European Extratropical Cyclones

IMPLICATIONS FOR LOCAL INSURERS

0 European Extratropical Cyclones | TransRe White Paper by James Rohman | May 2014

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Introduction " 1" 2 This paper highlights the climatology, characteristics, history and climate change projections of European Extratropical Cyclones. " January 2014" was the fourth warmest since record keeping began in 1880, yet it fell in the middle of the most extreme winter Europe (and the Northeast US) has experienced in 15 years. A nexus of storms brought high winds,

heavy rainfall and severe snowstorms for almost five months, causing flooding and damage to life onshore and at sea.

Such prodigious weather patterns indicate equally prodigious climate

disturbances. Many of those storms formed as Extratropical Cyclone s (ETC). The first3 section of this paper discusses the large-scale climate 4

variations and indices that encourage and energize ETC development.

Every year, roughly" 200 atmospheric disturbances form in the North 3 4" Atlantic. A handful of the disturbances, which develop as cold-core and 4

asymmetric cyclones, morph into organized storms and track east towards Europe. The second section summarizes the characteristics and dynamics " of these pandemonium-inducing windstorms. "

On average, European ETCs create $1.2 billion of insured loss per year. In extreme years with multiple tail events, they can be the leading cause of

global storm losses; in 1990 and 1999, they caused $18 billion and $14 billion of insured losses, respectively. The 2014 season is expected to exceed $5.5 billion in insured loss. Section three covers the finer points of the European ETC archival record. 5 6 ETCs that form in the North Atlantic are among nature’s most powerful cyclones on earth, deriving their energy from the interaction and difference between cold Arctic air masses and warm subtropical air masses. ETCs can 5" 6" produce storms with central low pressure as low as 916 mb, winds up 6

to 140 mph and wave heights over 100 feet. Given the recent record of IPCC reports and extreme weather, the final section offers predictions for

ETC development" and tracking. "

The disruption and loss to everyday life from extreme weather is calamitous. As the science of climatology continues to evolve, TransRe

strives to inform the insurance industry of advancements in the field. Our efforts come with the hope that property damage, loss of life and societal

interferences will diminish with greater knowledge of extreme natural Figurcatastrophes.e 1. An extratrop ical disturbance turns into an organized storm in the eastern North Atlantic. Over February 12-14, 2014, a series of storms struck the coastline of northe rn Europe, causing widespread flood and damage, resulting in approximate insured losses of $1.3B to residences, businesses and infrastructure. Fiigurre 1.. An exttrrattrropiicall diistturrbance tturrns iintto an orrganiized sttorrm iin tthe eastterrn N orrtth Atltlantitic.. Overr Febrruarry 12--14,, 2014,, a serriies off sttorrms sttrruck tthe coasttliline off norrttherrn Eurrope,, causiing wiidesprread flflood and damage,, rresultilting iin apprroxiimatte iinsurred llosses off $1..3B tto rresiidences,, busiinesses and iinffrrasttrructturre..

#! European Extratropical Cyclones | TransRe White Pape! r 3 European Extratropical Cyclones | TransRe White Paper 1 #! Eurropean Exttrrattrropiicall Cycllones || TrransRe Whitite Pape!! rr

Executive Summary

• Extratropical Cyclones (ETC) that form in the North Atlantic are, on average, the second most destructive insured natural catastrophe worldwide (after Atlantic hurricanes).

• They can generate winds up to 140mph and waves over 100ft.

• The wind-fields range from 120-1200 miles, with the spread of most damaging winds 50-150 miles from the storm center.

• On average, 200 ETCs form each year.

• They derive their energy from the horizontal temperature difference between warm, subtropical air masses and cold, polar air masses.

• The two most consistent factors driving European ETCs are the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO).

• AO and NAO shape the jet stream, funnel ETCs into various parts of Europe.

o Positive AO/NAO is associated with storms that have a west-southwest to east-northeast track across Europe.

o Negative AO/NAO is linked to a more south-to-north track.

o NAO positive (negative) years are associated with a 10- 15% increase (decrease) in losses during winter storm seasons.

• AO/NAO variation is linked to the dynamics of the Madden-Julian Oscillation (MJO), a global tropical weather anomaly.

• Climate projections indicate an increase in ETC wind gusts from the eastern North Atlantic Ocean into Northern and Central Europe.

• Three futuristic climate simulations show ETC frequency is expected to increase over many parts of Europe, from Scandinavia to the Adriatic, due to climate change.

• In addition to land based losses, ETCs will impact marine exposures. Post-Panamax containerships are more prone to a phenomenon known as ‘parametric rolling’ than smaller vessels due to hull shape.

• TransRe is working to support insurers to better understand the causes and implications of European Extratropical Cyclones.

2 European Extratropical Cyclones | TransRe White Paper 2

Introduction Introduction This paper highlights the climatology, characteristics, history and Thisclimate paper changehighlights projections the clima oftology, European characteristics, Extratropical history Cyclones. and climate change projections of European Extratropical Cyclones. January 2014 was the fourth warmest since record keeping began in 1880, Januaryyet it 2014fell in wathes middlethe fourth of the warmest most extreme since record winter keeping Europe began (and the in 1880, Northeast yet US)it fell h inas the experienced middle of thein 15 most years. extreme A nexus winter of storms Europe brought (and the high Northeast winds, US)heavy has experienced rainfall and insevere 15 years. snowstorms A nexus for of almoststorms fivebrought months, high causing winds, heavyflooding rainfall and and damage severe tosnowstorms life onshore for and almost at sea. five months, causing flooding and damage to life onshore and at sea. Such prodigious weather patterns indicate equally prodigious climate Suchdisturbances. prodigious weather Many of patterns those storms indicate formed equally as prodigious Extratropical climate Cyclone s disturbances.(ETC). The Many first section of those of thisstorms paper formed discusses as Extratropical the large-scale Cyclone climates (ETC)variations. The first and section indices of thatthis encouragepaper discusses and energize the large ETC-scale development. climate variations and indices that encourage and energize ETC development. Every year, roughly 200 atmospheric disturbances form in the North EveryAtlantic. year, roughlyA handful 200 of theatmospheric disturbances disturbance, which develops form as in cold the-core North and Atlantic.asymmetric A handful cyclones, of the morphdisturbances into organized, which develop storms andas cold track-core east and towards asymmetricEurope. Thecyclones, second morph section into summarizes organized storms the characteristics and track east and towards dynamics Europe.of these The pandemonium second section-inducing summarizes windstorms the characteristics. and dynamics of these pandemonium-inducing windstorms. On average, European ETCs create $1.2 billion of insured loss per year. On Iaverage,n extreme European years with ETCs multiple create tail events, $1.2 billion they canof insured be the leading loss per cause year of. In extremeglobal storm years losses; with multiple in 1990 tail and events, 1999, they they can caused be the $18 leading billion causeand $14 of globalbillion storm of insuredlosses; lossesin 1990, respectively.and 1999, they The caused 2014 season$18 billion is expectedand $14 to billionexceed of insured $5.5 billionlosses, inrespectively. insured loss. The Section 2014 season three covers is expected the finer to points of exceedthe European $5.5 billion ETC in archivalinsured record loss. .Section three covers the finer points of the European ETC archival record. ETCs that form in the North Atlantic are among nature’s most powerful ETCscyclones that form on inearth the, Northderiving Atlantic their energyare among from nature’s the interaction most powerful and difference cyclonesbetween on earthcold Arctic, deriving air masses their energy and wfromarm thesubtropical interaction air andmasses. difference ETCs can betweenproduce cold storms Arctic air with masses central and low warm pressure subtropical as low air asmasses. 916 mb, ETC windss can up produceto 140 storms mph and with wave central heights low pressureover 100 feet.as low Given as 916 the mb,recent winds record up of to 140IPCC mph reports and andwave extreme heights weather, over 100 the feet. final Given section the offers recent predictions record of for IPCCETC reports development and extreme and trackingweather,. the final section offers predictions for ETC development and tracking. The disruption and loss to everyday life from extreme weather is Thecalamitous. disruption andAs the loss science to everyday of climatology life from continues extreme to weather evolve, TransReis calamitous.strives to Asinform the thescience insurance of climatology industry ofcontinues advancements to evolve, in theTransRe field. Our strivesefforts to inform come withthe insurance the hope thatindustr propertyy of advancements damage, loss in of the life field. and societalOur effortsinterferences come with willthe diminishhope that with property greater damage, knowledge loss of of extreme life and societalnatural interferencescatastrophes. will diminish with greater knowledge of extreme natural catastrophes.

