www.nature.com/npjclimatsci MEETING REPORT OPEN Summary of workshop on sub-seasonal to seasonal predictability of extreme weather and climate 1 2 2,4 3 4 Andrew W. Robertson , Suzana J. Camargo , Adam Sobel , Frederic Vitart and Shuguang Wang This paper provides a summary of the Workshop on Sub-Seasonal to Seasonal (S2S) Predictability of Extreme Weather and Climate, held at Columbia University, December 6–7, 2016. The 2-day workshop was attended by over 100 people and took stock of recent developments in Sub-seasonal to Seasonal predictability, S2S extreme weather phenomena, and real world predictions and use of forecasts. Workshop motivations, new ﬁndings, and outstanding questions discussed are described. npj Climate and Atmospheric Science (2017) 1:8 ; doi:10.1038/s41612-017-0009-1 INTRODUCTION cause extreme weather and climate events with large societal impacts (ﬂoods, droughts, storms, heat, and cold waves), as well as Columbia University’s International Research Institute for Climate to improve their prediction for early warning. Two weeks to a and Society (IRI) and its Initiative on Extreme Weather and Climate, season ahead is a key forecasting time range, both from the together with the World Weather Research Program (WWRP) and physical perspective of the climatic drivers of extremes (MJO, World Climate Research Program (WCRP)’s Sub-Seasonal to blocking events, sudden stratospheric warmings, land/sea/ice Seasonal Prediction Project (S2S), with support from NOAA’s surface interactions), and for decision makers to have sufﬁcient Modeling, Analysis, Predictions and Projections (MAPP) program time to take preemptive actions. However, it is also a challenging and involvement of its S2S Prediction Task Force, held a 2-day range because it falls into a gap that currently exists between workshop on December 6–7, 2016 at the Columbia University weather and seasonal climate forecasts: weather is deterministi- Lamont-Doherty Earth Observatory campus in Palisades, New cally predictable as a function of atmospheric initial conditions up York. The workshop targeted all in academia, government, and the to a theoretical limit of a week or two, while climate acquires private sector with an interest in understanding the latest science probabilistic predictability from slowly varying atmospheric behind S2S predictability of extreme weather and climate and in boundary conditions (notably the ocean) on seasonal and longer developing early warning products. Some 115 people attended in scales. The ~2 weeks to a season range was previously thought to person, with another 150 joining the online conference livestream. be a “predictability desert”, where neither had much power. The workshop consisted of 23 talks, 33 posters, and discussion Recent research on sources of predictability and improved sessions, with the goal of sharing the latest research on extremes modeling has started to change that picture. The workshop using S2S models, the WWRP/WCRP S2S research initiative and summarized the key advances in this emerging area. From the data archive, operational S2S forecasting, private sector forecasts, user standpoint, sub-seasonal forecasts made on a near-weekly sources of S2S predictability of weather extremes, and risk basis, could ﬁll the gap between weather forecasts that are issued management perspectives. These talks are brieﬂy summarized every day, and seasonal climate forecasts that are typically only below, and a list of the presenters and presentation titles is made once a month; they have the potential to provide weekly provided in Table 1. Several of the talks have been developed into updated forecast information of intermediate speciﬁcity about the papers appearing in this special issue. The workshop program can risks of extremes up to several weeks ahead that may be be accessed at http://iri.columbia.edu/s2s-extremes-workshop- particularly amenable to a range of users. 