Recently, weather prediction models based on artificial intelligence (AI) have become equally to slightly more accurate than the established operational weather prediction models in terms of deterministic scores. The much lower computational cost of the AI-models facilitates the generation of large ensembles, hence it is important to assess if their error growth properties are realistic. Here, we investigate error growth from initial condition perturbations with varying amplitudes, simulated with the AI-weather prediction models (PANGU, GraphCast, FourCastNet) and with a “classic” fluid equation-based weather model (ICON). From past research and the global convection-permitting ICON simulations, it is expected that small-amplitude initial condition perturbations would grow very fast initially in areas with latent heat release, then spreading out to larger and larger scales, eventually setting a fixed and fundamental limit to the predictability of weather. This phenomenon is known as the butterfly effect. We find however, that in contrast to ICON, the AI-based models completely fail to reproduce the rapid initial growth rates. Instead their growth rates remain similar to those typically found on synoptic-scales, which incorrectly suggests an unlimited predictability of the atmosphere. In contrast, if the initial perturbations are large in amplitude and comparable to current uncertainties in the estimation of the initial state, the AI-based models basically agree with results from the “physically-based” ICON simulations, although some deficits are still present, mostly related to their particularly low effective resolution. This provides an example of how machine learning models can fail to reproduce a fundamental physical principle, even though they can accurately mimic many observed behaviors.

How to cite: Selz, T. and Craig, G.: Can AI-based weather prediction models simulate the butterfly effect?, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-556, https://doi.org/10.5194/ems2024-556, 2024.

We use a moist adjoint model to compute the initial-condition perturbations that minimize the significant 48-72-h synoptic-scale forecast errors associated with the 15 November 2018 cyclone over the East Coast of the United States. These adjoint-optimal perturbations, which have a maximum amplitude of about 2 K in potential temperature, are widespread, extending throughout the troposphere and across much of North America. We investigate the most impactful subsets of the perturbations by truncating them in physical and spectral space and rescaling them to be equal in an energy norm to the full, unmodified perturbations. When the perturbations are confined to localized target regions, including those with the highest adjoint sensitivity or the strongest convective instability, they have weaker impacts on the forecast than when the perturbations within the target regions are removed and the rest of the perturbations are retained. Additionally, when the perturbations are filtered to retain only wavelengths longer than 1000 km, they have greater impacts on the forecast than when the perturbations are filtered to retain only wavelengths shorter than 1000 km. These results suggest that the 15 November 2018 forecast bust was strongly sensitive to widespread, large-scale uncertainties in the initial conditions, rather than those in localized, small-scale regions that could more feasibly be reduced by targeted observations.

Left: 600-hPa potential-temperature adjoint perturbations (color fill every 0.5 K) for the (b) box, (d) full, and (e) hole experiments. GFS analysis for 600-hPa geopotential height (black contours, contoured every 15 dam). Green box in (b), (e) bounds the box and hole regions.

Right: 72-h SLP errors (color fill, contoured at 3.5 hPa) for (a) control (unperturbed) simulation and for the adjoint perturbations (b) box, (d) full, and (e) hole. SLP contours from the GFS analysis (black) and the forecast (magenta) contoured every 6 hPa.

How to cite: Durran, D., Lloveras, D., and Doyle, J.: Can Observation Targeting Be a Wild Goose Chase? An Adjoint-Sensitivity Study of a U.S. East Coast Cyclone Forecast Bust, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-205, https://doi.org/10.5194/ems2024-205, 2024.

How to cite: Hauser, S., Martin, J. E., Cavallo, S., and Parsons, D. B.: Characteristics of large-scale circulation "forecast busts” over Europe in ERA5 medium-range reforecasts, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-995, https://doi.org/10.5194/ems2024-995, 2024.

Over the past years, Europe was stricken by unprecedented heat and drought extremes. Heatwaves (HWs) pose an increasingly major threat to society, which requires to further our understanding of their predictability some days or weeks ahead. In the present study, we focus on medium-range lead times of 5-12 days, for which the successful prediction of HW onsets is strongly dependent on the adequate prediction of large-scale Rossby wave patterns and their dynamics.

Using ECMWF and GEFSv12 ensemble reforecasts, we statistically assess the medium-range predictability of HW onsets over multiple European regions for the period 2001-2018. Heatwaves are objectively diagnosed as time periods where the 90th percentile in 2m maximum temperatures is exceeded both grid point-wise and integrated over the region for at least 3 days, giving about 50 HWs for each region. Predictive skill is evaluated mainly for large-scale flow features but also in terms of the ensemble’s capability to predict the extremeness of near-surface temperatures. By adopting the concept of Euro-Atlantic weather regimes, we are further able to stratify HW onsets and their predictability by the concurrent large-scale atmospheric flow pattern.

For both the British and Central European region, we find that the medium-range predictive skill is significantly better for HW onsets associated with either Scandinavian or European blocking compared to cases with no pronounced regime. This skill advantage mostly concerns the large-scale flow (500hPa geopotential patterns) and to some extent 850hPa temperatures, particularly in times of a Scandinavian blocking. In contrast, near-surface maximum temperature forecasts do not exhibit increased skill for regimes associated with above-average large-scale predictability, which may point to the inherent difficulty of predicting smaller scales and boundary layer processes.

Finally it is explored how exceptionally good or poor forecasts at 10 days lead time are related to the atmospheric state during or shortly after forecast initialisation. For Central European HW onsets, poor large-scale flow predictive skill is associated with significantly increased baroclinicity further upstream and a more intense North Atlantic jet stream whereas particularly good forecasts are on average characterized by an initial atmospheric state close to climatology. Forecast skill for near-surface temperatures is not affected by such dynamical precursors, but rather by pre-existing soil moisture anomalies, with drier than normal soils being linked to better predictions and vice versa. For the British region, exceptionally good forecasts of both large-scale flow and near-surface temperatures are associated with an already established blocking over the continent. In comparison to Central Europe, antecedent soil moisture anomalies do have a same-sign, but less prominent effect on near-surface temperature forecasts.

How to cite: Lemburg, A. and Fink, A. H.: Investigating the medium-range predictability of European heatwave onsets in relation to weather regimes using ensemble reforecasts, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-740, https://doi.org/10.5194/ems2024-740, 2024.

The 2023/24 El Niño ranks as the 2nd strongest El Niño in the 21st century thus far. The intensity of the event was successfully predicted as early as March 2023, based on the buildup of upper ocean heat content in the western equatorial Pacific. Nevertheless, the unusual evolution pattern of this event, including the two-step warming tendency and two warming centers during the 2023/24 El Niño, and the influence from outside of the equatorial Pacific, remain to be explained. Here we show that the 2023/24 strong El Niño was mainly contributed to by three factors. Firstly, the buildup of heat content in the western equatorial Pacific in the early months of 2023 fostered a strong canonical El Niño, characterized by a steady increase in SST anomalies across the central-eastern equatorial Pacific, peaking in late 2023. Secondly, the intense sea surface temperature warming outside of the equatorial Pacific suppressed the El Niño and confined the warming mainly to the eastern basin. Lastly, the westerly wind bursts that occurred in the western-central equatorial Pacific in autumn induced another relatively weak warm center in the central equatorial Pacific toward the end of 2023. While the latter two factors coincidentally offset each other, leaving the buildup of heat content appearing as the primary cause of the strong 2023/24 El Niño, they explained the detailed structure of this El Niño.  Our results not only confirm the essential role of equatorial ocean heat recharge for El Niño development, but also demonstrate the necessity of accounting for multi-scale interactions from a global perspective to understand and predict El Niño.

How to cite: Lian, T., Liu, T., Hu, R., Wang, J., Song, X., and Chen, D.: Predicting the 2023/24 El Niño from a multi-scale and global perspective, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-55, https://doi.org/10.5194/ems2024-55, 2024.

Variability in the jet stream can have a significant influence on the distribution of extreme weather, with winter-long anomalies leading to extreme seasons. During winter of 2009/2010, the Atlantic and African jets were anomalously merged for most of the winter, resulting in a persistent zonally oriented single jet. At the same time, intense and prolonged negative phase of the North Atlantic Oscillation (NAO) and unusually cold and extreme weather conditions were reported over the Northern Hemisphere. Such a merging was only observed to occur for a whole winter during the winters of 1968-69 and 1969-70. Preliminary results indicate that such persistent winter merged jets could be more frequent in a future global warming scenario and thus it is important to understand this rare dynamical jet state and its effects on the weather patterns. In this study, we explore the associated distributions of extreme weather during merged-jet winter months, in comparison to the distribution of extremes during negative NAO months. We show that merged jet winter months have a signature weather pattern distribution that is different from the negative NAO phase, affecting Northern Africa, Europe and south-west Greenland, which is associated with synoptic storms tending to either propagate zonally towards the Mediterranean, with a secondary northward then westward branch propagating to Greenland. We further find a complex relation to Arctic-Eurasian temperature anomalies. Once we remove the NAO-related high-latitude cold surface temperature anomalies, we find that Atlantic-African jet merging, especially the winter-long persistent ones, occur during winters with unusually warm Arctic and cold Eurasia. This is consistent with the theory that winter-long jet merging represents a dynamical regime transition to a merged jet, which is more likely to occur when mid-latitude baroclinicity is weaker.