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The Science of European Extratropical The Science of European Extratropical Cyclones Cyclones Climatology Climatology The major factors driving European ETCs are the Arctic Oscillation and The major factors driving European ETCs are the Arctic Oscillation and North Atlantic Oscillation, two climate indices for the Northern Hemisphere. North Atlantic Oscillation, two climate indices for the Northern Hemisphere. Arctic Oscillation (AO) Arctic Oscillation (AO) The Arctic is home to a semi-permanent low pressure circulation The Arctic is home to a semi-permanent low pressure circulation known as the ‘Polar Vortex’, which rose to prominence in the first quarter known as the ‘Polar Vortex’, which rose to prominence in the first quarter of 2014. The Vortex is in constant opposition to (and therefore represents of 2014. The Vortex is in constant opposition to (and therefore represents opposing pressure to) the weather patterns of the northern middle latitudes opposing pressure to) the weather patterns of the northern middle latitudes (i.e. Northern Europe, Northern Asia and northern North America). (i.e. Northern Europe, Northern Asia and northern North America). AO measures the variation in the strength, intensity and size of the jet AO measures the variation in the strength, intensity and size of the jet stream as it expands and contracts to alter the shape of the fast-flowing stream as it expands and contracts to alter the shape of the fast-flowing air current. AO is measured by opposing sea-pressure anomalies, ranging air current. AO is measured by opposing seath-pressure anomalies, ranging from the Arctic Circle approximately to the 38 parallel. from the Arctic Circle approximately to the 38th parallel. During the ‘negative phase’ of AO, sea-level pressure is high in the During the ‘negative phase’ of AO, sea-level pressure is high in the The Arctic Oscillation helps define the Arctic and low in the northern middle latitudes. The ‘positive phase’ of The Arctic Oscillation helps define the Arctic and low in the northern middle latitudes. The ‘positive phase’ of jet stream, which can send cold, polar AO has the exact opposite pattern. jet stream, which can send cold, polar air far south with its large oscillations. AO has the exact opposite pattern. air far south with its large oscillations. This image compares December 2010 Dr. James E. Hansen of NASA describes AO thus: This image compares December 2010 Dr. James E. Hansen of NASA describes AO thus: to the previous 10-year average, "The degree to which Arctic air penetrates into middle latitudes is related to to the previous 10-year average, "The degree to which Arctic air penetrates into middle latitudes is related to showing that AO and the shape of the the AO index, which is defined by surface atmospheric pressure patterns. showing that AO and the shape of the the AO index, which is defined by surface atmospheric pressure patterns. jet stream altered the temperature When the AO index is positive, surface pressure is low in the polar region. jet stream altered the temperature When the AO index is positive, surface pressure is low in the polar region. gradient in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently gradient in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently Arctic air was sent as far south as the from west to east, thus keeping cold Arctic air locked in the polar region. Arctic air was sent as far south as the from west to east, thus keeping cold Arctic air locked in the polar region. Carolinas and Portugal. Red (blue) When the AO index is negative, there tends to be high pressure in the polar Carolinas and Portugal. Red (blue) When the AO index is negative, there tends to be high pressure in the polar indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into (colder) than average. Note how the jet middle latitudes." (colder) than average. Note how the jet stream’s strange shape can juxtapose middle latitudes." stream’s strange shape can juxtapose warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC extended warm and cold fronts. phases, the jet stream tends to be reinforced and prevented from phases, the jet stream tends to be reinforced and prevented from extended warm and cold fronts. shifting. This is an effect called eddy feedback. shifting. This is an effect called eddy feedback. By maintaining the shape and consistency of the jet stream, eddy feedback By maintaining the shape and consistency of the jet stream, eddy feedback increases the propensity for clustering of ETCs. Such clustering increases the propensity for clustering of ETCs. Such clustering patterns were seen during the destructive 1987, 1990, 2000 and 2014 patterns were seen during the destructive 1987, 1990, 2000 and 2014 European ETC seasons. This is a month-to-month phenomenon. It does . This is a month-to-month phenomenon. It does notEuropean persist on ETC a yearly seasons basis. not persist on a yearly basis.

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AO’sThe Effects Science on Northern of European Latitudes Extratropical Cyclones During the positive phase, a strong Polar Vortex confines cold Arctic air acroClimatologyss Polar Regions. A rctic air is kept in the north, resulting in brisk winters for Greenland, northern Scandinavia and Russia, but keeping mosThet of Emajorurop efactors and th drivinge main lEuropeanand US re ETClativesl yare wa therm. Arctic Oscillation and North Atlantic Oscillation, two climate indices for the Northern Hemisphere. Extratropical storms are flung further north. AO positive creates storArcticmy wea tOscillationher in the UK a n(AO)d Sca ndinavia, while Southwest Europe

and Mediterranean countries remai n dry. The Arctic is home to a semi-permanent low pressure circulation known as the ‘Polar Vortex’, which rose to prominence in the first quarter of 2014. The Vortex is in constant opposition to (and therefore represents opposing pressure to) the weather patterns of the northern middle latitudes (i.e. Northern Europe, Northern Asia and northern North America).

Storms are pushed further AO measures the variation in the strength, intensity and size of the jet north and affect the UK and stream as it expands and contracts to alter the shape of the fast-flowing Scandinavia. air current. AO is measured by opposing sea-pressure anomalies, ranging from the Arctic Circle approximately to the 38th parallel.

During the ‘negative phase’ of AO, sea-level pressure is high in the The Arctic Oscillation helps define the Arctic and low in the northern middle latitudes. The ‘positive phase’ of jet stream, which can send cold, polar AO has the exact opposite pattern. air far south with its large oscillations. This image compares December 2010 Dr. James E. Hansen of NASA describes AO thus: Figure 2. Positive phase of Arctic Oscillation. to the previous 10-year average, "The degree to which Arctic air penetrates into middle latitudes is related to showing that AO and the shape of the Duritheng tAOhe nindex,egati vwhiche pha iss edefined, the Po bylar surfaceVortex iatmospherics weakened bpressurey influxe patterns.s of jet stream altered the temperature warmWhen air f rtheom AOSib eindexria. C iso lpositive,d air mig surfacerates s opressureuth with is t hlowe m ine athend polarering region.jet gradient in the Northern Hemisphere. streThisam. helpsArctic the air middle masse latitudes freez ejet o vstreamer No rtoth eblowrn E stronglyurope a nandd N consistentlyorth Arctic air was sent as far south as the Amefromrica west, whi ltoe teast,he M thusedite keepingrranean cold and Ar Socticut hairw elockedst US inex theper ipolarence region. Carolinas and Portugal. Red (blue) storWhenmier wthein tAOers .index is negative, there tends to be high pressure in the polar indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into (colder) than average. Note how the jet middle latitudes." stream’s strange shape can juxtapose warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC extended warm and cold fronts. phases, the jet stream tends to be reinforced and prevented from Storms migrate further south shifting. This is an effect called eddy feedback. and affect southern Europe By maintaining the shape and consistency of the jet stream, eddy feedback and the Mediterranean. increases the propensity for clustering of ETCs. Such clustering patterns were seen during the destructive 1987, 1990, 2000 and 2014 European ETC seasons. This is a month-to-month phenomenon. It does not persist on a yearly basis.

Figure 3. Negative phase of Arctic Oscillation.

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NorthThe ScienceAtlantic Oscillation of European (NAO) Extratropical Cyclones Similar to AO, NAO is comprised of north-south dipole anomalies, but isClimatology centered over the North Atlantic Gyre. The low pressure center is located over Stykkisholmur/Reykjavík Iceland, while the high pressure anomalyThe major is factorscentered driving over theEuropean Ponta Delgada,ETCs are Azores. the Arctic Oscillation and North Atlantic Oscillation, two climate indices for the Northern Hemisphere. It is important to note that the centers of the low and high pressure systems migrateArctic on Oscillation a seasonal and (AO) yearly basis; other locations can also be used to measure the NAO index. The Arctic is home to a semi-permanent low pressure circulation Bothknown phases as the of ‘P NolarAO Vareortex associated’, which rose with to large prominence-scale alterations in the first quarterin strengthof 2014. Theand Vlocationortex is inof constantthe North opposition Atlantic tojet (and stream therefore and subsequent represents stormopposing tracks. pressure They to)also the both weather modulate patterns patterns of the of northern heat and middle moisture latitudes transportation,(i.e. Northern Europe,which affect Northern spatial Asia temperature and northern and North precipitation America). patterns from the Eastern Europe to the coast of North America. AO measures the variation in the strength, intensity and size of the jet Typically,stream as NAO it expands affects and weather contracts patterns to alter across the shape Northern of the Europe, fast-flowing Greenland,air current. AOIceland is measured and Eastern by opposing US/Canada sea-pressure. It also has anomalies, the capability ranging to extendfrom the into Arctic Scandinavia Circle approximately in winter and to S theouthern 38th parallel.Europe over the summer months. During the ‘negative phase’ of AO, sea-level pressure is high in the The Arctic Oscillation helps define the DuringArctic prolongedand low in periods the northern of either middle positiv latitudes.e or negative The NAO, ‘positive anomalous phase’ of jet stream, which can send cold, polar weatherAO has bandsthe exact can oppositebe seen as pattern. far east as central Russia and north Siberia. air far south with its large oscillations. The figure from Willis Re shows the NAO exhibits variability on a seasonal and yearly basis, with prolonged This image compares December 2010 Dr. James E. Hansen of NASA describes AO thus: effect of positive and negative NAO periods of either phase dominating Atlantic weather patterns. From 1954- to the previous 10-year average, phases on European windstorm 1978,"The degreea negative to which NAO Arctic dominated air penetrates Atlantic into circulation middle latitudes patterns is relatedfor over to showing that AO and the shape of the losses. NAO’s strong influence on the 75%the AO of theindex, time which. is defined by surface atmospheric pressure patterns. jetpath stream of ETCs altered has the a significant temperature effect When the AO index is positive, surface pressure is low in the polar region. gradienton loss structurein the Northern and distribution Hemisphere. in DuringThis helps the 24the-year middle period, latitude the jetnegative stream NAO to blow dominated strongly for and at consistentlyleast three ArcticEurope, air withwas asent 10- as15% far increase south as the yearsfrom weststraight, to east, with thuslittle keepingto no presence cold Ar cticof a airpositive locked phase. in the NAOpolar positiveregion. did Carolinas(decrease) and in aggregatePortugal. Red losses (blue) during notWhen appear the AOas aindex season is negative,al mean in there consecutive tends to years.be high pressure in the polar indiNAOcates positive areas (negative) that are warmer seasons. region, weaker zonal winds, and greater movement of frigid polar air into The winter of 2013-14 has had an overwhelmingly positive NAO index. (colder) than average. Note how the jet middle latitudes." stream’s strange shape can juxtapose This provided more energy in the North Atlantic for intense and warm and cold weather anomalies, clusteredAO has a dominantstorms, which role in helps determining explain thethe Nordicshape ofStorm the jetSeries stream, (November which in th th with less than 50 miles between 13turn to play Decembers a crucial 19 role, 2013) in steering and the European almost continuous ETCs. During series active of storms ETC th th extended warm and cold fronts. fromphases December, the jet 17 stream, 2013 tends through to be February reinforced 20 , and2014 prevented across Europe. from shifting. This is an effect called eddy feedback. Though they are two different climate indices, NAO and AO ultimately measureBy maintaining the same the shapephenomenon. and consistency They both of thecalculate jet stream, large eddy-scale feedback differencesincreases inthe sea propensity-level pressure for clustering in the Northern of ETCs. Hemisphere, Such clustering and the ramificationspatterns were for seentemperature during gradientsthe destructive and sto 1987,rm tracks 1990 between, 2000 and Europe 2014 andEuropean North America ETC seasons. . This is a month-to-month phenomenon. It does not persist on a yearly basis.

