62 research outputs found
Impact of strong winds, heavy snow loads and soil frost conditions on the risks to forests in Northern Europe
The aim of this work was to study the potential impacts of strong winds, heavy snow loads, and soil frost conditions on the risks to forests in northern Europe under the current and changing climate (until 2100), with the main focus on Finland. More specifically, the analyses concentrated on: i) changes in the occurrence of strong winds, heavy snow loads, and unfrozen soil conditions in Finland, ii) regional risks to Finnish forests from heavy snow loads and strong winds, iii) the mean and extreme geostrophic wind speeds in Northern Europe, and iv) estimation of windstorm-related timber losses in Europe in 1960-2011 using the geostrophic and ageostrophic isallobaric winds as a basis for the analyses. This work employed the meteorological measurements made by the Finnish Meteorological Institute (FMI) between 1961-2009, the datasets from the Finnish National Forest Inventory and the EFIATLANTIC storm catalogue, a number of global climate model (GCM) simulation runs using different greenhouse gas (GHG) emission scenarios (A1B, A2, and B1) and re-analyzed weather datasets ERA-40 and ERA-Interim. The occurrence and depths of soil frost were studied with a model that simulates the freezing of the snow-free ground. The occurrence of large snow loads was estimated with the cumulative snow load approach. The analyses of the risks from snow and/or wind to forests were based on simulations done by the ecosystem model SIMA, the mechanistic model HWIND, and a new regression fit between storm wind impact and timber losses in Europe. According to this work, the growth of Finnish forests is expected to increase under the changing climate. Concurrently, the tree species distribution may also change, which affects the potential risks to forests from wind and snow. In the current climate, strong mean wind speeds of about 17-18 m s-1 have occurred approximately once in 10 years in Finland (in October-February) and caused wind damages. In this work, wind speeds induced by intense cyclones were also found to correlate well with primary losses of timber. The annual maximum soil frost depth has also decreased 5-10% in the period 1980-2009 compared to the period 1971-2000 in southern and central Finland. Under the future climate projections, the soil is expected to hardly freeze at all in southern and central Finland by 2100. Mean and extreme geostrophic wind speeds are also projected to increase during September-April slightly by 2100. Furthermore, days with heavy snow loads may still occur in the near future. To conclude, wind-induced risks to forests in particular may increase in Finland and elsewhere in Northern Europe in the forthcoming decades. However, proper and timely management of forests could help reduce at least to some degree wind-induced risks to forests in the future.TĂ€mĂ€n työn tavoitteena oli tutkia voimakkaiden tuulten, lumikuormien ja roudan esiintymisen mahdollisia vaikutuksia metsien tuhoriskeihin Pohjois-Euroopassa. Tarkastelut tehtiin nyky- ja muuttuvalle ilmastolle aina vuoteen 2100 asti. PÀÀpaino tutkimuksessa oli Suomessa. TyössĂ€ analysoitiin i) voimakkaan tuulen, lumikuormien ja routaolojen esiintymisessĂ€ tapahtuvia muutoksia Suomessa, ii) Suomen metsiin kohdistuvia alueellisia tuuli- ja lumituhoriskejĂ€, iii) ilmastonmuutoksen vaikutusta keskimÀÀrĂ€isiin ja ÀÀrimmĂ€isiin geostrofisiin tuuliin Pohjois-Euroopassa ja iv) Euroopan primÀÀristen metsĂ€tuhojen ja myrskyissĂ€ esiintyneiden geostrofisten ja ageostrofisen isallobaaristen tuulten vĂ€listĂ€ yhteyttĂ€ 1960â2011. TyössĂ€ kĂ€ytettiin Ilmatieteen laitoksen havaintoaineistoa 1961â2009, MetsĂ€ntutkimuslaitoksen valtakunnan metsien inventointitietoja (METLA), Eurooppalaista metsien myrskytuhotietokantaa (EFIATLANTIC) ja lukuisia maailmanlaajuisia ilmastomalleja huomioiden eri skenaariot (A1B, A2 ja B1). TĂ€mĂ€n lisĂ€ksi kĂ€ytettiin uusanalysoituja meteorologisia aineistoja (reanalyysejĂ€) ERA-40 ja ERA-Interim. Roudan syvyyksiĂ€ tutkittiin mallilla, joka laskee lumettoman maan jÀÀtymistĂ€. Puiden lumikuormien esiintymistĂ€ arvioitiin