How to cite: Suresan, S., Harnik, N., and Caballero, R.: Winter-long Atlantic-African jet merging and the effect on extreme weather, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-931, https://doi.org/10.5194/ems2024-931, 2024.

Recent work has provided robust evidence for the systematic co-occurrence of wintertime cold spells in North America and wet and windy extremes in Europe, which we term compound pan-Atlantic extremes. Both cold spells and wet and windy extremes are individually highly impactful, and their concurrence further amplifies their effects for actors with international exposure who are vulnerable to correlated losses. This study aims to investigate further the atmospheric processes associated with compound pan-Atlantic cold, wet and windy extremes and how these processes are represented in CMIP6 models.

On aggregate, cold spells in different parts of North America statistically co-occur with wind and precipitation extremes in specific European regions. However, North American cold spells can arise from multiple dynamical pathways, altering the location and timing of the associated European extremes for individual cold spells. Here, we use ERA5 reanalysis data (1940-2014) to identify North American wintertime cold spells in three different regions as well as the co-occurring wet and windy extremes in northern and southern Europe. We relate these compound pan-Atlantic extreme occurrences to dynamical pathways based on Pacific and Atlantic weather regimes. We then compare the CMIP6 historical simulation model data (1940-2014) with the ERA5 results. First, we discuss the performance of the models in capturing the weather regime frequencies and dynamical pathways to cold spells. Second, we review the ability of the models to replicate the spatial and temporal pattern of pan-Atlantic extremes for the three cold spell regions.
The results of this study contribute to the evaluation of the model fidelity in reproducing pan-Atlantic compound extremes and the associated circulation, with direct implications for the assessment of climate projections.

How to cite: Leeding, R. and Messori, G.: Dynamics of pan-Atlantic winter compound extremes in ERA5 and CMIP6 models., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-265, https://doi.org/10.5194/ems2024-265, 2024.

Our work focuses on the dynamic drivers of cold spells in North America and wet and windy extremes in Europe, which we refer to as pan-Atlantic compound surface extremes. In particular, we consider the role of different atmospheric dynamical pathways in leading to an intensification of the Alaskan Ridge, which previous work has highlighted as a key precursor to the pan-Atlantic extremes. We specifically emphasise the role of Rossby wave activity acting on various spatial and temporal scales. The separation is performed on the ERA5 data projected into the normal-mode functions, as one of the available wave decomposition techniques. We select planetary (zonal wavenumbers 1-3) and synoptic (zonal wavenumbers 4-8) components to understand what processes are related (or dominant) at different Rossby wave scales. One potentially important mechanism for Alaskan Ridge amplification is related to stratospheric wave reflection events, which are associated with the vertical propagation of planetary-scale waves, and occur approximately two weeks before the peak of North American cold spells. On a shorter timescale (within 10 days), Rossby wave trains with smaller spatial scales are enhanced 5 days prior to the peak. Preliminary analysis suggests that the enhancement is likely to result from the merging of meridionally and zonally propagating waves within the modulation of the planetary waves. Our results highlight the intricate connection between different dynamical processes in the atmosphere and amplified Alaskan Ridges, which in turn are a precursor to pan-Atlantic compound extremes. The dynamical and statistical link to surface extremes could aid in predicting such events.

How to cite: Strigunova, I. and Messori, G.: Scale-Dependent Rossby Wave Dynamics and Alaskan Ridge Amplification: Impacts on Pan-Atlantic Compound Extreme Events, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-805, https://doi.org/10.5194/ems2024-805, 2024.

Mediterranean weather extremes are often driven by regional low-pressure systems, here under the name of Mediterranean cyclones. In this work we take a Lagrangian (cyclone-centred) and a Eulerian (geographical) perspectives on the association between bi-variate compounds of rain-wind and wave-wind extremes and Mediterranean cyclones. Our aim is to provide new insight on the processes driving the compounding of extreme weather conditions in the extended Mediterranean region. Three main research questions are adressed.
(i.) What is the average spatial distribution of compound extremes around cyclone centres?
(ii.) How is the regional compound frequency modulated by the presence of cyclones?
(iii.) Is the presence of cyclonic air-streams (warm conveyor belts and dry intrusions) and fronts relevant for triggering compound extremes?
The results are strongly dependent on the compound type, on the cyclone class and on the season and region of occurrence. We find that in transition seasons compound extremes are far rarer than in the winter season, and that they are mostly associated with a nearby low-pressure centre. The fraction of cyclone-related compounds is weaker during the winter season, even though winter baroclinic cyclones are associated with the highest frequency and spatial extension of compound extremes. Finally, the presence of warm conveyor belts (inflow and ascent), dry intrusion outflow and cold fronts around the cyclone centre is shown to be important for the occurrence of regional compound extremes across all seasons. Warm conveyor belt ascent regions show high occurrence of both types of compound extremes; wave-wind compounds are also favoured in regions of dry intrusion outflow.

How to cite: Portal, A., Rousseau-Rizzi, R., Raveh-Rubin, S., Catto, J., Givon, Y., and Martius, O.: Mediterranean compound extremes and their link to Mediterranean cyclones, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-812, https://doi.org/10.5194/ems2024-812, 2024.

In the beginning of September 2023, Europe experienced a pronounced atmospheric omega-blocking, marked by a persistent anticyclone in the center flanked by two low pressure systems to the southwest and southeast. By definition, an atmospheric block interrupts the mean westerly flow and leads to prolonged persistent conditions lasting for at least five days. The core of the omega-blocking in September 2023 was located over Central Europe and Southern Scandinavia, which experienced a heatwave in the first week of September 2023. On the other hand, areas situated on the southwestern (Spain) and southeastern flanks (Greece, Bulgaria, and later Libya) were hit by extreme precipitation leading to major flood events.

We employ the method of ensemble boosting to explicitly simulate omega blocking situations with spatially compounding extremes (heatwave and extreme precipitation) with the Community Earth System Model 2 (CESM2). Using this model re-initialization approach, slight perturbations are introduced to initial conditions 10 to 25 days prior to the event, leading to the generation of hundreds of coherent physical event trajectories. This allows to study two sets of research questions: the first focuses on assessing the capability of the climate model to reproduce the 2023 event in its severity, while the second deals with identifying the primary factors of the omega block that result in the most severe impacts on the ground.

In our presentation, we introduce the research concept and address the following research questions: Is the CESM2 model capable of reproducing an omega blocking event with spatially compounding extremes in the magnitude of the September 2023 event? Could the September 2023 event have been even more devastating by chance? What characteristics of the omega block precondition the occurrence of the most extreme impacts in terms of heatwaves and extreme precipitation within the boosted ensemble?

How to cite: Mittermeier, M., Guo, Y., Beyerle, U., Suarez-Gutierrez, L., Bevacqua, E., and Fischer, E.: Spatially compounding extremes under omega blocking, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-964, https://doi.org/10.5194/ems2024-964, 2024.

We investigate the representation of hot-dry compound extremes in decadal predictions and their correlations with the corresponding univariate extremes. We use a multi-model ensemble (MME) of 125 members from the CMIP6 Decadal Climate Prediction Project (DCPP) hindcast simulations and compare it with different observational references. Our analysis focuses on the average of forecast years 2 to 5, with the different forecasts initialised every year from 1960 to 2014. We analyse the skill of predicting hot, dry and compound hot-dry events in the MME. Specifically, we select the days above the 90th percentile of the daily maximum temperature for hot events (tx90p). For dry events, we use two indicators, based on the Standardised Precipitation Index (SPI) and the Standardised Precipitation Evapotranspiration Index (SPEI), with accumulation periods of 3, 6 and 12 months, and we consider a dry event a month when the SPI or SPEI value ≤ -1. Finally, we identify days that present both hot and dry conditions according to these criteria as compound hot-dry days (HDSPEI3,-6,-12 and HDSPI3,-6,-12).

For the univariate extremes, results show strong skill for hot extremes, to a large extent driven by the warming trend. On the contrary, dry extremes show less uniform skill, with isolated areas of significant correlations. We find regional skill in some areas of the globe for hot-dry compounds. Through a comparison with historical simulations, we find that initialisation of the predictions leads to additional skill in localised regions, with most areas showing non-significant differences. However, the models still appear to underestimate the connections between compound and univariate extremes, especially between hot-dry compounds and dry conditions. These results show the potential and the limitations of predicting compound events in decadal forecasts, highlighting the pivotal role of dry conditions in assuring a skilful prediction.

