Research
Heat waves can have devastating effects on societies and the frequency of their occurrence is projected to increase in a warming climate. Forecasting these events well in advance would be beneficial to a multitude of stakeholders in e.g. agriculture, public health services and the renewable energy sector. Heatwave predictability decreases with increasing forecast time; however, extended heatwave predictability exists for large-scale events and specific case studies (Wulff and Domeisen, 2019; Pyrina et al., 2022; Domeisen et al., 2023).
The potential for extending the prediction skill of heat waves is based on the relation of extreme air temperature to persistent atmospheric circulation patterns such as blockings. Furthermore, there are indications that certain configurations of the atmospheric circulation are in fact manifestations of larger scale teleconnection patterns that could, for example, be driven by anomalous diabatic heating in the tropics (e.g. Wulff et al., 2017). Additionally, in the case of summer heat extremes, desiccation of soils can favor the occurrence of high temperatures once land-atmosphere interactions are triggered. Since soil moisture varies on inherently longer time scales than the atmospheric circulation, it can be used as a predictor of 2-meter air temperature. These lagged connections to 2-meter temperature can provide predictability on subseasonal to seasonal time scales that can be exploited by Machine Learning models to extend the heatwave prediction skill (e.g., Weirich et al., 2023, Pyrina et al., 2020).
In our group, we evaluate the predictors of heat events and heat-related variables on subseasonal time scales and work towards understanding the sources of predictability of these events. This will allow us to draw conclusions on the components of the subseasonal forecasting systems necessary for successful predictions of heat events. Our group’s work also focuses on heatwave prediction and impacts on a more local scale. We explore whether the skill horizon for heatwave predictions in Switzerland can be extended with different physical-statistical pre- and post-processing methods and evaluate the potential benefits of early warning products for sectors that are directly linked to human lives and livelihoods (external page CROSS grant “HEATaware”).
Involved people: Dominik Bueeler, Maria Pyrina
Formerly involved people: Ole Wulff, Bernat Jiménez Esteve, Elizabeth Weirich Benet
References:
Domeisen, D.I.V., Eltahir, E.A.B., Fischer, E.M. et al. Prediction and projection of heatwaves. Nat Rev Earth Environ 4, 36–50 (2023). external page https://doi.org/10.1038/s43017-022-00371-z
Weirich-Benet, E., Pyrina, M., Jiménez-Esteve, B., Fraenkel, E., Cohen, J., & Domeisen, D. I. (2023). Subseasonal Prediction of Central European Summer Heatwaves with Linear and Random Forest Machine Learning Models. Artificial Intelligence for the Earth Systems, external page https://doi.org/10.1175/AIES-D-22-0038.1
Pyrina, M., & Domeisen, D. I. (2022). Subseasonal predictability of onset, duration, and intensity of European heat extremes. Quarterly Journal of the Royal Meteorological Society, 149(750), 84-101, external page https://doi.org/10.1002/qj.4394
Pyrina, M., Nonnenmacher, M., Wagner, S., & Zorita, E. (2021). Statistical seasonal prediction of European summer mean temperature using observational, reanalysis, and satellite data. Weather and Forecasting, 36(4), 1537-1560, external page https://doi.org/10.1175/WAF-D-20-0235.1
Wulff, C. O., D. Domeisen (2019), Higher subseasonal predictability of extreme hot European summer temperatures as compared to average summers, Geophysical Research Letters 46, external page https://doi.org/10.1029/2019GL084314
Wulff, C. O., R. Greatbatch, D. Domeisen, G. Gollan and F. Hansen (2017) Tropical Forcing of the Summer East Atlantic Pattern, Geophysical Research Letters 44 (21), external page https://doi.org/10.1002/2017GL075493
El Niño Southern Oscillation (ENSO) is the dominant mode of interannual climate variability in the tropics. Atmosphere-ocean feedbacks are responsible for changes in surface winds, pressure, and sea surface temperature. ENSO exhibits an irregular period between 2 and 7 years between its warm and cold phases, i.e. El Niño/La Niña.
The changes associated with ENSO in terms of the location and strength of tropical convection excite poleward propagating Rossby wave trains that can influence the extratropics through so-called “teleconnections”. In particular, the ENSO teleconnection to the North Atlantic and Eurasia is driven by several mechanisms, such as the stratospheric pathway and tropospheric pathways through the North Pacific (Jiménez-Esteve & Domeisen, 2018) and the Tropical Atlantic.
