This study documents the dynamics behind the first occurrence of a multi-year negative Indian Ocean Dipole (nIOD) event during 2021–2022, which lasted for an unprecedented 19 months! The event was sustained by the triple-dip La Niña event of 2020–2022. The conducive background state led to anomalous westerly wind activity and a record number and duration of westerly-wind bursts in the equatorial Indian Ocean. The resulting modulation of Wyrtki jets aided the sustenance of the nIOD event.

Severe thunderstorm outbreaks are a source of major damage across Europe. Using historical data, we analysed the large-scale weather patterns that lead to these outbreaks in eight different regions. Three types of regions emerge: those limited by temperature, limited by moisture and overall favourable for thunderstorms; consistent with their associated weather patterns and the general climate. These findings help explain regional differences and provide a basis for future forecast improvements.

Due to changes in atmospheric circulation some regions are warming quicker than others. Statistical methods are used to estimate how much of the local summer temperature trends are due to circulation changes. We evaluate these methods by comparing their estimates to special simulations representing only temperature changes related to circulation changes. By applying the methods to observations of 1979–2023 we find that half of the warming over parts of Europe is related to circulation changes.

Katabatic winds are a key feature of the climate of Antarctica, but substantial biases remain in their representation in atmospheric models. This study investigates a katabatic wind event in the ICOLMDZ model using in-situ observations. The framework allows to disentangle which part of the bias is due to horizontal resolution, to parameter calibration and to structural deficiencies in the model. We underline in particular the need to refine the physics of the model snow cover.

The high-frequent moist static energy (MSE) flux dominates poleward atmospheric heat transport in the Southern Hemisphere. In this study, we investigate how this high-frequent eddy MSE flux evolves over the life cycle of extratropical cyclones. By attributing eddy MSE fluxes to nearby cyclones, we assess the contribution of individual cyclones to zonally integrated atmospheric heat transport and discuss the relationship between cyclone numbers and atmospheric heat transport on a seasonal scale.

The need for information about climate change is ever increasing. Therefore, it is important to have knowledge about climate change, along with an understanding of the uncertainties of climate model ensembles. Here, climate change in Sweden and neighbouring countries and its relation to global warming is described. Global warming results in higher temperature, more warm days and fewer cold days. The local and global warming suggest that climate change in Sweden may currently be at its fastest.

In this study we use very high resolution numerical weather prediction model simulations of the Berlin, Germany region along with assessment of field campaign observations to understand better the impact of urban areas on the near-surface boundary layer. We find that there a clear affect of urban areas up to 15 kilometers downwind of the city centre in both the field campaign observations and the high resolution model.

Rossby wave breaking (RWB) influences weather at a large scale and can contribute to extreme weather events, but it is not known if climate change will have an effect on where and how often RWB occurs. We investigate how extreme sea ice loss and warming of the sea surface effect RWB. Our results show that sea surface temperatures significantly change local RWB frequencies and the closely related upper atmospheric jet streams, but that sea ice changes have no noticeable effect.

Hemispheric heatwaves have fundamental implications for ecosystems and societies. They are studied together with the large-scale atmospheric dynamics, through the lens of the poleward heat transports by planetary-scale waves. Extremely weak transports of heat towards the Poles are found to be associated with hemispheric heatwaves in the Northern Hemisphere mid-latitudes. Therefore, we conclude that heat transports are a clear indicator, and possibly a precursor of hemispehric heatwaves.

Turbulence can occur in clear-air conditions at cruising altitude. From around 5000 clear-air turbulence events identified using aircraft measurements, nonlinear breaking of large-scale waves and rapidly ascending airstreams associated with cyclones are found concurrent with 40 % and 30 % of them respectively. The results further show that these weather systems may trigger turbulence by generating highly deformed flow or flow instability, improving our understanding of clear-air turbulence.

This study uses Lagrangian moisture tracking and high-resolution weather simulations to trace moisture sources for Storm Ianos (Sept 2020). The analysis identified the Ionian Basin and southwestern Balkans as the primary sources, with secondary contributions from the surrounding seas. Large transport moisture traveled via three main pathways, with the Marmara-Black Sea route most significant. For record-breaking rainfall local evaporation over Greece and the Ionian Sea dominated moisture uptake.

