AS1.30
Presentations: Fri, 27 May | Room 0.11/12
Midlatitude synoptic-scale Rossby waves propagate along narrow bands of enhanced potential vorticity gradients co-located with the jet streams – the jet waveguides. These waveguides influence where and how efficiently the waves can propagate. The structure and location of the waveguides further affects how boundary wave forcing e.g., from the tropics or the surface forces and interacts with the midlatitudes waves. Very persistent waveguides can lead to persistent surface weather, a recent example is the flow situation over the Atlantic in summer 2021. Persistent waveguides potentially offer increased sub-seasonal predictability.
Alas, the story becomes more complicated as the jet waveguides do not exist in isolation, but rather form in response to midlatitude dynamics and boundary forcing and these two-way interactions need to be considered when investigating sub-seasonal predictability. This overview presentation will introduce the key characteristics of the jet waveguides, provide some illustrative examples and close with an overview of open questions and suggestions for ways forward.
How to cite: Romppainen-Martius, O., Kornhuber, K., Tuel, A., Wehrli, K., White, R., and Wirth, V.: Jet waveguides - links to persistent surface weather and sub-seasonal predictability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1142, https://doi.org/10.5194/egusphere-egu22-1142, 2022.
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 skill 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. Object-based methods tend to be highly sensitive to the choice of thresholds used to identify the objects and conceptual issues arise when assigning forecast to analysis objects. We assess RWP predictability using NOAA GEFSV12 ensemble reforecasts for RWPs that have been previously tracked in ERA5 data, due to the higher available temporal resolution.
A prominent result is that RWP predictability depends on the stage of the RWP lifecycle: The propagation stage exhibits higher predictability than the decay or genesis stage. A small seasonal dependence is found, with summer being the least predicable season. No significant dependence is found on size and amplitude of RWPs or their geographical location. The presentation will further discuss the link of RWP predictability to different MJO-Phases as one means to better understand the role of the MJO in midlatitude predictability.
How to cite: Prestel-Kupferer, I., Schmidt, S., Riemer, M., and Teubler, F.: Predictability of Rossby wave packets in the midlatitudes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6962, https://doi.org/10.5194/egusphere-egu22-6962, 2022.
Slower components of the climate system, such as the stratosphere or tropical convection, are potential sources for predictability in the midlatitudes on sub-seasonal time scales of 10-60 days. However, beyond two weeks the skill of current sub-seasonal numerical weather prediction models is generally low. The research project SPREADOUT aims to shed light on how physical and dynamical processes related to synoptic weather systems affect sub-seasonal forecast skill and therefore might hinder exploiting potential sources of predictability.
Large-scale flow patterns, so-called weather regimes, govern the character of weather in the midlatitudes over continent-size regions. In the Atlantic-European region these explain most of the atmospheric variability on sub-seasonal time scales. Many regimes are characterized by blocking anticyclones. Latent heat release in ascending air streams, so-called warm conveyor belts (WCBs), injects air into the upper troposphere. Such diabatic outflow often amplifies the waviness of the jet stream, resulting in ridge building and ultimately in blocking. In this presentation we elucidate the role of diabatic outflow for sub-seasonal predictability of Atlantic-European weather regimes. A specific focus lays on the European Blocking regime, with a pronounced block over western Europe and the North Sea region.
First, we discuss the representation and forecast skill of year-round Atlantic-European weather regimes in 20 years of ensemble reforecast from the Subseasonal-to-Seasonal prediction project data base. Next, we employ the novel AI-based diagnostic “ELIAS 2.0” to identify WCB footprints in the same reforecast dataset. This allows, for the first time, for a systematic evaluation of WCB forecast skill in a NWP model. Interestingly for both, regimes and diabatic WCB outflow, skill vanishes on average already in week 2. We find that European blocking has least skill and show that this is predominantly related to a poor representation of diabatic outflow that helps establishing and maintaining the regime life cycle. Finally we discuss sensitivities of WCB representation to the ensemble configuration and the potential role of WCB activity in establishing teleconnections from remote regions.
Büeler, D., L. Ferranti, L. Magnusson, J. F. Quinting, and C. M. Grams, 2021: Year-round sub-seasonal forecast skill for Atlantic–European weather regimes. Q. J. R. Meteorol. Soc., 147, 4283–4309, doi:10.1002/qj.4178.
Pickl, M., S. T. K. Lang, M. Leutbecher, and C. M. Grams, 2022: The effect of stochastically perturbed parametrization tendencies on rapidly ascending air streams, In revision for Q. J. R. Meteorol. Soc.
Quinting, J. F., and C. M. Grams, 2021a: Toward a Systematic Evaluation of Warm Conveyor Belts in Numerical Weather Prediction and Climate Models. Part I: Predictor Selection and Logistic Regression Model. Journal of the Atmospheric Sciences, 78, 1465–1485, doi:10.1175/JAS-D-20-0139.1.
Quinting, J. F., and C. M. Grams, 2021b: EuLerian Identification of ascending Air Streams (ELIAS 2.0) in Numerical Weather Prediction and Climate Models. Part I: Development of deep learning model. Geoscientific Model Development Discussions, 1–29, doi:10.5194/gmd-2021-276.
Wandel, J., J. F. Quinting, and C. M. Grams, 2021: Toward a Systematic Evaluation of Warm Conveyor Belts in Numerical Weather Prediction and Climate Models. Part II: Verification of Operational Reforecasts. Journal of the Atmospheric Sciences, 78, 3965–3982, doi:10.1175/JAS-D-20-0385.1.
How to cite: Grams, C. M., Büeler, D., Pickl, M., Quinting, J. F., and Wandel, J.: Sub-seasonal atmospheric predictability: understanding the role of diabatic outflow (SPREADOUT), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12039, https://doi.org/10.5194/egusphere-egu22-12039, 2022.