2016/ where recorded videos of the presentations can be accessed. The workshop was motivated by the increasing interest in SUB-SEASONAL TO SEASONAL PREDICTABILITY extreme weather and climate events, both in terms of developing The workshop began with a series of talks on S2S predictability of the potential for early warning systems for better societal large-scale atmospheric phenomena, beginning with an introduc- preparedness, as well as toward gaining a better understanding tion to the S2S project, which is a 5-year joint research project of of the potential impacts of climate change on disruptive weather. the WWRP and WCRP that began in 2013. A central pillar of the Recent scientiﬁc developments in sub-seasonal to seasonal S2S project is a large research data archive of S2S forecasts and prediction (from 2 weeks to a season ahead), together with the hindcasts from 11 operational forecasting centers around the establishment of the WWRP/WCRP S2S prediction project archive world. The talk was given by F. Vitart (ECMWF) who is one of the of operational model forecasts, provide a new opportunity to co-chairs of the S2S project, together with A. W. Robertson (IRI). better understand the physical phenomena and processes that The S2S project has already advanced research signiﬁcantly 1 2 International Research Institute for Climate and Society (IRI), Columbia University, Palisades, NY, USA; Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, 3 4 USA; European Centre for Medium Range Weather Forecasts (ECMWF), Reading, UK and Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, USA Correspondence: Andrew W. Robertson (email@example.com) Received: 31 May 2017 Revised: 1 September 2017 Accepted: 15 September 2017 Published in partnership with CECCR at King Abdulaziz University S2S Extremes Workshop 2016 AW Robertson et al. npj Climate and Atmospheric Science (2018) 8 Published in partnership with CECCR at King Abdulaziz University Table 1. List of workshop presentations Presenter & afﬁliation Presentation title Session 1: Sub-seasonal to seasonal predictability F. Vitart (ECMWF) Prediction of extreme events at sub-seasonal to seasonal lead times C. Jones (UCSB) Sub-seasonal forecast skill of precipitation in the contiguous United States during the 2015–16 winter E. Barnes (CSU) North Paciﬁc blocking anticyclones, S2S dataset H.-M. Kim (SUNY-SB) MJO prediction: current status and future challenges J. Perlwitz (ESRL) Role of stratospheric processes on ENSO-NAO connections on seasonal-to-subseasonal timescale R. Saravanan (TAMU) Does “oceanic weather” affect atmospheric weather in the midlatitudes? Session 2: S2S extreme weather phenomena M. DeFlorio (NASA JPL) Prediction of atmospheric rivers G. Vecchi (GFDL) Towards a uniﬁed system for prediction and understanding of regional and extreme tropical cyclone activity D. Farnham (Columbia University) Seasonal climate signals, regional daily intense precipitation H. Hendon (BoM) Attribution of extreme heat events using a seasonal forecast framework M. Tippett (Columbia University) Monthly predictions of severe weather indices in CFSv2 P. Dirmeyer (GMU) Predictability of heat waves following drought events in the United States in S2S models A. Lang (SUNY-Albany) The stratosphere-troposphere processes and their role in climate (SPARC) stratospheric network for the assessment of predictability (SNAP) E. Coughlan (RCCC) “Ready-Set-Go!” How S2S forecasts could help the Red Cross/Red Crescent Session 3: Real world prediction and use of forecasts M. Ventrice (The Weather Co.) Weeks 3–5, sub-monthly volatility, skillful, automated sub- seasonal guidance that can be used as a platform for all forms of business A. Kumar (CPC) The utility of seasonal hindcast data base for analyzing climate variability and extremes; an example of US west coast winter precipitation variability during strong El Niño events M. L’Heureux (CPC) Capability of model forecasts to predict the 2015–16 El Nino and similarly extreme ENSO events B. Kirtman (RSMAS) The subseasonal experiment (SubX) W. Norton (City Financial) Attributing predictable signals at subseasonal timescales. P. Roundy (SUNY-Albany) Propagation characteristics of the MJO in different intraseasonal mid to high latitude blocking states C. Lee (IRI) Relationship between tropical cyclones and the Madden-Julian oscillation in the S2S dataset A. Vintzileos (CPC) Enhancing resilience to heat extremes: multi-model forecasting of excessive heat events at subseasonal lead times S. Materia (CMCC) Seasonal forecast of droughts: providing useful information to water management and agribusiness sector Poster session J. Nie (Columbia University) Enhanced sensitivity of precipitation extreme on temperature in the 2015 Texas storm S. Wang (Columbia University) Subseasonal predictability of precipitation in the maritime continent V. Cheng (University of Toronto) Towards predicting