Preprints  

The dynamics of the equatorial troposphere is incredibly complex. Several types of waves constitute the core of the theoretical understanding of tropical dynamics. Signatures of these waves are observed in the spectrum of atmospheric variables. We decompose the atmospheric winds into their wave contributions. A spectral analysis of these fields reveal important departures from the linear theory of equatorial waves, suggesting a possible role of nonlinearity in the propagation of these waves.

Novel techniques from computer science are used to study extreme weather events. Inspired by the interactive 3-dimensional visual analysis of the most comprehensive climatological dataset available (ERA5), we improve commonly used metrics for measuring polar winter storms and outbreaks of cold air. The software (Met.3D) that we develop and apply is freely available and can be used generically for visual analysis of a broad variety of atmospheric processes in weather and climate data.

This work is assessing the forecast of the temperature over the Sahara, a key driver of the West African monsoon, at seasonal time scale. The seasonal models are able to reproduce the climatological state and some characteristics of the temperature during the rainy season in the Sahel. But, because of errors in the timing, the forecast skill scores are significant only for the first 4 weeks.

In this study, we introduce a novel method to objectively define and identify Arctic extreme seasons based on different surface variables. We find that such seasons are resulting from various combinations of unusual seasonal conditions. The occurrence or absence of different atmospheric processes strongly affects the character of Arctic extreme seasons. Further, changes in sea ice and sea surface temperature can strongly influence the formation of such a season in distinct regions.

Major sudden stratospheric warmings (SSWs) are some of the most dramatic events in the atmosphere, and are believed to help cause extreme winter weather events such as the 2018 Beast from the East in Europe and North America. Here, we use unique data from the European Space Agency's new Aeolus satellite to make the first-ever measurements at a global scale of wind changes due to an SSW in the lower part of the atmosphere, to help us understand how SSWs affect the atmosphere and surface weather.

The strongest cyclone intensification is associated with a strong dry-dynamical forcing. Moreover, strong forcing and strong intensification correspond to a tendency for poleward cyclone propagation, which occurs in distinct regions in the Northern Hemisphere. There is a clear spatial pattern in the occurrence of certain forcing combinations. This implies a fundamental relationship between dry-dynamical processes and the intensification as well as the propagation of extratropical cyclones.

We use an advanced statistical approach to investigate the dynamics of the development of sudden stratospheric warming events in the winter Northern Hemisphere. We identify distinct signals of these events and their event type at lead times beyond currently predictable lead times. These signals could potentially improve the predictability of sudden stratospheric warming events, leading to improved long-range weather prediction.

We re-examine the uncertainty of ENSO teleconnection to the North Atlantic by considering the November–December and January–February months in the cold season, in addition to the conventional DJF months. This is motivated by previous studies reporting varying teleconnected atmospheric anomalies and the mechanisms concerned. Our results indicate an improved confidence in the patterns of the teleconnection. The finding may also have implications on research in predictability and climate impact.

Given the recent focus on the influence of upper tropospheric structure in wind and temperature on midlatitude weather, we use an idealised model to investigate how structural modifications impact cyclone development. We find that cyclone intensification is less sensitive to these modifications than to changes in the amount of cloud condensation, suggesting that an accurate representation of the upper-level troposphere is less important for midlatitude weather than previously anticipated.

In this study we analyse the complex seasonal evolution of the East Asian summer monsoon. Using reanalysis data, we show the importance of the interaction between tropical and extratropical air masses converging at the monsoon front, particularly during its northward progression. The upper-level flow pattern (e.g. the westerly jet) controls the balance between the airstreams, and thus the associated rainfall. This framework provides a basis for studies of extreme events and climate variability.

Why do some monsoon seasons start early while others start late? For the Australian monsoon, some previous research suggested the El Niño–Southern Oscillation in the months before the onset influenced the monsoon timing, and we wanted to test if that was still correct, and if other large-scale climate patterns also influenced onset timing. We found that a strong La Niña pattern usually coincided with an early onset but a weak La Niña and El Niño patterns didn’t show a consistent relationship.