In a water-filled rotating annulus setup, a widely studied laboratory toy model of the mid-latitude atmospheric circulation, we investigated the statistical properties of pointwise temperature "station data" from a series of long experiment runs. Our aim was to explore how the thermal boundary conditions affect temperature fluctuations in a quasi-geostrophic flow system dominated by irregular Rossby waves. While the rotation rates and the geometrical dimensions of the experimental configuration were kept identical in all runs (yielding Taylor number Ta ≈ 3.3×1011), the values of the prescribed "meridional" temperature contrast ΔT – measured between the two coaxial lateral sidewalls of the annular tank – were different, thus Rossby numbers ranged from Ro = 0.009 to 0.026. We found that for larger values of ΔT the temperature distribution became wider, and the thermal fluctuations appeared more symmetric under time-reversal. Discrete AR(1) autoregressive processes were fitted to the time series of temperature fluctuations, and the resulting correlation timescales and average forecast errors of the AR(1) models also increased with ΔT. The persistence of the "weather" in the tank was also evaluated in terms of Hurst exponent spectra. Our results imply that in a realistic polar amplification scenario, where the meridional temperature contrast decreases over time, mid-latitude extreme temperatures (due to Rossby wave dynamics) are not expected to occur more frequently. However, the temporal autocorrelation and predictability of daily temperature signals may decrease, and sudden cooling events may become more likely to occur than sharp increases in temperature.
How to cite: Vincze, M., Hancock, C., and Speer, K.: The effects of meridional temperature contrast on local temperature fluctuations in the mid-latitude atmosphere: a laboratory experiment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10662, https://doi.org/10.5194/egusphere-egu22-10662, 2022.
Summer heatwaves often occur under persistent anticyclones, or blocking. Here we quantify the sole effect of blocking changes on the frequency of heatwaves under global warming in the CMIP6 climate models. We employ an optimized blocking index that best correlates with summer continental heatwaves. Our optimized index gives a Pearson correlation of 0.7 with continental heatwaves and a 1σ uncertainty of 0.20% in projecting 10-year-mean heatwave frequency. The index optimization is achieved by excluding blocking over the ocean, tuning thresholds, mandating perfect stationarity and detecting only anticyclonic anomalies. Counting only the effect of changes in blocking, we project a one fifth decrease in Northern Hemisphere heatwave frequency under SSP585 (a frequency decrease of 0.3% in comparison with the historical frequency of 1.7%). We also find that both the blocking–heatwave correlation and the index thresholds (amplitude and duration) giving the highest correlation, vary insignificantly over different continents and in the future. Therefore, the future increase of around 60% in heatwave frequency under SSP585 is not caused by blocking changes, but by factors, like those of thermodynamics, that boost blocking's capability in driving heatwaves.
How to cite: Chan, P. W., Catto, J., and Collins, M.: Less heatwave-driving blocking under global warming, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8089, https://doi.org/10.5194/egusphere-egu22-8089, 2022.
Atmospheric blocking describes a quasi-stationary weather pattern in midlatitudes which is characterised by a disruption of the westerly flow. Within the blocking anticyclone, large-scale subsidence of air effects dissipation of clouds. Intensive solar radiation during summer leads to a positive radiation balance whereas the balance is negative during winter. Therefore, a longer blocking event is often related to temperature extremes. On the one hand, the positive radiation balance in summer causes heatwaves which are intensified by adiabatic warming of subsiding air and warm air advection from the south on the western side of the block. On the other hand, cold spells occur in winter, especially on the eastern side of the block where advection of cold air from the north intensifies the effect of negative radiation balance. In case of longer persistence of the anticyclone, dry episodes can extend to droughts. The drought can be flanked by flood events due to quasi-stationary low-pressure systems. These weather phenomena have high socio-economic impacts and are good indicators to evaluate the accuracy of models. Thus, futural occurrence of blocking is of interest. We investigate the simulated blocking frequency by CMIP6 global climate models in different scenarios with focus on seasonal and regional trends. To assess the reliability of the models, we compare the historical simulations to centennial reanalyses. The comparison shows that the models underestimate the blocking frequency over Europe, the Northern Atlantic and the Northern Pacific. Furthermore, the models are apparently not able to represent variability and trends during the 20th century as detected by the reanalyses. The scenarios show a general decrease of blocking frequency in Northern Hemispheric midlatitudes during the 21st century. The decreasing trend in the simulations is stronger the stronger the emission of greenhouse gases is. This trend could be related to weakening of Atlantic Meridional Overturning Circulation (AMOC) which is expected to be stronger in high-emission scenarios. However, a closer analysis of the ensemble mean of the models indicates regional and seasonal differences. For example, a strong decrease is simulated over the Northern Atlantic during winter whereas the models simulate an increase of blocking frequency over Eastern Europe and Siberia during summer. Furthermore, we developed an approach for an ensemble mean weighted by mean square error (MSE) to improve the confidence in the simulations. The weighted ensemble mean confirms the trends of the non-weighted ensemble mean and shows only slight differences. Thus, the application of a weighted ensemble mean yields only small improvements.
How to cite: Lohmann, R., Purr, C., and Ahrens, B.: Atmospheric Blocking Trends and Variability in CMIP6-Simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1716, https://doi.org/10.5194/egusphere-egu22-1716, 2022.
Laboratory experiments are used to study the effect of Arctic warming on the amplitude and zonal phase speed of mid-latitude jet meanders. Our results show that a progressive decrease of the meridional temperature difference 1) slows down the eastward propagation of the jet stream, 2) complexifies its structure, and 3) increases the frequency of extreme events. Extreme events and temperature variability show a clear trend in relation to the Arctic warming only at latitudes influenced by the jet stream, whilst such trend reverses in the equatorial region south of the subtropical jet. Despite missing land-sea contrast in the laboratory model, we find similar trends of temperature variability and extreme events in the experimental data and the National Centers for Environmental Prediction (NCEP) reanalysis data. Moreover, our data qualitatively confirm the decrease in temperature variability due to the meridional temperature gradient weakening (which has been proposed recently based on proxy data). Probability distributions are weakly sensitive to changes in the temperature gradient, which is in accordance with recent findings from quasigeostrophic models.
How to cite: Harlander, U., Rodda, C., and Vincze, M.: A laboratory perspective of extreme events in a global warming scenario, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5050, https://doi.org/10.5194/egusphere-egu22-5050, 2022.
Most studies on blocking events have so far focused on changes in the frequency of blocking events and the resulting weather extremes. The duration of blocking events and its potential changes under climate change have received much less attention. Previous studies are uncertain and show slightly different changes in the duration of blocking events. Using three large-ensemble, fully coupled general circulation model simulations, and two different blocking indices, we show no noticeable change in the duration of blocks under future climate change. Based on the results from the 1D traffic jam, proposed by Nakamura and Huang in 2018, we have found that an increase in the mean flow (speed of the jet) or transient eddy forcing independently, cause a reduction in the duration of blocking events. However, in reality, the transient eddy forcing and the speed of the jet are varied dependently, and they are likely to covary through shear. To address this issue, we diagnosed the relationship between transient forcing and the meridional temperature gradient in the GFDL dry dynamical core. By substituting this relationship in the 1D model, we show no changes in the duration of blocking events consistent with a constant duration in the GFDL dry dynamical core simulations. The responses of transient forcing and mean flow under climate change in a comprehensive GCM explain the insignificant change in the duration of blocks. It is worth mentioning that we only focus on the duration of blocking events as one of the important factors in driving extreme weather events, however, the impacts of other essential factors such as thermodynamics and soil moisture are needed to be further investigated.