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The Science of European Extratropical Positive NAO Cyclones When NAO is in its positive phase, the Icelandic low and Azores high are strengthened.Climatology Heightened pressure systems lead to an increased pressure gradient across the North Atlantic Gyre. Because large barometric The major factors driving European ETCs are the Arctic Oscillation and differentials increase wind speeds between areas of high and low pressure, North Atlantic Oscillation, two climate indices for the Northern Hemisphere. Westerly winds intensify across the North Atlantic. This has the overall effectArctic of heavy Oscillation winds flowing (AO) into Europe.

The Arctic is home to a semi-permanent low pressure circulation known as the ‘Polar Vortex’, which rose to prominence in the first quarter of 2014. The Vortex is in constant opposition to (and therefore represents opposing pressure to) the weather patterns of the northern middle latitudes (i.e. Northern Europe, Northern Asia and northern North America). Just like a positive AO, positive NAO sends intense AO measures the variation in the strength, intensity and size of the jet storms in to North Europe stream as it expands and contracts to alter the shape of the fast-flowing and UK. air current. AO is measured by opposing sea-pressure anomalies, ranging from the Arctic Circle approximately to the 38th parallel.

During the ‘negative phase’ of AO, sea-level pressure is high in the The Arctic Oscillation helps define the Arctic and low in the northern middle latitudes. The ‘positive phase’ of jet stream, which can send cold, polar AO has the exact opposite pattern. air far south with its large oscillations. This image compares December 2010 Dr. James E. HansenFigure 4. of Positive NASA phasedescribes of North AO Atlanticthus: Oscillation. to the previous 10-year average, Negative"The degree NAOto which Arctic air penetrates into middle latitudes is related to showing that AO and the shape of the the AO index, which is defined by surface atmospheric pressure patterns. jet stream altered the temperature NegativeWhen the NAO AO indexis the isexact positive, opposite surface of positivepressure NAO. is low In in the the negative polar region. phase , gradient in the Northern Hemisphere. theThis difference helps the between middle latitude the Azores jet stream high andto blow Icelandic strongly low and lessen consistentlys, Arctic air was sent as far south as the decreasingfrom west to the east, sea thus pressure keeping gradient cold Arctic. The air lockedslackened in the gradie polarnt region. results in Carolinas and Portugal. Red (blue) weakerWhen the Westerlies AO index andis negative, less wind there in tendsEurope to .be high pressure in the polar indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into (colder) than average. Note how the jet middle latitudes." stream’s strange shape can juxtapose warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC phases, the jet stream tends to be reinforced and prevented from extendedThe negative warm and NAO cold keepsfronts. the shifting. This is an effect called eddy feedback. UK and North Europe dry and sends storms in to the By maintaining the shape and consistency of the jet stream, eddy feedback Mediterranean region. increases the propensity for clustering of ETCs. Such clustering patterns were seen during the destructive 1987, 1990, 2000 and 2014 European ETC seasons. This is a month-to-month phenomenon. It does not persist on a yearly basis.

Figure 5. Negative phase of North Atlantic Oscillation.

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RelationshipThe Science between of European Madden-Julian Extratropical Oscillation (MJO)Cyclones and NAO /AO AClimatology 2008 study by Hai Lin, Gilbert Brunet and Jacques Derome of Environment Canada and McGill University found an observed connection betweenThe major the factors NAO driving and M Europeanadden-Julian ETC Os scillationare the Arctic (MJO) Oscillation. and North Atlantic Oscillation, two climate indices for the Northern Hemisphere. MJO is a band of low-pressure that develops in the Indian Ocean before propagatingArctic Oscillation east as an anomalous (AO) and convective weather band. It dumps rain on Southeast Asia and Australia, continues east towards the Pacific, andThe ultimatelyArctic is homeimpacts to the a semi Atlantic-permanent Ocean before low pressure dissipating circulation near Africa . known as the ‘Polar Vortex’, which rose to prominence in the first quarter MJOof 2014. is associated The Vortex withis in constantlarge-scale opposition variations to (and in atmospheric/oceanic therefore represents indiceopposings, affecting pressure drought, to) the weather monsoon patterns and tropical of the cyclone northern patterns middle latitudesin every ocean(i.e. Northern basin. MJOEurope, can Northern also affect Asia ETC ands northernthrough Northa complex America). interaction with NAO and AO. AO measures the variation in the strength, intensity and size of the jet Thestream authors as it ofexpands the 2008 and study contracts found a tolong alter-distance the shape connection of the fast between-flowing tropicalair current. convection AO is measured of MJO and by opposingthe Northern sea -Hemisphere’spressure anomalies, NAO. When ranging MJOfrom theis in Arctic Phases Circle 2- 4,approximately NAO has a significantto the 38th parallel. positive amplification about 5-15 days after depressed convection of MJO reaches the tropicalDuring the central ‘negative Pacific phase’. of AO, sea-level pressure is high in the The Arctic Oscillation helps define the Arctic and low in the northern middle latitudes. The ‘positive phase’ of jet stream, which can send cold, polar Conversely,AO has the wexacthen MJOopposite is in pattern.Phases 6-8, there is a significant negative airMJO far south is a slowwith- itsmoving large atmosphericoscillations. amplification roughly 5-15 days after enhanced convection of MJO Dr. James E. Hansen of NASA describes AO thus: Thispattern image that compares impacts December global tropical 2010 reaches the tropical central Pacific. to the previous 10-year average, and sub-tropical weather. It can be MJO"The degreealso has to a which strong Arctic relationship air penetrates with AO. into When middle MJO latitudes related is related to showing that AO and the shape of the tracked through NOAA’s dynamic convectionthe AO index, is which enhanced is defined (depressed) by surface over atmospheric the Indian pressure Ocean, patterns.AO tends jet stream altered the temperature MJO forecast. toWhen favor the its AO positive index is (ne positive,gative) surface phase. pressure is low in the polar region. gradient in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently ArcticAbove, air was the forecastsent as farindex south is dividedas the Asfrom NAO west and to east,AO are thus intrinsically keeping cold related, Arctic we air see locked how inthe the tropical polar MJOregion. Carolinasinto octants, and Portugal. with each Red eighth (blue) of the influencesWhen the AONorth index Atlantic is negative, winter weather. there tends The to results be high indicate pressure that in MJO the polar indiindexcates corresponding areas that are to warmer a different forcesregion, haveweaker a directzonal winds,influence and ongreater the Atlantic movement sector’s of frigid NAO polar and air AOinto. (colder)global than tropical average. basin. Note how the jet middle latitudes." stream’s strange shape can juxtapose An additional strong connection has been observed between MJO and warmWhen and the cold index weather is within anomalies, the center theAO hasNorthern a dominant Hemisphere’s role in determining winter stratospheric the shape of Ptheolar jet V stream,ortex. Large which in withcircle, less MJOthan 50is considered miles between weak and variationsturn plays a in crucial the jet role stream in steering and sudden European warming ETCs. Duringevents activetend to ETC follow extendeddifficult warmto read. and MJO cold becomes fronts. certainphases ,MJO the jetphases stream. This tends provides to be furtherreinforced for support and prevented for the complex from and stronger as the index moves further importantshifting. Thislink between is an effect global called tropical eddy weather feedback. and North Atlantic winter from the center. storms. By maintaining the shape and consistency of the jet stream, eddy feedback Dueincreases to the observedthe propensity relationships for clustering between of MJO ETCs. con Suchvection clustering and NAO /AO behavior,patterns werethere seen exists during the potential the destructive to create 1987, medium 1990 to, 2000 long and range 2014 forecastsEuropean forETC NAO seasons and AO. This, given is a monthaccurate-to- monthpredicti phonsenomenon. of MJO Itphase does andnot persist future onmovements a yearly basis.. A predictive method of this sort would allow for more detailed and far- sighted forecasts for ETCs.

