two approaches to each other to assess their consistency, and then use them to relate the jet dynamics to a known driver of variability, Rossby wave breaking. Finally, we present preliminary results linking persistent jet dynamics to extreme heat events in Europe.

How to cite: Banderier, H., Tuel, A., Woollings, T., and Romppainen-Martius, O.: Complementary approaches to characterize the jet stream dynamics in summer and link them to extreme weather in Europe, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3439, https://doi.org/10.5194/egusphere-egu24-3439, 2024.

Both the circulation response to climate change as well as internal atmospheric variability are marked by a meridional displacement of the extratropical storm track. It remains to be quantified how such changes in the storm track modulate the occurrence of heatwaves. We combine a composite analysis of reanalysis data with idealized model experiments to investigate the response in heatwave frequency to variations in the storm track latitude in two different datasets. In the idealized model, a forced poleward storm track shift leads to an increase in upper-tropospheric Rossby wave phase speed, and vice versa, which in turn reduces and increases heatwave frequency and duration across the mid-latitudes. A similar relationship between storm track latitude, Rossby wave phase speed, and heatwave duration is found for internal variability in reanalysis data. However, in reanalysis, a reduction in phase speed does not necessarily lead to an increased heatwave frequency due to geographically phase-locked wave trains induced by zonal asymmetries. These results shed new light on the dynamical drivers for heat extremes.

How to cite: Wicker, W., Russo, E., and Domeisen, D.: Midlatitude heatwave variability modulated by a shifting storm track, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4019, https://doi.org/10.5194/egusphere-egu24-4019, 2024.

Despite decades of research, climate models have failed to produce a consistent picture about regional variations in atmosphere blocking. The lack of sufficient model outputs has impeded the progress in unraveling the sources of biases in the model simulations. To address this issue, we perform a systematic analysis of blocking events, from two state-of-the-art climate models  with high temporal and spatial resolution — Community Earth System Model, Large Ensemble Community Project 2 (CESM LENS2) and the Norwegian Earth System Model medium resolution (NorESM2-MM) —alongside an idealised simulation featuring regionally varying CO2 forcings. A benchmark dataset of blocking events is created using the finite-amplitude local wave activity metric since it captures the growth and decay of high-amplitude Rossby waves while conserving wave activity density up to higher accuracy.  We also perform a detailed analysis of the wave activity budget to quantify the dominant physical processes that lead to biases in historical runs and processes that shift blocking patterns in warming scenario model runs.

How to cite: Barpanda, P. and Li, C.: Sources of biases in blocking representation in climate models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17801, https://doi.org/10.5194/egusphere-egu24-17801, 2024.

Extreme weather events are often linked to atmospheric blocking. While a comprehensive theory of blocking is yet to be developed, the onset mechanism of these systems poses a major challenge. As a result, the representation of blocking in climate models is inadequate and consistently underestimated. Although successive improvements in the blocking representation are evident in climate models, they still underestimate the blocking frequency in the Northern Hemisphere (NH).

In this study, we observed an improvement in the representation of blocking in the Community Earth System Model Large Ensemble 2 (LENS2) during winter and a notable deficiency during summer. When compared with observations, the winter blocking bias (-4% to +2%) was found to be substantially reduced, while the summer blocking exhibited a significant bias (-12% to +12%) compared to previous studies. Under the SSP370 scenario, LENS2 suggests an overall decline in winter blocking (11%) and an increase in summer blocking (12%) by the year 2100 in the NH.

The underlying reason for the underestimation of blocking in climate models is often associated with the mean jet state and stationary waves. However, the lack of an onset theory causes challenges in identifying blocking systems. There is also a missing explanation of why temporal persistence is considered specifically as 5 days. Thus, we define less persistent blocking (LPB) as a blocking regime that satisfies the flow reversal criterion and persists for less than 5 days. We found a significant presence of the frequency of LPB (~15% to 25% maximum) in the NH in both models and observations. It means that we were ignoring the presence of these blocking systems, which may have a role in driving extreme events and can potentially emerge as stronger and more persistent blocking systems in the future.