How to cite: Nabizadeh, E., Lubis, S., and Hassanzadeh, P.: Lack of Change in Atmospheric Blocking Duration in a Warming Climate, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6509, https://doi.org/10.5194/egusphere-egu22-6509, 2022.
In the context of the accelerating global warming, surface air temperature in the Arctic region is increasing faster than the Northern Hemisphere or global average. This phenomenon is expression of the so-called Arctic Amplification (AA), and is related to the interaction of several processes, including the observed reduction in sea ice and the associated ice-albedo feedback. The AA potentially affects atmospheric circulation patterns in the Northern Hemisphere, namely by modifying the westerly jet stream dynamics and the occurrence of weather regimes. In particular, atmospheric blocking at mid-to-high latitudes may be subject to significant variations in formation, frequency, spatial patterns and intensity due to of the changes in the AA-induced changes in atmospheric circulation.
The objective of this study is to investigate the dynamics of atmospheric blocking in the Northern Hemisphere, by analysing the variability and frequency of the associated spatial patterns at decadal time scales. To this aim, the ERA5 reanalysis is used, and blocking events are detected based on geopotential height gradients between mid and high latitude regions.
Results highlight that blocking is associated with the occurrence of extreme events, in particular with increased likelihood of heatwaves and cold spells in the blocking high. Moreover, impacts are observed in the region adjacent to the blocking high, due to the persistent deflection of synoptic disturbances. The relationship between AA and blocking events is also analysed, to identify possible mechanisms controlling the variability of atmospheric blocking in the last decades.
How to cite: Cadau, M., Gaetani, M., Fosser, G., and Bordoni, S.: Atmospheric blocking and Arctic Amplification: climatology and associated impacts in ERA5 data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6610, https://doi.org/10.5194/egusphere-egu22-6610, 2022.
Changes in the Greenland Blocking Index (GBI) present a threat to both the mass loss of the Greenland ice sheet, potentially leading to increased sea level rise, and downstream regions, such as the European continent and Mediterranean Basin that may be affected by associated changes in the frequency and characteristics of weather patterns. While a consistent increase in the summer GBI has been detected in (several) observational and reanalysis datasets, global climate models and seasonal prediction systems so far have failed in capturing this historical trend. The question whether the observed trend is real and not reproduced by models or an artifact of multidecadal variability (and, hence, a transient phenomenon) has so far not been answered. Here we apply the Peter and Clark momentary conditional independence (PCMCI) causal discovery tool to (i) unravel essential causal drivers of the GBI in a set of selected atmospheric fields at intraseasonal (weekly and 3-day averages) time scales in observations and (ii) compare the latter with those found in the ECMWF SEAS5 seasonal forecasts. Observed causal links well reproduced by the dynamical model would be considered indicative of the trend in GBI found in observations more likely emerging because of the selected time frame. By contrast, any differences between the two sets of causal drivers in observations and models would support the hypothesis that some key mechanisms are not appropriately represented in the forecast model, preventing it from correctly reproducing the observed trend.
How to cite: Di Capua, G., Beckmann, J., Coumou, D., Davini, P., Donner, R. V., and Rousi, E.: Increasing trend in the Greenland blocking Index: can causal inference help to explain discrepancies between observations and models?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9825, https://doi.org/10.5194/egusphere-egu22-9825, 2022.
Consensus on the cause of recent midlatitude circulation changes toward a wavier manner in the Northern Hemisphere has not been reached, albeit a number of studies collectively suggest that this phenomenon is driven by global warming and associated Arctic amplification. Here, through a fingerprint analysis of various global simulations and a tropical heating-imposed experiment, we suggest that the suppression of tropical convection along the Inter Tropical Convergence Zone induced by sea surface temperature (SST) cooling trends over the tropical Eastern Pacific contributed to the increased summertime midlatitude waviness in the past 40 years through the generation of a Rossby-wave-train propagating within the jet waveguide and the reduced north-south temperature gradient. This perspective indicates less of an influence from the Arctic amplification on the observed mid-latitude wave amplification than what was previously estimated. This study also emphasizes the need to better predict the tropical Pacific SST variability in order to project the summer jet waviness and consequent weather extremities.
How to cite: Ding, Q., Sun, X., Wang, S.-Y. S., Topál, D., Li, Q., Castro, C., Teng, H., Luo, R., and Ding, Y.: Enhanced jet stream waviness induced by suppressed tropical Pacific convection during boreal summer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1126, https://doi.org/10.5194/egusphere-egu22-1126, 2022.
Circumglobal Rossby wave patterns (CRWPs) are large-scale configuration characterized by amplified Rossby waves stretching in the zonal direction across a broad portion of a latitude circle. Previous research considered CRWPs mainly from two points of view: either as particular type of planetary wave, undergoing quasi-resonant amplification when trapped between two turning latitudes, or as a single Rossby wave packet (RWP) being ducted zonally in presence of particularly strong meridional potential vorticity gradient, that acts as a waveguide. Although with different characteristics, a structured waveguide for Rossby waves is needed in both cases.
Space/time spectral analysis allows to study CRWPs from their spectral projection over a range of wavenumber/phase speed harmonics, with the implicit assumption that a CRWP would project only over few zonal wavenumbers and propagation speeds characterizing it over a broad portion of the hemisphere. Such spectra are calculated from meridional wind at 250hPa over a sliding time window of 61d, in order to consider the intraseasonal variability in spectral power. The application of principal component analysis to this data set allows for few leading variability modes in the spectral domain during boreal winter, and these modes appear to be related to CRWPs.
They correspond to the baroclinic propagation of amplified RWPs from the Pacific to the Atlantic storm track in a hemispheric flow configuration displaying enhanced meridional gradients of geopotential height over midlatitudes. The first CRWP is forced by tropical convection anomalies over the Indian Ocean and features the propagation of amplified Rossby wave packets over northern midlatitudes, while the second one propagates rapidly over latitudes between 35◦N and 55◦N and appears to have extratropical origin. An anomalous equatorward propagation of Rossby waves from the Atlantic eddy-driven jet to the North African subtropical jet is observed for both CRWPs These results support the perspective that CRWPs are substantially related with the circumglobal propagation of RWPs.