kumulatiivisella lumikuormamallilla. Metsiin kohdistuvia lumikuormien ja tuulten aiheuttamia riskejĂ€ analysoitiin SIMA-ekosysteemimallilla ja mekanistisella HWIND-mallilla. TĂ€mĂ€n lisĂ€ksi kehitettiin regressioyhtĂ€lö, jolla voitiin arvioida metsien puustotuhojen mÀÀrÀÀ myrskytuulten perusteella Euroopassa. Työn perusteella Suomen metsien kasvu tulee lisÀÀntymÀÀn ilmaston lĂ€mmetessĂ€ ja lisĂ€ksi puulajien jakauma voi muuttua. TĂ€llĂ€ voi olla vaikutusta siihen, millaisia lumi- ja tuulituhoja metsissĂ€ tulevaisuudessa mahdollisesti esiintyy. Nykyilmastossa, 10-minuutin keskituuli, joka esiintyy keskimÀÀrin kerran kymmenessĂ€ vuodessa, on voimakkaimmillaan noin 17â18 ms-1 maan etelĂ€- ja lĂ€nsiosan sisĂ€maassa. Tyypillisimmin voimakkaita tuulia esiintyy lokakuusta helmikuuhun ulottuvalla jaksolla. Euroopan myrskyjen laajuudella, tuulen nopeudella, ja primÀÀristen metsĂ€tuhojen suuruudella on selvĂ€ korrelaatio. Suomen etelĂ€- ja keskiosassa roudan syvyydet ovat pienentyneet talvella 5-10 % verrattaessa jaksoa 1980â2009 jaksoon 1971â2000. MikĂ€li ilmaston lĂ€mpeneminen jatkuu, maan etelĂ€- ja keskiosa muuttuvat vuosisadan loppuun mennessĂ€ vĂ€hitellen roudattomiksi. Lumikuormat voivat vielĂ€ lĂ€hivuosikymmeninĂ€ olla suuria maan etelĂ€- ja keskiosassa, mutta loppuvuosisadalla suuria lumikuormia esiintynee enÀÀ pohjoisessa. Suomessa keskimÀÀrĂ€iset ja ÀÀrimmĂ€iset geostrofiset tuulet voimistuvat syyskuu-huhtikuu vĂ€lisenĂ€ aikana vuoteen 2100 mennessĂ€. Muutos on kuitenkin vain muutaman prosentin luokkaa. Kaikki tulokset vahvistavat kĂ€sitystĂ€ siitĂ€, ettĂ€ erityisesti metsiin kohdistuva tuuliriski on mitĂ€ todennĂ€köisimmin kasvamassa niin Suomessa kuin muualla Pohjois-Euroopassa. TĂ€mĂ€ edellyttÀÀ tuhoriskien huomioimista metsien hoidossa
The Extratropical Transition of Hurricane Debby (1982) and the Subsequent Development of an Intense Windstorm over Finland
On 22 September 1982, an intense windstorm caused considerable damage in northern Finland. Local forecasters noted that this windstorm potentially was related to Hurricane Debby, a category 4 hurricane that occurred just 5 days earlier. Due to the unique nature of the event and lack of prior research, our aim is to document the synoptic sequence of events related to this storm using ERA-Interim reanalysis data, best track data, and output from OpenIFS simulations. During extratropical transition, the outflow from Debby resulted in a ridge building and an acceleration of the jet. Debby did not reintensify immediately in the midlatitudes despite the presence of an upper-level trough. Instead, ex-Debby propagated rapidly across the Atlantic as a diabatic Rossby wave-like feature. Simultaneously, an upper-level trough approached from the northeast and once ex-Debby moved ahead of this feature near the United Kingdom, rapid reintensification began. All OpenIFS forecasts diverged from reanalysis after only 2 days indicating intrinsic low predictability and strong sensitivities. Phasing between Hurricane Debby and the weak trough, and phasing of the upper- and lower-level potential vorticity anomalies near the United Kingdom was important in the evolution of ex-Debby. In the only OpenIFS simulation to correctly capture the phasing over the United Kingdom, stronger wind gusts were simulated over northern Finland than in any other simulation. Turbulent mixing behind the cold front, and convectively driven downdrafts in the warm sector, enhanced the wind gusts over Finland. To further improve understanding of this case, we suggest conducting research using an ensemble approach.Peer reviewe
Climatology, variability, and trends in near-surface wind speeds over the North Atlantic and Europe during 1979-2018 based on ERA5
