Python code used in the analysis for the paper: "Identifying controls of extratropical cyclone intensity at genesis time and during intensification in the North Atlantic and Europe". Cornér, J., Bouvier, C., and Sinclair, V. A.: Identifying controls of extratropical cyclone intensity at genesis time and during intensification in the North Atlantic and Europe, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-6335, 2025.

This study compares the vertical tilt and potential vorticity structure of average and extreme extra-tropical cyclones (ETCs) in the Northern Hemisphere during the cold season (October-March) in ERA5 reanalysis. Vertical tilt is quantified using a novel two-component (directional and lateral, aligned with and across cyclone motion) horizontal offset between the near-surface and upper-level ETC centers. Compared to average ETCs, extreme ETCs exhibit an amplified tilt magnitude during the intensification phase and a change in the sign of the lateral tilt prior to maximum intensity. Accounting for the tilted vertical structure, this study shows that average ETCs are associated with an upper-level positive anomaly, which is constant through the ETC lifecycle, while the intensifies from the bottom up. In contrast, extreme ETCs are characterized by a growing magnitude of upper-level anomaly, and a simultaneous intensification throughout the lower levels.

Abstract Tropospheric ozone has become a major air quality concern, with nonmethane volatile organic compounds (NMVOCs) as key precursors. Previous on-road emission studies suggest on-road evaporation emissions now rival or exceed exhaust emissions as NMVOC sources in urban areas, driving increased regulatory attention. However, the effectiveness of evaporation emission control in mitigating ambient ozone remains poorly quantified. Using chemical transport model simulations over eastern China during summer 2017, we investigate regional ozone responses to 100% reductions in on-road evaporation NMVOC emissions. Results reveal pronounced regional variation in O3 benefit tied to local photochemical regimes. In VOC-limited regions (Beijing–Tianjin–Hebei and the Yangtze River Delta), evaporation emission controls reduce maximum daily 8-h average ozone (MDA8h O3) by up to 8 ppbv, equivalent to a 10% relative decrease. In contrast, non-VOC-limited regions exhibit minimal response (<1 ppbv), indicating negligible O3 health benefits from evaporation controls alone. These findings demonstrate that uniform national evaporation emission standards may raise regional health inequality in ozone benefits. We recommend prioritizing evaporation emission regulations in these regions while pursuing concurrent NOx-VOC cocontrol strategies in transitional and NOx-limited areas to maximize ozone mitigation effectiveness.

Tropospheric ozone has become a major air quality concern, with nonmethane volatile organic compounds (NMVOCs) as key precursors. Previous on-road emission studies suggest on-road evaporation emissions now rival or exceed exhaust emissions as NMVOC sources in urban areas, driving increased regulatory attention. However, the effectiveness of evaporation emission control in mitigating ambient ozone remains poorly quantified. Using chemical transport model simulations over eastern China during summer 2017, we investigate regional ozone responses to 100% reductions in on-road evaporation NMVOC emissions. Results reveal pronounced regional variation in O-3 benefit tied to local photochemical regimes. In VOC-limited regions (Beijing-Tianjin-Hebei and the Yangtze River Delta), evaporation emission controls reduce maximum daily 8-h average ozone (MDA8h O-3) by up to 8 ppbv, equivalent to a 10% relative decrease. In contrast, non-VOC-limited regions exhibit minimal response (<1 ppbv), indicating negligible O-3 health benefits from evaporation controls alone. These findings demonstrate that uniform national evaporation emission standards may raise regional health inequality in ozone benefits. We recommend prioritizing evaporation emission regulations in these regions while pursuing concurrent NO x -VOC cocontrol strategies in transitional and NO x -limited areas to maximize ozone mitigation effectiveness.