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The Science of European Extratropical Characteristics of Extratropical Cyclones CharacteristicsCyclones of Extratropical Cyclones Dynamics DynamicsClimatology Every year, approximately 200 extratropical disturbances form in the North Every year, approximately 200 extratropical disturbances form in the North EveryAtlantic.The year, major They approximately factors derive driving their 200 energyEuropean extratropical from ETC thes disturbances arehorizontal the Arctic temperature form Oscillation in the North and Atlantic. They derive their energy from the horizontal temperature Atlantic.differentialNorth TheyAtlantic between derive Oscillation their warm, energy, two subtropical climate from indicesthe air horizontal masses for the and Northern temperature cold, Hemisphere.Arctic air differential between warm, subtropical air masses and cold, Arctic air differentialmasses. Most between storms warm, develop subtropical during the airwinter masses months, and when cold, the Arctic air massesArctic. Most Oscillation storms develop (AO) during the winter months, when the massestemperature. Most differential storms develop between during the tropicsthe winter and months, poles are when the greatest.the temperature differential between the tropics and poles are the greatest. The Arctic is home to a semi-permanent low pressure circulation Cold core disturbances that organize into larger storms and track east We can see the importance of Coldknown core asdisturbances the ‘Polar V thatortex organize’, which roseinto tolarger prominence storms inand the track first quartereast We can see the importance of are steered by the jet stream. With its undulating movements and an in-depth knowledge of the areof steered 2014. The by theVortex jet streamis in constant. With itsopposition undulating to (andmovements therefore and represents an in-depth knowledge of the sometimes erratic oscillations, the jet stream’s shape helps define storm science and workings of the sometimesopposing erratic pressure oscillations, to) the weather the jet stream’spatterns ofshape the northernhelps define middle storm latitudes science and workings of the track. Because the AO and NAO help define the extremes of the NAO and AO indexes, which track.(i.e. Because Northern the Europe, AO and Northern NAO help Asia define and northern the extremes North of America). the NAO and AO indexes, which eccentricities in the jet stream, a refined understanding of the development track that exact temperature and eccentricities in the jet stream, a refined understanding of the development track that exact temperature and of AOAO /NAOmeasures is imperative the variation to understand in the strength, both past intensity and future and trends. size of the jet pressure gradient, as well as the of AO/NAO is imperative to understand both past and future trends. pressure gradient, as well as the stream as it expands and contracts to alter the shape of the fast-flowing presshapesure and gradient, location as of well the asNorth the Storms that affect Europe have tracked across the North Atlantic with the shape and location of the North Stormsair current. that affect AO isEurope measured have by tracked opposing across sea the-pressure North Atlantic anomalies, with rangingthe Atlantic jet stream. sinusoidal, upper-atmospheric jet stream. With anth average forward motion of Atlantic jet stream. sinusoidalfrom the, upperArctic- atmosphericCircle approximately jet stream. to theWith 38 an parallel.average forward motion of sinusoidal35mph, an, upper ETC’-satmospheric asymmetrical jet stream.windfield With creates an average the greatest forward swath motion of of 35mph, an ETC’s asymmetrical windfield creates the greatest swath of damaDuringge along the ‘negative its southeast phase’ quadrant of AO, sea near-level the pressurefrontal wave is high. in the damage along its southeast quadrant near the frontal wave. The Arctic Oscillation helps define the Arctic and low in the northern middle latitudes. The ‘positive phase’ of European ETCs can, at times, produce gusts in excess of 150 mph. jet stream, which can send cold, polar EuropeanAO has ETCsthe exact can, opposite at times, pattern.produce gusts in excess of 150 mph. EuropeanThese storms, ETCs (which can, atinclude) times, Xynthia produce or gustsKyrill, experiencedin excess of rapid 150 mph. air far south with its large oscillations. These storms, (which include) Xynthia or Kyrill, experienced rapid Theseintensification storms, (whichafter interacting include) Xynthiawith a trough or Kyrill of, lowexperienced pressure rapidhigh in the This image compares December 2010 intensificationDr. James E.after Hansen interacting of NASA with describes a trough ofAO low thus: pressure high in the intensificationatmosphere. after interacting with a trough of low pressure high in the atmosphere. to the previous 10-year average, atmosphere."The degree to which Arctic air penetrates into middle latitudes is related to showing that AO and the shape of the the AO index, which is defined by surface atmospheric pressure patterns. Phasejet stream I- A alteredperturbation the temperature forms Phase I- A perturbation forms When the AO index is positive, surface pressure is low in the polar region. Phasealonggradient aI- baroclinic Ain perturbation the Northern zone formsbetween Hemisphere. along a baroclinic zone between This helps the middle latitude jet stream to blow strongly and consistently lowArctic and air high was pressure sent as farsystems. south as the low and high pressure systems. from west to east, thus keeping cold Arctic air locked in the polar region. Carolinas and Portugal. Red (blue) Phase II- The wave amplifies, When the AO index is negative, there tends to be high pressure in the polar Phaseindicates II- The areas wave that amplifies, are warmer Phasescale contractsII- The wave and amplifies,the fronts of region, weaker zonal winds, and greater movement of frigid polar air into scale(colder) contracts than average. and the Notefronts how of the jet middle latitudes." the storm form. thestream’s storm strangeform. shape can juxtapose

Phasewarm andIII-The cold fronts weather ‘T-bone’ anomalies, and AO has a dominant role in determining the shape of the jet stream, which in Phase III-The fronts ‘T-bone’ and Phasewaveswith less IIIamplify.- Thethan fronts 50 Storm miles ‘T -forcebone’ between winds and turn plays a crucial role in steering European ETCs. During active ETC waves amplify. Storm force winds wavesformextended between amplify. warm theStorm and warm cold force and fronts. winds phases, the jet stream tends to be reinforced and prevented from form between the warm and shifting. This is an effect called eddy feedback. occluded fronts. occluded fronts. Phase IV-Full seclusion of the By maintaining the shape and consistency of the jet stream, eddy feedback Phase IV-Full seclusion of the Phasewarm front.IV-Full Storm seclusion and hurricane of the increases the propensity for clustering of ETCs. Such clustering warm front. Storm and hurricane patterns were seen during the destructive 1987, 1990, 2000 and 2014 force winds found on cold side of force winds found on cold side of European ETC seasons. This is a month-to-month phenomenon. It does the occluded front. the occluded front. not persist on a yearly basis. the occluded front.

Figure 6. The Shapiro-Keyser Extratropical Cyclone Model, developed in 1990 shows Figure 6. The Shapiro-Keyser Extratropical Cyclone Model, developed in 1990 shows Figurethe life -6.cycle The of Shapiro an ETC.-Keyser Extratropical Cyclone Model, developed in 1990 shows the life-cycle of an ETC. 4 9 9 European Extratropical Cyclones | TransRe White Paper 9 European Extratropical Cyclones | TransRe White Paper 9

European Extratropical Cyclones | TransRe White Paper

Structure,The Science Shape and of EuropeanSize Extratropical Cyclones ETCs differ in structure, shape and size from Atlantic, east Pacific, westClimatology Pacific and Indian Ocean hurricanes/typhoons. The latter are warm core tropical cyclones, deriving their energy from a vertical temperature differentiThe majoral between factors the driving upper European and lower ETC atmosphere.s are the Arctic Oscillation and North Atlantic Oscillation, two climate indices for the Northern Hemisphere. Because ETCs are fueled by a horizontal temperature differential, they do notArctic lose strength Oscillation as they track (AO) over land, unlike tropical cyclones. ETCs keep their energy throughout landfall, affecting a single location anywhereThe Arctic between is home 6 and to 24 a semihours.-permanent low pressure circulation known as the ‘Polar Vortex’, which rose to prominence in the first quarter Anotherof 2014. difference The Vortex between is in constant ETCs and opposition tropical to cyclones (and therefore is the represents distinct frontsopposing. The commapressure-shaped to) the weatherappearance patterns of ETCs of the identifies northern middlethe location latitudes of warm(i.e. Northern and cold Europe, air masses Northern that Asia feed and the northern circulation North of America). the storm . The distinct warm and cold fronts pack the greatest punch, with the highest surfaceAO measures winds and the heaviest variation precipitation in the strength, along their intensity boundaries. and size of the jet stream as it expands and contracts to alter the shape of the fast-flowing ETCsair current.become AOasymmetric is measured when by mature opposing because sea-pressure the tropical anomalies, and polar ranging air massesfrom thethat Arctic provide Circle the approximatelyenergy are not touniform the 38 inth parallel.temperature and pressure. Full-blown storms vary greatly in size, with diameters ranging fromDuring 120-1200 the ‘negative miles. The phase’ spread of ofAO, most sea -damaginglevel pressure winds is tends high into the stay The Arctic Oscillation helps define the withinArctic 50 -and150 lowmiles in ofthe the northern storm center. middle latitudes. The ‘positive phase’ of jet stream, which can send cold, polar AO has the exact opposite pattern. air far south with its large oscillations. This image compares December 2010 Dr. James E. Hansen of NASA describes AO thus:

to the previous 10-year average, "The degree to which Arctic air penetrates into middle latitudes is related to showing that AO and the shape of the the AO index, which is defined by surface atmospheric pressure patterns. jet stream altered the temperature When the AO index is positive, surface pressure is low in the polar region.

gradient in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently Arctic air was sent as far south as the from west to east, thus keeping cold Arctic air locked in the polar region. Carolinas and Portugal. Red (blue) When the AO index is negative, there tends to be high pressure in the polar indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into (colder) than average. Note how the jet middle latitudes." stream’s strange shape can juxtapose warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in

with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC extended warm and cold fronts. phases, the jet stream tends to be reinforced and prevented from shifting. This is an effect called eddy feedback.

By maintaining the shape and consistency of the jet stream, eddy feedback increases the propensity for clustering of ETCs. Such clustering patterns were seen during the destructive 1987, 1990, 2000 and 2014 FigureEuropean 7. This showsETC seasons the windstorm. This footprints is a month of Xynthia-to-month (Feb ph 2010)enomenon. on the left It anddoes Kyrillnot (Jan persist 2007) on on a the yearly right. basis.Although each storm had a slightly different storm track, one can see how the storms did not lose energy over land and were able to maintain strength deep into the European continent. Xynthia traveled over 1700 miles into Europe, Kyrill covered over 1800 miles.

4 10 European Extratropical Cyclones | TransRe White Paper 10 European Extratropical Cyclones | TransRe White Paper

