TY - JOUR A1 - Totz, Sonja Juliana A1 - Tziperman, Eli A1 - Coumou, Dim A1 - Pfeiffer, Karl A1 - Cohen, Judah T1 - Winter precipitation forecast in the European and mediterranean regions using cluster analysis JF - Geophysical research letters N2 - The European climate is changing under global warming, and especially the Mediterranean region has been identified as a hot spot for climate change with climate models projecting a reduction in winter rainfall and a very pronounced increase in summertime heat waves. These trends are already detectable over the historic period. Hence, it is beneficial to forecast seasonal droughts well in advance so that water managers and stakeholders can prepare to mitigate deleterious impacts. We developed a new cluster-based empirical forecast method to predict precipitation anomalies in winter. This algorithm considers not only the strength but also the pattern of the precursors. We compare our algorithm with dynamic forecast models and a canonical correlation analysis-based prediction method demonstrating that our prediction method performs better in terms of time and pattern correlation in the Mediterranean and European regions. KW - precipitation anomaly KW - seasonal forecast KW - cluster analysis Y1 - 2018 U6 - https://doi.org/10.1002/2017GL075674 SN - 0094-8276 SN - 1944-8007 VL - 44 SP - 12418 EP - 12426 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Kretschmer, Marlene A1 - Coumou, Dim A1 - Donges, Jonathan Friedemann A1 - Runge, Jakob T1 - Using Causal Effect Networks to Analyze Different Arctic Drivers of Midlatitude Winter Circulation JF - Journal of climate N2 - In recent years, the Northern Hemisphere midlatitudes have suffered from severe winters like the extreme 2012/13 winter in the eastern United States. These cold spells were linked to a meandering upper-tropospheric jet stream pattern and a negative Arctic Oscillation index (AO). However, the nature of the drivers behind these circulation patterns remains controversial. Various studies have proposed different mechanisms related to changes in the Arctic, most of them related to a reduction in sea ice concentrations or increasing Eurasian snow cover. Here, a novel type of time series analysis, called causal effect networks (CEN), based on graphical models is introduced to assess causal relationships and their time delays between different processes. The effect of different Arctic actors on winter circulation on weekly to monthly time scales is studied, and robust network patterns are found. Barents and Kara sea ice concentrations are detected to be important external drivers of the midlatitude circulation, influencing winter AO via tropospheric mechanisms and through processes involving the stratosphere. Eurasia snow cover is also detected to have a causal effect on sea level pressure in Asia, but its exact role on AO remains unclear. The CEN approach presented in this study overcomes some difficulties in interpreting correlation analyses, complements model experiments for testing hypotheses involving teleconnections, and can be used to assess their validity. The findings confirm that sea ice concentrations in autumn in the Barents and Kara Seas are an important driver of winter circulation in the midlatitudes. Y1 - 2016 U6 - https://doi.org/10.1175/JCLI-D-15-0654.1 SN - 0894-8755 SN - 1520-0442 VL - 29 SP - 4069 EP - 4081 PB - American Meteorological Soc. CY - Boston ER - TY - JOUR A1 - Coumou, Dim A1 - Lehmann, Jascha A1 - Beckmann, Johanna T1 - The weakening summer circulation in the Northern Hemisphere mid-latitudes JF - Science N2 - Rapid warming in the Arctic could influence mid-latitude circulation by reducing the poleward temperature gradient. The largest changes are generally expected in autumn or winter, but whether significant changes have occurred is debated. Here we report significant weakening of summer circulation detected in three key dynamical quantities: (i) the zonal-mean zonal wind, (ii) the eddy kinetic energy (EKE), and (iii) the amplitude of fast-moving Rossby waves. Weakening of the zonal wind is explained by a reduction in the poleward temperature gradient. Changes in Rossby waves and EKE are consistent with regression analyses of climate model projections and changes over the seasonal cycle. Monthly heat extremes are associated with low EKE, and thus the observed weakening might have contributed to more persistent heat waves in recent summers. Y1 - 2015 U6 - https://doi.org/10.1126/science.1261768 SN - 0036-8075 SN - 1095-9203 VL - 348 IS - 6232 SP - 324 EP - 327 PB - American Assoc. for the Advancement of Science CY - Washington ER - TY - JOUR A1 - Lehmann, Jascha A1 - Coumou, Dim T1 - The influence of mid-latitude storm tracks on hot, cold, dry and wet extremes JF - Scientific reports N2 - Changes in mid-latitude circulation can strongly affect the number and intensity of extreme weather events. In particular, high-amplitude quasi-stationary planetary waves have been linked to prolonged weather extremes at the surface. In contrast, analyses of