Interestingly, the hotspot of LPB includes drought-prone regions such as the western coast of the United States and well-known blocking centers such as the Euro-Atlantic and Pacific regions. This leads us to propose a new potential avenue for studying the precursors of LPB dissipation, which will provide insights into the longstanding problem of the blocking onset mechanism.

How to cite: Shelke, P., Jendersie, S., and Golledge, N.: Are we missing future extreme events by ignoring less persistent blocking?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10982, https://doi.org/10.5194/egusphere-egu24-10982, 2024.

How to cite: Pyrina, M., Wicker, W., de Vries, A. J., Fragkoulidis, G., and Domeisen, D. I. V.: Rossby wave packets driving concurrent and non-concurrent heatwaves in the Northern and Southern Hemisphere mid-latitudes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10165, https://doi.org/10.5194/egusphere-egu24-10165, 2024.

The North Pacific High is a dominant circulation system that governs the weather in the East Asian region during the summer, and its western boundary serves as a waveguide for the propagation of Rossby waves from the equatorial to mid-latitudes. The deep convection over the equatorial western Pacific usually creates Rossby waves that propagate northward along this waveguide. This meridional Rossby wave, known as the Pacific-Japan (PJ) pattern, is the dominant teleconnection pattern in the vicinity of East Asia, and it often accompanies Heatwaves.

In this study, the circulation and thermodynamic characteristics of the PJ pattern were investigated based on a daily timescale to better understand their relationship with the likelihood of heatwaves in East Asia. According to thermodynamic budget calculations, horizontal heat advection crossing the climatological flow pattern is the key factor for the observed surface air warming. The circulation pattern associated with a PJ pattern largely explains the enhanced warm advection. The overall findings of this study provide valuable insights into the development mechanisms of heatwaves on an intraseasonal timescale.

How to cite: Noh, E. and Kim, J.: The Role of the Meridional Rossby wave for Extreme Heatwaves Over East Asia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14709, https://doi.org/10.5194/egusphere-egu24-14709, 2024.

The Eddy-driven jet meandering has been hypothesized to increase due to climate change. This meandering frequently induces slow-moving patterns of low and high pressure anomalies, potentially causing extreme weather events such as droughts, flooding, heat waves and cold spells. However, the quantitative link between the jet’s meandering and storms development is still lacking, as well as a conclusive mechanism for the effect of climate change on the jet’s meandering. In this study, we first separate the physical components in the atmospheric complex system using an idealized global circulation model. We outline the connection between the decreasing equator-to-pole temperature gradient due to Arctic amplification and the meandering of the jet. As the meridional temperature gradient decreases, the eddy-driven jet slows and its meridional layout widens. By Lagrangian tracking cyclones and anticyclones, we link the jet meandering to the formation of cyclones and anticyclones. Looking at more realistic simulations, using CMIP6 data and applying a similar analysis we find analogous linkage between the meandering of the jet and storm genesis under the SSP585 scenario. We will present both our new methodology and results connecting jet meandering and extreme events.

How to cite: Aviv, E. and Kaspi, Y.: Quantifying the relation betwen jet meandering and extreme events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17739, https://doi.org/10.5194/egusphere-egu24-17739, 2024.

Cold waves are characterized by a sharp drop in air temperatures that lasts for a few days, impacting various sectors of society, including human health, agriculture, and transportation. In India, the northern parts of the country witness the majority of cold wave events during boreal winter seasons starting from November to February. In this study, we identify the extreme cold wave events over north India during the period 1951-2020 and investigate their occurrence with rapid Arctic warming through the Quasi Resonant Amplification (QRA) fingerprint. Our findings reveal that a warm Arctic, double zonal jet formation, and amplification of (baroclinic) wave numbers 6 to 7 were observed during extreme cold waves of north India. The upper tropospheric double jet acts as waveguides, trapping the 6-7 wavenumbers, leading to the amplification of Rossby waves, resulting in the persistence of extreme cold wave conditions. Moreover, the sea ice retreat over the Barents-Kara Sea observed during the extreme cold wave events induced by the Arctic warming weakens the equator-to-pole temperature gradient. This leads to the meandering of the jetstream, and promotes the formation of atmospheric blocks. Consequently, an Omega block emerges over the Ural region leading to the advection of cold air from higher latitudes to the northern parts of the country. Hence our study concludes that the Arctic warming which is confirmed through the QRA fingerprint results in highly persistent and anomalous winter weather conditions in north India.