Extratropical cyclones (ETCs) are an important part of the atmospheric circulation, cause most of the day-to-day weather variability, and have societal impacts through strong winds and heavy precipitation in the mid-latitudes. Therefore, from both weather forecasting and climate change perspectives it is crucial to understand how they develop and intensify. In this study we aim to identify which environmental background conditions, here called ETC precursors, have the most control on the intensity of ETCs in the North Atlantic and Europe in the cold season. We apply an ensemble-based statistical method with ERA5 reanalysis data to associate climatologically typical perturbations in multiple ETC precursor fields at genesis time to distributions of five ETC intensity measures at time of maximum ETC intensity. We find that higher ETC wind intensity is associated with a stronger jet stream, especially downstream of the ETC centre, and increased meridional temperature gradients, with an emphasis on warmer upper levels south of the ETC centre. Precipitation is controlled by temperature and moisture throughout the tropospheric column, with higher values associated with more precipitation. We perform the same analysis for four groups of ETCs with different average intensities and show that while differences exist in the controlling precursors among the groups, no clear patterns are observed. Due to the non-linear growth of ETC intensity, the precursor fields at genesis time offer limited explanations about differences in maximum ETC intensity. Through analysing the temporal evolution of the four ETC intensity groups, we conclude that to understand differences in ETC intensity it is necessary to investigate multiple ETC precursor fields and their evolution through time.

Although well-researched in the present climate, it is poorly understood how Rossby wave breaking (RWB) may change in a warmer future climate. In this study, we examine how large changes in sea ice cover (SIC) and sea surface temperature (SST) affect the frequency and spatial distribution of Rossby wave breaking in the Northern Hemisphere during the boreal winter (December-February) and summer (June-August) seasons. Our experiment setup consists of eight 40-year atmosphere-only simulations from two models (OpenIFS and EC-Earth) that use different combinations of prescribed present-day and future SIC and SST values under the SSP5-8.5 scenario.We find present-day RWB frequencies that correspond well with previous literature. Our models are generally in good agreement with regards to the spatial distribution of RWB. The effects of SSP5-8.5 SST on RWB are substantial, while simulations using future SIC and present-day SSTs do not exhibit statistically significant changes compared to the present. In simulations with SST changes, anticyclonic wave breaking (AWB) frequencies show large decreases during both winter and summer, while the primary changes to cyclonic wave breaking (CWB) are small increases of varying magnitude in winter. The winter changes are notably collocated with changes in the strength and location of jet streams. The largest changes occur over the North Pacific, where winter AWB decrease by 60 %-70 % over the East Pacific and summer AWB decrease by roughly 50 % over the West Pacific and East Asia. Over the western North Atlantic, decreases of 10 %-30 % in winter AWB are collocated with a stronger eddy-driven jet, which may suggest an eastward shift in AWB. In summer, AWB decreases by about 50 % over North America but increases slightly over Europe. As with related previous studies of future changes in blocking and jet stream waviness, there are uncertainties in our results, and especially determining the impact of SIC changes likely requires longer simulations than those used in this study. This study demonstrates that particularly SST changes are an important component for changes to RWB in future climates.

Using a novel set of coordinated simulations from four different models, the response of the wintertime (December-February) North Atlantic jet stream and storm track to prescribed sea surface temperature increases and sea ice loss is analysed and the underlying physical mechanisms investigated. Three out of the four models show a southward shift of the upper-level jet stream with an increase in jet speed over Europe, where the contribution of sea surface temperatures dominates over the effects of sea ice loss. However, the remaining model lacks the increase in jet speed over Europe, which originates from opposite responses of similar magnitude due to the future sea surface temperatures and sea ice cover. The jet stream responses are primarily driven by the change in the meridional temperature gradient and, as a consequence, baroclinicity. At the same time, momentum flux convergence acts as a secondary amplifying and dampening factor. The same three models see a significant eastward shift of the extratropical cyclone track density, which is equally driven by changes to sea surface temperatures and sea ice cover. A consistent feature across all models is a decrease in the frequency of extratropical cyclones in the Mediterranean. The responses of extratropical cyclones to future sea ice cover and sea surface temperatures do not exceed the inter-model climatological differences. Notable differences in the future response of the jet stream and storm track occur, and thus considerable uncertainty remains in how the European climate will respond to a warmer climate.

