Matthias Röthlisberger

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Röthlisberger

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Matthias

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0000-0003-0904-9495

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Publications 1 - 10 of 22

Recurrent Rossby waves and south-eastern Australian heatwaves
Ali, S. Mubashshir; Röthlisberger, Matthias; Parker, Tess; et al. (2022)
Weather and Climate Dynamics
In the Northern Hemisphere, recurrence of transient synoptic-scale Rossby wave packets in the same phase over periods of days to weeks, termed RRWPs, may repeatedly create similar surface weather conditions. This recurrence can lead to persistent surface anomalies. Here, we first demonstrate the significance of RRWPs for persistent hot spells in the Southern Hemisphere (SH) using the ERA-Interim (ERA-I) reanalysis dataset and then examine the role of RRWPs and blocks for heatwaves over south-eastern Australia (SEA). A Weibull regression analysis shows that RRWPs are statistically associated with a significant increase in the duration of hot spells over several regions in the SH, including SEA. Two case studies of heatwaves in SEA in the summers of 2004 and 2009 illustrate the role of RRWPs in forming recurrent ridges (anticyclonic potential vorticity – PV – anomalies), aiding in the persistence of the heatwaves. Then, using a weather-station-based dataset to identify SEA heatwaves, we find that SEA heatwaves are more frequent than climatology during days with extreme RRWPs activity over SEA (high RSEA). On days with both high RSEA and heatwaves, circumglobal zonal wavenumber 4 and 5 (WN4, WN5) anomaly patterns are present in the composite mean of the upper-level PV field, with an anticyclonic PV anomaly over SEA. The Fourier decomposition of the PV and meridional wind velocity fields further reveals that the WN4 and WN5 components in the suitable phase aids in forming the ridge over SEA for days with high RSEA. In addition, we find anomalous blocking over the Indian and the South Pacific oceans during SEA heatwaves, which may help to modulate the phase of RRWPs.
Breaking Rossby waves drive extreme precipitation in the world’s arid regions
De Vries, Andries-Jan; Armon, Moshe; Klingmüller, Klaus; et al. (2024)
Communications Earth & Environment
More than a third of the world’s population lives in drylands and is disproportionately at risk from hydrometeorological hazards such as drought and flooding. While weather systems governing precipitation formation in humid regions have been widely explored, our understanding of the atmospheric processes generating precipitation in arid regions remains fragmented at best. Here we show, using a variety of precipitation datasets, that Rossby wave breaking is a key atmospheric driver of precipitation in arid regions worldwide. Rossby wave breaking contributes up to 90% of daily precipitation extremes and up to 80% of total precipitation amounts in arid regions equatorward and downstream of the midlatitude storm tracks. The relevance of Rossby wave breaking for precipitation increases with increasing land aridity. Contributions of wave breaking to precipitation dominate in the poleward and westward portions of arid subtropical regions during the cool season. Our findings imply that Rossby wave breaking plays a crucial role in projections and uncertainties of future precipitation changes in societally vulnerable regions that are exposed to both freshwater shortages and flood hazards. (Figure presented.)
Quantifying the physical processes leading to atmospheric hot extremes at a global scale
Röthlisberger, Matthias; Papritz, Lukas (2023)
Nature Geoscience
Heat waves are among the deadliest climate hazards. Yet the relative importance of the physical processes causing their near-surface temperature anomalies (T′)—advection of air from climatologically warmer regions, adiabatic warming in subsiding air and diabatic heating—is still a matter of debate. Here we quantify the importance of these processes by evaluating the T′ budget along air-parcel backward trajectories. We frst show that the extreme near-surface T′ during the June 2021 heat wave in western North America was produced primarily by diabatic heating and, to a smaller extent, by adiabatic warming. Systematically decomposing T′ during the hottest days of each year (TX1day events) in 1979–2020 globally, we fnd strong geographical variations with a dominance of advection over mid-latitude oceans, adiabatic warming near mountain ranges and diabatic heating over tropical and subtropical land masses. In many regions, however, TX1day events arise from a combination of these processes. In the global mean, TX1day anomalies form along trajectories over roughly 60 h and 1,000 km, although with large regional variability. This study thus reveals inherently non-local and regionally distinct formation pathways of hot extremes, quantifes the crucial factors determining their magnitude and enables new quantitative ways of climate model evaluation regarding hot extremes.
Understanding the vertical temperature structure of recent record-shattering heatwaves
Hotz, Belinda; Papritz, Lukas; Röthlisberger, Matthias (2024)
Weather and Climate Dynamics