How to cite: D'Andrea, F., Riboldi, J., Rousi, E., Lott, F., and Riviere, G.: Insights into circumglobal Rossby wave patterns from space/time spectral analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7073, https://doi.org/10.5194/egusphere-egu22-7073, 2022.
Recurrence of Rossby wave packets in a short time, termed as RRWPs, can lead to persistent and extreme weather events. Here, we study top RRWP events in the two basins: the North Atlantic and the East Pacific basins during 1979–2018 using ERA-5 reanalysis data. With the help of composite maps, we answer questions such as what are the characteristics of these events? Do they have a preferred phase? Is a particular flow configuration conducive to such events? What is the role of atmospheric blocks? And, whether these events are initiated by a common pathway such as tropical forcings or have several different pathways? We find that these events have a preferred phase configuration which varies with season. Both North Atlantic and East Pacific events are dominated by specific wavenumbers. In winter, dominant wavenumbers are 3, 4, and 5. In contrast, wavenumbers 5, 6, and 7 dominate in JJA. Furthermore, we address the role of low frequency zonal flow, blocks, and tropical convection during RRWP events using a causal network framework.
How to cite: Ali, S. M., Meyer, L., Röthlisberger, M., Methven, J., Zscheischler, J., and Martius, O.: Recurrent Rossby wave packets in the Northern Hemisphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3420, https://doi.org/10.5194/egusphere-egu22-3420, 2022.
Recently, Nakamura and Huang proposed a simplified model of blocking onset in which blocks form due to an obstruction of the zonal propagation of wave activity on the mid-latitude waveguide analogous to how traffic jams emerge on a highway. The theory is derived from the budget of finite-amplitude local wave activity which can quantify blocking even during the non-linear and high-amplitude stage accurately.
Using the local wave activity framework, we investigate the development of a winter European block and asssess the possible role of the "traffic jam" mechanism in the flow transition. We determine processes contributing to the blocking onset by evaluating the terms of the wave activity budget with data and carry out an ensemble sensitivity analysis to track precursor Rossby wave activity through time. Complementing previous reanalysis-based composite studies which found a large case-to-case variability, the ensemble approach enables us to apply the wave activity framework to individual blocking events, yielding a flow-dependent analysis.
We find evidence for a “traffic jam” blocking onset on 18 December 2016. Block development is sensitive to upstream precursor Rossby wave activity up to 2.5 days prior to the onset date. However, threshold behavior as implied by the idealized theory is not detected. The relationship of finite-amplitude local wave activity and its zonal flux as mapped by the ensemble exhibits the established characteristics of a traffic jam. We therefore suggest that the traffic jam mechanism may play an significant role in some cases of blocking onset and discuss the implications for the predictability of blocking.
How to cite: Polster, C. and Wirth, V.: Evidence for a "Traffic Jam" Onset of Blocked Flow from Ensemble Sensitivity Analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1703, https://doi.org/10.5194/egusphere-egu22-1703, 2022.
A full understanding of the dynamical behaviour of atmospheric blocking is still lacking, despite the influence of blocking towards hazardous mid-latitude weather extremes. Using geopotential height anomalies, relative to the zonal mean flow and persistent longitude-dependent eddies, and an objective feature tracking algorithm, the climatologies and lifecycles of anticyclones that contribute to blocking events are explored. Case studies and a climatology for blocking are presented using this process, and results show that this method performs favourably in relation to existing block detection methods since most blocking configurations are successfully detected. Then, blocking events are classified according to location of occurrence and persistence, and characteristics including intensity and areal extent are examined. The anticyclonic features contributing to blocks are also studied in terms of their genesis and lysis regions, along with anomaly strength and speed. It is found that many of the anticyclonic features that enter a block form a long way upstream, before travelling along the Rossby wave guide and intensify in the block. Furthermore, anticyclonic features that leave a block can then proceed to re-intensify further downstream and be part of a further blocking episode in a new location. This shows that there is an inherent interaction between transient waves and stationary blocks, and these results provide evidence for the previously-proposed selective absorption mechanism (SAM) for block maintenance.
How to cite: Suitters, C., Martinez-Alvarado, O., Hodges, K., Schiemann, R., and Ackerley, D.: Atmospheric blocking and the contribution of transient waves to its onset, decay and maintenance , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12203, https://doi.org/10.5194/egusphere-egu22-12203, 2022.
The North Atlantic eddy-driven jet (EDJ) is the most outstanding component of the Euro-Atlantic atmospheric circulation. Classically, the winter EDJ variability has been described in terms of latitude and intensity. However, this classification has recently been shown as incomplete to fully characterize more complex EDJ states. For instance, the three well established latitudinal states of the EDJ do not match the four preferred flow regimes of the Euro-Atlantic sector. A possible solution to this trouble would be the additional consideration of other EDJ characteristics, such as the tilt, waviness or longitudinal extension among others, but this has not been investigated yet.
This study presents a set of daily parameters of the North Atlantic EDJ that allows for a dissection of the EDJ structure beyond the well-established trimodality in reanalysis data. The set is composed by intensity, sharpness, position, latitudinal boundaries, longitudinal extension, tilt and zonal asymmetries, which provides a manageable treatment of EDJ configurations without the need of high-dimensional 2-D fields. The parameters are computed through new developed diagnostics based on winter North Atlantic daily (zonal) wind, averaged between 925-700 hPa. The multi-parametric approach allowso deepen the structure of the EDJ, as well as the impacts and drivers of its variability.
Recurrent states of the EDJ are assessed via multiparametric clustering, which is able to reconstruct the trimodality of the EDJ and more complex EDJ configurations described in the literature. Furthermore, the considered parameters turn useful for the study of regional surface impacts of the EDJ, which are better explained by a combination of parameters rather than changes in individual parameters such as the jet latitude.
Lastly, the influence of a selected number of potential external drivers of the EDJ is analysed by means of linear regressions and composite analyses based on reanalysis data. The winter stratospheric vortex and previous late summer North Atlantic horseshoe are identified as the most influential drivers of different EDJ parameters, in agreement with some previous studies. However, winter snow, ice cover or sea surface temperature patterns can also affect to some extent specific parameters. These results suggest some potential predictability of the winter EDJ.