Future projections of the European climate suffer from uncertainties in the changes of the North Atlantic jet stream. Most climate models simulate an annual-mean poleward shift of the jet in response to global warming, but the responses have a large spread. Previous studies comparing climate models suggest that the projected meridional shift of the jet is negatively correlated with its simulated present-day mean latitude. Model basic-state biases are a possible cause for the uncertainty, but their effect is difficult to assess because the spread in simulations may be caused by any intermodel differences. In this study, the effect of model biases on future projections was isolated by modifying the basic state of a single atmospheric model with a runtime correction method aiming to adjust the model climatology toward those of three other models and a reanalysis. The effect of model biases was found to be strongly seasonal. In winter, changes in the frequencies of two of the three preferred positions of the jet were found to be sensitive to the model biases, causing considerable uncertainty in the jet shift and contributing to the anticorrelation between the present-day jet latitude and the future shift, but in summer, the impact of biases is small relative to the magnitude of the changes. While the anticorrelation of jet latitude and shift is only partly caused by biases, our results demonstrate there is potential to reduce uncertainty in the projected jet stream changes by improving model basic states.

Observational data collected in December 2014 at the base camp of Mount Everest, Nepal, indicated frequent new particle formation events of pure biogenic origin. Those events were speculated to be controlled by the along-valley winds forming in the valley connecting the Indo-Gangetic plain to the observational site, the Nepal Climate Observatory-Pyramid. The valley winds funnel highly oxygenated organic molecules of biogenic origin to higher elevations where they nucleate. The mechanism was referred to as "The Himalayan aerosol factory". Its geographical extent and climate implications are currently unknown. In view of this, we conducted numerical chemical model simulations to corroborate the presence of the mechanism, and to quantify its geographical extent. Our numerical simulations confirmed that biogenic emissions located in the valleys can be converted into ultra-low volatility organic compounds, transported to the observational site by the along-valley winds, and therein nucleate. The overall time scale of the process, from the release of biogenic emissions to the conversion to ultra-low volatile organic compounds to the arrival time at the observational site, was found to be around 4 hours, consistent with the predicted along-valley winds intensity and the geographical distribution of biogenic emissions. A first estimation of the maximum injection height of biogenic particles, and highly oxygenated organic molecules, indicated the presence of efficient nucleating gases and biogenic particles at an elevation as high as 5000-6000 m a.s.l. These results suggest that the Himalayan chain, under specific weather conditions, is a main contributor to the biogenic aerosol loads in the free troposphere. Considering these findings, field campaigns, especially at the entrance of the valley's floors, and research consortia supporting atmospheric research in Asian mountain regions, are highly encouraged.