How to cite: Aranyossy, A., Donat, M., De Luca, P., Delgado-Torres, C., and Solaraju-Murali, B.: Hot, dry and compound hot-dry extremes in decadal predictions, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-507, https://doi.org/10.5194/ems2024-507, 2024.

Eastern China experienced persistent regional extreme heatwaves in the summer of 2022, with disparate spatial features and formation mechanisms in different months. We quantitatively assessed the relative contributions of three oceans, i.e., tropical Indian Ocean and Pacific and North Atlantic, and the local soil moisture–temperature feedback using linear regression. The results showed that the monthly mean atmospheric circulation anomalies failed to explain the extreme heatwave in June 2022. The combined contribution of the tropical Indo-Pacific and North Atlantic sea surface temperature anomalies (SSTAs), together with the local soil moisture–temperature feedback, explaining approximately 10% of the temperature anomalies. In July, the tropical Indo-Pacific SSTAs promoted anomalous atmospheric circulation and extreme heat via meridional circulation originating in the Maritime Continent, accounting for approximately 10% of the temperature anomalies, with North Atlantic SSTAs contributing the same percentage by a mid-latitude steady Rossby wave. Local soil moisture–temperature feedback accounted for 42% of the anomalies. The tropical Indo-Pacific SSTAs produced a strong western North Pacific anticyclone in August, but their direct contribution to the temperature anomalies was negligible. The North Atlantic SSTAs contributed 9% of the total via the mid-latitude steady Rossby wave. Local soil moisture–temperature feedback contributed 66%, suggesting that the July heatwave and drought exerted a significant impact on the subsequent August extreme heatwave. Global warming has greatly facilitated extreme heatwaves, accounting for about 30%–40% of these events in summer 2022. These results also suggest that the climatic effects of tropical Indo-Pacific and North Atlantic SSTAs on Eastern China are evident in the month-to-month variation in summer. Our results thus contribute to the understanding and prediction of extreme heatwaves in Eastern China.

How to cite: Jiang, J., Liu, Y., Mao, J., and Wu, G.: Extreme heatwave over Eastern China in summer 2022: the role of three oceans and local soil moisture feedback, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-220, https://doi.org/10.5194/ems2024-220, 2024.

Storm Ciarán was an impactful weather system that affected western Europe in the first days of November 2023. The main threat from Ciarán were extreme winds, that particularly hit the areas around the English Channel: the highest wind gust recorded was 207 km/h at Pointe du Raz, in Bretagne (France), just as the storm was rapidly deepening. In this work, we discuss some dynamical characteristics of storm Ciarán related to the occurrence of those extreme winds: its origin as a diabatic Rossby wave, and its evolution as a warm-seclusion cyclone with the development of a sting jet.

Diabatic Rossby waves (or vortices) are a particular type of cyclones driven by latent heat release along a baroclinic zone. These shallow systems have a smaller scale than usual extratropical cyclones, and are driven by the low-level vorticity generation that results from intense, localized condensational heating. Ciarán’s evolution resembled the infamous storm Lothar (1999), because both underwent explosive intensification starting from the diabatic Rossby wave stage, and both resulted in extreme winds.

Rapidly deepening cyclones that develop a warm seclusion are also associated with strong surface winds, as they can provide a favorable environment to the development of sting jets. A sting jet is an airstream that accelerates as it descends out of the tip of the hooked cloud head, generating strong low-level winds into the frontal-fracture region of the cyclone. Storm Ciarán indeed originated as a diabatic Rossby wave and then developed into a rapidly deepening warm-seclusion cyclone containing a sting jet. We thus illustrate its evolution, highlighting the occurrence of a “diabatic Rossby wave-sting jet” dynamical pathway leading to the generation of high-impact winds.

How to cite: Riboldi, J. and Volonté, A.: The dynamics of Storm Ciarán (2023): from diabatic Rossby wave to warm-seclusion cyclone with a sting jet, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-572, https://doi.org/10.5194/ems2024-572, 2024.

Downslope windstorm (DW) is a mesoscale phenomenon with considerable impact on nature and human activities in mountainous regions. It is characterised not only by the descent of air down the lee slopes but also by wind acceleration on the windward side of a mountain. Climate studies of such complex and local events on larger scale are rare. In this work, a high-resolution climate model is used to study the DW characteristics in the present and future climate in the Scandinavian mountains.

The HARMONIE-Climate (HCLIM) regional climate model is used on two nested grids with horizontal grid spacing of 12 and 3 km. The DW’s are identified using a two-step algorithm. First the favorable terrain for DW’s is marked, followed by the identification of DW events in these areas based on stability, horizontal and vertical wind speed. Synoptic circulation types (CT) are used to distinguish DW’s according to different large-scale environments. The Scandinavian mountains are divided into four subregions, and the analysis is focused on winter, which exhibits the highest frequency of DW’s.

The simulation at 3-km grid spacing agrees considerably better with observations in DW-prone regions, indicating that such high resolution is needed to reproduce strong DW’s. The CT’s with highest frequency of DW occurrence are those with strong cross-mountain pressure gradient. For every CT, DW’s were additionally grouped as either cold or warm winds in the lee of a mountain, distinguishing between bora-like (cold) and foehn-like (warm) DW’s. Due to the considerable terrain complexity of the Scandinavian Mountains, a straightforward link between CT’s and the DWs’ effect on local warming or cooling is not evident. Results show that even in a single grid point DW’s can cause warming and cooling regardless of a CT or wind direction. We focus on one such exemplary grid point to highlight how different synoptic settings can lead to very similar local conditions but with opposite DW character. Additionally, insights into future climate projections of DW’s are presented.

How to cite: Jureša, P., Belušić, D., Hansen, F., and Marimbordes, S.: Downslope windstorms in the Scandinavian mountains from a kilometer-scale regional climate model, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-719, https://doi.org/10.5194/ems2024-719, 2024.

This study investigates convective wind gusts measured by National Meteorological Institute (INMET, in Portuguese) operational automated surface weather stations network in southern Brazil from 2005 to 2015. Only events unequivocally associated with deep convective storms were selected, as evaluated with a set of criteria that include the employment of geostationary satellite data. Wind gusts exceeding 25 ms ^-1 are categorized as severe, while non-severe gusts with moderate intensity range from 15 ms ^-1 to 24.9 ms ^-1, and weaker gusts from 10 ms ^-1 to 14.9 ms ^-1.

Convective parameters computed from the Fifth Generation of the European Centre for Medium-Range Weather Forecasts Reanalysis (ERA5) are used to investigate the atmospheric conditions conducive to severe wind gusts. Emphasis is placed on assessing the ability of convective parameters to discriminate between severe and sub-severe wind gust environments. The results indicate that the mean wind computed from the surface to the 6 km layer was the only parameter that showed some discriminatory ability. Overall, convective parameters alone are unable to distinguish environments conducive to the formation of storms capable of generating intense winds from those generating weak wind gusts. This result aligns with ingredient-based analysis, as individual parameters alone do not encompass all the necessary ingredients required for the development of these storms.

Therefore, we evaluate the combined product of ERA5-based convective available potential energy (CAPE) and bulk wind difference in the 0-6 km layer (BWD). The results indicate that as the gust intensity increase, the high CAPE high shear space gradually becomes more preferable. In other words, there is a tendency for more severe storms to cluster in the sector of the diagram where both CAPE and BWD values are moderate to high.

How to cite: Ferreira, V., de Lima Nascimento, E. D. L., and de Oliveira dos Santos, L.: Atmospheric environmets of convectively generated wind gusts in Southern Brazil, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-833, https://doi.org/10.5194/ems2024-833, 2024.

Extreme storm events have a profound impact on aviation operations, often leading to significant damages and hazardous flying conditions. This study focuses on four prominent storms that affected both Europe and the USA between February 2022 and June 2023, resulting in widespread disruptions to aviation services.
By leveraging historical and contemporary data on sea level pressure (SLP), this research applies the attribution methodology outlined by Yiou \cite{Yiou} to compute weather analogues. By means of 6-hourly ERA5 data-sets spanning from 1950 to 2023, split into two distinct 35-year periods—[1950–1984], denoting a factual period,  and [1988–2023], representing a counterfactual period — we identify the cyclone time for each event, designated as the time when cyclones associated with the storms reach their lowest SLP. Thirty best analogue cyclones are selected within defined spatial domains for each period, encompassing all months of the year in one case and only the months of the involved season in the other.
Composite maps of SLP and geopotential height anomalies are computed for the time-steps corresponding to the found analogues, assessing differences between the two periods. The analysis extends to precipitation and eddy dissipation rate (EDR), a crucial indicator to assess the level of air turbulence during flights.
These comprehensive analyses aim to provide a comprehensive assessment of atmospheric conditions during the studied extreme events, to enhance understanding of the atmospheric dynamics of extreme storm events and their implications for aviation safety, flight operations, and air traffic management. The findings are crucial for developing strategies to mitigate risks and improve adaptation for future storm events in the aviation industry.