In southeast Austria, summer thunderstorms often cause severe damage but are very hard to accurately forecast. With data from the WegenerNet 3D Open-Air Laboratory, we study these storms from beginning to end in multiple atmospheric parameters, like temperature, cloud properties, and wind speed. The characteristic features we find in these parameters expand our understanding of intense storms and can improve their prediction.

In a changing climate, hot days and humidity are expected to rise, increasing heatwave frequency and impact. Most heatwave indices focus only on temperature or are region-specific, limiting their ability to assess human thermal comfort. To address this, we developed a novel index that integrates temperature and relative humidity—two readily available variables—to detect heatwaves. We test it on record-breaking heatwaves, evaluating its ability to assess humidity conditions.

Atmospheric circulation changes contribute significantly to climate projection uncertainty, which is compounded by the limited understanding of how upper-tropospheric circulations influence near-surface events like heatwaves. This study develops a methodology to trace the imprint of upper-tropospheric circulations on the lower-tropospheric energy budget, and introduces a classification system enabling statistical summaries of the energetics that respect underlying circulation patterns.

In East Africa, rain is more likely on days with westerly winds. We have made the first timeseries showing when and where westerly winds occur in East Africa. We found that westerlies are most common in January and February over Tanzania, where we estimate they cause up to 60 % of rainfall, and are up to 3 times more likely on days with a tropical cyclone in the Indian Ocean. This work improves our understanding of what causes rain in East Africa, which should lead to better forecasting.

Extratropical cyclones feature rapidly ascending airstreams known as warm conveyor belts, which influence upper-level flow dynamics. This study classifies interactions of warm conveyor belts with the jet stream into four types: no interactions, ridges, blocks, and tropospheric cutoffs. We use reanalysis data to show that the interaction type depends more on the structure of the ambient flow than on the WCB properties, which improves the understanding of extratropical flow variability.

Summertime North Atlantic storms bring heavy rainfall, especially near their centers and along their fronts. By tracking precipitating air parcels back in time we find that the moisture comes from areas of strong ocean evaporation, with hotspots in the Gulf Stream region. We also find that sometimes evaporation in a previous storm can contribute to rainfall in the next. Unlike in winter, summer storms also draw moisture from land, and their properties are partly shaped by former tropical storms.

In this study, we investigate a particularly long-lived example of a Siberian summer cyclone, which originates during a heat wave in Kazakhstan in July 2021 and propagates into the Arctic, where it leads to heavy precipitation and alters the Arctic tropopause. Although a rare event in current climate, this case reveals how compounding events may be linked by one weather system and portrays a type of cyclone event which is likely to become more frequent in a warmer climate.

Here we identify the drivers of Rio de Janeiro’s record-breaking November 2023 heatwave. We find that springtime extreme heat in the city is becoming more frequent and heat events of the magnitude experienced in November 2023 may become significantly more likely with continued climate change. These results characterizing the evolving risk for extreme heat in Rio de Janeiro are essential for the city’s development of targeted hazard management plans.

To identify sources of the model systematic errors, we investigate northern hemisphere mid-latitude wind errors at short- to medium-range timescale using the atmospheric zonal-mean budgets analysis and the relaxation technique. These process-based diagnostics specify to what extent the individual components in the budgets contribute to the total tendency of the corresponding variable. This study shows disentanglements of compensating errors caused by mechanical and thermal forcings.

The study examines the delayed withdrawal of the Mainland Indochina Southwest Monsoon by exploring spatial trends. The new Cumulative Change-Point Monsoon index effectively describes seasonal shifts. Results indicate stronger subtropical westerly jets and weaker tropical easterly jets in recent years, impacting wind patterns and delaying monsoon withdrawal.

Quasi-stationary Rossby waves, lasting weeks, can be linked to persistent extreme weather. The mechanisms of these quasi-stationary waves may be impacted by stationary forcings like topography, heating, and land surface. The presence of these forcings extends the duration of strong quasi-stationary wave events. Our climate model experiments give insights into the mechanisms of quasi-stationary waves, highlighting the importance of a combination of transient eddies and background flow conditions.