In this study, we investigate atmospheric new particle formation (NPF) across 65 d in the Bolivian central Andes at two locations: the mountaintop Chacaltaya station (CHC, 5.2 km above sea level) and an urban site in El Alto–La Paz (EAC), 19 km apart and at 1.1 km lower altitude. We classified the days into four categories based on the intensity of NPF, determined by the daily maximum concentration of 4–7 nm particles: (1) high at both sites, (2) medium at both, (3) high at EAC but low at CHC, and (4) low at both. These categories were then named after their emergent and most prominent characteristics: (1) Intense-NPF, (2) Polluted, (3) Volcanic, and (4) Cloudy. This classification was premised on the assumption that similar NPF intensities imply similar atmospheric processes. Our findings show significant differences across the categories in terms of particle size and volume, sulfuric acid concentration, aerosol compositions, pollution levels, meteorological conditions, and air mass origins. Specifically, intense NPF events (1) increased Aitken mode particle concentrations (14–100 nm) significantly on 28 % of the days when air masses passed over the Altiplano. At CHC, larger Aitken mode particle concentrations (40–100 nm) increased from 1.1 × 103 cm−3 (background) to 6.2 × 103 cm−3, and this is very likely linked to the ongoing NPF process. High pollution levels from urban emissions on 24 % of the days (2) were found to interrupt particle growth at CHC and diminish nucleation at EAC. Meanwhile, on 14 % of the days, high concentrations of sulfate and large particle volumes (3) were observed, correlating with significant influences from air masses originating from the actively degassing Sabancaya volcano and a depletion of positive 2–4 nm ions at CHC but not at EAC. During these days, reduced NPF intensity was observed at CHC but not at EAC. Lastly, on 34 % of the days, overcast conditions (4) were associated with low formation rates and air masses originating from the lowlands east of the stations. In all cases, event initiation (∼ 09:00 LT) generally occurred about half an hour earlier at CHC than at EAC and was likely modulated by the daily solar cycle. CHC at dawn is in an air mass representative of the regional residual layer with minimal local surface influence due to the barren landscape. As the day progresses, upslope winds bring in air masses affected by surface emissions from lower altitudes, which may include anthropogenic or biogenic sources. This influence likely develops gradually, eventually creating the right conditions for an NPF event to start. At EAC, the start of NPF was linked to the rapid growth of the boundary layer, which favored the entrainment of air masses from above. The study highlights the role of NPF in modifying atmospheric particles and underscores the varying impacts of urban versus mountain top environments on particle formation processes in the Andean region.

In mountainous regions, diurnal thermally driven winds impact daily weather and air quality. This study investigates how the inclination of idealised valleys affects these winds and the transport of passive tracers using high-resolution numerical simulations with the Weather Research and Forecasting (WRF) model. We explore a range of valley inclinations from 0 to 2.28°, bridging the gap between previous studies on flat and moderately inclined (up to 0.86°) idealised valleys and steeper (2–5°) real Himalayan valleys. We find that during daytime in the inclined valleys, up-valley winds penetrate deeper into the valleys and become stronger, up to a critical inclination beyond which the winds weaken. The flat-floored valley exhibits the strongest night-time down-valley winds overall, but surface-based down-valley winds are more prominent in inclined valleys. Steeper valleys enhance the vertical transport of passive tracers, resulting in ventilation at higher altitudes compared to the flat-floored valley. Despite stronger overall tracer outflow in the flat-floored valley, this occurs at lower altitudes, leading to most of the ventilated tracers being accumulated in the lowest few kilometres of the atmosphere. Consequently, steeper valleys are more efficient at ventilating tracers to the upper troposphere, which would, for example, lead to higher potential for long-range transport. These findings underscore the critical role of valley geometry in shaping wind patterns and pollutant transport, providing valuable insights for improving transport modelling in mountainous regions.

The question of how to quantify the intensity of extratropical cyclones (ETCs) does not have a simple answer. To offer some perspective on this issue, we analyse multiple measures of intensity for North Atlantic and European ETCs for the extended winter season between 1979 and 2022 using ERA5 reanalysis data. The most relevant intensity measures are identified by investigating relationships between them and by performing a sparse principal component analysis on the set of measures. We show that dynamical intensity measures correlate strongly with each other, while correlations are weaker for impact-relevant measures. Based on the correlations and the sparse principal component analysis, we find that five intensity measures, namely 850 hPa relative vorticity, 850 hPa wind speed, wind footprint, precipitation, and a storm severity index, describe ETC intensity comprehensively and non-redundantly. Using these five measures as input, we objectively classify the ETCs with a cluster analysis based on a Gaussian mixture model. The cluster analysis is able to produce four clusters between which ETCs differ in terms of their intensity, life cycle characteristics such as deepening rate and lifetime, and geographical location. A fourth of all ETCs belong to the weakest cluster and occur mostly over Europe and in the Mediterranean area. Nearly half of all ETCs belong to the average-intensity cluster and occur mostly at the northeastern parts of the main North Atlantic storm track. A fifth of all ETCs belong to the second most intense cluster and occur mostly at the start of the North Atlantic storm track. Finally, less than a 10th of all ETCs belong to the most intense cluster and occur almost equally everywhere. This last cluster includes a clear majority of a set of investigated impactful storms (17 out of 21), which demonstrates the ability of the method to identify potentially damaging ETCs.