How to cite: Rapella, L., Alberti, T., and Faranda, D.: Quantifying the Impact of Extreme Storm Events on Aviation Operations over Europe and the USA, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-303, https://doi.org/10.5194/ems2024-303, 2024.

Event identification and tracking tools have become essential in weather and climate research due to the rapid growth of weather and climate model data. These algorithms automate the detection of extreme weather-related events and hazardous phenomena and their corresponding features, thereby enhancing predictive capabilities. Our study introduces an open and adaptable framework designed for the automated identification and tracking of various hazard types from a Lagrangian perspective, spanning multiple spatial and temporal scales and providing insights into the dynamics and behavior of hazardous events. This framework is versatile and applicable to a range of scenarios from historical hazard analyses to future climate scenarios evaluation. Additionally, this work facilitates the investigation of weather patterns that give rise to the hazards and enables the assessment of their impacts. The algorithm relies on connected components and distance thresholds to identify and track events in space and time, permitting event merging and splitting dynamics over time. Furthermore, it characterizes events based on diverse spatio-temporal features such as duration, volume, intensity, and trajectory. The framework includes visualization tools for displaying event trajectories. It enables statistical analyses of detected time series derived from historical records and climate model outputs (e.g., identifying trends, exploring correlations between feature pairs, and analyzing feature distributions). To prove the framework's efficacy and versatility, we applied it to detect major hazards including heatwaves, cold spells, heavy precipitation events, hydrological droughts, and even combinations of these events identified as compound events. Leveraging ERA5 reanalysis, satellite data, and radar data we constructed an extended historical catalog of these hazards in France and conducted statistical analyses to characterize event properties and structures. Subsequently, applying the framework to Cordex data allowed us to assess potential future hazard scenarios.

How to cite: Cazzaniga, G., Bonhomme, L., Burq, A., Vrac, M., and Faranda, D.: Automated identification and tracking framework for hazard cataloging, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-697, https://doi.org/10.5194/ems2024-697, 2024.

We examine the complex meteorological processes involved in the development of multiple dust storms in the middle of March 2022 over North Africa by means of multi-level high-resolution reanalysis datasets, and satellite and surface observations. These dust storms ultimately transported a remarkable amount of dust into Europe, and in particular to the Iberian Peninsula.

The analyses show that baroclinic Rossby Wave Breaking (RWB) events over two regions: the eastern North Atlantic and the western Eurasian border, resulted in the creation of a downscale confluence of low-level jets that ablated dust in the Sahara Desert. One of these active baroclinic regions involved multiple wave-breaks over western Russia and the Middle East that transported unseasonably cold air southwestward into Egypt and Sudan. A second active baroclinic region occurred along the North African Atlantic Coast where rapid downstream dispersion of energy was focused in response to very intense cyclogenesis over Labrador. The cold pool which propagated into northwestern Africa and cold air over northeastern Africa effectively sandwiched hot Saharan air in between the Hoggar and Atlas Mountain Ranges. The fronts separating these air masses became juxtaposed with the two mountain ranges creating ideal conditions for blocking and multiple barrier jets’ genesis. These barrier jets each combined with larger scale semi-geostrophic low-level jet formation, resulted in the ablation of dust into two separate large plumes both of which were transported poleward out of North Africa into Europe around a upper-level cutoff-low near Iberia slowly displacing downstream.

The blocked RWB forcing the penetration of cold air over eastern Africa constitutes a substantial difference with respect to other more frequent events where RW trains intensify, penetrate equatorward into North Africa and propagate eastward.

Work funded by MCIN/AEI/ 10.13039/501100011033 under Grant PID2020-115153RB-I00 (rROSSETA Project).

How to cite: G. Orza, J. A. and Kaplan, M. L.: Multi-scale analysis of the meteorological processes forcing African dust ablation and transport in the extreme mid-march 2022 case, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-986, https://doi.org/10.5194/ems2024-986, 2024.

High-concentration Asian dust storm (ADS) events significantly increase atmospheric particulate matter concentrations, adversely affecting respiratory health and causing substantial economic losses in environmental and agricultural productivity. Understanding the mechanisms, pathways, and behavior patterns of ADS is essential for providing early warnings for periods and regions where severe dust events are anticipated, enabling appropriate preventive measures. This research aims to identify key meteorological variables and classify synoptic patterns of high-concentration sand dust events (dust warnings) during the beginning stages at the source regions and the peak concentration stages observed over Korea, using unsupervised learning methods such as Principal Component Analysis (PCA) and K-means clustering. The study analyzed the ADS cases observed from 2002 to 2022, utilizing the ECMWF reanalysis data (ERA5). The results indicate that during the origin stage, the primary meteorological variables were the geopotential height and temperature at lower layers (900-1000hPa), while temperature and humidity were key variables during the peak concentration observation stage. Each stage was classified into five cluster patterns. Temporal analysis of these patterns revealed that most ADSs occurred more frequently at night, except for pattern 4 that was predominantly observed during daytime, and pattern 5 that was exclusively observed at night. During the peak concentration stage, three out of five patterns showed more frequent occurrences at night, while patterns 1 and 4 had more frequent daytime occurrences, and pattern 5 was predominantly observed at night. This research contributes to the improvement of high-concentration ADS prediction by classifying and analyzing the synoptic patterns related to their occurrences.

How to cite: Lee, S. and Park, S. K.: Identification of Key Meteorological Variables and Synoptic Patterns of High-Concentration Asian Dust Storms, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-771, https://doi.org/10.5194/ems2024-771, 2024.

The region of Emilia Romagna in northeastern Italy, with its capital in Bologna, experienced a sequence of flooding events on May 2nd, 10th, and 16th, 2023. These three events were linked to the passage of three extratropical cyclones, identified as Mediterranean cyclones, over the same region within a short period. We investigate the influence of climate change on each of these cyclones using an Extreme Event Attribution framework.

We use the analogues approach to identify similar cyclones in two periods characterized by weak and strong climate change. Cyclones are defined using a multivariate method based on sea level pressure, surface wind speed, and precipitation rate. We first normalize the fields via quantile normalization. We then calculate the spatially averaged Euclidean distance between the cyclones and other time steps within the phase space defined by the normalized sea level pressure, wind speed, and precipitation rate. Analogues are identified as those with the minimum Euclidean distance.

We assess detected changes using daily ERA5 reanalysis data, comparing the meteorological hazards of the analogues of the past period (1950–1985) with the present (1987–2022). To evaluate future changes, we conduct a multi-model study with high-resolution regional models within the EURO-CORDEX program, comparing historical (1970–2000) with future projections (2070–2100). We found an increase in precipitation associated with two of the cyclones during the present period compared to the past. Future projections also indicate an increase in precipitation compared to the present climate. Hence, our findings suggest a possible role of climate change in increasing the severity of such cyclones.

How to cite: Ginesta, M., Lu, C., Coppola, E., Yiou, P., and Faranda, D.: Climate change fingerprint on the 2023 Emilia Romagna floods, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-448, https://doi.org/10.5194/ems2024-448, 2024.

This study investigates the dynamics and thermodynamics driving sub-hourly heavy precipitation events (SHHPs) associated with Regional Low-pressure (RegL) systems in mainland Portugal. SHHPs are significant natural hazards, impacting various sectors such as agriculture, particularly viticulture. Using observations from automated surface weather stations from the Portuguese Weather Service (IPMA) from 2000 to 2022, with a 10-minute temporal resolution, this research focuses on three representative stations across different climatic regions. Viana do Castelo station represents the northernmost region, known for high precipitation and strong Atlantic influence. Castelo Branco station, in the central region, is less exposed to Atlantic systems and more influenced by convective systems from western Spain. Faro station, in the south, experiences less influence from maritime polar air masses and more from maritime tropical air masses or Mediterranean/North African systems. The southern region of Portugal demonstrates higher precipitation variability, characterized by intense events occurring on fewer rainy days. The study establishes a link between SHHPs and low-pressure systems west of the Iberian Peninsula (IP). These systems exhibit a cold core, particularly evident at mid-levels, and feature a positive vorticity anomaly extending from the upper troposphere to lower levels. This configuration induces differential positive vorticity advection, especially increasing with height, developing upward motion east of the low-pressure systems over western Iberia. Moreover, these systems facilitate moisture advection over western Iberia at lower levels, evident in positive anomalies of 2-meter dew point temperatures, and promote instability conditions, as diagnosed by various instability indices. Additionally, higher values of total column cloud ice water during heavier precipitation events suggest its potential as a predictor for such occurrences. The study also suggests a potential association of some SHHPs with cut-off lows, though this hypothesis requires further validation. Overall, this research provides a systematic assessment of atmospheric conditions driving SHHPs, contributing to a better understanding of these hazardous events in mainland Portugal.