This study presents the monthly 10âm wind speed climatology, decadal variability and possible trends in the North Atlantic and Europe from ERA5 reanalysis from 1979 to 2018 and investigates the physical reasons for the decadal variability. Additionally, temporal time series are examined in three locations: the central North Atlantic, Finland and Iberian Peninsula. The 40âyear mean and the 98th percentile wind speeds emphasize a distinct landâsea contrast and a seasonal variation with the strongest winds over the ocean and during winter. The strongest winds and the highest variability are associated with the storm tracks and local wind phenomena such as the mistral. The extremeness of the winds is examined with an extreme wind factor (the 98th percentile divided by mean wind speeds) which in all months is higher in southern Europe than in northern Europe. Mostly no linear trends in 10âm wind speeds are identified in the three locations but large annual and decadal variability is evident. The decadal 10âm wind speeds were stronger than average in the 1990s in northern Europe and in the 1980s and 2010s in southern Europe. These decadal changes were largely explained by the positioning of the jet stream and storm tracks and the strength of the northâsouth pressure gradient in the North Atlantic. The 10âm winds have a positive correlation with the North Atlantic Oscillation in the central North Atlantic and Finland on annual scales and during cold season months and a negative correlation in Iberian Peninsula mostly from July to March. The Atlantic Multiâdecadal Oscillation has a moderate negative correlation with the winds in the central North Atlantic but no correlation in Finland and Iberian Peninsula. Overall, our results emphasize that while linear trends in wind speeds may show a general longâterm trend, more information on the changes is obtained by analysing longâterm variability.This study presents the monthly 10-m wind speed climatology, decadal variability and possible trends in the North Atlantic and Europe from ERA5 reanalysis from 1979 to 2018 and investigates the physical reasons for the decadal variability. Additionally, temporal time series are examined in three locations: the central North Atlantic, Finland and Iberian Peninsula. The 40-year mean and the 98th percentile wind speeds emphasize a distinct land-sea contrast and a seasonal variation with the strongest winds over the ocean and during winter. The strongest winds and the highest variability are associated with the storm tracks and local wind phenomena such as the mistral. The extremeness of the winds is examined with an extreme wind factor (the 98th percentile divided by mean wind speeds) which in all months is higher in southern Europe than in northern Europe. Mostly no linear trends in 10-m wind speeds are identified in the three locations but large annual and decadal variability is evident. The decadal 10-m wind speeds were stronger than average in the 1990s in northern Europe and in the 1980s and 2010s in southern Europe. These decadal changes were largely explained by the positioning of the jet stream and storm tracks and the strength of the north-south pressure gradient in the North Atlantic. The 10-m winds have a positive correlation with the North Atlantic Oscillation in the central North Atlantic and Finland on annual scales and during cold season months and a negative correlation in Iberian Peninsula mostly from July to March. The Atlantic Multi-decadal Oscillation has a moderate negative correlation with the winds in the central North Atlantic but no correlation in Finland and Iberian Peninsula. Overall, our results emphasize that while linear trends in wind speeds may show a general long-term trend, more information on the changes is obtained by analysing long-term variability.Peer reviewe
The projected 21st century forest-fire risk in Finland under different greenhouse gas scenarios
Tykkylumen alueellinen esiintyminen Suomessa kahden laskentamenetelmÀn perusteella
TutkimusselosteSeloste julkaisusta: Lehtonen, I., Hoppula, P., Pirinen, P. & Gregow, H. 2014. Modelling crown snow loads in Finland: a comparison of two methods. Silva Fennica 48(3), article id 1120
Ilmastonmuutoksen vaikutukset Suomessa metsÀnhoidon nÀkökulmasta
Kasvavat metsÀt sitovat hiiltÀ ilmakehÀstÀ, ja metsillÀ on siten tÀrkeÀ rooli ilmastonmuutoksen hillinnÀssÀ. MetsÀt ovat myös tÀrkeitÀ virkistysalueita, ja ennen kaikkea luonnontilaisia metsiÀ tarvitaan pyrittÀessÀ suojelemaan luonnon monimuotoisuutta. Toisaalta metsÀteollisuus on yksi Suomen tÀrkeimmistÀ teollisuudenaloista, joten myös metsien taloudellinen merkitys Suomessa on suuri.