The impact of ENSO onto the North Pacific exhibits important non-linearities with respect to the tropical Pacific ENSO sea surface temperature anomalies, with a stronger impact during El Niño, compared to La Niña (Jiménez-Esteve & Domeisen, 2019). In the the North-Atlantic and Europe, non-linarities with respect to the ENSO forcing are weaker and more difficult to detect using observations than in the North Pacific. However, model experiments indicate that the tropospheric pathway of ENSO to the North Atlantic exhibits significant nonlinearity with respect to the tropical SST forcing, both in terms of the location and amplitude of the impacts (Jiménez-Esteve & Domeisen, 2020).
The pathway through the stratosphere leads to improved predictability of the North Atlantic / Europe region for years that exhibit both an El Nino and a sudden stratospheric warming event (Domeisen et al., 2015), especially for high-top models (Butler et al., 2016).
The ENSO teleconnection towards the North Atlantic may also travel through, or be modulated by, the tropical Atlantic sea surface temperature (SST) anomalies and is also a major area of our focus. Such a connection may allow ENSO to influence the North Atlantic by proxy of the Atlantic SST, increasing predictability into the decaying boreal spring and summer seasons. Using an idealized Atmospheric Global Circulation Model (AGCM), we hope to expand upon the importance of the tropical Atlantic.
Formerly involved people: Bernat Jiménez Esteve and Jake Casselman
References:
Jiménez-Esteve, B. and D.I.V. Domeisen, 2018: The Tropospheric Pathway of the ENSO–North Atlantic Teleconnection. J. Climate, 31, 4563–4584, external page https://doi.org/10.1175/JCLI-D-17-0716.1
Jiménez‐Esteve, B. and D.I.V. Domeisen, 2019: Nonlinearity in the North Pacific atmospheric response to a linear ENSO forcing. Geophysical Research Letters, 46, 2271– 2281. external page https://doi.org/10.1029/2018GL081226
Jiménez-Esteve, B., & Domeisen, D. I. V. (2020). Nonlinearity in the tropospheric pathway of ENSO to the North Atlantic. Weather and Climate Dynamics, 1(1), 225–245. external page https://doi.org/10.5194/wcd-1-225-2020
Butler, A. H. et al. 2016. The Climate-system Historical Forecast Project: do stratosphere-resolving models make better seasonal climate predictions in boreal winter? QJRMS, 142(696), 1413–1427. external page https://doi.org/10.1002/qj.2743
Domeisen, D.I. et al. 2015: Seasonal Predictability over Europe Arising from El Niño and Stratospheric Variability in the MPI-ESM Seasonal Prediction System. J. Climate, 28, 256–271, external page https://doi.org/10.1175/JCLI-D-14-00207.1
In addition to studying ENSO teleconnections towards the North Atlantic European region, inter-basin connections are important for understanding the larger picture of ENSO teleconnections, as the SSTs retain a "memory" of an ENSO event long after it has receded. The SSTs over the Tropical North Atlantic (TNA) are well known to experience a positive (negative) anomaly following an El Niño (La Niña) event, which can further influence regions around the world. Our group investigates the linearity of this connection by breaking the inter-basin teleconnection into the tropical and extratropical connection. The tropical pathway is dominated by a tropospheric temperature mechanism and Walker cell perturbation, while the Pacific North American pattern represents the extratropical teleconnection. We find that the TNA SST is nonlinear to ENSO, that the tropical pathway is more linear than the extratropical pathway, and that overall, both the extratropical and tropical pathways cannot explain this nonlinearity. Other aspects that may lead to this nonlinearity include the background climatology of the TNA SST and differences in the tropospheric and stratospheric teleconnection to the North Atlantic.
Formerly involved people: Jake Casselman
References:
Casselman, J. W., Taschetto, A. S., Domeisen, D. I. V. (in-press). Non-linearity in the pathway of El Niño-Southern Oscillation to the tropical North Atlantic. Journal of Climate, DOI: 10.1175/JCLI-D-20-0952.