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! TheHist oSciencerical Re cofo rEuropeand Extratropical Cyclones Climate Indices Chosen: An inherent necessity of any Chief Risk Officer or Chief Catastrophe Officer Climatologyis the ability to minim ize risk, maximize opportunity and maintain business AAO- Antarctic Oscillation cHonitsintuoityr.i cUandle Rrstaencdiongr dlo ss history and large-scale climate indices plays AO- Arctic Oscillation Thean i mmajorporta factorsnt role idrivingn the a nEuropeanalysis an dETC calcsu arelatio then o fArctic busin Oscillationess opport uandnitie s. Climate Indices Chosen: NorthAn inh Atlanticerent ne cOscillationessity of an,y two Chi eclimatef Risk O indicesfficer or Cforhi ethef C aNortherntastrophe Hemisphere.Officer AMM- Atlantic Meridianal Mode is the ability to minimize risk, m aximize opportunity and maintain business - Antarctic Oscillation Accuweather Study AAO Arcticcontinuity .Oscillation Understanding lo (AO)ss histo ry and large-scale climate indices plays AMOA-O A-t lAarnctic OMsuclitlila-Dtioenc adal Oscillation Iann Aimppriol r2ta0n1t4 r,o lDe ainn t hKeo attnlaolwyssisk ai nodf cAaclccuulawtioena tohf ebru scinoemssp loeptpeodr tuan sittieusd. y on our Thebeh aArcticlf on t hise homegenes tois ,a d semievelo-ppermanentment, inte nlowsity pressure, track an circulationd clusterin g of MEI-A MMuMlt-i vAatlraiantteic EMNerSidOia nInadl eMxo de Accuweather Study knownEurope asan theET C‘Psolar usin Vgortex vario’,u whichs clim roseate i ntod prominenceices and sig inn athels. firstThe quarterstudy is - Ma-d Adtelann Jticu lMiaunl tiO-Dseccilaladtaiol On scillation MJOAMO ofiInn t 2014.eAnpdrile 2d 0The t1o4 ,s DVhoaortexnw Khoo tiswtlo inrwe asconstantdkie orsf Amcicg oppositionuhwt uesaeth celri mc otoamt e(andp ilnedtei cdthereforee as satnud yo otrepresentshne oru sr ignals to NAOM- ENIo- rMthu lAtivtalarniattiec EONsScOill aIntidoenx opposingabnehtiaclifp oante pressuret haec tgiveen eEs uto)isro, pdtheeave nweatherl oEpTmCe nset ,patternsa insotennss.i ty of, t rtheack northernand clus tmiddleering o flatitudes (i.e.Eur oNorthernpean ET CEurope,s using vNorthernarious c liAsiamate and ind icnorthernes and s Northignals .America). The study is MJO- Madden Julian Oscillation ONI- Oceanic Nino Index Tinhten sdteudd tyo fsohcouws ehdo wo nre 1ad3e hrsi gmhig-ihmt upsaec ct lismtoatrem insd bicetsw aenedn o t1h9e5r 3si gannadls 2to0 13. NAO- North Atlantic Oscillation AOaVnat rimeasurescioipuaste c alicmtiavet ethe E inudr ovariationicpeasn ( tEhToCs e ins eu athesseodn strength,sm. ost ofte nintensity by and size of the jet PDO- Pacific Decadal Oscillation Accuweather to assess streamfuture w asea tith eexpandsr trends) andwere contracts evaluated tofo ralter eac hthe st oshaperm. L oofn gthete fastrm a-flowingnd sho rt ONI- Oceanic Nino Index The study focused on 13 high-impact storms between 1953 and 2013. SOI- Southern Oscillation Index airtVea rcurrent.rmio utsre cnlimd AOsa twe ieisnrd emeasuredic iedse (nthtiofiseed .uby sWe opposingdh imleo ssot moftee sea no fb ty-h pressureAec icnudwiceeast hanomalies,esrh toow aesdse lsitst lranging e - Pacific Decadal Oscillation th PDO fromor no the pre Arcticdictiv eCircle skill, oapproximatelythers did. to the 38 parallel. QBO- Quasi-Biennial Oscillation future weather trends) were evaluated for each storm. Long term and short SOI- Southern Oscillation Index term trends were identified. While some of the indices showed little ! DuringToar bnloe p1r .ethe Sdeicv t‘negativeeivne cslkimilla, toet hi nphase’edrsic edsid a. r ofe li sAO,ted tseahat f-olevelretell a pressurective ETC sise ahighsons ainn dthe eve nts. ClimQaBteO V- aQruiasbil-eBsie nCnhiaol sOesnc:il lation The Arctic Oscillation helps define the ArcticStorms andare l ilowsted inby theinsu rnortherned loss, in dmiddleexed to Ulatitudes.SD 2012. IThensure ‘positived loss data phase’ for the of jet stream,! which can send cold, polar AO1T9a5b 3lhase f1lo. o Sthede vise nuexact nclaimvatiel aopposite ibnldei;c easn aeres tlpattern.iimsteadte t hwaat sf o prertoevlli daectdiv eb yE TthCe sUeaKs oMnest aOndff iecvee innt sa. 60 year 850mCblim Zaotnea Vl aWriianbdle Asn Cohmoasleyn : air far south with its large oscillations. rSettorromssp eacreti vliest esdtu bdyy i.n Tsuhree d1 9lo5s3s, sintodremxe wd otou UldS hDa 2v0e1 l2ik. eInlys utorepdp leodss D daartiaa foinr tah em odern 300m85b0 Zmobn Zaol nWali nWdi nAdn Aonmoamlya ly Dr.195 James3 flood is E. un aHansenvailable; aofn eNASAstimate wdescribesas provided AO by t hthus:e UK Met Office in a 60 year This image compares December 2010 climactic and economic scenario. ‘X’ means no data available, ‘POS’ means positive retrospective study. The 1953 storm would have likely topped Daria in a modern toP therec 3previousip00itmabl Zeo Wn10aal- tWyeareri nAdn Aaverage,onmomalayl y phase, ‘NEG’ means negative phase, ‘NEU’ means a Neutral ENSO, ‘LA’ means La "Theclima cdegreetic and e ctoon whichomic sc eArcticnario. ‘Xair’ m penetrateseans no data aintovaila middleble, ‘POS latitudes’ means po issit irelatedve to showing that AO and the shape of the Nina, ‘EL means El Nino, ‘MOD’ means moderate MJO, ‘WK’ means weak MJO, SurfPacreec iPprietasbsluer We aAtneor mAnaolmy aly thepha AOse, ‘ Nindex,EG’ me whichans neg aisti vdefinede phase, ‘byNE Usurface’ means aatmospheric Neutral ENSO ,pressure ‘LA’ means patterns. La jet stream altered the temperature ‘NSiTnaG, ’‘ EmLe maneasn sst rEoln Ngi nMo,J ‘OM.O! D’ means moderate MJO, ‘WK’ means weak MJO, Surface Pressure Anomaly When the AO index is positive, surface pressure is low in the polar region. gradient! in the Northern Hemisphere. ‘STG’ means strong MJO.! Storms Chosen: This helps the middle latitude jet stream to blow strongly and consistently Arctic !air was sent as far south as the Storms Chosen: from west to east, thus keeping cold Arctic air locked in the polar region. Carolinas1953 N S andea F Portugal.lood- Ja nRed 195 (blue)3 When the AO index is negative, there tends to be high pressure in the polar indicates195 3areas N Se athat Flo oared- Jwarmeran 1953 Capella (Jan 1976 Gale) - Jan 1976 region, weaker zonal winds, and greater movement of frigid polar air into (colder)Ca thanpella average.(Jan 1976 Note Gale )how - Ja nthe 19 7jet6 middle latitudes." Great Storm of ’87- Oct 1997 stream’sGre strangeat Storm shape of ’87- canOct 1juxtapose997 AO has a dominant role in determining the shape of the jet stream, which in warm and cold weather anomalies, - Jan 1 990 DariaD a(Briau r(Bnsu’r nDsa’ yD aSyt oSrtmor)m) - Jan 1990 with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC extendedViviaVniv- i Fawarmne-b F 1e9b and9 109 9 0cold fronts. phases, the jet stream tends to be reinforced and prevented from shifting. This is an effect called eddy feedback. WiebWkiee-bFkeeb-F 1e9b9 109 90 By maintaining the shape and consistency of the jet stream, LothLaort-h Daer-c D 1e9c9 109 90 eddy feedback increases the propensity for clustering of ETCs. Such clustering AnatAonl-aDtoelc-D 1e9c9 199 99 patterns were seen during the destructive 1987, 1990, 2000 and 2014 Martin- Dec 1999 Martin- Dec 1999 European ETC seasons. This is a month-to-month phenomenon. It does Kyrill- Jan 2007 not persist on a yearly basis. Kyrill- Jan 2007 Klaus- Jan 2009 Klaus- Jan 2009 Xynthia- Feb 2010 Xynthia- Feb 2010 St. Jude- Oct 2013 St. J!ude- Oct 2013 ##! ! European Extratropical Cyclones | TransRe White Pa! per #4# !

EuropeanEuropean Extratropical Extratropica lCyclones Cyclones | | TransRe TransRe White White PaperPa! per 11

For storms with a west-southwest to east-northeast track, at least two ofFor theThe storms QBO, Science withNAO a and west AO-southwest ofindices European are to positiveeast-northeast. StormsExtratropical track, with a at south least-to two- northofFor theCyclones trackstorms QBO, have NAOwith at a andleast west AO -twosouthwest indices of the three areto east positive QBO,-northeast NAO. Storms and track, AO with at indices aleast south two -to - negative.northof the track QBO, A havenegative NAO at andleast AAO AO two correlates indices of the arethree closely positive QBO, with. NAOStorms the southand with AO- toa -southindicesnorth- totrack. - negative.northClimatology track A negativehave at least AAO two correlates of the three closely QBO, with NAO the and south AO indices-to-north track. Annegative. important A negativeobservation AAO from correlates the study closely is thewith importance the south-to- northof NAO track. in the intensificationAn Theimportant major observationfactors of storm driving developmentfrom European the study ETC. Whenis thes are importanceNAO the changesArctic Oscillationof signNAO beforein andthe An important observation from the study is the importance of NAO in the aintensification stormNorth develops Atlantic of Oscillation ,storm usually development from, two negative climate. When indicesto positive, NAO for the therechanges Northern is potential sign Hemisphere. before for intensification of storm development. When NAO changes sign before strongera storm develops European, usually ETC creation from negative. This finding to positive, suggests there an is increasepotential of for the a storm develops, usually from negative to positive, there is potential for jetstronger stream’sArctic European powerOscillation, which ETC supportcreation (AO)s .the This storm finding intensification suggests an theory increase. of the stronger European ETC creation. This finding suggests an increase of the jet stream’s power, which supports the storm intensification theory. jetThe stream’s Arctic power is home, which to support a semis- thepermanent storm intensification low pressure theory circulation. known as the ‘Polar Vortex’, which rose to prominence in the first quarter

of 2014. The Vortex is in constant opposition to (and therefore represents opposing pressure to) the weather patterns of the northern middle latitudes (i.e. Northern Europe, Northern Asia and northern North America).

AO measures the variation in the strength, intensity and size of the jet stream as it expands and contracts to alter the shape of the fast-flowing air current. AO is measured by opposing sea-pressure anomalies, ranging from the Arctic Circle approximately to the 38th parallel.