Acknowledgments: Research funded by National Funds by FCT – Portuguese Foundation for Science and Technology, under the project UIDB/04033/2020 and LA/P/0126/2020 (https://doi.org/10.54499/UIDB/04033/2020). Vine & Wine Portugal—Driving Sustainable Growth Through Smart Innovation, PRR & NextGeneration EU, Agendas Mobilizadoras para a Reindustrialização, Contract Nb. C644866286-011.

How to cite: Cruz, J., Belo-Pereira, M., Fonseca, A., and Santos, J.: Exploring dynamic and thermodynamic drivers associated with heavy sub-hourly precipitation events in Portugal, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-140, https://doi.org/10.5194/ems2024-140, 2024.

The impact of El Niño on the Hadley Circulation (HC) has been a topic of previous studies, but the results have been inconclusive. We study how El Niño affects the HC during different stages of its cycle. In development years, the HC anomaly shows an equatorial quasi-symmetric pattern, while in decay years, it shows an asymmetric pattern. And it is shown a transition in the variability of the first two modes of the HC during these stages, with the first mode exhibiting a larger explained variance in the decaying stage. The regime change in HC variability corresponds to underlying anomalous SST distributions, as confirmed by sensitive experiments. The differences in tropical SST during different stages of El Niño cause differences in SST meridional gradients, which determine the location of convergence. This explains why the HC anomalies have different spatial structures during El Niño development and decay years. Quantitative assessment reveals stronger HC-SST response amplitudes during the decaying stage compared to the development stage. Employing the Kuo-Eliassen (KE) equation, diabatic heating anomalies during the decaying stage explain the difference in air-sea response intensity between the two stages. Diabatic heating variations are identified as the primary contributor to amplification or reduction of air-sea response intensity during the respective El Niño stages, providing insights into the different air-sea processes throughout the El Niño lifespan. Our results show that the meridional distribution of SST during different El Niño stages has significantly distinct impacts on meridional circulation and clarify the differences in El Niño's effects on climate.

How to cite: Feng, J., Ji, X., Li, J., Chen, X., and Wang, C.: A quantitative explanation for the different Impacts of El Niño Development and Decay Stages on the Hadley Circulation, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-402, https://doi.org/10.5194/ems2024-402, 2024.

The tropical Atlantic, as a vital component of the tropical oceans, exerts significant influence on both local and global climates through its sea surface temperature variability and interactions among tropical basins. We found a significant influence of the sea surface temperature anomaly over tropical south Atlantic (TSA-SSTA) on ENSO Events and the Meiyu onset date. A significant negative correlation between the preceding TSA-SSTA and subsequent summer ENSO events was revealed. The warm (cold) TSA-SSTA corresponds to the following summer La Niña (El Niño)-like SSTA in the tropical Pacific. Warm TSA-SSTA can trigger local anomalous upward motion, resulting in precipitation and positive Walker circulation anomalies in the equatorial Pacific. This configuration facilitates the strengthening of easterly anomalies in the lower level of the equatorial Pacific, further promoting the development of La Niña-like SSTA in the central to eastern equatorial Pacific. These mechanisms and conclusions remain valid even after linearly removing the signal of the preceding winter ENSO. At the same time, when TSA-SSTA is warm the Meiyu onset date tends to be early. This is because that warm TSA-SSTA triggers significant positive Gill responses in the equatorial Pacific, further generating local anticyclonic circulation anomalies over the Maritime Continent of the northwest Pacific. Under the combined effects of large-scale forcing and the "precipitation-circulation Sverdrup positive feedback" mechanism in the northwest Pacific, anomalous anticyclones can develop and be sustained, resulting in an earlier Meiyu onset date. Furthermore, the interannual relationship between the TSA-SSTA and ENSO also exhibits decadal variability, closely related to the Pacific Decadal Oscillation (PDO).

How to cite: Liu, Y., Zhang, S., Ma, T., Sheng, C., and Dong, B.: The connection between sea surface temperature anomaly over tropical south Atlantic and ENSO and the associated impact on East Asian climate, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-403, https://doi.org/10.5194/ems2024-403, 2024.

Most tropical cyclones (TCs) generated over the eastern North Pacific (ENP) do not make landfall. Consequently, TCs in this basin have received less attention, especially those that occur away from the mainland. Furthermore, there have been few studies of the climatic effects of ENP TCs. This study explores the feedback relationship between ENP TCs and the intensity of the El Niño–Southern Oscillation (ENSO), including El Niño and La Niña events, from the perspective of accumulated cyclone energy (ACE). Observational and modeling results indicate that the ENP ACE 3 months earlier can still affect the intensity of El Niño and La Niña events, although the SST persistence is main contributor. Thereinto, the impact of ENP TCs on El Niño appears to be approximately equal to that on La Niña. Moreover, this impact is independent of the persistence of the sea surface temperature (SST) in the Niño 3.4 region and the Madden–Julian Oscillation. Generally, the greater the ENP ACE, the stronger the El Niño, and the smaller the ENP ACE, the stronger the La Niña, this is especially the case for those TCs that develop over the July‒September period. In addition, results show that the ENP TCs modulate ENSO intensity by changing anomalous zonal wind at the low-level atmospheric layer. And the joint impacts of the low-level zonal wind anomalies on the Walker circulation and the east-west thermocline gradient lead to the time characteristics that ENP TCs lead ENSO intensity by about 3 months. This study proposes a cross-time-scale role for tropical cyclones in the development of ENSO, further expanding the theoretical framework for ENSO's development, and offering significant scientific relevance and practical value for its prediction.

How to cite: Wang, Q. and Tan, Z.-M.: Impact of tropical cyclones over the eastern North Pacific on El Niño–Southern Oscillation intensity, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-513, https://doi.org/10.5194/ems2024-513, 2024.

Tibetan Plateau (TP) has the highest elevation worldwide, and functions as an important modulator of both regional and global climates. Known as Asia’s water tower, TP is the headwater area of some major rivers, including the Yangtze River, Yellow River, Mekong River, Brahmaputra River, Ganges River, and Indus River. Precipitation over the TP exerts a great impact not only on local water resources but also on those downstream. Thus, variations in TP precipitation, especially their spatial pattern, are worth further study. In recent decades, the dipole pattern trends in precipitation over the TP has raised concerns, i.e., opposite precipitation trends in the northern TP (NTP) and in southern TP (STP). However, the physical processes of the variations in TP precipitation, especially the roles of local mesoscale systems, Tibetan Plateau vortices (TPVs), are not yet clear. In this work, dipole pattern variations are found in TPVs-associated precipitation, which experienced an interdecadal shift in the last two decades, that is, increases in NTP and decreases in STP first and then varies opposite trends. The interdecadal shift in the TPVs-associated precipitation trends greatly contributes to the interdecadal shift in the TP precipitation trends. Furthermore, the causes of variations in TPV frequency are explored, and find that the changes in zonal winds over the TP and the meridional winds across the northwestern TP boundary are closely related to the regionally different variations in TPV frequency. Subsequently, the changes in the sea surface temperature of India Ocean are demonstrated to be responsible for the wind variations.

How to cite: Li, L. and Zhang, R.: Interdecadal shift in dipole pattern precipitation trends over the Tibetan Plateau: Roles of local vortices, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-99, https://doi.org/10.5194/ems2024-99, 2024.

The source of potential vorticity (PV) substance (PVS) of the atmosphere is at the earth’s surface, whereas the PVS variation within a limited region depends on the sum of the effective PV flux crossing the boundary surface surrounding the region. Because the PV circulation (PVC) is the time- integral of effective PV flux, the monthly mean gross PVS in the northern troposphere is determined by the sum of PVC penetrating the enveloped three boundaries, e. i., the PVC cross- the equatorial vertical section CEPVC, the PVC cross- the tropopause CUPVC, and the PVC cross- the earth’s surface CBPVC. Because the CBPVC is associated with surface potential temperature and circulation, for an equilibrium gross PVS in the hemisphere, the change in CEPVC or/and CUPVC can induce the climate change near the surface through the adjustment of the atmospheric internal PVC changes.

Based on reanalysis data and by employing statistics and diagnostics, the characteristics of the July mean PVS and PVC in the northern troposphere, and the relationship between the Asian monsoon and the PVC crossing its three boundaries in July are investigated. Results demonstrate that, corresponding to a CBPVC pattern with negative (positive) PVC over the north (south) on the Tibetan Plateau, strong positive PV anomalies and westerly flows develop in the troposphere over the plateau， leading to increased precipitation in the downstream area along the Meiyu front. The combination effects of the CEPVC and CUPVC can trigger the adjustment of the troposphere internal PVC, resulting in PVS and circulation redistribution within NH and forming a meridional seesaw of precipitation over the East Asian summer monsoon area. Attribution diagnoses reveal that the tropical sea surface temperature anomaly contributes significantly to the CEPVC, whereas the meridional gradient of PVS over the northwestern Tibetan Plateau on the tropopause contributes more to the CUPVC.