Large scale variations in the tropics impact energy, momentum and moisture transport between the tropics and the mid-latitudes. On subseasonal timescales, the Madden-Julian Oscillation (MJO) is the dominant mode of tropical variability. The MJO consists of eastward moving band of rainfall and circulation anomalies in the tropics with a time period of ~ 30-90 days from the Indian Ocean to the Pacific Ocean.
The MJO has significant impact on the circulation and weather patterns in the mid-latitude regions. The MJO can impact Euro-Atlantic weather via two pathways – tropospheric and stratospheric pathways. The tropical diabatic heating associated with the phase of the MJO convection lead to a direct Rossby wave response into the extratropical troposphere. The MJO can also influence the extratropical stratosphere (Domeisen et al. 2020b), thereby effecting the polar vortex and mid-winter sudden stratospheric warmings and impacting tropospheric weather. The enhanced MJO convection over the west Pacific (phase 5 and 6) lead to weak stratospheric vortex events at 4-week lag. Thus, the MJO is considered as a potential source of predictability on subseasonal time scales.
We aim to better understand the role of stratosphere in the strong North Atlantic Oscillation (NAO) events due to the slowly propagating MJO events (Yadav and Straus, 2017). We are also evaluating the impact of the MJO teleconnections on stratospheric polar vortex in the Sub-seasonal to Seasonal (S2S) Project database forecasting models to understand if the models can capture the stratospheric pathway of MJO teleconnections.
Formerly involved people: Priyanka Yadav
References:
Yadav, P., & Straus, D. M. (2017). Circulation Response to Fast and Slow MJO Episodes, Monthly Weather Review, 145(5), 1577-1596. external page https://doi.org/10.1175/MWR-D-16-0352.1
Domeisen, D. I., Butler, A. H., Charlton-Perez, A. J., Ayarzagüena, B., Baldwin, M. P., Dunn-Sigouin, E. et al. (2020). The role of the stratosphere in subseasonal to seasonal prediction: 2. Predictability arising from stratosphere-troposphere coupling. Journal of Geophysical Research: Atmospheres, 125, e2019JD030923. external page https://doi.org/10.1029/2019JD030923
The Indian Ocean Dipole (IOD) is defined by the difference in sea surface temperature between Arabian Sea (western Indian Ocean) and the eastern Indian Ocean south of Indonesia. The IOD can influence the weather systems in both the hemispheres causing temperature and precipitation extremes. We aim to understand the mechanisms of the IOD influence on the NAO in a changing climate, and what role the stratosphere will play in influencing the NAO events. The influence of IOD is not limited to NAO, it also impacts precipitation and temperature extremes in both the hemispheres. Using the observational data and model experiments, we are interested in addressing how the warming of the Indian ocean and intensity of IOD will influence the extreme events.
Formerly involved people: Priyanka Yadav
The stratosphere, the layer above the troposphere or “weather layer” starting at around 10km above the Earth’s surface, has over the past decades been shown to exhibit a detectable downward influence, adding predictability to surface weather. Downward coupling of extreme stratospheric events, e.g. sudden stratospheric warming (SSW) events, can be communicated through a range of different waves, among them synoptic-scale eddies (Domeisen et al., 2013). In order to understand how stratospheric events come about in the first place, we need to understand wave generation and propagation into the stratosphere. In the absence of large-scale asymmetries in the troposphere, waves exhibit a phase speed in the zonal (East-West) direction. Before SSW events, phase speeds of waves propagating upward into the stratosphere show a significant tendency to propagate predominantly eastward in re-analysis (Domeisen et al., 2018) and an idealized model (Domeisen et al., 2012). Zonal asymmetric heating in the tropics, such as e.g. during ENSO events, lead to a strengthening of the shallow Brewer-Dobson circulation and a weakening of the polar vortex (Yang et al., 2014).
Sudden stratospheric warming events can have a significant impact on surface weather, often associated with a negative phase of the North Atlantic Oscillation (NAO). The tropospheric response after SSW events is strongest in the North Atlantic and Europe; however, not all events exhibit the same surface response. Only two thirds of the SSW events are dominated by an equatorward shift of the jet in the North Atlantic, consistent with the canonical SSW response of a negative NAO. For the remaining third of SSW events a poleward shift of the jet is found. The Pacific is found to contribute to the sign of the North Atlantic response, as synoptic wave propagation from the Eastern Pacific links the Pacific and Atlantic storm tracks for both equatorward and poleward jet responses (Afargan-Gerstman and Domeisen, 2020).