Extreme heatwaves are one of the most impactful natural hazards, posing risks to human health, infrastructure, and ecosystems. Recent theoretical and observational studies have suggested that the vertical temperature structure during heatwaves limits the magnitude of near-surface heat through convective instability. In this study, we thus examine in detail the vertical temperature structure during three recent record-shattering heatwaves, the Pacific Northwest (PNW) heatwave in 2021, the western Russian (RU) heatwave in 2010, and the western European and UK (UK) heatwave in 2022, by decomposing temperature anomalies (T 0) in the entire tropospheric column above the surface into contributions from advection, adiabatic warming and cooling, and diabatic processes. All three heatwaves exhibited bottom-heavy yet vertically deep positive T 0 extending throughout the troposphere. Importantly, though, the T 0 magnitude and the underlying physical processes varied greatly in the vertical within each heatwave, as well as across distinct heatwaves, reflecting the diverse synoptic storylines of these events. The PNW heatwave was strongly influenced by an upstream cyclone and an associated warm conveyor belt, which amplified an extreme quasi-stationary ridge and generated substantial mid-to upper-tropospheric positive T 0 through advection and diabatic heating. In some contrast, positive upper-tropospheric T 0 during the RU heatwave was caused by advection, while during the UK heatwave, it exhibited modest positive diabatic contributions from upstream latent heating only during the early phase of the respective ridge. Adiabatic warming notably contributed positively to lower-tropospheric T 0 in all three heatwaves, but only in the lowermost 200–300 hPa. Near the surface, all three processes contributed positively to T 0 in the PNW and RU heatwaves, while near-surface diabatic T 0 was negligible during the UK heatwave. Moreover, there is clear evidence of an amplification and downward propagation of adiabatic T 0 during the PNW and UK heatwaves, whereby the maximum near-surface T 0 coincided with the arrival of maximum adiabatic T 0 in the boundary layer. Additionally, the widespread ageing of near-surface T 0 over the course of these events is fully consistent with the notion of heat domes, within which air recirculates and accumulates heat. Our results for the first time document the four-dimensional functioning of anticyclone–heatwave couplets in terms of advection, adiabatic cooling or warming, and diabatic processes and suggest that a complex interplay between large-scale dynamics, moist convection, and boundary layer processes ultimately determines near-surface temperatures during heatwaves.
Recurrent Rossby Wave Packets Modulate the Persistence of Dry and Wet Spells Across the Globe
Ali, S. Mubashshir; Martius, Olivia; Röthlisberger, Matthias (2021)
Geophysical Research Letters
Persistent dry and wet spells can arise from stationary weather situations or recurrent flow patterns and result in significant socio-economic impacts. Here, we study the effects of recurrent synoptic-scale transient Rossby wave packets (RRWPs) on the persistence of dry and wet spells using the ERA-Interim reanalysis data. RRWPs significantly alter (decrease and increase) dry and wet spell persistence across the globe. Spatial patterns of statistically significant links between RRWPs and spell durations arise from the superposition of a zonally symmetric component and a wave-like component that is modulated by local factors such as orography and the position relative to major moisture sources. The zonally symmetric component is apparent during the Northern Hemisphere winter and dominates the Southern Hemisphere signal in winter and summer. The wave-like component appears primarily in the Northern Hemisphere, changes its wavenumber with the season and is thus, conceivably related to stationary wave dynamics. © 2021. American Geophysical Union.
Meteorological history of low-forest-greenness events in Europe in 2002-2022
Hermann, Mauro; Röthlisberger, Matthias; Gessler, Arthur; et al. (2023)
Biogeosciences
Forest dieback in Europe has recently intensified and has become more extensive. This dieback is strongly influenced by meteorological variations of temperature, T2m, and precipitation, P, and can be monitored with forest greenness. This study quantitatively investigates the 3-year meteorological history preceding events of reduced forest greenness in Europe's temperate and Mediterranean biome with a systematic approach. A specific focus lies in the timing of unusually persistent and unusually strong anomalies of T2m and P, as well as their relation to synoptic weather systems. A pragmatic approach based on remote sensing observations of the normalized difference vegetation index (NDVI) serves to identify low-forest-NDVI events at the 50km scale in Europe in June to August 2002-2022. We quantify the impact of the hottest summer on record in Europe in 2022, which, according to our criteria, negatively affected 37% of temperate and Mediterranean forest regions, and thereby reduced forest greenness more extensively than any other summer in 2002-2022. The low-NDVI events occurred in particularly dry and hot summers, but their meteorological histories also featured significant anomalies further in the past, with clear differences between the temperate and Mediterranean biome. A key feature is the anomalous accumulation of dry periods (i.e., periods with a P deficit) over the preceding 26 and 34 months in the temperate and Mediterranean biome, respectively. In the temperate biome only, T2m was anomalously persistent during almost the same 26-month period and featured distinctive peaks late in the past three growing seasons. While anomalously strong hot-dry conditions were characteristic of temperate low-NDVI events already in the previous summer, we find hardly any other systematic meteorological precursor in the Mediterranean prior to the event year. The identified dry periods went along with reduced cyclone activity in the Mediterranean and positive