How to cite: Barriopedro, D., García-Burgos, M., Ayarzagüena, B., and García-Herrera, R.: Multiparametric perspective of the North Atlantic eddy-driven jet: configurations, regional impacts and remote drivers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4621, https://doi.org/10.5194/egusphere-egu22-4621, 2022.
The distinction between eddy-driven and subtropical jets is conceptually important and well-founded through different driving mechanisms and dominant types of variability. Despite this clear conceptual distinction between the jet types, it is not straightforward to distinguish between these jet archetypes in reanalysis data of individually detected jets.
Inspired by EOF and cluster analyses exploring the variablity between different jets, we here propose a novel and easy-to-apply criterion to distinguish between subtropical and eddy-driven jets. We document that for most ocean basins and seasons, there is a clear bimodality in the occurrence of jets in a potential temperature/wind speed space. These two jet modes in this phase space align well with the conceptual expectations for subtropical and eddy-driven jets regarding their vertical structure as well as their regional and seasonal occurrence.
Surprisingly, the bimodality in phase space is most pronounced in regions such as the western North Pacific during winter, where jets are typically regarded as “merged”, a mixture of eddy-driven and subtropical. Our results thus call into question this typical intepretation of the Pacific jet, and rather suggest that the Pacific winter jet becomes more “merged” in character towards the eastern end of the storm track.
How to cite: Spensberger, C., Li, C., and Spengler, T.: Separating eddy driven and subtropical jets in reanalyses, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6945, https://doi.org/10.5194/egusphere-egu22-6945, 2022.
The incoherent Hadley cell (HC)-edge and jet-latitude change is found during the Last Glacial Maximum (LGM), whereas the robust coherent shifts of HC edge and eddy-driven jet latitude are evident in the present and future climate, especially in the Southern hemisphere. By performing parameter sweep experiments, here we investigate this incoherent HC–jet change in the LGM-like and global warming-like experiments with a dry dynamical core of the general circulation model where cooling or heating is imposed in tropical upper-troposphere and polar surface. The LGM-like experiments reveal that an incoherent HC–jet change, i.e., a poleward shift of the jet latitude but an equatorward shift of the HC edge, appears when the polar forcing is substantially stronger than the upper-tropospheric tropical cooling, indicating the broadened baroclinic zone. This broadened baroclinic zone is explained by the separate roles of fast and slow waves. As polar cooling is enhanced, fast waves contribute to the poleward shifted jet in the midlatitudes. However, slow waves in the subtropics do not change much in response to polar cooling due to an equatorward shifted critical latitude by the upper tropospheric cooling, resulting in the HC edge located on the equatorward side compared to the control simulation. Such an incoherent HC–jet change is also found in the global warming-like experiments, suggesting that a poleward HC shift but an equatorward jet shift may occur in the future climate with accelerated Arctic amplification.
How to cite: Kim, S.-Y. and Son, S.-W.: Incoherent Hadley cell and jet change in the Last Glacial Maximum: a parameter sweep study using a dynamic-core GCM, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-442, https://doi.org/10.5194/egusphere-egu22-442, 2022.
Numerical solutions of the eigenvalue equation associated with zonally propagating waves of the Linearized Rotating Shallow Water Equations are derived in a channel on the equatorial β-plane in the presence of a uniform mean zonal flow. The meridionally varying mean height field is in geostrophic balance with the prescribed mean zonal flow. In addition to the trivial Doppler shift of the free waves’ phase speeds, the mean state causes the dispersion curves of each of the free Rossby and Poincaré waves to coalesce in pairs of modes when the zonal wavenumber increases. For large zonal wavenumber or large mean flow, the latitudinal variation of the waves’ amplitudes differs from that of free waves i.e., Hermite Functions (in wide channels) and Harmonic Functions (in narrow channels) do not describe the amplitude structure. For large mean speed and for large zonal wavenumber the eigenvalue problem loses its Sturm-Liouville structure and the eigenfunctions have multiple extrema between successive zeros of the function itself. In contrast to free Kelvin waves, in the presence of a mean flow the meridional velocity component of these waves does not vanish identically. For zonal stratospheric winds of order 20 m s-1 and for gravity wave speed of order 25 m s-1 the phase speed with mean wind can be twice that of the classical theory with no mean zonal wind.
How to cite: De-Leon, Y., Paldor, N., and Garfinkel, C. I.: Equatorial waves on the β-plane in the presence of a uniform zonal flow: Beyond the Doppler shift, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1205, https://doi.org/10.5194/egusphere-egu22-1205, 2022.
Potential vorticity (PV) analysis plays a central role in studying atmospheric dynamics and in particular in studying the life cycle of weather systems. The three-dimensional (3-D) structure and temporal evolution of the associated PV anomalies, however, are not yet fully understood. An automated technique to objectively identify 3-D PV anomalies can help to shed light on 3-D atmospheric dynamics in specific case studies, as well as facilitate statistical evaluations within climatological studies. Such a technique to identify PV anomalies fully in 3-D, however, does not yet exist.
This study presents a novel algorithm for the objective identification of PV anomalies. The algorithm is inspired by morphological image processing techniques and can be applied to both two-dimensional (2-D) and 3-D fields on vertically isentropic levels.
The method maps input data to a horizontally stereographic projection and relies on an efficient computation of horizontal distances within the projected field. Candidates for PV anomaly features are filtered according to heuristic criteria, and feature description vectors are obtained for further analysis. The generated feature descriptions are well suited for subsequent case studies of 3-D atmospheric dynamics, or for generation of climatologies of feature characteristics.
We evaluate our approach by comparison with an existing 2-D technique, and demonstrate the full 3-D perspective by means of meteorological case studies comprising tropical cyclogenesis and a subtropical extreme rainfall event. These case studies demonstrate the complexity and variations in the 3-D structure of the detected PV anomalies. Such anomalies are often insufficiently captured by a 2-D method. We discuss further advantages of using a 3-D approach, including elimination of temporal inconsistencies in the detected features due to 3-D structural variation, and elimination of the need to manually select a specific isentropic level on which the anomalies are assumed to be best captured.
How to cite: Fischer, C., Fink, A. H., Schömer, E., van der Linden, R., Maier-Gerber, M., Rautenhaus, M., Chung, S. Y., Kriening, M., and Riemer, M.: A novel method for objective identification of weather-relevant 3-D potential vorticity anomalies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4671, https://doi.org/10.5194/egusphere-egu22-4671, 2022.