fast-traveling synoptic-scale waves and their direct influence on heat and cold extremes are scarce though changes in such waves have been detected and are projected for the 21st century. Here we apply regression analyses of synoptic activity with surface temperature and precipitation in monthly gridded observational data. We show that over large parts of mid-latitude continental regions, summer heat extremes are associated with low storm track activity. In winter, the occurrence of cold spells is related to low storm track activity over parts of eastern North America, Europe, and central-to eastern Asia. Storm tracks thus have a moderating effect on continental temperatures. Pronounced storm track activity favors monthly rainfall extremes throughout the year, whereas dry spells are associated with a lack thereof. Trend analyses reveal significant regional changes in recent decades favoring the occurrence of cold spells in the eastern US, droughts in California and heat extremes over Eurasia. Y1 - 2015 U6 - https://doi.org/10.1038/srep17491 SN - 2045-2322 VL - 5 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Totz, Sonja Juliana A1 - Eliseev, Alexey V. A1 - Petri, Stefan A1 - Flechsig, Michael A1 - Caesar, Levke A1 - Petoukhov, Vladimir A1 - Coumou, Dim T1 - The dynamical core of the Aeolus 1.0 statistical-dynamical atmosphere model BT - validation and parameter optimization JF - Geoscientific model development : an interactive open access journal of the European Geosciences Union N2 - Here, we present novel equations for the large-scale zonal-mean wind as well as those for planetary waves. Together with synoptic parameterization (as presented by Coumou et al., 2011), these form the mathematical description of the dynamical core of Aeolus 1.0. The regions of high azonal wind velocities (planetary waves) are accurately captured for all validation experiments. The zonal-mean zonal wind and the integrated lower troposphere mass flux show good results in particular in the Northern Hemisphere. In the Southern Hemisphere, the model tends to produce too-weak zonal-mean zonal winds and a too-narrow Hadley circulation. We discuss possible reasons for these model biases as well as planned future model improvements and applications. Y1 - 2018 U6 - https://doi.org/10.5194/gmd-11-665-2018 SN - 1991-959X SN - 1991-9603 VL - 11 IS - 2 SP - 665 EP - 679 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Kretschmer, Marlene A1 - Cohen, Judah A1 - Matthias, Vivien A1 - Runge, Jakob A1 - Coumou, Dim T1 - The different stratospheric influence on cold-extremes in Eurasia and North America JF - npj Climate and Atmospheric Science N2 - The stratospheric polar vortex can influence the tropospheric circulation and thereby winter weather in the mid-latitudes. Weak vortex states, often associated with sudden stratospheric warmings (SSW), have been shown to increase the risk of cold-spells especially over Eurasia, but its role for North American winters is less clear. Using cluster analysis, we show that there are two dominant patterns of increased polar cap heights in the lower stratosphere. Both patterns represent a weak polar vortex but they are associated with different wave mechanisms and different regional tropospheric impacts. The first pattern is zonally symmetric and associated with absorbed upward-propagating wave activity, leading to a negative phase of the North Atlantic Oscillation (NAO) and cold-air outbreaks over northern Eurasia. This coupling mechanism is well-documented in the literature and is consistent with the downward migration of the northern annular mode (NAM). The second pattern is zonally asymmetric and linked to downward reflected planetary waves over Canada followed by a negative phase of the Western Pacific Oscillation (WPO) and cold-spells in Central Canada and the Great Lakes region. Causal effect network (CEN) analyses confirm the atmospheric pathways associated with this asymmetric pattern. Moreover, our findings suggest the reflective mechanism to be sensitive to the exact region of upward wave-activity fluxes and to be state-dependent on the strength of the vortex. Identifying the causal pathways that operate on weekly to monthly timescales can pave the way for improved sub-seasonal to seasonal forecasting of cold spells in the mid-latitudes. Y1 - 2018 U6 - https://doi.org/10.1038/s41612-018-0054-4 SN - 2397-3722 VL - 1 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Kornhuber, Kai A1 - Petoukhov, Vladimir A1 - Karoly, D. A1 - Petri, Stefan A1 - Rahmstorf, Stefan A1 - Coumou, Dim T1 - Summertime Planetary Wave Resonance in the Northern and Southern Hemispheres JF - Journal of climate Y1 - 2017 U6 - https://doi.org/10.1175/JCLI-D-16-0703.1 SN - 0894-8755 SN - 1520-0442 VL - 30 SP - 6133 EP - 6150 PB - American Meteorological Soc. CY - Boston ER - TY - JOUR A1 - Totz, Sonja Juliana A1 - Petri, Stefan A1 - Lehmann, Jascha A1 - Coumou, Dim T1 - Regional Changes in the Mean Position and Variability of the Tropical Edge JF - Geophysical research letters N2 - Recent studies indicate that the tropical belt has been expanding during recent decades, which can significantly influence precipitation in subtropical climates. Often the location of the tropical border is identified using the Hadley cell edge (HCE) or the subtropical jet stream (STJ), but most studies concentrated on the zonal-mean state, thereby missing regional impacts. Here we detect longitudinal-resolved trends in STJ cores and HCEs over 1979-2016 in both hemispheres at a higher spatial and temporal resolution than previous studies. Besides pronounced regional trend differences in both sign and magnitude, we show that winter HCE and STJ variability increased in the Mediterranean region and decreased over the American and Asian continents. Rainfall variability in these regions changed likewise, and most of those changes can be explained by changes in HCE/STJ variability. This highlights the importance of understanding future tropical belt changes both regionally and in terms of variability. Plain Language Summary We applied a new network-based method to detect motion of the tropical climate border with longitudinal resolution. Depending on the longitudinal position, there are differences in both direction and magnitude of the border motion. In addition, we demonstrate that the rainfall variability is increasing in the Mediterranean region and decreasing over the American and Asian continents, which can be explained by the variability of the tropical belt location. Y1 - 2018 U6 - https://doi.org/10.1029/2018GL079911 SN - 0094-8276 SN - 1944-8007 VL - 45 IS - 21 SP - 12076 EP - 12084 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Stadtherr, Lisa A1 - Coumou, Dim A1 - Petoukhov, Vladimir A1 - Petri, Stefan A1 - Rahmstorf, Stefan T1 - Record Balkan floods of 2014 linked to planetary wave resonance JF - Science Advances N2 - In May 2014, the Balkans were hit by a Vb-type cyclone that brought disastrous flooding and severe damage to Bosnia and Herzegovina, Serbia, and Croatia. Vb cyclones migrate from the Mediterranean, where they absorb warm and moist air, to the north, often causing flooding in central/eastern Europe. Extreme rainfall events are increasing on a global scale, and both thermodynamic and dynamical mechanisms play a role. Where thermodynamic aspects are generally well understood, there is large uncertainty associated with current and future changes in dynamics. We study the climatic and meteorological factors that influenced the catastrophic flooding in the Balkans, where we focus on large-scale circulation. We show that the Vb cyclone was unusually stationary, bringing extreme rainfall for several consecutive days, and that this situation was likely linked to a quasi-stationary circumglobal Rossby wave train. We provide evidence that this quasi-stationary wave was amplified by wave resonance. Statistical analysis of daily spring rainfall over the Balkan region reveals significant upward trends over 1950-2014, especially in the high quantiles relevant for flooding events. These changes cannot be explained by simple thermodynamic arguments, and we thus argue that dynamical processes likely played a role in increasing flood risks over the Balkans. Y1 - 2016 U6 - https://doi.org/10.1126/sciadv.1501428 SN - 2375-2548 VL - 2 PB - American Assoc. for the Advancement of Science CY - Washington ER - TY - JOUR A1 - Mann, Michael E. A1 - Rahmstorf, Stefan A1 - Kornhuber, Kai A1 - Steinman, Byron A. A1 - Miller, Sonya K. A1 - Petri, Stefan A1 - Coumou, Dim T1 - Projected changes in persistent extreme summer weather events BT - The role of quasi-resonant amplification JF - Science Advances N2 - Persistent episodes of extreme weather in the Northern Hemisphere summer have been associated with high-amplitude quasi-stationary atmospheric Rossby waves, with zonal wave numbers 6 to 8 resulting from the phenomenon of quasi-resonant amplification (QRA). A fingerprint for the occurrence of QRA can be defined in terms of the zonally averaged surface temperature field. Examining state-of-the-art [Coupled Model Intercomparison Project Phase 5 (CMIP5)] climate model projections, we find that QRA events are likely to increase by similar to 50% this century under business-as-usual carbon emissions, but there is considerable variation among climate models. Some predict a near tripling of QRA events by the end of the century, while others predict a potential decrease. Models with amplified Arctic warming yield the most pronounced increase in QRA events. The projections are strongly dependent on assumptions regarding the nature of changes in radiative forcing associated with anthropogenic aerosols over the next century. One implication of our findings is that a reduction in midlatitude aerosol loading could actually lead to Arctic de-amplification this century, ameliorating potential increases in persistent extreme weather events. Y1 - 2018 U6 - https://doi.org/10.1126/sciadv.aat3272 SN - 2375-2548 VL - 4 IS - 10 PB - American Assoc. for the Advancement of Science CY - Washington ER -