Ilmastonmuutoksen edetessÀ ja erilaisten metsien kÀyttöön liittyvien intressien ristipaineessa korostuu kysymys siitÀ, miten metsiÀ voidaan hyödyntÀÀ kestÀvÀllÀ tavalla. MetsÀnhoidon suositusten uudistamisen pohjaksi tarvitaan tietoja niin ilmastonmuutoksen suuruudesta kuin sen vaikutuksistakin. TÀssÀ raportissa esitetÀÀn tÀmÀnhetkisen tietÀmyksen mukaiset arviot ilmastonmuutoksesta Suomessa sekÀ siitÀ, millaisia vaikutuksia muutoksella on Suomen metsiin ja metsÀtaloudelle.
Viimeisten noin 150 vuoden aikana keskilĂ€mpötila on Suomessa jo kohonnut pari astetta. TĂ€llĂ€ hetkellĂ€ lĂ€mpötila nousee Suomessa vajaat puoli astetta vuosikymmenessĂ€. Kuluvan vuosisadan puolivĂ€liin mennessĂ€ lĂ€mpötilan odotetaan kohoavan nykyisestĂ€ noin 1â1,5 astetta lisÀÀ. Kuinka paljon lĂ€mpötila nousee edelleen vuosisadan jĂ€lkipuoliskolla, riippuu suuresti kasvihuonekaasupÀÀstöjen tulevasta maailmanlaajuisesta kehityksestĂ€. Suomessa lĂ€mpötilan nousu on noin kaksi kertaa nopeampaa kuin maapallolla keskimÀÀrin. LĂ€mpenemisen lisĂ€ksi sateiden odotetaan lisÀÀntyvĂ€n etenkin talvikuukausina. Toisaalta kesĂ€llĂ€ kuivuus voi vaivata aiempaa useammin.
LÀmpeneminen ja ilmakehÀn kohonnut hiilidioksidipitoisuus ovat jo omalta osaltaan kiihdyttÀneet metsien kasvua, ja tulevaisuudessa metsiemme ennustetaan kasvavan yhÀ rivakammin. Toisaalta lisÀÀntyvÀt metsÀtuhot voivat osittain neutralisoida tÀmÀn kehityksen. Erityisesti kuusikot ovat alttiita paitsi monille tuhonaiheuttajille, niin EtelÀ-Suomessa myös lisÀÀntyvÀlle kuivuudelle. TuhohyönteisistÀ lÀmpeneminen hyödyttÀÀ muun muassa kaarnakuoriaisiin lukeutuvaa kirjanpainajaa. Talvella roudan vÀheneminen vaikeuttaa puiden korjuuolosuhteita, mikÀ lisÀÀ juuristovaurioiden riskiÀ korjuun yhteydessÀ ja haittaa puunkorjuun logistiikkaa. Myös tuulituhojen riski kasvaa roudan vÀhentyessÀ.
KansainvÀlisesti tavoitteeksi on asetettu lÀmpenemisen rajaaminen maailmanlaajuisesti alle kahteen asteeseen esiteolliseen aikaan eli 1700-luvun puolivÀlin ilmastoon verrattuna. TÀmÀ edellyttÀisi nopeaa maailmanlaajuista kasvihuonekaasupÀÀstöjen hillintÀÀ. Toistaiseksi kasvihuonekaasujen pÀÀstöjen kasvua ei ole pystytty rajoittamaan siinÀ mÀÀrin, ettÀ tavoitteen toteutuminen nÀyttÀisi todennÀköiseltÀ, joten on syytÀ varautua voimakkaampaan lÀmpenemiseen. Pahimman skenaarion toteutuessa lÀmpötila voi maailmanlaajuisesti kohota jopa yli neljÀ astetta kuluvan vuosisadan aikana.
YksittÀisen metsÀnomistajan kannalta keskeistÀ on huolehtia metsien kasvusta ja elinvoimasta sekÀ pyrkiÀ myös tunnistamaan metsiÀ uhkaavat riskit, puuston ja maaston vaihtelevuus huomioon ottaen. Riskien hallinnan tueksi verkossa on saatavilla runsaasti avoimia ilmaston vaihteluita ja sÀÀn ÀÀri-ilmiöiden esiintymistÀ kuvaavia tietoaineistoja sekÀ ennustepalveluita.Growing forests sequester carbon from the atmosphere, and hence forests have Aan important role in mitigating climate change. Forests are also important as recreational areas, and natural forests are needed in maintaining biodiversity. On the other hand, the economic importance of forests is substantial in Finland as the forest industry is a major contributor to wellbeing in the country.