the interannual variability of tornado occurrences In North America A. Anichowski (Columbia University) The roles of tropical forcing and atmospheric noise in winter 2016 California precipitation A. Munoz (GFDL) Cross-timescale diagnostics of coupled general circulation models J. Williams (McGill University) Dynamic preconditioning of the September sea-ice extent minimum N. Cohen (Columbia University) Spectral trend estimation using raw data: application for the solar activity and climatological variability over India N. Vigaud (Columbia University) Multi-model ensembling of sub-monthly precipitation forecasts over North America R. Burgman (Florida International University) Understanding mechanisms of predictability of extreme North American drought in the North American multi-model ensemble project A. Subramanian (Oxford University) Stochastic multi-scale modeling for subseasonal-to-seasonal prediction B. Bradford (US Naval Academy) Subseasonal variability associated with the MJO: propagating MJO events and US severe weather activity S2S Extremes Workshop 2016 AW Robertson et al. Published in partnership with CECCR at King Abdulaziz University npj Climate and Atmospheric Science (2018) 8 Table 1. continued Presenter & afﬁliation Presentation title C. Baggett (Colorado State University) An assessment of the strike probabilities of atmospheric rivers along the west coast of North America as a function of MJO phase in S2S hindcast models E. LaJoie (NOAA CPC) Frequency and intensity of extreme weather events will increase in response to rising greenhouse gas concentrations D. Collins (NOAA CPC) Combining forecasts from multiple models of the NMME to identify extreme seasonal temperatures J. Doss-Gollin (Columbia University) Physical mechanisms and subseasonal-to-seasonal predictability of persistent intense rainfall and Paraguay river ﬂooding during the austral summer 2015/2016 J. Baijnath (University of Waterloo) A historical spatiotemporal evaluation of statistical extremes in lake effect snowfall over the Ontario snowbelt region of the Laurentian Great lakes M. Pena (NOAA EMC) Increasing S2S ensemble prediction capacity at NCEP: experiments with one-way and two-way interaction models. N. Klingaman (University of Reading) S2S predictions of climate extremes in developing countries during ENSO S. Shah (University of Colorado) Subseasonal forecasting skill of a global icosahedral ocean–atmosphere model using an ALE vertical coordinate T. Nakazawa (Korea Meteorological Administration) Attribution of extreme events to the global warming? W. Li (NCEP) The impact of the SST on the sub-seasonal forecast using the NCEP global ensemble forecast system A. Winters (SUNY Albany) Weather regime-dependent predictability: sequentially linked high-impact weather events over the United States during March 2016 A. Handayani (BMKG Indonesia) Trailing the northerly cold surge propagation B. Green (University of Colorado) Evaluation of MJO predictive skill in multi-physics and multi-model global ensembles B. Mundhenk (Colorado State University) Sub-seasonal to seasonal modulation of landfalling atmospheric river activity by Northeast Paciﬁc height anomalies H. Attard (SUNY Albany) Three types of synoptic events and their associated troposphere–stratosphere coupling H. Chang (University of Arizona) Evaluating changes in extreme weather during the North American monsoon in the southwest US using high resolution, convective- permitting regional atmospheric modeling J.-H. Qian (University of Massachusetts, Lowell) Winter weather regimes and extreme precipitation in the Northeast United States J. Blufer (SUNY Albany) Examining sources of variability in model skill during the 7 January 2013 sudden stratospheric warming event W. Li (University of Chicago) Periodic behavior in the Southern Hemisphere storm track on hemispheric and regional scales M. Osman (University of Buenos Aires) Evaluation of subseasonal forecast models for a strong heat wave over southern South America O. Tuinenburg (Utrecht University) Can we see the drought coming? S. Ratna (Japan Agency for Marine-Earth Science The unusual wet summer (July) of 2014 in southern Europe and its predictability using NCEP CFSv2 model and Technology) T. Wu (Beijing Climate Center) The impact of SST initial state on MJO prediction Y. Takaya (Meteorological Research Institute, Japan) Lingering effects of preceding strong El Niño