In the Northern-Hemisphere mid latitudes the winter land-sea thermal contrast is expected to decrease with increasing CO2 in the atmosphere, due to a faster warming of the continents with respect to the oceans. Moreover, the reduction of the winter thermal contrast is basin dependent, as it is influenced by regional warming patterns specific of the Atlantic and Pacific sectors, e.g. by the North-Atlantic Warming Hole.
In this work we run a set of idealised perpetual-winter numerical experiments made with the simplified atmospheric circulation model SPEEDY where the extratropical land-sea thermal contrast is reduced by means of warm temperature anomalies over the continents. The reduction of the thermal contrast is performed first over the whole Northern Hemisphere, then over individual basins - Atlantic and Pacific - by warming, in turn, the land temperature of East Asia and North America. The impact of the reduced winter contrast on the mid-latitude tropospheric circulation is analysed with a focus on stationary planetary waves, the jet streams and the associated storm tracks. From the individual-basin approach we find that the Pacific land-sea thermal contrast is particularly important for the shape and amplitude of the stationary planetary waves and that it affects the whole Northern-Hemisphere circulation, reaching the North Atlantic storm track and jet. The role of the stratospheric pathway in the tropospheric response to reduced thermal contrast is also investigated, and shows nearly opposite features with respect to reduced Atlantic or reduced Pacific thermal contrast.
How to cite: Portal, A., Pasquero, C., D'Andrea, F., Davini, P., Hamouda, M., and Rivière, G.: The effects of reduced Atlantic and Pacific land-sea thermal contrast on the extratropical circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6394, https://doi.org/10.5194/egusphere-egu22-6394, 2022.
The heat wave that enveloped the Pacific Northwest from late June through early July 2021 delivered unprecedented temperatures to the normally cool region --- 108°F (42°C) in Seattle, 116°F (47°C) in Portland --- and claimed over 1000 lives mostly in British Columbia. We investigate the meteorological and dynamical conditions that led to this extreme heat event. The extreme surface temperatures (29-30 June) were preceded by the formation of an upper-level atmospheric blocking that snatched a warm pool of air from lower latitudes (25-27 June). A heat-trapping stable stratification ensued within the block, raising the surface temperatures significantly. The block itself was initiated by an upper-tropospheric wave breaking and the concomitant surface cyclogenesis off the coast of Alaska a few days prior (22-24 June). The regional local wave activity budget reveals that a localized diabatic source associated with this storm critically contributed to the zonal wave activity flux downstream, whose convergence over Canada drove the blocking. A simple wave activity-based reconstruction predicts a 41 percent reduction in strength and a 10-degree eastward displacement of the block when the upstream diabatic source is reduced by just 30 percent. Our work complements previous trajectory-based studies to gain insight on the role of diabatic heating in blocking episodes.
How to cite: Nakamura, N.: The 2021 Pacific Northwest heat wave and associated blocking: meteorology and the role of an upstream cyclone as a diabatic source of wave activity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1930, https://doi.org/10.5194/egusphere-egu22-1930, 2022.
This work investigates systematic changes of the global atmospheric circulation during midlatitude heat waves in spectral space. The basis functions for the expansion of global data are the eigensolutions of the linearised primitive equations; here, the Rossby waves are represented in terms of Hough harmonics that are defined by their zonal wavenumber, meridional modes and vertical structure functions. Their orthogonality allows diagnostics in terms of energy of the zonal mean flow and wave energies. The diagnostic provides a holistic dynamical picture of the variability spectrum of Rossby waves and allows scale-selective filtering of modes of variability in physical space. The method is applied to reanalysis datasets starting in 1980.
The reconstructed circulation (the inverse projection onto wind and geopotential fields based on selective filtering) during heat waves is dominated by large-scale anticyclonic systems in agreement with previous studies, thereby demonstrating the physical meaningfulness of the applied method. Probability distribution functions of Rossby wave energies are evaluated separately for the zonal mean flow, for the planetary and synoptic zonal wavenumbers with the tropospheric barotropic structure. Time series of wave energies are characterised by a chi-square distribution. The chi-square skewness shows a reduction in the number of involved degrees of freedom for planetary-scale circulation by 50% during the heat waves. This reduction yields a quantitative estimate for the coarse large-scale structure of blocking events and its barotropic structure in the midlatitude troposphere. The robustness of the results is ensured by considering the four reanalyses: ERA5, ERA-Interim, JRA-55 and MERRA datasets.
How to cite: Strigunova, I., Blender, R., Lunkeit, F., and Žagar, N.: Signatures of midlatitude heat waves in the global atmospheric circulation , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5137, https://doi.org/10.5194/egusphere-egu22-5137, 2022.
Persistent dry winter events over the Eastern Mediterranean (EM) disrupt the precipitation patterns in the rainy (winter) season and dramatically reduce water availability in the region. Self-organizing map classification of atmospheric profile data over Israel has shown that the most persistent (over 4 days) dry and warm winter extreme events are induced by a stagnant ridge over the EM, pronounced trough/cutoff low over the western/central Mediterranean and blocking over the north Atlantic. It is however, unclear how this Rossby wave pattern emerges and what are the atmospheric mechanisms that govern the associated development of dry and warm surface conditions. Here we objectively identify persistent dry and warm winter events over Israel, and focus on three case studies, aiming to understand the relation between the baroclinic synoptic setting, precursor Rossby waves, and how the dry and warm conditions emerge using a Lagrangian approach to study the history of the involved airmasses. We found large day-to-day variability within events and identified the leading mechanisms of the warm and dry conditions to be: adiabatic heating during slantwise subsidence, heating by sensible heat fluxes from the surface, and advection of warm and dry continental air. In addition, the Atlantic blocking and EM ridge are supported by upstream diabatic heating in warm conveyor belts (WCB) of north Atlantic cyclones and Mediterranean cyclones, respectively. A tripole flow during these events demonstrates the sequential relation between Atlantic ridge (or block), trough over Europe and ridge over EM and/or West Russia. These results place local persistent warm and dry anomalies as an outcome of a stationary Rossby-wave pattern, providing new opportunities for understanding such extremes and their predictability.
How to cite: Raveh-Rubin, S. and Berkovic, S.: Persistent warm and dry extremes over the Eastern Mediterranean during winter: the role of upstream Rossby wave pattern, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-683, https://doi.org/10.5194/egusphere-egu22-683, 2022.
Dust storm genesis is one of several high-impact weather phenomena that may result from Rossby wave breaking (RWB). The wave propagation into low latitudes over North Africa facilitates instability through upper-level divergence and the reduction of the static stability ahead of the cyclonic side of the wave train, and therefore modulates the low-level tropospheric conditions in dust source areas.