During the ‘negative phase’ of AO, sea-level pressure is high in the The Arctic Oscillation helps define the Arctic and low in the northern middle latitudes. The ‘positive phase’ of jet stream, which can send cold, polar AO has the exact opposite pattern. air far south with its large oscillations. Dr. James E. Hansen of NASA describes AO thus: This image compares December 2010 to the previous 10-year average, "The degree to which Arctic air penetrates into middle latitudes is related to showing that AO and the shape of the the AO index, which is defined by surface atmospheric pressure patterns. jet stream altered the temperature When the AO index is positive, surface pressure is low in the polar region.

gradient in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently Arctic air was sent as far south as the from west to east, thus keeping cold Arctic air locked in the polar region. Carolinas and Portugal. Red (blue) When the AO index is negative, there tends to be high pressure in the polar indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into (colder) than average. Note how the jet middle latitudes." stream’s strange shape can juxtapose warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in

with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC Figure 8. The various storm tracks of those listed in Table 1. The normal path of most extended warm and cold fronts. phases, the jet stream tends to be reinforced and prevented from Figurestorms 8. is The from various the west storm-southwest tracks to of the those east listed-northeast. in Table The 1.remaining The normal storms path without of most shifting. This is an effect called eddy feedback. stormsFigurethe aforementioned is8. from The thevarious west path storm-southwest take tracks unusual to of the routesthose east throughlisted-northeast. in the Table North The 1. remainingAtlanticThe normal and storms Europe. path ofwithout most thestorms aforementioned is from the west path-southwest take unusual to the routes east through-northeast. the TheNorth remaining Atlantic andstorms Europe. without By maintaining the shape and consistency of the jet stream, eddy feedback theData aforementioned suggests that path fast take moving unusual storms routes throughwith a generalthe North west Atlantic-to- eastand Europe. Dataorientationincreases suggests track the that propensitywith fast a movingstrong for west stormsclustering-to-east with jetof a streamETCs. general .Such A weststrong, clustering-to -east orientationDatahorizontalpatterns sugg ests polartrack were thatjet withseen stream fast a during strongmoving positioned the west storms destructive across-to-east with North jet a 1987, America generalstream 1990 .and westA, strong,2000the-to North- eastand 2014 horizontalorientationAtlanticEuropean is polar found track ETC jet with with streamseasons a positivea strong positioned. This NAO west is and aacross- monthto AO,-east andNorth-to jet- favorsmonth streamAmerica a ph strong.enomenon. A and strong, mid the- North It does Atlantichorizontallatitudenot persist is jet foundpolar stream, on jetwith a streamyearlyfueling a positive basis. developmentpositioned NAO and across of AO,strong North and ETCs. favors America This a strongpattern and the mid North- encourages the development of storm clusters. latitudeAtlantic jetis foundstream, with fueling a positive development NAO and of AO, strong and ETCs. favors This a strong pattern mid - encourageslatitude jet stream, the development fueling development of storm of clusters strong ETCs.. This pattern encourages the development of storm clusters. 12 European Extratropical Cyclones | TransRe White Paper 12 4 European Extratropical Cyclones | TransRe White Paper European Extratropical Cyclones | TransRe White Paper 12 12 European Extratropical Cyclones | TransRe White Paper

The Science of European Extratropical The positive phase of QBO enhances this jet pattern and aids in the What is QBO? CyclonesThe positive phase of QBO enhances this jet pattern and aids in the What is QBO? developmentdevelopment of ETC of ETC clustering clustering with westwith- southwestwest-southwest to east- northeastto east-northeast tracks. The quasi-biennial oscillation (QBO) tracks. The quasi-biennial oscillation (QBO) Climatology is a slow moving pattern (average The more severe ETCs track through or near the English Channel, with is a slow moving pattern (average TheThe major more factors severe driving ETCs European track ETC throughs are the or Arctic near Oscillation the English and Channel, with cycle period is 28.5 months) of a directional path towards the German Bight. cycleatmospheric period is waves.28.5 months) These waves of Northa directional Atlantic Oscillation path toward, two climates the indices German for theBight Northern. Hemisphere. atmospheric waves. These waves Storms with southerly paths bring high impacts to South France and Spain, start in the tropical troposphere and Arctic Oscillation (AO) starttravel in the higher tropical before troposphere being dissipated and whileStorms more with northerly southerly storms paths impact bring the UK.high Areas impacts east to of South the UK, France and Spain, travelin the higher stratosphere, before being and cause dissipated Theincludingwhile Arctic more France,is home northerly the to aNetherlands, semistorms-permanent impact Belgium, the low UK. pressure Luxembourg Areas circulation east and of Germanythe UK, in thesuccessive stratosphere, waves ofand easterly cause and knownareincluding heavily as the impacted France,‘Polar V ortex bythe both Netherlands,’, which south rose-to -tonorth prominenceBelgium, and west Luxembourg in- southwestthe first quarter to and Germany . successivewesterly wind wave anomalies.s of easterly and ofeast 2014.are-northeast heavily The Vortex impactedstorms is in constant by both opposition south -toto (and-north therefore and west represents-southwest to opposingeast-northeast pressure to) storms the weather. patterns of the northern middle latitudes westerly wind anomalies. However, there does not seem to be an efficient long-term predictor for The phases of QBO alter the strength (i.e. Northern Europe, Northern Asia and northern North America). the strength and path of storms. This is not very surprising, as highly- of the Atlantic jet stream: westerly However, there does not seem to be an efficient long-term predictor for The phases of QBO alter the strength AOfunded measures research the in variation forecasting in tropicalthe strength, storm intenintensitysification and showssize of only the jet (negative) phases weaken the the strength and path of storms. This is not very surprising, as highly- of the Atlantic jet stream: westerly modest improvements. Predicting the path and strength of non-tropical Atlantic jet stream, while easterly stream as it expands and contracts to alter the shape of the fast-flowing stormsfunded is evenresearch more indifficult. forecasting tropical storm intensification shows only (negative)(positive) phases phases weakenstrengthen the the jet air current. AO is measured by opposing sea-pressure anomalies, ranging modest improvements. Predicting the pathth and strength of non-tropical Atlanticstream jet and stream, tend to while cause easterly stormier from the Arctic Circle approximately to the 38 parallel. Withstorms potential is even impacts more fromdifficult. the UK into Scandinavia, storms with west- (positive)European phases winters. strengthen the jet Duringsouthwest the ‘negative to east-northeast phase’ of orientations AO, sea-level can pressure track far is into high Eur in opethe . streamThe Arctic and Oscillation tend to cause helps definestormier the ArcticWhileWith noand potential long low-term in the impactsclimate northern signal from middle can the predict latitudes.UK into paths, Scandinavia,The a negative‘positive NAOphase’ storms and of with west-

Eujetropean stream, winters.which can send cold, polar AOnegativesouthwest has the AO exact strongly to eastopposite -suggestsnortheast pattern. a moreorientations southerly can storm track track far, into Europe. air far south with its large oscillations. influencingWhile no longETCs-term to impact climate France signal and can Spain predict, with paths, possible a negative tracks into NAO and Dr. James E. Hansen of NASA. Nevertheless describes AO, these thus: cl imate signs can also This image compares December 2010 Italynegative and Eastern AO strongly Europe suggests a more southerly storm track, to the previous 10-year average, support storms that impact much larger areas if they track south-to-north, as "Theinfluencing degree to which ETCs Arctic to impact air penetrates France into and middle Spain latitudes, with ispossible related to tracks into showing that AO and the shape of the with the case of the 1987 storm. theItaly AO index,and Eastern which is definedEurope by. Neverthelesssurface atmospheric, these pressure climate patterns. signs can also jet stream altered the temperature When the AO index is positive, surface pressure is low in the polar region. Reanalysissupport storms data from that the impact NCEP/ muchNCAR larger database areas gives if they evidence track south that -ato -north, as gradient in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently Arctic air was sent as far south as the strongwith thejet streamcase of with the anomalously1987 storm. strong winds correlates with the from west to east, thus keeping cold Arctic air locked in the polar region.. Carolinas and Portugal. Red (blue) development of stronger and more frequent high impact storms WhenReanalysis the AO index data is fromnegative, the there NCEP/ tendsNCAR to be database high pressure gives in the evidence polar that a indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into Specifically,strong jet yearsstream with with a higher anomalously-than-normal strong number winds of storms correlates with the (colder) than average. Note how the jet middleclustered latitudes." together shows anomalously powerful wind flow at the development of stronger and more frequent high impact storms. stream’s strange shape can juxtapose 300mb level. The 1999-2000 and 2013-2014 European ETC seasons are AO has a dominant role in determining the shape of the jet stream, which in warm and cold weather anomalies, particularly good examples, as shown in Figure 9. with less than 50 miles between turnSpecifically, plays a crucial years role in with steering a higher European-than ETCs-normal. During number active ofETC storms extended warm and cold fronts. phases clustered, the jet together stream tends shows to beanomalously reinforced and powerful prevented wind from flow at the shifting300mb. This level is .an The effect 1999 called-2000 eddy and feedback. 2013-2014 European ETC seasons are particularly good examples, as shown in Figure 9. By maintaining the shape and consistency of the jet stream, eddy feedback

increases the propensity for clustering of ETCs. Such clustering patterns were seen during the destructive 1987, 1990, 2000 and 2014

European ETC seasons. This is a month-to-month phenomenon. It does not persist on a yearly basis.

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European Extratropical Cyclones | TransRe White Paper 13 13 European Extratropical Cyclones | TransRe White Paper

13 European Extratropical Cyclones | TransRe White Paper

The Science of European Extratropical Cyclones

Climatology

The major factors driving European ETCs are the Arctic Oscillation and

North Atlantic Oscillation, two climate indices for the Northern Hemisphere. Arctic Oscillation (AO)

The Arctic is home to a semi-permanent low pressure circulation known as the ‘Polar Vortex’, which rose to prominence in the first quarter of 2014. The Vortex is in constant opposition to (and therefore represents opposing pressure to) the weather patterns of the northern middle latitudes (i.e. Northern Europe, Northern Asia and northern North America).