events on the typhoon activity in early summer: case study of sub-seasonal and seasonal predictions in 2016 S2S Extremes Workshop 2016 AW Robertson et al. through the S2S database, and the workshop highlighted some of physical basis for sub-seasonal TC prediction. Encouraging analysis these developments. The database offers an unprecedented of forecasts from S2S database shows that the models can capture resource for scientists, modeling groups, and interested parties this modulation, and several speakers showed examples of to better study, understand, and ultimately improve S2S predic- successful TC activity forecasts, even from the lower-resolution tion—in the same way that the Coupled Model Intercomparison models, while high-resolution models like the 25 km GFDL HiFLOR Project (CMIP) archives have advanced climate change research— model are able to capture intense category 4 and 5 storms, by making output from many models available publicly using a which are not simulated by the lower resolution models. Although uniﬁed protocol. Several oral and poster presenters shared early small-scale severe weather events, such as hail storms and results from the S2S database, which became fully operational in tornados cannot be represented explicitly in the current genera- 2015, often highlighting the potential for future research. Several tion of S2S forecast models, we may still be able to predict workshop presentations also featured the activities of the MAPP their aggregate statistics several weeks into the future. While the S2S Prediction Task Force, a research initiative to advance NOAA’s best skill is naturally found during the ﬁrst week of the forecasts, capability to model and predict sources of S2S predictability. M. Tippett (Columbia University) showed examples of index-based Speciﬁc S2S phenomena discussed included the MJO (H.-M. Kim, skillful prediction of tornado and hail activity over the US in the SUNY-SB), the role of stratospheric processes on ENSO-NAO week 2–4 range using techniques developed for seasonal teleconnections (J. Perlwitz, NOAA-ESRL), “oceanic weather” forecasting. impacts on atmospheric midlatitude weather (R. Saravanan, Several talks addressed the predictability of heat waves. Work TAMU), North Paciﬁc blocking anticyclones (E. Barnes, CSU), and over the US also highlighted the often strong relationship sub-seasonal forecast skill of precipitation in the contiguous between preceding drought conditionals and heat waves (P. United States during the 2015–16 winter (C. Jones, UCSB). A study Dirmeyer). Results from ECMWF indicate that the devastating of how MJO propagation in the tropics can be affected by Russian heat wave of 2010 could be have predicted up to 3 weeks midlatitude blocking was also presented (P. Roundy, SUNY- in advance with today’s models. The workshop also highlighted Albany). that S2S forecasts are now being used in extreme events The MJO is recognized as one of the most important sources of attribution to physical phenomena. Results presented by F. Vitart predictability for sub-seasonal forecasts in the tropics, analogous showed that the extreme cold wave that occurred over Europe in to ENSO for seasonal forecasts, and a majority of the operational March 2013 could partly be attributed to an MJO event over the models now reach a bivariate anomaly correlation skill of the Indian Ocean. The question of attributing the record breaking heat Wheeler–Hendon RMM indices exceeding 0.5 at 2 weeks lead over Australia in October 2015 was addressed by H. Hendon time, quantiﬁed using the S2S database. This is remarkable (BoM), who found that most of the predictability was supplied advance over the situation even a decade ago when models were through atmospheric initial conditions up to 2 weeks in advance, then not able to adequately represent and forecast the MJO. with little role found for the ocean and land surface in that case. However, the MJO is often still too weak and too slowly There is still much to be learned about the importance of propagating in models, and their forecasts still typically under- atmospheric initial conditions near the classical deterministic estimate the forecast uncertainty. The impact of the MJO on predictability limit of Lorenz at 7–15 days. weather conditions—particularly in the midlatitudes through teleconnections—is still not adequately captured by models, REAL WORLD PREDICTION AND USE OF