The analysis of three dust storm case studies that strongly impacted the Iberian Peninsula and another one impacting the Cape Verde Islands indicates that a double RWB process within the Polar Jet (PJ) stream over the eastern North Atlantic and western Europe is a common large-scale upper-level precursor to the formation of the large North African dust storms and subsequent transport of dust (Orza et al, 2020; Dhital et al, 2020).
The synoptic and larger subsynoptic scale features were analyzed using ERA-Interim reanalysis while mesoscale features were studied by high-resolution WRF-CHEM simulations. There are substantial differences between the case studies in the location and geometry of upper- and low-level features following the first break (Dhital et al, 2020; 2021). However, in all case studies (1) RWB within the PJ stream is observed; (2) there is a substantial poleward expansion of the upper-level subtropical ridge; and (3) the secondary RWB is amplified by nonlinear wave reflection from the first break.
Preliminary results from a larger number of cases study indicate the relevance of secondary RWB in the PJ stream. The dynamics may also result in cutoff lows and amplified low-PV ridging accompanying cyclogenesis and heatwaves, respectively, which emphasizes the need to improve our understanding of the involved processes.
- Dhital S, ML Kaplan, JAG Orza, S Fiedler (2020). Atmospheric dynamics of a Saharan dust outbreak over Mindelo, Cape Verde Islands, preceded by Rossby wave breaking: Multiscale observational analyses and simulations. J. Geophys. Res. Atmos. 125, e2020JD032975.
- Dhital S, ML Kaplan, JAG Orza, S Fiedler (2021). Poleward Transport of African Dust to the Iberian Peninsula Organized by a Barrier Jet and Hydraulic Jumps: Observations and High-Resolution Simulation Analyses. Atmos. Environ. 261, 118574.
- Orza JAG, S Dhital, S Fiedler, ML Kaplan (2020). Large Scale Upper-level Precursors for Dust Storm Formation over North Africa and Poleward Transport to the Iberian Peninsula. Part I: An Observational Analysis. Atmos. Environ. 237, 117688.
How to cite: G. Orza, J. A., Dhital, S., Fiedler, S., and Kaplan, M. L.: Large-scale forcing of extreme African dust storms by double Rossby wave breaking, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10592, https://doi.org/10.5194/egusphere-egu22-10592, 2022.
The Gulf of Genoa is one of the most cyclogenetic places on the globe, mostly due to Alpine lee-cyclogenesis. The mistral wind is an inherent feature in this process, known for its positive contribution to the deepening of the lee-cyclone. The mistral is recognized by many as one of the most dangerous weather regimes in the Mediterranean and has been associated to extreme weather events in the region. While extensive research has focused on the lower-tropospheric mistral and lee cyclogenesis, the features of the Rossby waves that drive the process are not generally known. Here, the isentropic potential vorticity (PV) structures governing the occurrence of the mistral wind are classified using a self-organizing map (SOM) clustering algorithm. A 36-year (1981–2016) climatological classification of Rossby waves generating a mistral wind is performed based on daily ERA-Interim isentropic PV data. 16 distinct mistral associated PV structures are identified, where each classified flow pattern corresponds to a different type or stage of the Rossby wave life cycle. From broad troughs to thin PV streamers to distinguished cutoffs, each of these PV patterns exhibits a distinct surface impact. A clear seasonal separation between the clusters is evident, and transitions between the clusters correspond to different Rossby wave- breaking processes. This analysis provides a new perspective on the surface impact of Rossby waves throughout their lifetimes, and their influence on extreme weather events.
How to cite: Givon, Y., Keller Jr., D., Silverman, V., Pennel, R., Drobinski, P., and Raveh-Rubin, S.: The mistral wind from a Rossby-wave perspective: a climatological classification of RWB, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-690, https://doi.org/10.5194/egusphere-egu22-690, 2022.
Remote influences on extreme anomalous rainfall over North and South Vietnam during extended summer (May-October) are explored using a 38-year (1979-2016) ERAi global dataset. Composite WET and DRY events with lags of up to two weeks are assembled for rainfall indices over subregions of Vietnam. The large-scale dynamical precursors to these events are explored through diagnostics of the extratropical flow.
Rainfall extremes of opposing signs show asymmetrical large scale precursors and different pathways of influence. Both WET and DRY events over North Vietnam are seen to originate from Europe and propagate either at high latitudes or along the Asian Jet. A persistent Siberian High is linked with WET events over North Vietnam. Anomalies also develop along the Asian Jet for both regions, but only for DRY events over South Vietnam. Events over South Vietnam are also much more influenced by tropical precursors.
The Dynamical Research Empirical Atmospheric Model (DREAM) was used to examine pathways of influence to the circulation over Vietnam sub-regions in the medium range. A set of stationary wave experiments was conducted with artificial heat sources in different locations around the world, using ERAi summer climatology as a basic state. Influence functions with respect to upper-level divergent flow over the target regions of North and South Vietnam indicate two different pathways: extratropical wave-trains and tropical waves. For North Vietnam, heat sources over Europe give the most influence after 15 days. For South Vietnam, the influence is much weaker and significant precursors are more likely to be tropical Kelvin wave sources.
How to cite: Le, H.-H., Hall, N., and Ngo-Duc, T.: Rossby wave teleconnections to rainfall anomalies over Vietnam, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2285, https://doi.org/10.5194/egusphere-egu22-2285, 2022.
This study analyzes the seasonal evolution and trends of atmospheric blocking and their influences on temperature and moisture transport around the Antarctic Peninsula for the period 1979-2020. A geopotential height-based method based on the ECMWF’s ERA5 and its predecessor ERA-Interim was applied over two domains, one located to the west (150-90W, 50-70S) and the other over and to the east (90-30W, 50-70S) of the Antarctic Peninsula. Spatial patterns of geopotential heights on days with blocking feature well-defined ridge axes over and west of much of South America, and days with the most extreme blocking (above the 99th percentile) show upper-tropospheric ridge and cut-off low features that have been associated with extreme weather patterns. Blocking days are found to be more frequent in the first half of the period (1979-1998) than the second (1999-2018) in all seasons in the west domain, whereas they seem to be more common over the eastern (Peninsula) domain in 1999-2018 for austral winter, spring, and autumn, although all these differences are not statistically significant. Meteorological observations in the Antarctic Peninsula indicate colder conditions than the observed climatology in the Antarctic Peninsula during blocking days over the western domain. On the contrary, mean blocking days over and to the east of the Peninsula yield warmer conditions than the climatology. Similar to the observed pattern, ERA5 also shows colder and warmer conditions during blocking days over the western and eastern domains, respectively. A further analysis with ERA5 indicates that blocking days over the Drake Passage and to the east of the Peninsula are associated with positive moisture transport anomalies towards the Peninsula coinciding with atmospheric river events, which trigger warm and humid conditions over the Peninsula, particularly in austral autumn. These results suggest that blocking patterns around the Antarctic Peninsula can have notable impacts on moisture transport and extreme temperature events affecting the cryospheric processes, particularly over the leeward side of the Peninsula and Larsen C Ice Shelf.