How to cite: Wu, G., Liu, Y., He, B., Sheng, C., and Li, Y.: Potential Vorticity Circulation Crossing the Boundaries of Northern Troposphere and the East Asian Monsoon in July, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-224, https://doi.org/10.5194/ems2024-224, 2024.

Arctic sea ice has undergone a significant decline in the Barents–Kara Seas since the late 1990s. Previous studies have shown that the decrease in sea ice caused by increased poleward moisture transport is modulated by tropical sea temperature changes (mainly referring to La Niña events). The occurrence of multi-year La Niña events has increased significantly in recent decades, and their impact on Arctic sea ice needs to be further explored. In this study, we investigate the relationship between sea ice variation and different atmospheric diagnostics during multi-year La Niña and other La Niña  years. The decline in BKS sea ice during multi-year La Niña winters is significantly stronger than that during other La Niña years. It is because the multi-year La Niña tends to accompany the warm Arctic-cold continent pattern with a barotropic high-pressure blocked over the Ural region. Consequently, more frequent northward atmospheric rivers intrude into the BKS, intensifying long-wave radiation downward to the underlying surface and melting sea ice in the Barents–Kara Seas. However, in the early mature winter of other La Niña cases, negative North Atlantic Oscillation presents in the North Hemisphere high latitudes, which obstructs the atmospheric rivers to the south of Iceland. We infer that such a different response of sea ice decline in Barents–Kara Seas to different La Niña is related to stratospheric processes. Considering likely future climate change, more frequent multi-year La Niña events may account for substantial Arctic sea ice loss in recent decades. In particular, it was proposed that the multi-year La Niña probably occur more frequently in a warming climate.

How to cite: Zhong, W.: Wintertime Arctic Sea Ice Decline Related to Multi-Year La Niña Events, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-89, https://doi.org/10.5194/ems2024-89, 2024.

Societies across the Mediterranean and wider European region can be significantly affected by weather extremes such as heatwaves, cold spells, windstorms and intense rainfall events. Temperature extremes are connected to atmospheric blocking, while heavy precipitation and windstorms have been associated with Mediterranean Cyclones (MCs). However, the connection between atmospheric blocking and MCs is still understudied, despite evidence suggesting their mutual importance for cyclone development and for exacerbating and synchronizing surface extremes. The aim of the present study is the systematic investigation of the frequency of MC development downstream of atmospheric blocks over the Euro-Atlantic region, and to examine how different cyclone track characteristics along with cyclone-attributed-precipitation might be modulated under such conditions. To this end we employ the combined ‘best tracks’ MCs dataset with objectively identified (using potential vorticity anomalies) blocking features in ERA5 for the 1970-2020 period. We find that in the presence of atmospheric blocks, MCs that develop downstream tend to be more intense and to be associated with increased precipitation compared to other MCs. Moreover, under blocking conditions, the distribution of precipitation varies geographically between the north-west and south-east Euro-Mediterranean region, with moisture transport contributing to this difference. MCs developing under this scenario form particular subsets of MCs, with preferred seasonality and geographical distribution, compared to all MCs. Lastly, certain MCs tend to be more static, while their mobility exhibits variability under blocking conditions. These results have important implications for the predictability of MCs and their various impacts in the region on both weather and climate time scales.

How to cite: Loizou, P. and Raveh-Rubin, S.: Atmospheric blockings and downstream cyclones in the Euro-Mediterranean sector, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-102, https://doi.org/10.5194/ems2024-102, 2024.

Rossby Wave Breaking (RWB) events describe the last stage in the life cycle of baroclinic atmospheric disturbances. These breaking events can strongly influence the large-scale circulation and are tightly related to low-frequency weather regimes. In addition, RWBs are often associated with weather extremes such as heat waves, blockings, and extreme precipitation events. Here we examine the three-way interaction between RWB, weather regimes, and surface weather in the North Atlantic. This is done by combining a RWB detection algorithm, a storm-tracking routine, and a clustering technique to identify low-frequency circulation regimes in the North-Atlantic. We find that regardless of weather regime, most cyclones and anticyclones are associated with an Anticyclonic Wave Breaking (AWB) and/or a Cyclonic Wave Breaking (CWB) at some point during their lifetime, while very few storms do not involve any upper-level wave breaking (~11%). Moreover, storm characteristics (e.g., actual and relative positions, intensities, and displacements) differ depending on the associated breaking type. In “same-pairing” cases (i.e., cyclones with CWB and anticyclones with AWB) the surface system is positioned so that its associated upper-level winds would enhance the breaking (the anomalous circulation is in the same direction as the background shear). In “opposite-pairing” cases (i.e., cyclones with AWB and anticyclones with CWB), the upper-level winds associated with the surface system do not act to enhance the breaking which occurs in the direction of the background shear. In addition, we find that the surface storm characteristics are significantly altered with the weather regime, with distinct and clearly preferred storm paths in each cluster. We suggest a picture in which the resulting RWB frequencies and positions in each cluster are modified by the corresponding tracks of cyclones and anticyclones, with the maximum breaking found where there is a constructive interaction with the low-frequency flow. The positions of RWBs, in turn, shape the overall cluster structure and contributes to the persistence of the weather regime. An improved understanding of the relation between weather systems, RWB events, and weather regimes can also help us improve our understanding of and confidence in projected future circulation changes (e.g., by relating changes in the frequency and positions of RWB events, storm-tracks, and the North-Atlantic jet).

How to cite: Tamarin Brodsky, T. and Harnik, N.: The three-way feedback between North-Atlantic circulation regimes, Rossby wave breaking, and surface weather., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-917, https://doi.org/10.5194/ems2024-917, 2024.

After establishing the link between Gulf Stream heat transport and subsequent atmospheric blocking in Mathews and Czaja (2024), revealing nearly double the amount of blocking over Greenland during DJF following increased heat transport through the Florida Straits in late summer, and confirming this connection using the ECMWF IFS in Mathews et al. (2024)—where surface latent heat fluxes over the Gulf Stream were suppressed, resulting in a reduction of blocking by up to 30% across the entire northern hemisphere, attributed to the diminished transport of boundary layer air into the upper troposphere along warm conveyor belts—we now adopt a playful exploration of this mechanism through a heuristic model.

This simple three-box model connects a basic slab ocean to an atmospheric boundary layer via heat fluxes, which then connects to the upper troposphere through slantwise convection following Emanuel (1983). Mimicking real-world dynamics, the model shows increased (decreased) ocean heat content before (after) an atmospheric block. Additionally, convection is triggered only when negative potential vorticity is present in the atmospheric boundary layer as theorised by Bennetts and Hoskins (1973).  

By adjusting various feedback parameters the system bifurcates, leading to a series of period doublings before entering a chaotic regime. By utilising the instantaneous dynamical systems properties of the attractor as described by Faranda et al. (2017), we infer the predictability of the system with different arrangements of control parameters.

This simple model underscores the importance of accurately representing the oceanic pathway to atmospheric blocking in more sophisticated climate models for precise predictions of how atmospheric blocking changes in a warmer climate.

How to cite: Mathews, J., Czaja, A., Vitart, F., Roberts, C., and Deshmukh, V.: Chaos, Coupling and Climate: The Oceanic Pathway to Atmospheric Blocking, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1053, https://doi.org/10.5194/ems2024-1053, 2024.

Rossby Wave Packets (RWPs) are atmospheric perturbations located at the upper levels in mid‐latitudes which, under certain circumstances, terminate in Rossby Wave Breaking (RWB) events. When RWB events are sufficiently persistent and spatially extended, they are synoptically identical to atmospheric blockings, which are linked to heatwaves and droughts. Therefore, studying RWB events that occur after RWPs propagation and their link with blocking is key to enhance extreme weather events detection between 10–30 days in advance. In this study we assessed the occurrence of RWB events after the propagation of transient RWPs, whether long‐lived RWPs (RWPs with a lifespan > 8 days, or LLRWPs) are linked to large‐scale RWB events that could form a blocking event, and the proportion of blocking situations that occur near large-scale RWB events. To do so, we applied a tracking algorithm to detect and follow transient RWPs in the southern hemisphere during summertime between 1979-2021, developed a wave breaking algorithm to identify RWB events, and searched for blocking events with different intensities. Results show that LLRWPs and the other RWPs displayed large‐scale RWB events around 40% of the time, and most of the RWB events in both distributions last around 1–2 days, which is not long enough to identify them as blocking situations. Nearly 17% of blockings showed a RWB event nearby, but barely 5% of these blockings were linked to RWPs, thus suggesting that transient RWPs are not strongly linked to blocking development. Lastly, large‐scale RWB events associated with RWPs that lasted less than 8 days are influenced by El Niño‐Southern Oscillation

How to cite: Perez, I., Barreiro, M., Ehstand, N., Hernández‐García2, E., and Lopez, C.: Wave breaking events and their Link to Rossby wave packets and atmospheric blockings during Southern Hemisphere Summer, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-466, https://doi.org/10.5194/ems2024-466, 2024.