Storm activity in the tropics is one of the key phenomena that provide weather predictability on extended time scale of about 10–40 days. The influence of tropical storminess on places like North America is sensitive to the overall average state of the climate system. In this study, we show that global warming will likely lead to decreases in the influence of these storms on weather over the North Pacific, but that there is a lot of uncertainty in how this influence will change.

We present a ten year analysis on the occurrence of enhanced wind shear in the northern hemisphere, focussing on the region around the transport barrier that separates the first two layers of the atmosphere. The major result of our analysis is that enhanced wind shear above a certain threshold occurs frequently and nearly exclusively in this region, which, as an indicator for turbulent mixing, might have major implications concerning the separation efficiency of the transport barrier.

Mistral wind is a renowned phenomenon in the Mediterranean, yet, its large-scale controlling mechanisms have not been systematically mapped. Here, using a new mistral database for 1981–2016, the upper-tropospheric flow patterns are classified by a self-organizing map algorithm, resulting in 16 distinct patterns related to Rossby wave life cycles. Each pattern has unique surface impact, having implications to understanding mistral predictability, air-sea interaction and their future projections.

The amount of precipitation over Europe largely varies from year to year, with impacts on crop yields and energy production. In this study, we show that it is possible to infer much of the winter precipitation and temperature signal over Europe by only knowing the frequency of occurrence of certain atmospheric circulation patterns. The results highlight the importance of (daily) weather to understand and interpret seasonal signals.

Air stagnation occurs when an air mass becomes settled over a region and precipitation is suppressed. The large-scale influence on European air stagnation is explored here. We show that around 60 % of the monthly variability in air stagnation can be explained by dynamical indices describing the large-scale circulation. The weather systems most related to stagnation are different for regions across Europe. The findings can be used to assess the representation of air stagnation in climate models.

Using a large sample of Atlantic tropical cyclone tracks, we diagnose the mean circulations in the extratropics that are associated with rapid changes in the storm speeds. We show that rapid accelerations and decelerations are associated with distinct phasing between the tropical cyclone and weather waves of the extratropics. We also show that over the past 5 decades, rapid acceleration and deceleration of tropical cyclones have reduced in magnitude.

This study investigates the development processes of extratropical cyclones in East Asia, particularly when they are passing through the Korean Peninsula. These extratropical cyclones originate from Mongolia and East China and travel towards the Korean Peninsula. The quantitative analyses in the potential vorticity perspective demonstrate that the former is mostly driven dry-dynamically and the latter is influenced significantly by moist processes.

Atmospheric blocking events are complex large-scale weather patterns which block the path of the jet stream. They are associated with heat waves in summer and cold snaps in winter. Blocking is poorly understood, and the effect of climate change is not clear. Here, we present a new method to study blocking using unsupervised machine learning. We show that this method performs better than previous methods used. These results show the potential for unsupervised learning in atmospheric science.

We classify by wave geometry the stratospheric polar vortex during the final warming that occurs every spring in both hemispheres due to a combination of radiative and dynamical processes. We show that the shape of the vortex can have impacts on both total column ozone and surface weather following the final warming. These results have implications for prediction and our understanding of stratosphere-troposphere coupling processes in springtime.

Stationary, long-lasting blocked weather patterns can lead to extreme conditions. Within this study the temporal evolution of the occurrence probability is analysed and the onset, offset, and transition probabilities of blocking within the past 30 years are modeled. Using Markov models combined with logistic regression we found large changes in summer, where the probability of transitions to so-called Omega blocks increase strongly, while the unblocked state becomes less probable.

Weather variability controls the transport of ozone from the stratosphere to the Earth's surface and water vapor from oceanic source regions to continental land masses. Forecasting these types of transport has high societal value because of the negative impacts of ozone on human health and the role of water vapor in governing precipitation variability. We use upper level wind forecasts to assess the potential for predicting ozone and water vapor transport 3–6 weeks ahead of time.