How to cite: Bozkurt, D., Marín, J. C., and Barrett, . S.: Atmospheric blocking patterns around the Antarctic Peninsula and their influences on temperature and moisture transport, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10555, https://doi.org/10.5194/egusphere-egu22-10555, 2022.
A better understanding of the dynamics in the upper troposphere and lower stratosphere (UTLS) is important for e.g., improving surface weather predictability. This study focuses on wave propagation in the UTLS and its role in the connection between blocking and stratospheric variability associated with sudden stratospheric warming (SSW) events. We analyse the properties of the quasi-geostrophic Eliassen-Palm (EP) flux in the UTLS using vertically high-resolved Global Navigation Satellite System (GNSS RO) radio occultation observations.
The results show that GNSS RO observations provide detailed information on the vertical structure in the UTLS and are suitable for observing the atmospheric circulation. The EP flux obtained with the GNSS RO is in good agreement with theory and previous studies. We find that SSW is preceded by enhanced EP flux propagating upward in the UTLS, associated with blocking events. In the case of wave reflection, downward propagation leads to blocking over the North Pacific.
How to cite: Yessimbet, K., Ossó, A., Ladstädter, F., and Steiner, A.: Observed connection between blocking and sudden stratospheric warming events using GNSS radio occultation observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10562, https://doi.org/10.5194/egusphere-egu22-10562, 2022.
The Mediterranean Basin has several features that enhance cyclonic activity, such as its complex topography and sharp land-sea temperature differences. However, some processes occurring outside the basin can influence its seasonal variability. In this work, we highlight the relationship between the North Atlantic Storm Track (NAST) and cyclones passing through the Eastern Mediterranean (EM). We use Empirical Orthogonal Function (EOF) analysis of upper-level meridional wind variance to inspect monthly NAST regimes and their influence on wintertime flow in the EM.
We find that the higher functions (EOF4-5) show stronger links to the region and specifically, to anomalies in synoptic fields associated with cyclones (upper-level potential vorticity, sea level pressure) and in monthly precipitation. These functions manifest as a pair of orthogonal zonal waves, reminiscent of subseasonal teleconnections that were previously linked to seasonal precipitation extremes in the EM (the South Levant pattern). It is hypothesized that this connection is mainly brought about by large-scale adiabatic advection of PV in the upper levels. Meanwhile, the two leading NAST modes (“pulsing” and latitudinal “shifting”) were found to produce a comparatively smaller effect.
On the daily scale, we investigate the NAST-EM connection through the lens of zonally propagating Rossby wave packets, in both reanalysis and CMIP6 models (which capture the patterns well).
How to cite: Sandler, D., Ziv, B., Saaroni, H., Rostkier-Edelstein, D., and Harnik, N.: Atmospheric Rossby Waves as a Link Between North Atlantic Storm Track Variability and Eastern Mediterranean Cyclones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7072, https://doi.org/10.5194/egusphere-egu22-7072, 2022.
Midlatitude atmospheric blockings largely influence the usual synoptic flows propagating eastward in the atmosphere. They are characterized by long-lived, large-scale high-pressure systems, which have a significant impact on the trajectories of jet streams, and thus on the distribution of stormtracks and weather systems they drive. As a result of their formation, long-standing extreme weather can occur in different forms and during different seasons: heat waves, cold waves, drought, floods... Therefore, the study of their future activity and related climate extremes is highly important in the context of a changing climate. Meanwhile, the literature shows that coupled General Circulation Models (GCMs) of climate highly underestimate blocking frequencies in a wide variety of regions, thereby largely limiting the use of GCMs for climate extremes forecasts. This is notable for blockings occurring in regions and during seasons where they are prominent and have the largest societal impacts, such as boreal Winter European Atmospheric Blockings (WEABs). A wide variety of causes have been detected by former studies such as the horizontal resolution of the atmospheric model grid or GCM biases in tropical convection and North Atlantic current position. However, WEAB frequencies remain underestimated by all GCMs of the last three generations of the Coupled Model Intercomparison Project (CMIP3, CMIP5, and CMIP6).
Here, we first use an ensemble of historical simulations from 28 CMIP6 GCMs based on very different numerical schemes and grid resolutions and check their ability to simulate WEABs compared with the ERA5 reanalysis dataset. Using k-means clusterings methods based on blocking patterns, we show that a single type of observed WEABs, namely blockings occurring over the British Isles/North Sea regions, is largely underestimated by all GCMs, whose ensemble mean is nearly 65% lower than the frequency observed in ERA5. On the other hand, ERA5 frequencies of blockings occurring in Greenland, western Europe, and Scandinavia, all fall within the GCM range. This result indicates that GCM biases in simulating WEABs found by former studies are essentially affecting a specific location and, therefore, narrow their panel of potential causes. In addition, considering a subset of 8 out of the 28 GCMs with mesoscale eddy-permitting ocean resolution (i.e. higher than 0.25°x0.25°) from the HighResMIP ensemble, we find that the eddy-permitting parametric scheme allows to significantly increase the frequency of simulated WEABs. For the HighResMIP simulations, the frequency of British Isles/North Sea blockings is doubled for GCMs with eddy-permitting ocean resolutions but is still underestimated by an order of 50% compared with ERA5 reanalysis data. This improvement is here hypothesized to be related to a less biased position of the North Atlantic current, which was identified by former studies as a reason for WEABs underestimation by GCMs.
How to cite: Michel, S., von der Heydt, A., Baatsen, M., van Westen, R., and Dijkstra, H.: A single type of winter European atmospheric blocking underestimated by GCMs, and improvements found for eddy-permitting ocean parametrization , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6335, https://doi.org/10.5194/egusphere-egu22-6335, 2022.