AO measures the variation in the strength, intensity and size of the jet to alter the shape of the fast-flowing Figure stream 9. The NCEP/NCARas it expands reanalysis and contracts of 300mb zonal wind in meters per second. On Figure 9. The NCEP/NCAR reanalysis of 300mb zonal wind in meters per second. On the leftair is current.the 1999 -AO2000 is season, measured on the by right opposing is the 2013 sea-2014-pressure season. anomalies, For the 1999 ranging- theFigure left is 9. the The 1999 NCEP/NCAR-2000 season, reanalysis on the ofright 300mb is the zonal 2013th wind-2014 in season. meters perFor second.the 1999 On- 2000 season,from the the Arctic anomalous Circle wind approximately band extends to over the 70 38 degrees parallel. longitude. 2000the left season, is the the1999 anomalous-2000 season, wind on band the extendsright is the over 2013 70 -degrees2014 season. longitude. For the 1999- In 2000bothDuring season,scenarios, the the ‘negative surfaceanomalous storms phase’ wind band tend of AO,extends to formsea over- levelin the70 pressuredegrees left front longitude. iquadrants high in the The Arctic Oscillation helps define the andIn both theArctic rightscenarios, and rear low quadrant surface in the northern stormsof the oval tend middle shaped to form latitudes. wind in anomaly. the The left ‘positive frontThis meansquadrant phase’ of jet stream, which can send cold, polar thatandIn twobothAO the separate has scenarios,right the rear exact storms surfacequadrant opposite can storms formof the pattern. at ovaltend nearly shaped to formthe samewind in the anomaly.time left within front This quadrantthis means air far south with its large oscillations. highthatand leveltwo the separaterightwind flowrear storms quadrantpattern can and of form maintainthe oval at nearly shaped parallel the wind tracks same anomaly. intotime Europe within This means,this This image compares December 2010 creatinghighthatDr. twolevel aJames storm separate wind E.family flowHansen storms or pattern storm of canNASA cluster.and form maintaindescribes at nearly parallelAO the thus: same tracks time into within Europe this, creatinghigh level a storm wind family flow pattern or storm and cluster. maintain parallel tracks into Europe, to the previous 10-year average, A good"The example degree of to these which ‘clustered Arctic air ETCs’ penetrates struck intoEurope middle over latitudes February is 10 relatedth- to creating a storm family or storm cluster. showing that AO and the shape of the th th 14A good ofthe 2014. AOexample iFigurendex, of which 1 these (inside is ‘clustered defined front page) by ETCs’ surface shows struck atmospheric how Europe some over pressurepatterns February patterns. 10 - jet stream altered the temperature th th combined14A goodWhen of 2014. example andthe Figure AOintensified ofindex these1 (inside is into positive,‘clustered storms.front surface page)ETCs’ showspressurestruck Europehow is low some over in the patterns February polar region. 10 - gradient in the Northern Hemisphere. th combined14 This of 2014. helps and Figure the intensified middle 1 (inside latitude into front storms. jet streampage) showsto blow howstrongly some and patterns consistently Arctic air was sent as far south as the The cover image of this paper shows waves striking a lighthouse and a piercombined atfrom Douro’s west and riverto east,intensified mouth thus in keepingPorto, into storms.Portugal cold Ar onctic Monday, air locked February in the polar 10th, region. Carolinas and Portugal. Red (blue) TheWhen cover the image AO indexof this is papernegative, shows there waves tends striking to be high a lighthouse pressure andin the a polar 2014. This particular cluster put the entire coast of Portugal on the th indicates areas that are warmer pierTheregion, at cover Douro’s weaker image river zonalof mouth this winds, paper in Porto, and shows Portugalgreater waves movement on striking Monday, aof lighthouse Februaryfrigid polar 10and air, intoa highest scale of alert as storm Stephanie rolled in. th (colder) than average. Note how the jet 2014.piermiddle at This Douro’s latitudes."particular river mouth cluster in Porto,put the Portugal entire coaston Monday, of Portugal February on the10 , stream’s strange shape can juxtapose Thehighest2014. 300mb This scale high particular oflevel alert winds ascluster stormcan be put Stephaniepredicted the entire with rolled coast a certain in. of Portugal level of skill on inthe warm and cold weather anomalies, currenthighestAO climate has scale a modeldominant of alert software. roleas storm in The determining StephanieECMWF theclimate rolled shape model in. of the that jet stream,has which in The 300mb high level winds can be predicted with a certain level of skill in with less than 50 miles between outputturn to playnines monthsa crucial could role in help steering diagnose European and predictETCs. Duringhigh-impact active ETC The 300mb high level winds can be predicted with a certain level of skill in extended warm and cold fronts. seasonscurrentphases .climate Additionally,, the model jet stream QBO software., which tends The enhances to ECMWF be reinforced west climate-to-east and model storm prevented thattracks, has from can beoutputcurrent prshiftingedicted to climate nine .several This monthsmodel is monthsan software. effectcould in called helpadvance The diagnose ECMWFeddy. feedback. climateand predict model high that-impact has seasonsoutput to. Additionally, nine months QBO could, which help enhances diagnose west and- topredict-east storm high -tracks,impact can beseasons prByedicted maintaining. Additionally, several the monthsshape QBO, andwhich in advanceconsistency enhances. ofwest the- tojet-east stream, storm eddy tracks, feedback can be increasespredicted theseveral propensity months for in clusteringadvance. of ETCs. Such clustering patterns were seen during the destructive 1987, 1990, 2000 and 2014 European ETC seasons. This is a month-to-month phenomenon. It does not persist on a yearly basis.

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14 4 European Extratropical Cyclones | TransRe White Paper 14 European Extratropical Cyclones | TransRe White Paper 14 European Extratropical Cyclones | TransRe White Paper

Predictions Under A Changing Climate The Science of European Extratropical PredictionsThe 2013-2014 European Under winter A storm Changing season had someClimate of the most severeCyclones sequences of storms since 1990. Given that some of this may be TheduePredictions to2013 climate-2014 change European Under and winterresultant A stormChanging altered season weather ha Climate dpatterns, some of studying the most the severeclimateClimatology sequenceschange impact of stormson European since ETCs1990. becomesGiven that a somehigh priority of this. may be dueThe to 2013 climate-2014 change European and winter resultant storm altered season weather had some patterns, of the studying most the climateSchwierzsevereThe majorchangesequences et al. factors impactconducted of drivingstorms on European aEuropean since study 1990 ETCsthat ETC. Givenused becomess are thatthree the some aArctic different high of thisOscillationpriority mayclimate. be and scenariosdueNorth to climate Atlantic for change Europe Oscillation and to resultantcreate, two climatean altered insurance indicesweather loss for patterns, the model Northern studying. State Hemisphere. -theof- the-art Schwierzglobalclimate models change et al. were impact conducted utilized on European toa studyprovide ETCs that estimates becomes used three of a highthe different uncertainties. priority. climate scenariosArctic for Oscillation Europe to create (AO) an insurance loss model. State-of-the-art OverSchwierz the past et al. 30 conducted years, insured a study natural that used catastrophe three different losses climatehave increased globalscenarios models for Europewere utilized to create to provide an insurance estimates loss of model the uncertainties.. State-of-the- art significantlyThe Arctic. While is home factors to athat semi contribute-permanent to the low rise pressure include s ocioeconomiccirculation global models were utilized to provide estimates of the uncertainties. Overdevelopmentknown the past as alongthe30 years, ‘P coastlines,olar insured Vortex the ’natural, which frequency catastrophe rose toof prominenceweather losses-related have in the increased first quarter significantlycatastrophesOverof 2014.the past .The While has30 Vyears, ortexincreased factors insured is in that constantat naturalacontribute corresponding catastropheopposition to the raterise tolosses (and .include Schwierz have therefore sincreasedocioeconomic et representsal.’s developmentstudysignificantlyopposing takes. While pressurethealong demographic factors coastlines, to) thatthe contributeweather the shifts frequency patterns intoto the mind riseof of weather includethewhen northern -analyzingsrelatedocioeconomic middle the latitudes development along coastlines, the frequency of weather-related catastrophesimpacts(i.e. Northern of climate has Europe, increased change Northern at on a Europeancorresponding Asia and ETCs northern rate. . Schwierz North America). et al.’s catastrophes has increased at a corresponding rate. Schwierz et al.’s study takes the demographic shifts into mind when analyzing the studyAO takesmeasures the demographic the variation shifts in the into strength, mind when intensity analyzing and the size of the jet ECHAM5-­‐CHRM impacts of climateHC change-­‐CHRM on European ETCs. HC-­‐CLM impactsstream of asclimate it expands change and on Europeancontracts ETCs to alter. the shape of the fast-flowing

ECHAM5ECHAM5-­‐CHRM-­‐CHRM air current. AOHCHC is-­‐ CHRM-­‐measuredCHRM by opposing sea-pressure anomalies,HC-­‐CLMHC-­‐CLM ranging from the Arctic Circle approximately to the 38th parallel.

During the ‘negative phase’ of AO, sea-level pressure is high in the

The Arctic Oscillation helps define the Arctic and low in the northern middle latitudes. The ‘positive phase’ of jet stream, which can send cold, polar AO has the exact opposite pattern.

air far south with its large oscillations. Dr. James E. Hansen of NASA describes AO thus: This image compares December 2010

to the previous 10-year average, "The degree to which Arctic air penetrates into middle latitudes is related to

showing that AO and the shape of the the AO index, which is defined by surface atmospheric pressure patterns. jet stream altered the temperature When the AO index is positive, surface pressure is low in the polar region.

gradient in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently

Arctic air was sent as far south as the from west to east, thus keeping cold Arctic air locked in the polar region. Carolinas and Portugal. Red (blue) When the AO index is negative, there tends to be high pressure in the polar

indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into Expected climate change impact on ETC gust fields. Shown is the (colder) than average. Note how the jet FigureFiguremiddle 10. Expectedlatitudes." climate change impact on ETC gust fields. Shown is the difference between historical CTL scenario (Oct 1961-Mar 1990) and the futuristic A2 stream’s strange shape can juxtapose difference between historical CTL scenario (Oct 1961-Mar 1990) and the futuristic A2 scenarioAO has (Oct a 2071 dominant-Mar 2100) role set in forth determining by the IPCC. the Each shape box representsof the jet astream, different which in warm and cold weather anomalies, Expected climate change impact on ETC gust fields. Shown is the Figurescenariocomparison 10 (Oct. of 2071mean- Margust 2100)fields, givenset forth the bythree the global IPCC. climate Each modelbox represents simulation sa different with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC differencecomparison(from left to between right,of mean ECHAM5 historical gust -fields,CHRM, CTL given HCscenario-CHRM, the three (Oct HC -1961globalCLM).-Mar climate 1990) model and thesimulation futuristics A2 phases, the jet stream tends to be reinforced and prevented from extended warm and cold fronts. scenario(from left (Octto right, 2071 ECHAM5-Mar 2100)-CHRM, set forth HC -byCHRM, the IPCC. HC-CLM). Each box represents a different Theshifting most striking. This climate is an effect impact called is an increase eddy feedback. in wind gusts over a comparison of mean gust fields, given the three global climate model simulations broad band that stretches from the eastern North Atlantic, over Great (fromThe mostleft to strikingright, ECHAM5 climate-CHRM, impact HC is -anCHRM, increase HC-CLM). in wind gusts over a BritainBy maintaining and into Northern the shape and and Central consistency Europe. ofThe the ECHAM5 jet stream,-CHRM eddy feedback broad band that stretches from the eastern North Atlantic, over Great climateincreases model the extends propensity the wind for field clustering all the way of ETCs.to Eastern Such Europe clustering. TheBritain most and striking into Northernclimate impact and Central is an increase Europe in. The wind ECHAM5 gusts over-CHRM a Thispatterns area of increasedwere seen wind during is circumscribed the destructive by two 1987,parallel 1990 bands, 2000 of and 2014 broadclimatereduced band modelgusts that from extends stretches Spain theto the from wind Eastern the field eastern Mediterranean all the Northway toSea Atlantic, Eastern (in the over South)Europe Great . European ETC seasons. This is a month-to-month phenomenon. It does BritainThisand thearea andNorthern of intoincreased AtlanticNorthern wind to Scandinaviaand is circumscribed Central (in Europe the North)by .two The. parallel ECHAM5 bands-CHRM of not persist on a yearly basis. climatereduced modelgusts from extends Spain the to windthe Eastern field all Mediterranean the way to Eastern Sea (in Europethe South). Thisand thearea Northern of increased Atlantic wind to isScandinavia circumscribed (in the by twoNorth) parallel. bands of reduced gusts from Spain to the Eastern Mediterranean Sea (in the South)15 European Extratropical Cyclones | TransRe White Paperand the Northern Atlantic to Scandinavia (in the North). 15