Most of the day-to-day variability in weather in Europe, including damaging events, is caused by extratropical cyclones (ETCs). ETCs are very different from one another and to more easily study their development, intensity, and structure, various ETC classification schemes have been proposed. Here, we propose an intensity-based scheme in which we first identify necessary ETC intensity measures to describe ETC intensity comprehensively from both dynamical and impact-relevant perspective, and then use them to produce an ETC classification.ERA5 reanalysis data from 1979 to 2022 was used to track ETCs and compute their intensity measures in the extended winter season (October-March). A total of 7361 ETC tracks were identified in the North Atlantic and Europe. Eleven intensity measures were analysed including 850-hPa relative vorticity, mean sea level pressure, wind speeds at various levels, wind gust, wind footprint, precipitation, and storm severity index. Among the 11 intensity measures, relevant ones were identified by analysing their correlation with each other combined with a sparse principal component analysis (sPCA). The selected measures were used to classify the ETCs by performing a cluster analysis with Gaussian mixture modelling.Based on the sPCA and relationships between the intensity measures, the set was reduced to 5 measures: 850-hPa relative vorticity, 850-hPa wind speed, wind footprint, precipitation, and storm severity index. Therefore, to describe ETC intensity comprehensively, one needs to use more than one or two intensity measures. The cluster analysis with these 5 measures as input produced 4 discernible clusters. Between these clusters ETCs differed in terms of their intensity, life cycle characteristics, and geographical location. Despite only 9 % of all ETCs belonging to the most intense cluster, it contained 17 out of 21 investigated impactful named storms, which demonstrates the relevance of the classification and its ability to identify potentially impactful ETCs.

We integrated Enviro-HIRLAM (Environment-High Resolution Limited Area Model) meteorological output into FLEXPART (FLEXible PARTicle dispersion model). A FLEXPART simulation requires meteorological input from a numerical weather prediction (NWP) model. The publicly available version of FLEXPART can utilize either ECMWF (European Centre for Medium-range Weather Forecasts) Integrated Forecast System (IFS) forecast or reanalysis NWP data, or NCEP (U.S. National Center for Environmental Prediction) Global Forecast System (GFS) forecast or reanalysis NWP data. The primary benefits of using Enviro-HIRLAM are that it runs at a higher resolution and accounts for aerosol effects in meteorological fields. We compared backward trajectories generated with FLEXPART using Enviro-HIRLAM (both with and without aerosol effects) to trajectories generated using NCEP GFS and ECMWF IFS meteorological inputs, for a case study of a heavy haze event which occurred in Beijing, China in November 2018. We found that results from FLEXPART were considerably different when using different meteorological inputs. When aerosol effects were included in the NWP, there was a small but noticeable difference in calculated trajectories. Moreover, when looking at potential emission sensitivity instead of simply expressing trajectories as lines, additional information, which may have been missed when looking only at trajectories as lines, can be inferred.