Weather and climate is determined by large-scale air pressure patterns, which can be grouped into a small number of representative patterns. We use two different statistical methods and data from 20 climate models to detect significant changes in a future scenario of strong warming. We focus on the summer season as this is important for both human activities and natural ecosystems. We find an increase in high pressure above Europe and low pressure above the Arctic ocean.

Afforestation is a key strategy for climate change mitigation, yet the impacts on tropical hydroclimate remain uncertain. We find that future afforestation would increase evaporation and precipitation in the tropics, especially over Africa. It would also reduce net precipitation (precipitation minus evaporation) in these regions, which determines water availability. This happens because trees slow near-surface winds, while their influence on the energy budget strengthens convection.

Understanding how projected urbanization and climate change affect typhoons, which may cause the most destructive natural catastrophes, is crucial. Based on numerical simulations of five landfalling typhoons in Shanghai, China, our results highlight that warming sea surface temperatures significantly shift typhoon tracks with intensified structures (increased size, intensity, and affected time) on the big scale. At the meantime, urbanization further enhances local rainfall intensity.

Heatwaves are becoming more frequent, but the contribution by atmospheric circulation changes is unclear. Experiments with an idealized model that simulates atmospheric dynamics, but excludes clouds, radiation, and moisture, show how a poleward storm track shift increases the eastward phase speed of Rossby waves and reduces mid-latitude heatwave frequency. New evidence suggests that this mechanism is already active in the Southern Hemisphere and may soon emerge in the Northern Hemisphere.

Km-scale climate models have large added value for modeling precipitation, but their benefits for winds are less studied. We show that the km-scale model better reproduces strong winds in complex terrain, which are up to twice stronger than in a coarser model, and can also capture downslope glacier winds in higher terrain. Future changes in mean and strong winds are governed by the large-scale circulation change, while for weak winds by the temperature change, which is less uncertain.

This study examines links between the stratospheric Quasi-Biennial Oscillation (QBO) and large-scale atmospheric circulations in the tropics, subtropics, and polar regions. The QBO teleconnections and their modulation by the El Niño-Southern Oscillation (ENSO) are investigated through a series of climate model experiments. While QBO teleconnections are qualitatively reproduced by the multi-model ensemble, they are not consistent due to modelled QBO bias and other systematic model biases.

Extreme weather events pose increasing challenges for aviation, including flight disruptions and infrastructure damage. This study examines the influence of anthropogenic climate change on four recent major storms across Europe, the USA, and East Asia. Our research underscores the growing intensity of extreme storms, driven by human-induced climate change, underscoring the need to adapt aviation strategies to an increasingly hazardous environment.

We used the variable-resolution CESM to simulate present-day and future temperature and precipitation extremes in the Arctic by applying global grids (~111 km) with and without regional refinement (~28 km) and following a storyline approach. We found that global grids with (without) regional refinement generally perform better in simulating present-day precipitation (temperature) extremes, and that future high (low) temperature and wet precipitation extremes are projected to increase (decrease).

As the climate warms, extreme weather is becoming more frequent in mid-latitudes. A key factor is the jet stream, shaped by atmospheric waves that influence wind and storm patterns. This study presents a simplified model showing how swirling air currents (eddies) maintain the jet stream and impact weather. As global warming alters these patterns, this research helps improve predictions of future weather changes.

Extreme cold temperatures have widespread impacts on health, agriculture, infrastructures and the economy. We develop for the first time a methodology to build a catalogue of cold spell events, tracked in space and time. This catalogue is used to examine the behaviour of cold spells and its climatology. The results reveal specific pathways through which cold air affect midlatitudes.

Representation of severe convection is a challenge for the numerical weather prediction models. We show that an explicit stochastic convection initiation scheme allows numerical representation of the isolated bow echo of severe social impact, showing its cold-pool-driven dynamics, formation of the rear inflow jet and strong surface winds. In moist convection context, we polemize with the idea of horizontal sizes of model perturbations being no less than the effective model’s resolution.