Key words: Extreme cold wave, Quasi Resonant Amplification, Atmospheric blocking, Arctic warming

How to cite: k s, A. and Attada, R.: Role of Arctic Warming on Extreme Cold Weather Conditions in North India, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-279, https://doi.org/10.5194/egusphere-egu24-279, 2024.

Rossby Wave Packets (RWPs) are linked to extreme weather events and exert a strong influence on the predictability of weather systems in the midlatitudes. Considering the whole wave packet, in the sense of the packet envelope, RWPs can be viewed as entities that describe variability of the atmosphere beyond the synoptic scale. We here examine the predictability of RWPs as such entities. As a verification metric we used the so-called Displacement and Amplitude Score (DAS) applied to the envelope field of the midlatitude flow. The DAS is based on a field deforming method and, as one of its major advantages, avoids the “double-penalty” verification problem without the need to identify single RWP objects. We assess RWP predictability using a 19-year period of NOAA GEFSV12 ensemble reforecasts for RWPs that have been previously tracked in reanalysis data.

Forecast errors defined by this metric asymptote towards saturation but do not completely reach saturation within the 10 days lead time available to this study. Corresponding error growth rates maximize during the medium range, in contrast to common error-growth models, in which growth rates are a maximum initially and monotonically decrease with lead time. We hypothesize that this difference relates to the lead-time dependence of error-growth mechanism. Variations in RWP predictability are dominated by the stage of the RWP life cycle, with higher predictability found for the propagation stage than the onset and decay stages. In addition, RWP predictability exhibits a seasonal cycle, with higher predictability in winter than in summer. Controlling for seasonality and the stage of the life cycle, we find i) that high-amplitude RWPs exhibit higher predictability than low-amplitude RWPs for medium-range forecasts and ii) that there is a general pattern of higher predictability over Eurasia than over the ocean basins, with some more detailed variations according to different lead times and life- cycle stages. Finally, predictability of the propagating stage is higher if forecasts are initialized after RWP onset than if initialized before onset. RWP onset thus acts as a partial predictability barrier to the subsequent propagation stage.

How to cite: Riemer, M., Prestel-Kupferer, I., Schmidt, S., and Teubler, F.: Predictability of midlatitude Rossby wave packets , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18176, https://doi.org/10.5194/egusphere-egu24-18176, 2024.

The underlying dynamics responsible for the climatological position of jet streams are complex. In a warming world, there is mounting evidence from modelling and observational studies that amplified upper‐level tropical warming will have a poleward impact on the latitude of the tropospheric jet streams, which will continue across this century. However, existing research has also created confusion over these exact movements/trends, and as such they remain without consensus at any scale nor in any region. Here, we argue that this is in part due to the wide variety of statistics that have been used to define ‘jet latitude’ – one such method of quantifying the jet position, from which to calculate climatological shifts.

In this talk, trends associated with the latitude of the lower tropospheric jet streams are examined over the North Pacific using seven unique jet latitude statistics, four modern climate reanalysis products and CMIP6 historical simulations and future projections. Using these, we assess the relative importance of various associated uncertainties arising from choice of data, scenario, or statistic. The results show that the winter North Pacific Jet is moving polewards within both the reanalysis and climate models. The climatological trend of the North Pacific jet is found to vary by season in the reanalysis, and is most robust to choice of statistic and reanalysis dataset in winter. Finally, a poleward end-of-century shift of the jet position is shown that is robust to choice of statistic and model for autumn.

How to cite: Keel, T., Brierley, C., Frame, T., and Edwards, T.: Exploring uncertainty of trends in the lower-tropospheric North Pacific Jet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3591, https://doi.org/10.5194/egusphere-egu24-3591, 2024.

How to cite: Filippucci, M. and Bordoni, S.: Recent Greenland warming in early summer and its link to atmospheric blocking trends in the Northern Atlantic sector, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2474, https://doi.org/10.5194/egusphere-egu24-2474, 2024.