This study investigates the global drivers of sudden temperature changes from one day to the next using observational data and trajectory analysis. In extratropical regions, these shifts are mainly driven by air mass movements linked to circulation patterns. In tropical areas, local factors like cloud cover play a key role. Understanding these mechanisms improves predictions of extreme temperature events, aiding in better preparation and mitigation strategies.

We study the impacts of globally increasing sea surface temperatures and sea-ice loss on the atmosphere in wintertime. In future climates, the jet stream shifts southward over the North Atlantic and extends further over Europe. Increasing sea surface temperatures drive these changes. The region of high activity of low-pressure systems is projected to move east towards Europe. Future increasing sea surface temperatures and sea-ice loss contribute with similar magnitude to the eastward shift.

Storms strongly resembling hurricanes are sometime observed to form well outside the tropics, even in polar latitudes. They behave capriciously, developing very rapidly and then dying just as quickly. We show that strong dynamical processes in the atmosphere can sometimes cause it to become locally much colder than the underlying ocean, creating the conditions for hurricanes to form, but only over small areas and for short times. We call the resulting storms "cyclops".

The Indian summer monsoon season runs from June to September. However, the exact onset date of monsoon varies from year to year. The onset marks the arrival of copious rainfall over India, and its accurate prediction has crucial societal implications. Comparing early versus late monsoon onsets, we note that warmer oceanic surface water in the Bay of Bengal advances monsoon onset over India. We validated our hypothesis by examining a climate model response to warm anomalies in the Bay of Bengal.

This work presents the identification of Saharan warm air intrusions in the Western Mediterranean, which are the displacement of air masses formed over the Sahara desert toward the West of the Mediterranean region. We focus on the recent past and obtain a catalogue of intrusion days. The results show: the existence of different types of intrusions, important impacts on extremely high temperatures in the Mediterranean and Europe and the dynamic mechanisms that can onset these events.

As tropical cyclones move poleward, they can transform into extratropical cyclones, a process known as extratropical transition. These storms can pose serious risks to human lives and cause damage to infrastructure along the northeastern coasts of the U.S. & Canada. Our study investigates the impacts of climate change on the frequency, intensity, and location of extratropical transitions, revealing that transitioning storms may become more destructive in the future but may not be more frequent.

This study investigates the Laseyer, a local windstorm in a narrow Swiss valley, characterized by strong south-easterly winds during north-westerly ambient flow. Using large-eddy simulations (LES) with 30 m grid spacing, this is the first study to reveal that the extreme gusts in the valley are caused by an amplifying interplay of two recirculation regions. Modifying terrain and ambient wind conditions affects the windstorm's intensity and highlights the importance of topographic details in LES.

The swift succession of storms, referred to as cyclone clustering, is often associated with weather extremes. We introduce a detection scheme for these events and subdivide these into two types. One type is associated with storms that follow each other in space, whereas the other type requires a proximity over time. Cyclone clustering is more frequent during winter and the first type is associated with stronger storms, suggesting that the two types emerge due to different mechanisms.

This study presents a comprehensive dynamic and statistical analysis of mixed Rossby-gravity waves during the austral summer. It highlights the significant influence of upper-level lateral forcing on wave dynamics and the critical role of lower-tropospheric humidity in modulating asymmetric convection, providing new insights into the behavior of equatorial waves and their impact on regional weather patterns.

This study identifies five warm-season weather regimes (WRs) to investigate the link between tornado activity and large-scale atmospheric circulations. Certain WRs strongly affect tornado activity due to their relationship with the environment. When a WR that positively affects tornado activity persists over multiple days, it further increases the tornado activity probability. The links found highlight the potential application of WRs for developing better seasonal prediction of tornadoes.

Extreme meteorological and climatological events properties are changing under human caused climate change. Extreme events attribution methods seek to estimate the contribution of global warming in the probability and intensity changes of extreme events. Here we propose a procedure to estimate these quantities for the flow analogues method which compare the observed event to similar events in the past.

The Quasi-Biennial Oscillation (QBO) of the tropical stratospheric mean winds has been relatively steady over the 7 decades it has been observed, but there are always cycle-to-cycle variations. This study used several global atmospheric models to investigate systematic modulation of the QBO by the El Niño/La Niña cycle. All models simulated shorter periods during El Niño, in agreement with observations. By contrast, the models disagreed even on the sign of the El Niño effect on QBO amplitude.

Rainfall in the tropics is strongly related to the amount of moisture in the atmosphere. In this work, we examine a measure of moisture called the ‘moist margin’, which describes a transition between rainy and clear regions of the tropics. We show that movement of the moist margin is often related to a variety of weather systems. Cases where tropical moisture moves far poleward are often related to low-pressure systems or large-scale waves in the extratropics.

As extremely dry and warm conditions over the Amazon basin can cause huge damages, we study the role of atmospheric transport into the western Amazon for such events. We show that the physical processes depend on the climate variability El Niño Southern Oscillation (ENSO). While warm and dry air from the Atlantic Ocean is transported to the western Amazon for extreme events during the warm ENSO phase, we expect continental regions to have a stronger impact for the other extreme events we found.

The jet stream is the main feature of the upper level flow, and drives the weather at the surface. It is stronger and better defined in winter, and has mostly been studied in that season. However, it is very important for (extreme) weather in summer. In this work, we improve and use two existing and complementary methods to study the jet stream(s) in the Euro-Atlantic sector, with a focus on summer. We find that our methods can verify each other and agree on interesting signals and trends.