Ongoing climate change and the multiple contradictory interests towards forests expressed from different sectors in society make it important to study how forests can be exploited in a sustainable way. Information on the magnitude and impacts of climate change are needed in revising the forest management recommendations. In this report, we present an assessment of climate change in Finland based on current knowledge and describe the expected effects of the change on forests and forestry.
Over the last 150 years, the mean temperature in Finland has already risen by about 2 °C. Presently, the temperature continues to increase with a rate of almost 0.5 °C per decade, and by the mid-century, temperatures in Finland are expected to be approximately 1â1.5 °C higher than at present. The rate of warming during the latter half of the 21st century will largely depend on the future evolution of global greenhouse gas emissions. In Finland, the rate of warming is about twice as large as the global average. In addition to warming, precipitation levels, particularly in winter, are expected to increase in the future. On the other hand, drought may occur in summer more frequently than at present.
Increasing temperature and rising atmospheric carbon dioxide concentration have already contributed to accelerating forest growth. In the future, our forests are projected to grow even more rapidly. On the other hand, an increasing frequency of forest damages may partly overrule this development. Particularly, spruce forests are vulnerable to many insect pests but in southern Finland also to drought. An example of a pest benefitting from the warming is the European spruce bark beetle. In winter, reduction of soil frost complicates the logistics of forest harvesting and increases the risk of root damage during the harvest. The risk of wind damage also increases as the soil frost decreases.
Internationally, the goal is to limit global warming to less than 2 °C compared to pre-industrial era, i.e., the mid-18th century. This would require rapid global mitigation of greenhouse gas emissions. So far, limiting the increase in global greenhouse gas emissions has not been adequately successful so that reaching the target would seem likely. Thus, there is a need to be prepared for a more severe warming. In the worst case, the temperature could rise even by more than 4 °C globally by the end of the 21st century.
From the forest owner viewpoint, it is important to take care of the growth and vitality of the forest stands and to identify the risks threatening the stands, taking into account the variability of the stands and the terrain. To support risk management, there are available several open data sets on climate variability and the occurrence of extreme weather events, as well as forecasting services.VĂ€xande skogar binder kol frĂ„n atmosfĂ€ren och spelar dĂ€rmed en viktig roll för att mildra klimatförĂ€ndringarna. Skogar Ă€r ocksĂ„ viktiga som rekreationsomrĂ„den, och framför allt behövs naturskogar för att skydda den biologiska mĂ„ngfalden. Ă
andra sidan Àr skogsindustrin en av Finlands viktigaste industribranscher och skogsbrukets ekonomiska betydelse Àr dÀrmed ocksÄ betydande.
PÄgÄende klimatförÀndringar, samt delvis till och med motstridiga intressen gentemot skogen som uttrycks frÄn olika sektorer i samhÀllet, gör det viktigt att studera hur skogen kan utnyttjas pÄ ett hÄllbart sÀtt. Information om bÄde omfattningen och konsekvenserna av klimatförÀndringarna behövs för att revidera skogsvÄrdsrekommendationerna. I denna rapport presenterar vi en bedömning av klimatförÀndringarna i Finland baserat pÄ nuvarande information och beskriver de förvÀntade effekterna pÄ skog och skogsbruk.
Under de senaste 150 Ă„ren har medeltemperaturen i Finland redan stigit med cirka 2 °C. För nĂ€rvarande stiger temperaturen i Finland med nĂ€stan 0,5 °C per Ă„rtionde. I mitten av 2000-talet förvĂ€ntas temperaturerna vara cirka 1â1,5 °C högre Ă€n för nĂ€rvarande. UppvĂ€rmningshastigheten under den senare hĂ€lften av 2000-talet beror till stor del pĂ„ den framtida utvecklingen av de globala utslĂ€ppen av vĂ€xthusgaser. I Finland Ă€r uppvĂ€rmningen ungefĂ€r dubbelt sĂ„ snabb som det globala genomsnittet. Förutom uppvĂ€rmningen förvĂ€ntas nederbörden att öka i framtiden, sĂ€rskilt pĂ„ vintern. Ă
andra sidan kan torka förekomma pÄ sommaren oftare Àn för nuförtiden.