FORECASTS reducing the current value of S2S forecasts. Encouraging results A large part of the workshop was devoted to the emerging area of are nonetheless starting to emerge, including predictability of the positive phase of the NAO up to 2–3 weeks in advance, which may S2S forecasting for real-world applications. Besides promoting be associated with MJO teleconnections. research to improve understanding and forecast skill, the WWRP/ The talk by C. Jones highlighted the challenges in forecasting WCRP’s S2S project strives to spur forecast uptake by operational precipitation beyond a week in advance over the US during the centers and exploitation by the applications community. The 2015–16 winter season. Blocking events over the North Paciﬁcwere workshop witnessed considerable S2S interest in the private sector, shown by E. Barnes to steer Atmospheric Rivers into Alaska and the with companies actively developing sub-seasonal forecasts and Paciﬁc Northwest, which she was able to track to predictable MJO seeking to attribute skill to speciﬁc predictable phenomena, such activity over the tropical Indio-Paciﬁc Ocean, particularly during the as their MJO (M. Ventrice, The Weather Co.; W. Norton, City easterly phase of the stratospheric quasi–biennial oscillation. The Financial). Such companies have previously used statistical models stratosphere is increasingly being recognized as an important for energy forecasts and are eager to tap into new capabilities, player in forecasting on the S2S time scale and today’s models including making weekly updates to established seasonal forecasts often have a large number of vertical levels, allowing them to that are only issued once a month; the ability of sub-seasonal better resolve the stratosphere. The talk by J. Perlwitz drew forecasts to provide weekly updates was particularly highlighted, attention to the role of sudden warmings of the polar stratosphere as was the interest in temperature and wind forecasts. in mediating the impact of ENSO on the NAO. Midlatitude ocean Three presentations were made by members of NOAA’s Climate eddies have monthly time scales and are garnering increasing Prediction Center (CPC), which has been issuing week 3 + 4 attention as a potential source of the S2S predictability of US west- probabilistic climate outlooks experimentally since September coast weather, as discussed by R. Saravanan. 2015 (A. Kumar, M. L’Heureux, A. Vintzileos). El Niño/La Niña events are important sources of predictability at sub-seasonal (multi- week), as well as seasonal (multi-month) forecast lead times, and S2S EXTREME WEATHER PHENOMENA the 2015–16 El Niño in particular was discussed. This event did not The second set of talks continued with the theme of predicting result in the expected wet conditions over California, and sub- different types of extreme weather events, and how the latter are seasonal forecasts provide a new tool for analyzing failures in related to sub-seasonal predictable dynamical phenomena like the seasonal forecasts by analyzing more frequently updated shorter- MJO. These included Atmospheric Rivers (M. DeFlorio, NASA JPL), term forecast evolutions. tropical cyclones (G. Vecchi, GFDL; C. Lee IRI), intense precipitation S2S forecast products are under development in several and severe weather (D. Farnham, Columbia University; M. Tippett, application sectors, and talks were presented on multi-model Columbia University), heat waves and drought (H. Hendon, BoM; P. forecasting of excessive heat events (A. Vintzileos, CPC), lessons Dirmeyer, GMU). Tropical cyclone (TC) activity over Indo-Paciﬁc learned from seasonal forecast of droughts for the water Ocean is strongly modulated by the MJO, which provides a management and agribusiness sectors (S. Materia, CMCC), and npj Climate and Atmospheric Science (2018) 8 Published in partnership with CECCR at King Abdulaziz University S2S Extremes Workshop 2016 AW Robertson et al. “Ready-Set-Go!”, or how S2S forecasts could help the Red Cross/ the workshop are grateful for the support provided by Columbia University’s Extreme Weather & Climate Initiative, WMO, the NOAA Climate Program Ofﬁce Modeling, Red Crescent (E. Coughlan, RCCC). Sub-seasonal forecasting of Analysis, Predictions, and Projections (CPO-MAPP) program, and Columbia University’s extremes will require more categories of the forecast distribution International Research Institute for Climate and