anticyclone frequency in the temperate biome. The occurrence of these two weather systems is locally more nuanced, showing, e.g., consistently increased and decreased cyclone frequency over western and northern Europe, respectively, in all event summers. Finally, the systematic meteorological histories are useful to test whether locally observed meteorological impacts, e.g., structural overshoot, systematically influenced the investigated events. In summary, systematic investigations of the multi-annual meteorological history provided clear evidence of how surface weather and synoptic-scale weather systems over up to 3 years can negatively impact European forest greenness. The observation of the record-extensive low-NDVI event in the summer of 2022 underlines that understanding the forest-meteorology interaction is of particular relevance for forest dieback in a changing climate.
How relevant are frequency changes of weather regimes for understanding climate change signals in surface precipitation in the North Atlantic-European sector? A conceptual analysis with CESM1 large ensemble simulations
Fischer, Luise; Bresch, David N.; Büeler, Dominik; et al. (2025)
Weather and Climate Dynamics
Climate change affects the climatology of surface precipitation in spatially inhomogeneous ways, and it is challenging to identify and quantify the contribution of atmospheric circulation changes to this pattern. Various methods have been developed to characterize the large-scale atmospheric circulation and assess its changes, e.g., by classifying the flow into so-called weather regimes or circulation types. Several studies have then related frequency changes of these regimes due to global warming to changes in surface weather parameters. However, even without regime frequency changes, the climatology of surface parameters may change due to so-called regime intensity changes (e.g., a particular regime becomes on average wetter or drier). In this study, the question of how relevant frequency changes of weather regimes are for understanding climate change signals in surface precipitation is addressed with a novel conceptual framework. For every regime i, a spatially varying parameter γ3i(P) is introduced, which corresponds to the ratio of the contributions from regime frequency vs. regime intensity changes to the climate change signal of precipitation P. Conceptual considerations show that γ3i(P) is (i) proportional to the relative change of regime frequency, (ii) proportional to the regime-specific anomaly of precipitation, and (iii) inversely proportional to the climate change effect on regime intensity. The combination of these independent and competing factors makes the study of γ3i(P) interesting and insightful. As a specific example application of this framework, we consider a 7-category weather regime classification in the North Atlantic-European sector and large ensemble simulations with the CESM1 climate model under the RCP8.5 emission scenario for the periods 1990-1999 and 2091-2100. Considering γ3i(P) for surface precipitation, P in this simulation setup reveals that (1) γ3 values are typically less than 0.25 and therefore, to first order, frequency changes of weather regimes (WRs) are of secondary importance for explaining climate change signals in P - in contrast, the intensity changes dominate, which are to a large degree, but not entirely, related to the so-called thermodynamic effects of global warming; (2) the main reason for the generally low values of γ3 is the comparatively small WR frequency changes and the limited regime-specific anomalies of P, in particular over continental Europe; and (3) γ3 values tend to be slightly larger for precipitation variables that are less constrained by thermodynamic arguments, i.e., γ3 for the number of wet days is larger than γ3 for the number of heavy-precipitation days. In summary, this study provides a generally applicable framework to quantify climate change effects of regime frequency changes on surface parameters, it illustrates the key conditions that must be fulfilled such that these frequency changes can become relevant, and, at least in our application, it shows that these conditions are generally not fulfilled.
Extreme wet seasons – their definition and relationship with synoptic-scale weather systems
Flaounas, Emmanouil; Röthlisberger, Matthias; Boettcher, Maxi; et al. (2021)
Weather and Climate Dynamics
An extreme aggregation of precipitation on the seasonal timescale, leading to a so-called extreme wet season, can have substantial environmental and socio-economic impacts. This study has a twofold aim: first to identify and statistically characterize extreme wet seasons around the globe and second to elucidate their relationship with specific weather systems. Extreme wet seasons are defined independently at every grid point of ERA-Interim reanalyses as the consecutive 90 d period with the highest accumulated precipitation in the 40-year period of 1979–2018. In most continental regions, the extreme seasons occur during the warm months of the year, especially in the midlatitudes. Nevertheless, colder periods might be also relevant, especially in coastal areas. All identified extreme seasons are statistically characterized in terms of climatological anomalies of the number of wet days and of daily extreme events. Results show that daily extremes are decisive for the occurrence of extreme wet seasons in regions of frequent precipitation, e.g., in the tropics. This is in contrast to arid regions where wet seasons may occur only due to anomalously frequent wet days. In the subtropics and more precisely within the transitional zones between arid areas and