Ice nucleating particles (INPs) play a crucial role in initiating ice crystal formation in clouds, influencing the dynamics and optical properties of clouds and their impacts on precipitation and the climate system. Despite their importance, there is limited knowledge about the vertical distribution of INPs. This study focuses on aircraft measurements conducted during spring 2018 above the boreal forest of Hyyti & auml;l & auml;, Finland. Similarities between INP concentrations, activated fractions, particle concentrations, and size distributions observed at ground level and in the boundary layer aloft indicate that surface particles and INPs are efficiently transported and mixed within the boundary layer. INP concentrations observed in the boundary layer are best predicted by a parameterization describing near-surface INP concentrations driven by the abundance of biogenic aerosol in the Finnish boreal forest, suggesting that biogenic INPs are dominant in the boundary layer above the same environment. Most of the INP concentrations and activated fractions observed in the free troposphere are notably lower than in the boundary layer, and the distinct particle size distributions suggest that different aerosol populations, likely resulting from long-range transport, are present in the free troposphere. However, we show one case where higher INP concentrations are observed in the free troposphere and where a homogeneous particle population exists from the surface to the free troposphere. This indicates that surface particles and INPs from the boreal forest can occasionally reach the free troposphere, which is particularly important as the INPs in the free troposphere can further travel horizontally and/or vertically and impact cloud formation.

Sources and transport processes of aerosols over the Italian northern Apennines are investigated with a focus on the area of the WMO/GAW global station of Mt. Cimone (CMN, 2165 m a.s.l.). The site is characterized by complex orography, representing a challenge for chemical transport model (CTM) applications when simulating processes controlling advection and diffusion of air pollutants within and above the planetary boundary layer (PBL). First, we extensively evaluated the skills of the WRF-CHIMERE (v2020r3) coupled CTM in reproducing both the meteorological conditions observed at the surface level of multiple weather stations and the sub-micrometre aerosol mass concentrations from intensive in situ measurements performed at CMN during July 2017. The analysis of the meteorological fields revealed that the local thermally-driven flows occurring over the adjacent coastal and mountainous regions are very well reproduced by the model. The accuracy is less at higher altitudes in proximity of CMN and on the slopes facing the Po valley, where also fewer observational meteorological data were available. The discrepancies between the model output and observations, especially in the near-surface wind dynamics, are mainly associated with the smoothed topography of the terrain as represented in the model: at the resolution of 1 km small-scale orographic features and related meteorological phenomena cannot be adequately reproduced. Our results indicate that the modeled particle mass concentrations and its chemical composition are in good agreement with observational data, with organic aerosol contributing to about 60% of the total sub-micrometer aerosol load during the investigated time period and sulphate being the most important inorganic component. Additionally, a model-based source apportionment analysis revealed that organic aerosol, and specifically secondary organic aerosol (SOA), were mostly of biogenic origin (contributing up to 66% of the secondary organic aerosol fraction). We further analyze the transport of organic aerosol particles associated with the typical wind pattern developing at the interface between plains, valleys and ridges of the northern Apennines mountains. Despite uncertainties in source areas and formation mechanisms, the model results indicated that the upslope valley winds might sustain the funneling of biogenic aerosol particles to higher elevations up to the Apennines ridge, eventually to above the diagnosed PBL height. For biogenic organic aerosol this process is more effective on the south-western slope of the Apennines range. This may result from either more favourable meteorological conditions or larger availability of aerosol particles over the lowlands. This work represents the first high-resolution (1 km) CTM study investigating the region of Mt. Cimone and is intended to provide original insights on the vertical transport of aerosols particles into the free troposphere in regions characterized by a complex orography, such as the Alpine range, the European Alps, and the Apennines. This work represents the first high-resolution chemical transport modelling study investigating the region of Mt. Cimone and is intended to provide original insight on the transport of aerosols particles in regions characterized by complex orography.