Högre temperaturer och stigande koldioxidkoncentration i atmosfĂ€ren har redan bidragit till att förbĂ€ttra skogstillvĂ€xten. I framtiden berĂ€knas vĂ„ra skogar vĂ€xa Ă€nnu snabbare. Ă
andra sidan kan en ökande frekvens av skogsskador delvis radera denna utveckling. Det Àr sÀrskilt granskogar som Àr sÄrbara för mÄnga skadedjur, men ocksÄ för torka i södra Finland. Ett exempel pÄ ett skadedjur som kan dra nytta av uppvÀrmningen Àr granbarkborren. PÄ vintern komplicerar en minskning av tjÀle logistiken för skogsavverkning och ökar risken för rotskador under avverkningen. Risken för vindskador ökar ocksÄ nÀr tjÀlen minskar.
Internationellt har mÄlet satts att begrÀnsa den globala uppvÀrmningen till mindre Àn 2 °C jÀm-fört med den förindustriella tiden, dvs mitten av 1700-talet. För att uppnÄ mÄlet skulle det krÀvas en snabb minskning av de globala utslÀppen av vÀxthusgaser. Hittills har man inte lyckats begrÀnsa ökningen av de globala utslÀppen av vÀxthusgaser i en sÄdan utstrÀckning att det skulle verka troligt att mÄlet kan uppnÄs. DÀrför finns det ett behov av att förbereda sig för en större uppvÀrmning. I vÀrsta fall kan den globala medeltemperaturen öka med mer Àn 4 °C till slutet av 2000-talet.
Det Àr viktigt för den enskilda skogsÀgaren att ta hand om skogens tillvÀxt och livskraft, samt att försöka identifiera riskerna som hotar skogarna, med hÀnsyn till variationer i bestÄnd och terrÀng. För att stödja riskhanteringen finns det flera öppna datauppsÀttningar om klimatvariabilitet och förekomsten av extrema vÀderhÀndelser, sÄvÀl som prognostjÀnster
Overadaptation to Climate Change? The Case of the 2013 Finnish Electricity Market Act
In this paper, we put forward a definition of over-adaptation in disaster risk reduction (DRR) and climate change adaptation (CCA) projects. We detail an illustrative case in which the response to extreme weather risk while aligned with the goals of CCA, is implemented beyond the economically efficient scale. We undertake a cost-benefit analysis of the 2013 Finnish Electricity Market Act, enacted partially as a reaction to long, storm-induced electricity blackouts experienced after 2000. The Act imposes strict requirements on electricity distribution companies as regards the duration of blackouts. Meeting these requirements entails investments amounting to billions of euros. As a benefit, we quantify the avoided cost from the blackouts for households and producers. Our results, derived from Monte-Carlo simulations, show that for urban areas, the net expected value is positive. However, in rural areas less strict requirements could have been economically more efficient. Our results indicate that distributional impacts and correspondence between those who benefit and those who pay the costs should be taken into account in DRR and CCA policies that require large-scale investments. We also note that the population affected by a disaster may not accept DRR and CCA that are successful in terms of regulation and implementation. This applies when societal and individual preferences do not coincide.Peer reviewe
Characteristics of extratropical cyclones and precursors to windstorms in northern Europe
Peer reviewe
Adding value to Extended-range Forecasts in Northern Europe by Statistical Post-processing Using Stratospheric Observations
The strength of the stratospheric polar vortex influences the surface weather in the Northern Hemisphere in winter; a weaker (stronger) than average stratospheric polar vortex is connected to negative (positive) Arctic Oscillation (AO) and colder (warmer) than average surface temperatures in northern Europe within weeks or months. This holds the potential for forecasting in that timescale. We investigate here if the strength of the stratospheric polar vortex at the start of the forecast could be used to improve the extended-range temperature forecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) and to find periods with higher prediction skill scores. For this, we developed a stratospheric wind indicator (SWI) based on the strength of the stratospheric polar vortex and the phase of the AO during the following weeks. We demonstrate that there was a statistically significant difference in the observed surface temperature in northern Europe within the 3-6 weeks, depending on the SWI at the start of the forecast. When our new SWI was applied in post-processing the ECMWF's 2-week mean temperature reforecasts for weeks 3-4 and 5-6 in northern Europe during boreal winter, the skill scores of those weeks were slightly improved. This indicates there is some room for improving the extended-range forecasts, if the stratosphere-troposphere links were better captured in the modelling. In addition to this, we found that during the boreal winter, in cases where the polar vortex was weak at the start of the forecast, the mean skill scores of the 3-6 weeks' surface temperature forecasts were higher than average.Peer reviewe
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