Much of the rainfall in tropical regions comes from organised cloud bands called convergence zones (CZs). These bands have hundreds of kilometers. In South America (SA), they cause intense rain for long periods of time. To study these systems, we need to define and identify them with computer code. We propose a definition of CZs based on the the pathways of air, selecting regions where air masses originated in separated regions meet. This method identifies important systems of rain in SA.

Tropical Pacific sea surface temperatures (El Nino) effect the global climate. The Pacific-to-Europe connection relies on interactions of large atmospheric waves, with winds and surface pressure. We looked at how mean errors in a climate model effect its ability to simulate the Pacific-Europe connection. We found that even large errors in the seasonal winds did not affect the response of the model to an En Nino event, which is good news for seasonal forecasts which rely on these connections.

High-resolution measurements of stable isotopes in near-surface vapour and precipitation show a W-shaped evolution during a 24-h landfalling atmospheric river event in southern Norway. We distinguish contributions from below-cloud processes, weather system characteristics, and moisture source conditions during different stages of the event. Rayleigh distillation models need to be expanded by additional processes to accurately predict isotopes in surface precipitation from stratiform clouds.

On 14–15 October 2018, in the Aude department (France), a heavy precipitation event produced up to about 300 mm of rain in 11 h. Simulations carried out show that the former hurricane Leslie, while involved, is not the first supplier of moisture over the entire event. The location of the highest rainfall appears to be driven primarily by the location of a quasi-stationary cold front and secondarily by the location of precipitation bands downwind of the mountains bordering the Mediterranean Sea.

Rossby Wave packets impact all aspects of midlatitude weather systems, from their climatological distribution to predictability. Case studies suggest an important role of latent heat release in clouds. We investigate thousands of wave packets with a novel diagnostic. We demonstrate that, on average, the impact of moist processes is substantially different between troughs and ridges and that dry conceptual models of wave packet dynamics should be extended.

Comparing simulations with realistic and smoothed SSTs, we find that the intensification of individual cyclones in the Gulf Stream and Kuroshio regions is only marginally affected by reducing the SST gradient. In contrast, we observe a reduced cyclone activity and a shift in storm tracks. Subdividing the winter climatology into dates with/without cyclones, we find that cyclones play only a secondary role in explaining the mean states differences between the SST experiments.

This work examines the fidelity to reproduce regional and global monsoons climatological features using the Met Office Unified Model (MetUM) third and fourth generations Global Atmosphere (GA3.0) and (GA4.0), two configurations of the HadGEM3 system developed for use across climate and weather time scales. GA3.0 largely captures global monsoon features, including the monsoon precipitation patterns. GA4.0 and GA3.0 results display a close similarity, and compares reasonably well against CMIP5.

We show that the persistence of summer weather has increased throughout the mid-latitudes over the last 40 years, in both observations and CMIP6 models. Our results provide solid evidence that the weakening of the summer jet-stream has already made weather more persistent. We also show that future greenhouse-gases emissions will further increase weather persistence, creating risks from high-impact, stalling weather extremes like persistent heat waves and stalling cyclones.

While the key role of moist processes in blocking has recently been highlighted, their moisture sources remain unknown. Here, we investigate moisture sources for wintertime Euro-Atlantic blocks using a Lagrangian method. We show that the Gulf Stream and Kuroshio and their extensions, along with the northeast of Hawaii, act as the primary moisture sources and springboards for particle ascent. We find that the evolution of the particles’ properties is sensitive to the moisture sources.

Motivated by the European heat wave occurrences of 2015 and 2018, this study evaluates the influence of cold North Atlantic SST anomalies on European heat waves by using the ERA-5 reanalysis product. Our findings show that widespread cold North Atlantic SST anomalies may be a precursor for a persistent jet stream pattern and are thus important for the onset of high European temperatures.

In this study, we investigate the three-dimensional life cycle of cyclonic structures (so-called PV cutoffs) in the upper troposphere. PV cutoffs are frequent weather systems in the extratropics and contribute substantially to high impact weather in many regions of the globe. However, many unknowns exist regarding their evolution. Using a new method to track PV cutoffs in reanalysis data, their life cycle is studied in detail and a classification into three life cycle types is proposed.