Dramatic sea ice loss has recently occurred at both poles. Multiple studies have suggested that changes to sea ice can impact weather in both the polar regions and mid-latitudes. However, the current generation of climate models disagrees on the rate and location of sea ice loss, and on the rate of warming in the polar regions.Thus, the atmospheric response to sea ice loss within and outside the polar regions remains highly uncertain. To reduce this uncertainty, we have performed a set of coordinated simulations with four different atmospheric general circulation models (AGCMs) within the project “Climate Relevant interactions and feedbacks: the key role of sea ice and Snow in the polar and global climate system” (CRiceS). A baseline simulation and six perturbation simulations were performed, all of which were 40-years long and had prescribed sea surface temperatures (SSTs) and sea ice concentration. In the perturbation simulations, the SSTs and sea ice concentration were changed independently, and then both were changed together. The SST and sea ice concentrations were obtained from CMIP6 simulations with the Australian Earth system model ACCESS-ESM1.5. Monthly-mean SST and sea-ice area averaged over 20 years of simulation were taken from 1) the historical simulation (years 1950-1970, Baseline simulation), 2) the scenario SSP1-2.6 simulation (years 2080-2100), and 3) the scenario SSP5-8.5 simulation (years 2080-2100) and were then used as perpetual monthly average values of SSTs and sea ice fraction in our model simulations, thus eliminating inter-annual variability in SSTs and sea ice. This array of perturbation experiments, performed with four AGCMs, allows us to isolate atmospheric responses in polar regions and mid-latitudes that are due to SST or sea ice changes, examine the linearity of these feedbacks, and investigate the robustness of the atmospheric responses. The results of this coordinated modelling experiment show that the models agree well on the magnitude and spatial distribution of the 2-m temperature and precipitation response. Increasing SSTs has a larger and more spatially extensive impact on the overall response than decreases in sea ice, which primarily only cause a localised response in regions where sea ice disappears (most notably, a strong warming over the Arctic ocean in winter). The models agree less well on the magnitude and spatial distribution of the mean sea level pressure response, in particular over northern Europe and Antarctica, suggesting that modelled uncertainties associated with atmospheric circulation are larger than uncertainties associated with thermodynamics. These results and others, along with information about the openly available dataset, will be presented.

The effect of modified equator-to-pole temperature gradients on the jet stream by low-level polar warming and upper-level tropical warming on jet streams is not fully understood. We perform four aquaplanet simulations to quantify the impact of different sea surface temperature distributions on jet stream strength, wave amplitudes and jet stream waviness, quantified by a modified Sinuosity Index. A large-scale uniform warming scenario increases the jet strength whereas decreases in jet strength occur in two scenarios where the meridional temperature gradient is reduced. However, all scenarios indicate substantial decreases in the magnitude of large wave amplitudes, jet stream extreme waviness and reduced variability of these diagnostics, suggesting a relationship with weakened baroclinicity. Our findings contradict the earlier proposed mechanism that low-level polar warming weakens the jet stream and increases wave amplitudes and jet stream waviness. We conclude that a weaker jet stream does not necessarily become wavier.

Explosive volcanic eruptions are well known to influence Earth’s temperature. Changes in Earth’s temperature can affect temperature gradients which in turn could affect the isentropic slope and hence Northern Hemisphere high and mid-latitude weather. Yet, the possible influence of volcanic eruptions on these atmospheric circulation patterns and the potential spatial extent are not well understood. To address this issue, we pursue two independent lines of evidence. Firstly, we simulate volcanic eruptions with the MPI-ESM1.2 Earth System Model and use the TRACK algorithm to explore how extra-tropical cyclone (ETC) frequency is affected in the model experiments. Secondly, we query the Greenland ice core NEEM-2011-S1 for indications of increased Northern Hemisphere ETC frequency correlating with evidence for explosive volcanism by comparing the storm proxies sodium and calcium; with the eruption proxy sulphur. Both the model and proxy evidence suggest that large explosive volcanic eruptions increase storminess around the location of the ice core. Furthermore, the simulations indicate that the number of ETCs increases in the subtropics and at high latitudes, while they decrease in the mid-latitudes. A detailed interrogation of the simulated eruptions reveals that increases in cyclone frequency are linked to steepening of the isentropic slope due to a larger meridional temperature gradient and to a lower tropopause. The steepening is driven by a combination of warming of the tropical stratosphere from absorption of longwave radiation by volcanic aerosols and surface cooling due to the scattering of sunlight by the same aerosols, whereas the lower tropopause may be attributed to a warmer stratosphere.