Involved people: Hilla Afargan-Gerstman
Formerly involved people: Bernat Jiménez Esteve
References:
Afargan‐Gerstman, H., & Domeisen, D. I. (2020). Pacific modulation of the North Atlantic storm track response to sudden stratospheric warming events. Geophysical Research Letters, 47(2), e2019GL085007. external page https://doi.org/10.1029/2019GL085007
Domeisen, D. I. V., Martius, O., & Jiménez‐Esteve, B. (2018). Rossby wave propagation into the Northern Hemisphere stratosphere: The role of zonal phase speed. Geophysical Research Letters, 45, 2064–2071. external page https://doi.org/10.1002/2017GL076886
Yang, H., Chen, G. und Domeisen, D. I. V. (2014). Sensitivities of the Lower-Stratospheric Transport and Mixing to Tropical SST Heating. J. Atmos. Sci., 71, 2674-2694, external page http://doi.org/10.1175/JAS-D-13-0276.1
Domeisen, D. I. V., L. Sun, and G. Chen (2013), The role of synoptic eddies in the tropospheric response to stratospheric variability, Geophys. Res. Lett., 40, 4933–4937, external page doi: 10.1002/grl.50943.
Domeisen, D. I. V., and R. A. Plumb (2012), Traveling planetary‐scale Rossby waves in the winter stratosphere: The role of tropospheric baroclinic instability, Geophys. Res. Lett., 39, L20817, external page doi: 10.1029/2012GL053684.
Extreme events in the stratosphere, such as Sudden Stratospheric Warming (SSW) events, have a significant impact on surface weather in the North Atlantic / Europe sector. These events provide a key for extending the lead times of reliable predictions and improving weather forecasting beyond the current prediction limit.
In our group, we investigate the role of the stratosphere as a predictor for surface weather events and the involved mechanisms by which the stratosphere may affect the frequency of extreme events in the troposphere, such as the occurrence of cold air outbreaks (CAOs) in the Arctic. By exploring the role of the Arctic troposphere and stratosphere on predictability of cold air outbreaks, we aim to understand, for example, how well can we predict cold air outbreaks at timescales of weeks to months and which regions will be affected the most by changes in the strength of the stratospheric polar vortex. This research is part of "Blue-Action", a major European research effort for investigating the effect of a changing Arctic on weather and climate.
Involved people: Hilla Gerstman-Afargan
References:
Afargan-Gerstman, H., Polkova, I., Papritz, L., Ruggieri, P., King, M. P., Athanasiadis, P. J., Johanna Baehr & Domeisen, D. I. (2020). Stratospheric influence on North Atlantic marine cold air outbreaks following sudden stratospheric warming events. Weather and Climate Dynamics, 1(2), 541-553. external page https://doi.org/10.5194/wcd-1-541-2020
The knowledge on remote connections in the climate system can be used in order to infer and improve the predictability of extratropical systems that are otherwise less predictable. The North Atlantic Oscillation (NAO), which is associated with the weather over Europe, exhibits an intrinsic predictability of up to almost 3 weeks (Domeisen et al., 2018), while operational weather models are generally still limited to making reliable predictions on timescales of several days — it will be interesting to see if this theoretical limit can be attained. On longer timescales of weeks to months, the skill for the NAO is generally small, but can be significantly improved by adding a statistical forecast using remote influences of more slowly varying systems such as sea ice, snow cover, sea surface temperatures, and the stratosphere. The variability of the NAO and the stratosphere shows similarities, i.e. both exhibit deviations from Gaussianity, which is likely due to the seasonal transition between summer and winter (Badin & Domeisen, 2014a,b). Extratropical stratospheric variability may be possible to describe as a low-dimensional chaotic system (Badin & Domeisen, 2014a,b).
References:
Domeisen, D.I., G. Badin, and I.M. Koszalka, 2018: How Predictable Are the Arctic and North Atlantic Oscillations? Exploring the Variability and Predictability of the Northern Hemisphere. J. Climate, 31, 997–1014, external page https://doi.org/10.1175/JCLI-D-17-0226.1
Dobrynin, M., Domeisen, D. I. V., ....& Baehr, J, 2018: Improved teleconnection-based dynamical seasonal predictions of boreal winter.