Society (IRI) which co-organized and than the standard terciles used in seasonal forecasting, for hosted the event in its Monell Auditorium in Palisades, New York. instance, forecasts for extreme thresholds, such as the 85th or 90th percentile, or the use of extremes indices. The choice of suitable metrics for veriﬁcation of large numbers of quantiles were AUTHOR CONTRIBUTIONS discussed, as well as the difﬁculty of identifying suitable “danger All authors researched, collated, and wrote this paper. level” thresholds, for which the “cost-loss” model framework may be informative. For real-world forecast applications, it will be necessary to have ADDITIONAL INFORMATION access to S2S forecasts in real time. While the S2S database is Competing interests: The authors declare no competing ﬁnancial interests. purely for research purposes and only provides these forecasts with a 3-week delay, at least two initiatives will soon provide the Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims forecasts in real time. The WMO is working through its Lead Centre in published maps and institutional afﬁliations. for long-range forecasting to create multi-model products in real time from the S2S models that will be made accessible to national meteorological services and the WMO’s regional climate centres. REFERENCES The second initiative is the NOAA program new Subseasonal 1. Vitart, F., Robertson, A. W. and S2S Steering Group. Sub-seasonal to seasonal Experiment (SubX; http://cola.gmu.edu/kpegion/subx) (presented prediction: linking weather and climate. In Seamless Prediction of the Earth System: From Minutes to Months, WMO-1156 (eds Brunet, G., Jones, S. & Ruti, P. M.) 385–401 by B. Kirtman, RSMAS) that will provide sub-seasonal real-time (World Meteorological Organization, Geneva, Switzerland, 2015). forecasts and hindcasts from North American modeling centers 2. Vitart, F., Robertson, A. W. & Anderson, D. L. T. Subseasonal to seasonal prediction (NCEP, Environment Canada, US Navy, NASA, NOAA-ESRL, and project, 2012: bridging the gap between weather and climate. WMO Bull. 61(2), NCAR/COLA/RSMAS) in a publicly accessible database housed at 23–28 (2012). IRI-Columbia University. SubX predictions will not only be real- 3. Vitart, F. et al. The Subseasonal to Seasonal (S2S) prediction project database. Bull. time but will also adhere to a common forecasting protocol, which Am. Meteor. Soc. 98(1), 163–176 (2017). will facilitate both research and real-world applications, extending 4. Zhang, C. Madden–Julian Oscillation: bridging weather and climate. Bull. Am. what is possible based on the S2S database. Meteor. Soc. 94, 1849–1870 (2013). 5. Vitart, F. Madden-Julian Oscillation prediction and teleconnections in the S2S database: MJO prediction and teleconnections in the S2S database. Q. J. R. Meteor. CLOSING REMARKS Soc. 143, 2210–2220 (2017). The S2S time scale is a frontier in our efforts to gain understanding and improve prediction of weather and climate, and progress will Open Access This article is licensed under a Creative Commons naturally require collaboration among individuals from diverse Attribution 4.0 International License, which permits use, sharing, communities and viewpoints. S2S research is an emerging area adaptation, distribution and reproduction in any medium or format, as long as you give where there are ample opportunities for young researchers to appropriate credit to the original author(s) and the source, provide a link to the Creative contribute to advancing scientiﬁc understanding, as well as the Commons license, and indicate if changes were made. The images or other third party possibility to have meaningful impacts on the business, govern- material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the ment, and nonproﬁt sectors. article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons. ACKNOWLEDGEMENTS org/licenses/by/4.0/. We wish to thank all the presenters and participants at the workshop, and give special thanks to Jaclyn Rabinowitz and Dannie Dinh whose excellent local organization and logistics enabled a successful workshop. It is a pleasure to thank two anonymous © The Author(s) 2018 reviewers whose comments improved the quality of the manuscript. The organizers of Published in partnership with CECCR at King Abdulaziz University npj Climate and Atmospheric Science (2018) 8
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