The Rossby Wave Sources (RWS), RWS trigger wave-like patterns arising from the upper troposphere of the north-eastern Pacific region, causing a response around the Northern Hemisphere with alternating regions of positive and negative correlation values between RWS and geopotential height at 500 hPa (Z500). Increased RWS intensity during summer is related to negative temperature anomalies over western North America, and positive
temperature anomalies over eastern North America, concurrently with increased precipitation over the western subtropical Atlantic and Northern Europe during summer. Colder than normal conditions in the North Pacific Ocean intensify the RWS and its impact on the global atmospheric circulation. We use 7 CMIP6 large ensembles for the period 1950-2014 to analyse how they reproduce the response in the global atmosphere to RWS over the North-eastern Pacific Ocean. The waveguides that RWS produce from the North-eastern Pacific around the globe show that CMIP6 large ensembles have displaced crests and troughs from the North-eastern Pacific compared to the locations of ERA5, although all of them show the spread of the signal worldwide.
This location displacement of crests and troughs relies on how intense compared to ERA5 is the bias of the modelled westerly subpolar jet stream intensity reaching Europe from the North American continent. Models with a stronger bias in intensity show that the negative RWS-Z500 correlations over Scandinavia are weakened. Correspondingly, the same applies to the south-to-north component of the bias at 200 hPa. As in ERA5, the CMIP6 large ensembles show that warmer than normal SST over the North Atlantic, constraint the teleconnection only to the North American continent.

How to cite: Fuentes Franco, R., Scaife, A., Lockwood, J., Dunstone, N., and Koenigk, T.: Rossby waves sources over the Eastern Northern Pacific and their impact on the global atmosphere from a set of CMIP6 large ensembles, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-583, https://doi.org/10.5194/ems2024-583, 2024.

Model simulations show a robust increase in El Niño-Southern Oscillation (ENSO)-related precipitation variability in a warmer climate, but there remains uncertainty in whether the characteristics of ENSO events themselves may change in the future. Our study aims to disentangle these effects by addressing how the global impacts of observed large El Niño events would change in under present and future background climate conditions.

Pacemaker simulations with the EC-Earth3-CC model were performed replaying the 3 strongest observed El Niño events from the historical record (1982/83, 1997/98, 2015/16). Model tropical Pacific sea surface temperature (SST) anomalies were restored towards observations, while imposing different background states, mimicking present and future climate conditions (following the SSP2-4.5). All variables outside the restoring region evolve freely in a coupled-atmosphere ocean transient simulation. For each start date, 30 ensemble members with different initial conditions were run for 2 years. Using this approach we ask ‘what impacts would arise if the observed El Niño occurred in the past or future’?

In response to the same imposed El Niño SST anomalies, precipitation anomalies are shifted towards the Eastern equatorial Pacific in the future compared to the present day, leading to changes in the extratropical response to El Niño. Some examples are an amplification of the surface temperature response over north-eastern North America, northern South America and Australia in boreal winter. We link the changes of El Niño related tropical Pacific precipitation to a decrease in the climatological zonal SST gradient in the equatorial Pacific, as we move from past to future climatologies, which potentially leads to a higher convection sensitivity to SST anomalies over the Central and Eastern equatorial Pacific in the future. Changes in the future climate response to extreme El Niño events are not homogeneous among regions. For example, cold and hot anomalies driven by extreme El Niño over North America and Australia in the future are amplified, whereas the response over southern Africa gets muted due to shifts in atmospheric circulation.

Our study aims to attribute the absolute future climate response to extreme El Niño events to either changes in El Niño teleconnections or climate change.

How to cite: Trascasa-Castro, P., Ruprich-Robert, Y., and Maycock, A.: Future changes in the global teleconnections of extreme El Niño events, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-239, https://doi.org/10.5194/ems2024-239, 2024.

Climate change is altering the frequency and intensity of extreme events globally, such as heat waves, droughts, flooding, and tropical storms. It is, therefore, of primary importance to quantify the influence of climate change on the properties of specific extreme events. The scientific community has developed different methodologies to answer that question. One extended approach, commonly known as the pseudo-global-warming (PWG) approach, consists of removing the anthropogenic climate change signal from event-constrained initial conditions of a physics-based weather model. However, performing state-of-the-art atmospheric model simulations of every single extreme event requires significant computational resources. Artificial Intelligence (AI)-based weather models offer a chance to speed up this process due to its much lower computational cost.

In this study, we apply the PGW approach to the AI-based FourCastNet model forecasts of a selection of extreme events. To account for the uncertainty associated with the influence of climate change, we remove the signal in thermodynamic variables from several CMIP6 models independently from the AI model's initial conditions. At the same time, we also test the sensitivity to the initialisation date. Using this approach, we quantify the impact of climate change on a heatwave event and tropical storm. Our results indicate that climate change significantly contributed to the intensity and characteristics of these two extreme events. We also discuss the strengths as well as limitations of the AI-based model in the simulation and attribution of extreme events by comparing our results to reanalysis and to more classical attribution methods. This research contributes to our understanding of the impacts of climate change on extreme events and highlights the potential of AI-based models for climate attribution studies.

How to cite: Jiménez-Esteve, B., Barriopedro, D., Johnson, J. E., and García-Herrera, R.: Climate Change Attribution of Extreme Events Using AI-based Weather Models, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-280, https://doi.org/10.5194/ems2024-280, 2024.

Extra-tropical cyclones (ETCs) are an important part of the climate system and are responsible for much of the weather in the mid-latitudes. The most extreme of these weather systems can cause damaging winds and heavy precipitation and can have adverse impacts on society. While the current generation of climate models are starting to agree on how the number, intensity and location of ETCs will change in the future, considerable uncertainty remains in how impact-relevant parameters such as wind gusts and precipitation will change. Here we use a novel combination of CMIP6 model projections and idealised model simulations of baroclinic waves to identify how precipitation and 10-m winds and wind gusts associated with ETCs may change in the future. We have performed a large ensemble (~6500 members) of baroclinic life cycle simulations with OpenIFS, which is a version of ECMWF’s Integrated Forecast System. The ensemble was created by varying 7 parameters with Latin hypercube sampling: average surface temperature; surface relative humidity; jet width, height and strength; lapse rate; and surface roughness. CMIP6 projections indicate an increase in mean temperature, lapse rate, jet width, and upper-level jet wind strength, and little change in relative humidity, jet height or surface roughness. Using CMIP6 projections from different scenarios, we map future climate projections onto the same 7-dimensional parameter space and thus identify the baroclinic wave simulations which are closest to the mean background states predicted in CMIP6 in the future. As our baroclinic wave simulations produce considerably more output fields at much higher temporal resolution compared to CMIP6, this approach enables a more thorough approach for examining how ETCs, and in particular, their precipitation and wind gusts, will change in the future. Initial results from this analysis will be presented.

How to cite: Sinclair, V., Bouvier, C., Cornér, J., van den Broek, D., Köhler, D., and Ekblom, M.: Future changes to extra-tropical cyclones: combining CMIP6 projections and idealised modelling., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-691, https://doi.org/10.5194/ems2024-691, 2024.

The present study investigates the uncertainties in future projections of summer convective afternoon rainfall (CAR), a key meteorological feature in Taiwan that significantly contributes to over 40-50% of the total summer rainfall in Taiwan. The analyses employ a dynamical downscaling approach utilizing the Weather Research and Forecasting Model (WRF) and the High-Resolution Atmospheric Model (HiRAM). The projections were driven by four different sea surface temperature (SST) categories derived from CMIP5 model simulations under the Representative Concentration Pathway (RCP) 8.5 scenario. All projections indicate a consistent decrease in CAR frequency but an increase in intensity by the end of the 21st century. However, the extent of changes in CAR varies across four simulations with different degrees of Pacific SST warming. These variations in potential future changes in CAR frequency and intensity are linked to variations in daytime thermal instability, local inland wind convergence, and moisture flux convergence over Taiwan. The study further investigates the relationship between local thermodynamic conditions and projected large-scale atmospheric circulation patterns, emphasizing a decrease in East Asian monsoonal low and a decrease in subtropical Pacific high in the future. The study also clarifies the potential impact of ENSO on modulating the changes in CAR activities over Taiwan under global warming, where La Niña-like circulation patterns induce more CAR activities over Taiwan compared to El Niño-like circulation patterns. In summary, the study offers valuable insights into the potential implications of climate change on CAR events in Taiwan, highlighting the importance of understanding the underlying physical mechanisms of the factors governing CAR event characteristics.

How to cite: Huang, W.-R., Chien, Y.-T., Cheng, C.-T., Hsu, H.-H., and Koralegedara, S. B.: Exploring Uncertainty in Future Changes of Convective Afternoon Rainfall in Taiwan: The Influence of Sea Surface Temperature, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-59, https://doi.org/10.5194/ems2024-59, 2024.