regions of frequent precipitation, both an anomalously high occurrence of daily extremes and of wet days are related to the formation of extreme wet seasons. A novel method is introduced to define the spatial extent of regions affected by a particular extreme wet season and to relate extreme seasons to four objectively identified synoptic-scale weather systems, which are known to be associated with intense precipitation: cyclones, warm conveyor belts, tropical moisture exports and breaking Rossby waves. Cyclones and warm conveyor belts contribute particularly strongly to extreme wet seasons in most regions of the globe. But interlatitudinal influences are also shown to be important: tropical moisture exports, i.e., the poleward transport of tropical moisture, can contribute to extreme wet seasons in the midlatitudes, while breaking Rossby waves, i.e., the equatorward intrusion of stratospheric air, may decisively contribute to the formation of extreme wet seasons in the tropics. Three illustrative examples provide insight into the synergetic effects of the four identified weather systems on the formation of extreme wet seasons in the midlatitudes, the Arctic and the (sub)tropics.
The substructure of extremely hot summers in the Northern Hemisphere
Röthlisberger, Matthias; Sprenger, Michael; Flaounas, Emmanouil; et al. (2020)
Weather and Climate Dynamics
In the last decades, extremely hot summers (here- after extreme summers) have challenged societies worldwide through their adverse ecological, economic and public-health effects. In this study, extreme summers are identified at all grid points in the Northern Hemisphere in the upper tail of the June–July–August (JJA) seasonal mean 2 m temperature (T2m) distribution, separately in ERA-Interim (ERAI) re- analyses and in 700 simulated years with the Community Earth System Model (CESM) large ensemble for present-day climate conditions. A novel approach is introduced to characterise the substructure of extreme summers, i.e. to elucidate whether an extreme summer is mainly the result of the warmest days being anomalously hot, of the coldest days being anomalously mild or of a general shift towards warmer temperatures on all days of the season. Such a statistical characterisation can be obtained from considering so-called rank day anomalies for each extreme summer – that is, by sorting the 92 daily mean T2m values of an extreme summer and by calculating, for every rank, the deviation from the climatological mean rank value of T2m. Applying this method in the entire Northern Hemisphere reveals spatially strongly varying extreme-summer substructures, which agree remarkably well in the re-analysis and climate model data sets. For example, in eastern India the hottest 30 d of an extreme summer contribute more than 65 % to the total extreme-summer T2m anomaly, while the colder days are close to climatology. In the high Arctic, however, extreme summers occur when the coldest 30 d are substantially warmer than they are climatologically. Furthermore, in roughly half of the Northern Hemisphere land area, the coldest third of summer days contributes more to extreme summers than the hottest third, which highlights that milder-than-normal coldest summer days are a key ingredient of many extreme summers. In certain regions, e.g. over western Europe and western Russia, the substructure of different extreme summers shows large variability and no common characteristic substructure emerges. Furthermore, we show that the typical extreme-summer substructure in a certain region is directly related to the region’s overall T2m rank day variability pattern. This indicates that in regions where the warmest summer days vary particularly strongly from one year to the other, these warmest days are also particularly anomalous in extreme summers (and analogously for regions where variability is largest for the coldest days). Finally, for three selected regions, thermodynamic and dynamical causes of extreme-summer substructures are briefly discussed, indicating that, for instance, the onset of monsoons, physical boundaries like the sea ice edge or the frequency of occurrence of Rossby wave breaking strongly determines the substructure of extreme summers in certain regions.
Drastic increase in the magnitude of very rare summer-mean vapor pressure deficit extremes
Hermann, Mauro; Wernli, Heini; Röthlisberger, Matthias (2024)
Nature Communications
Summers with extremely high vapor pressure deficit contribute to crop losses, ecosystem damages, and wildfires. Here, we identify very rare summer vapor pressure deficit extremes globally in reanalysis data and climate model simulations, and quantify the contributions of temperature and atmospheric moisture anomalies to their intensity. The simulations agree with reanalysis data regarding these physical characteristics of historic vapor pressure deficit extremes, and show a +33/+28% increase in their intensity in the northern/southern mid-latitudes over this century. About half of this drastic increase in the magnitude of extreme vapor pressure deficit anomalies is due to climate warming, since this quantity depends exponentially on temperature. Further contributing factors are increasing temperature variability (e.g., in Europe) and the expansion of soil moisture-limited regions. This study shows that to avoid amplified impacts of future vapor pressure deficit extremes, ecosystems and crops must become more resilient not only to an increasing mean vapor pressure deficit, but additionally also to larger seasonal anomalies of this quantity.

Publications 1 - 10 of 22

Matthias Röthlisberger

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