Geophysical Research Letters, 45, external page http://doi.org/10.1002/2018GL077209
Badin, G. and D.I. Domeisen, 2014: A Search for Chaotic Behavior in Stratospheric Variability: Comparison between the Northern and Southern Hemispheres. J. Atmos. Sci., 71, 4611–4620, external page https://doi.org/10.1175/JAS-D-14-0049.1
Badin, G. and D.I. Domeisen, 2014: A Search for Chaotic Behavior in Northern Hemisphere Stratospheric Variability. J. Atmos. Sci., 71, 1494–1507, external page https://doi.org/10.1175/JAS-D-13-0225.1
Sudden Stratospheric Warming (SSW) events are driven by large-scale waves and can have an impact on the weather globally. New techniques of 3D computer graphics are used to develop alternative visualizations of stratospheric and tropsopheric atmospheric fields in order to explore new perspectives on El Niño, Sudden Stratospheric Warmings and their impact on surface weather. The first visualisation below received a distinction at the external page SNSF scientific image competition.
Formerly involved people: Alexander Wollert
The winter stratosphere is featured by strong westerly circumpolar winds with large variability in the extratropics of both hemispheres. The large variability and the interaction with the planetary waves can lead to dynamical extreme events in the stratosphere, whose anomalies later can propagate downward to the troposphere and influence the surface weather from weeks up to months after the onset of the extreme events. One of the dominant extreme events is stratospheric sudden warming (SSW), which occurs on average in 6 out of 10 years irregularly in observation. Even though SSWs contribute significantly to the predictability of tropospheric weather, the predictability of SSWs is only around 1-2 weeks in state-of-the-art sub-seasonal prediction systems (Domeisen et al. 2020a). Therefore, understanding what influences the predictability of the stratospheric extreme events, especially SSW events, in both real world and complex forecast systems and improving the prediction of SSWs are important and significantly benefit forecasts at the surface (Domeisen et al. 2020b).
In our group, we investigate mechanisms of the predictability of SSWs and seek for extended-range (S2S time scale) of predictability of SSWs using various approaches, including model analysis, dynamical diagnostics, advanced statistics, and machine learning. For example, by applying an advanced statistical method on the dynamical equation, we are able to find signals indicating the occurrence of SSWs with lead times of around 3-4 weeks, which is beyond the current two-week predictability (Wu et al. 2021).
We also work on understanding the differences in predictability between different stratospheric extreme events in S2S models through investigating the dependence of predictability on various dynamic precursors. We aim to locate the sources of predictability in the extreme events and specify biases in the models that lead to the incorrect prediction of these¬ events. This will shed new lights on the future development of the forecast systems and the prediction of stratospheric and tropospheric weather in S2S timescales.
Involved people: Rachel Wu
Formerly involved people: Zheng Wu
References:
Domeisen, D. I. V., Butler, A. H., Charlton-Perez, A. J., Ayarzagüena, B., Baldwin, M. P., Dunn-Sigouin, E., et al (2020). The role of the stratosphere in subseasonal to seasonal prediction: 1. Predictability of the stratosphere. Journal of Geophysical Research: Atmospheres, 125, e2019JD030920. external page https://doi.org/10.1029/2019JD030920
Domeisen, D. I., Butler, A. H., Charlton-Perez, A. J., Ayarzagüena, B., Baldwin, M. P., Dunn-Sigouin, E. et al. (2020). The role of the stratosphere in subseasonal to seasonal prediction: 2. Predictability arising from stratosphere-troposphere coupling. Journal of Geophysical Research:Atmospheres, 125, e2019JD030923. external page https://doi.org/10.1029/2019JD030923
Wu, Z., Jiménez-Esteve, B., de Fondeville, R., Székely, E., Obozinski, G., Ball, W. T., and Domeisen, D. I. V.: Extended-range predictability of sudden stratospheric warming events suggested by mode decomposition, Weather Clim. Dynam. Discuss. [preprint], external page https://doi.org/10.5194/wcd-2021-14, in review, 2021.