Extratropical cyclones are essential for redistributing energy, moisture, and momentum from lower latitudes to higher latitude regions. Although extratropical cyclones during the winter season are relatively well studied, less is known about summer cyclones and their moisture sources. Therefore, this research focuses to enhance our understanding of how summertime extratropical cyclones in the Northern Hemisphere shape the characteristics of the global water cycle. More specifically, the study focused on determining the moisture sources of these storms and analyses how precipitating air parcels are transported to the cyclone center. 

For this purpose, 8-day backward trajectories were calculated for a subset of the 20 % most intense summertime cyclones over the North-Atlantic and for all air parcels within the vicinity of cyclone center, using the Lagrangian Analysis Tool LAGRANTO. Subsequently, moisture uptakes along the trajectories of precipitating air parcels were identified using the moisture source diagnostic WaterSip. Using this approach, we find that the bulk of the precipitation associated with summertime cyclones falls close to the cyclone center within the WCB, mainly during the intensification phase. The origins of this moisture correspond to areas of high evaporation, with significant hotspots over the Gulf Stream region and its northeastern extension, and continental sources for cyclones in the Labrador Sea. During the early stages of cyclone development, moisture is delivered to the cyclone center, and this changes to more remote sources when intensity is increasing and when precipitation is at its peak. Local evaporation becomes again more dominant as cyclones generate less precipitation and start to decay. Therefore, the source distance is largest during the intensification phase and decreases thereafter. We lastly discuss differences between extratropical cyclones that undergo an extratropical transition. 

How to cite: Stoffels, R., Weijenborg, C., and Benedict, I.: Moisture sources of summertime intense extratropical cyclones in the North-Atlantic, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-707, https://doi.org/10.5194/ems2024-707, 2024.

Cyclones handle extreme weather events across the Mediterranean and its neighboring continents, affecting the lives of hundreds of millions. Despite many studies addressing Mediterranean cyclones (MCs) in the last decades, their correct simulation and prediction remain a significant challenge to the present day. This may be attributed to the large variability between MCs, which differ greatly from each other in many aspects. Past classifications of MCs are primarily based on geographical and seasonal separations, yet recent advances and the appearance of “Medicanes” – devastating tropical-like Mediterranean cyclones - emphasize the need for a dynamical classification, focusing on the cyclone deepening mechanisms. A variety of processes alternately govern Mediterranean cyclones' evolution, including diabatic and adiabatic processes, topographic influences, surface temperature anomalies, and land-sea contrasts. Fortunately, each process bears a distinct signature on the potential vorticity (PV) field. Therefore, a PV perspective is called upon to distinguish among the driving mechanisms of the different “types” of Mediterranean cyclones. Here, a combined cyclone tracking algorithm is used to detect and track Mediterranean cyclones in ECMWF ERA5 from 1979-2020. Cyclone-centered, upper-level isentropic PV structures in the peak time of each cyclone track are classified using the Self Organizing Map (SOM). The SOM analysis reveals 9 classes of Mediterranean cyclones, with distinct cyclone characteristics, lifecycles, associated hazards, and long-term trends. Though classified by upper-level flow structures, each class shows different flow structures down to the surface. Unique synoptic, thermal, dynamical, seasonal, and geographical features indicate dominant processes in the evolution of each Mediterranean cyclone subset. Furthermore, the tropopause-surface coupling is explored and reveals the importance of topographically induced Rossby-wave breaking to the generation of the most intense Mediterranean cyclones. These results enhance our understanding of Mediterranean cyclones' predictability, by linking predictable large-scale Rossby wave formations and life cycles to under-predicted cyclonic variability and associated hazards.

How to cite: Givon, Y., Hess, O., Flaounas, E., Catto, J. L., Sprenger, M., and Raveh-Rubin, S.: Process-based classification of Mediterranean cyclones using potential vorticity, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-439, https://doi.org/10.5194/ems2024-439, 2024.

Mediterranean tropical‐like cyclones (TLCs) are damaging weather systems, which form over the Mediterranean Sea, resembling tropical cyclones. These cyclones can drive important socio‐economic losses in coastal areas. However, due to their small size and the relatively recent investigation of these cyclones, there is currently no robust categorization of which Mediterranean cyclones can be considered TLC. . Therefore, in this work, we propose a method to differentiate cyclones that attain actual tropical‐like characteristics in part of their lifetime, as they develop a warm core through intense convective processes. Several warm‐core cyclones in the Mediterranean, which were analyzed in the literature, are studied using ERA5 reanalysis, to identify the environment where they develop and distinguish tropical‐like cyclones from non‐tropical warm‐core cyclones. Initially, the cyclone phase space is analyzed to distinguish the cyclones that have a symmetrical deep warm core. Subsequently, the temporal evolution of several parameters is considered, including the distance between the area of maximum tangential wind speed and the cyclone center. The main results of this study show that part of the analyzed cyclones have features similar to tropical cyclones. Some differences are observed between the cyclones analyzed: one category of cyclones develops in areas of moderate‐low baroclinicity and intense convective processes, as occurs in tropical cyclones. Another group of cyclones develops in a strongly baroclinic environment with weak convective processes and intense vertical wind shear, as occurs in warm seclusions. Two cyclones, showing similarities with polar lows, are also identified. The results of this study propose a key to identify the Mediterranean cyclones that have tropical‐like characteristics.

How to cite: Gutiérrez-Fernández, J., Miglietta, M. M., González-Alemán, J. J., and Gaertner, M. Á.: A New Refinement of Mediterranean Tropical‐LikeCyclones Characteristics, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-360, https://doi.org/10.5194/ems2024-360, 2024.

The release of condensation latent heat associated with water phase changes is the most important non-adiabatic energy source in tropical cyclones (TCs) besides sea surface temperature, and will affect the energy conversion process inside TCs. Based on the derived moist atmospheric energy tendency equation in cylindrical coordinates,  the numerical simulation results were used in this paper to analyze the distribution and evolution characteristics of the multi-scale energy of Typhoon "Meranti".

The results showed that the high-value center of symmetric kinetic energy was always located at the Radius of maximum wind(RMW), distributed in a columnar shape and centrifugally inclined, whereas the high-value areas of asymmetric kinetic energy at the vortex scale and sub-vortex scale were mainly located in the strong updraft area within the eyewall and the low-level inflow area. At the strongest stage of the typhoon "Meranti", there also existed large kinetic energy in the middle-level inflow layer and the high-level outflow layer away from the vortex center. The high values of symmetric moist available potential energy were mainly distributed in the warm core and wet core within the eyewall, and their vertical stretching increased with the enhancement of the warm core structure and low-level moisture transport.

The moist available potential energy of symmetric vortices and asymmetric perturbations at each scale was generated by the non-adiabatic heating process at the corresponding scale, and was further converted into kinetic energy at the corresponding scale through vertical heat transfer.The research in this paper will help to analyze how the multi- water phase affect the energy and intensity of tropical cyclones from the energy perspective.

How to cite: Huang, H., Wang, J., Wang, T., Wang, Z., Jiang, C., Fan, Y., and Wang, X.: The Distribution and Evolution of Multi-scale Moist energy within the Typhoon "Meranti", EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-769, https://doi.org/10.5194/ems2024-769, 2024.

This research discusses the influence of tropical cyclone (TC) activities in the summer tropical northwest Pacific on the East Asian midlatitude westerly jet through statistical analysis and numerical experiments, and the mechanism is also analyzed.

The statistical results indicate that TC activities in the northwest Pacific can induce abnormal westerlies in the midlatitude region of East Asia, and the axis of the westerly jet shifts toward regions with larger anomalous westerlies induced by TCs.

Numerical research on the case of TC Maria shows that the thermal effects of the abnormal disturbances near Japan induced by TC activities in the tropical northwest Pacific can result in abnormal temperatures in the midlatitude region of East Asia. Under the constraint of thermal wind relation, the abnormal westerlies form and the position of the westerly jet changes, correspondingly. During this process, the abnormal phase of the P-J teleconnection induced by TC activities plays a major role in causing the subsidence motion in the midlatitude region of East Asia, leading to abnormal changes of the atmospheric temperatures, which affect the meridional temperature gradient in the midlatitude region. Due to the vertical variation of zonal winds and horizontal temperature gradients in the midlatitude region satisfying the thermal wind relationship overall, the zonal winds in the midlatitude region undergo abnormal vertical variations correspondingly, which further changes the position of area with maximum value of the East Asian westerly, and the meridional position of westerly jet axis changes as well.

Results of this study demonstrate that TC-induced remote disturbances have thermal effects and can also affect midlatitude systems.

How to cite: Wang, T., Wang, J., Huang, H., Wang, X., and Jiang, C.: Impacts of Thermodynamics Disturbances Induced by Tropical Cyclone on the East Asian Subtropical Westerly Jet, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-890, https://doi.org/10.5194/ems2024-890, 2024.