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TheThe figure Science from Schwierz of Europeanet al’s study shows Extratropical that regional gust models have Cyclonesminor deviations over oceans, but considerable differences over land. Wind gust projections differ by country and region, depending on climate model. Climatology Frequency of events are expected to increase over Southern TheScandinavia, major factors Northern driving European Great Britain, ETCs theare Norththe Arctic Sea Oscillation and the English and NorthChannel, Atlantic Benelux, Oscillation Germany, two climate (particularly indices for over the NorthernNorthern Hemisphere.Germany), extending towards Poland and the Baltic States, with an additional Arcticincrease Oscillation over the Alps, (AO) Southern Italy and the Adriatic Sea.

TheEvents Arctic are is homeexpected to a to semi increase-permanent average low annual pressure loss circulation over the 10 -year known(23%), as 30 the-year ‘Polar (50%) Vortex and ’100, which-year rose (104%) to prominence timeframe in. Thethe firstexpected quarter loss of for2014. rare The high V-orteximpa ctis eventsin constant is disproportionately opposition to (and large, therefore where represents Germany opposing(114%) pressureand Denmark to) the (116%) weather see patterns the greatest of the northern increase middle in insured latitudes loss (i.e.in Northernthe 100- yearEurope, model Northern. Asia and northern North America).

AOL ossmeasures potentials the variationfor Great inBritain the strength, and Germany intensity increase and size by 37%of the and jet stream21%, asrespectively it expands, indicat and contractsing a much to alterbroader the shapeloss distribution of the fast-flowing with Schwierz et al. generated a probabilistic airpotential current. AO for is more measured tail events. by opposing sea-pressure anomalies, ranging th event sets for different countries, using from the Arctic Circle approximately to the 38 parallel.

Swiss Re’s insurance loss model as During the ‘negative phase’ of AO, sea-level pressure is high in the event set inputs. The Arctic Oscillation helps define the Arctic and low in the northern middle latitudes. The ‘positive phase’ of jet stream, which can send cold, polar AO has the exact opposite pattern. The resulting climate change impact for air far south with its large oscillations. wintertime windstorms is calculated and This image compares December 2010 Dr. James E. Hansen of NASA describes AO thus: analyzed by country. to the previous 10-year average, "The degree to which Arctic air penetrates into middle latitudes is related to showingAnalysis that shows AO and average the shape expected of the loss the AO index, which is defined by surface atmospheric pressure patterns. jet stream altered the temperature increases in all countries except When the AO index is positive, surface pressure is low in the polar region. gradientIreland. in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently Arctic air was sent as far south as the from west to east, thus keeping cold Arctic air locked in the polar region. CarolinasLargest increasesand Portugal. are inRed Denmark (blue) and When the AO index is negative, there tends to be high pressure in the polar Figure 11. Climate change impact on annual expected loss, for selected regional indiGermany,cates areas which that are are both warmer situated region, weaker zonal winds, and greater movement of frigid polar air into markets given Swiss Re’s insurance loss model. Error bars encompass results of the (colder)within thethan band average. of strongest Note how gust the jet middle latitudes." stream’sincreases strange over theshape North can Sea. juxtapose three model chains considered. (CTL=Control). warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in In the A2 scenario simulation, the maximum wind increase is of 6-8% with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC from the English Channel to the German Bight. The general areas of extended warm and cold fronts. phases, the jet stream tends to be reinforced and prevented from increased gusts extend that corridor, from Western to Central Europe. shifting. This is an effect called eddy feedback. A2 simulations also show an increase in the overall number of ETC By maintaining the shape and consistency of the jet stream, eddy feedback events, assuming climate change and global warming scenarios. increases the propensity for clustering of ETCs. Such clustering patternsWe can were expect seen an increaseduring the of intensitydestructive and 1987,frequency 1990 of, 2000 wintertime and 2014 ETC s. EuropeanThis will correspondETC seasons to .more This iswindstorms a month-to -trackingmonth ph alongenomenon. the corridor It does notfrom persist Great on aBritain yearly tobasis. Germany via the Netherlands

In other words, the regions with the highest total gross premiums written can expect more detrimental activity from European ETCs.

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Insurance Case Study: Parametric Rolling and Post-Panamax Containerships InsuranceTheSinceInsurance theScience 1950’s, Case containerships ofCase Study: European haveStudy: Parametric grown inExtratropicalParametric size with the Rolling expansion Rolling of Cyclonesglobal trade. 14,000 containerships pass through the Panama Canal andevery Postand year, Post- carryingPanamax- grain,Panamax oil and Containers manufactured Containers goods.hips hips Climatology SinceThe theSince expansion 1950’s, the containerships1950’s, of the Panamacontainerships have Canal grown hashave resultedin grown size with inin sizemassive the withexpansion dredgingthe expansion of of global trade.global 14,000 trade. containerships14,000 containerships pass through pass through the Panama the Panama Canal Canal Theprojects major and factors the installationdriving European of larger ETC loads aresystems the Arctic at ports Oscillation around theand world every year,every carrying year, carrying grain, oil grain, and manufactured oil and manufactured goods. goods. Northas they Atlantic prepare Oscillation for the arrival, two climate of post -indicesPanamax for andthe NorthernNew Panamax Hemisphere. ships. The expansionThe expansion of the Panama of the Panama Canal has Canal resulted has resulted in massive in massive dredging dredging ArcticHow does Oscillation this relate to European (AO) ETCs? A significant portion of global projectstrade projectsand cross the esandinstallation the the North installation of Atlantic larger of load largerOcean systems load. If containerships systems at ports ataround ports take thearound that world route the world as theyThebetween Arcticprepareas they November is preparefor home the arrival toandfor a the Masemi of arrivalrch, post-permanent they- Panamaxof postrun the-Panamax low andrisk pressure Newof experiencing and Panamax New circulation Panamax ships.high winds ships. knownand waves as the during ‘Polar the V winterortex’ ,windstorm which rose season. to prominence in the first quarter How doesHow this does relate this to relate European to European ETCs? AETCs significant? A significant portion poof rtionglobal of global of 2014. The Vortex is in constant opposition to (and therefore represents tradeIn cross1998trade,es APL cross the China Northes the lost Atlantic North a number Atlantic Ocean of containers. OceanIf containerships. If overboard containerships take when that takeit routewas that route Global trade routes emphasize the opposing pressure to) the weather patterns of the northern middle latitudes betweenstruckbetween November by Typhoon November and Babs, Ma rch,and demonstrating theyMarch, run they the that riskrun postofthe experiencing risk-Panamax of experiencing shipshigh winds are high winds passage across the North Atlantic, even (i.e. Northern Europe, Northern Asia and northern North America). and vulnerablewavesand during waves to thea during phenomenon winter the windstorm winter known windstorm season. as ‘parametric season. rolling’. This occurs during the winter months when ETCs AOwhen measures waves above the variation a certain in threshold the strength, or critical intensity height comeand size exactly of the twice jet are more likely to form, intensify and In 1998,In APL 1998 China, APL lost China a number lost a numberof containers of containers overboard overboard when it waswhen it was Global tradeGlobal routes trade emphasizeroutes emphasize the the streamas fast asas ita ship’sexpands roll andperiod. contracts to alter the shape of the fast-flowing cluster. struck bystruck Typhoon by Typhoon Babs, demonstrating Babs, demonstrating that post that-Panamax post-Panamax ships are ships are passagepassage across acrossthe North the Atlantic, North Atlantic, even evenair current. AO is measured by opposing sea-pressure anomalies, ranging vulnerableParametricvulnerable to a rolling phenomenon to cana phenomenon occur known in stern asknown-quartering ‘parametric as ‘parametric or bow rolling’-quartering .rolling’ This occursseas.. This occurs during theduring winter the monthswinter months when ETCs when ETCs from the Arctic Circle approximately to the 38th parallel. whenCargo waveswhen on above wavespost-Panamax a above certain a ships thresholdcertain is particularlythreshold or critical or vulnerablehe criticalight come he ightto exactlybe come lost intwiceexactly these twice are moreare likely more to likely form, to intensify form, intensify and and as fastseas, asas becausea fast ship’s as aroll of ship’s theperiod. U roll-shape, period. flared bow and wide beam of the oversized, cluster.cluster. During the ‘negative phase’ of AO, sea-level pressure is high in the The Arctic Oscillation helps define the Arcticpost-Panamax and low inhull. the northern middle latitudes. The ‘positive phase’ of ParametricParametric rolling can rolling occur can in occurstern -inquartering stern-quartering or bow- quarteringor bow-quartering seas. seas. jet stream, which can send cold, polar AO has the exact opposite pattern. CargoIn APLonCargo post China -onPanamax postlitigation-Panamax ships, the isexpert shipsparticularly witnesses is particularly vulnerable proved vulnerable thatto be in lost certain to in be these lostsea in these air far south with its large oscillations. seas,conditions becauseseas, because, ofpost the- PanamaxU -ofshape, the U -flaredshipsshape, boware flared especiallyand bow wide and beam prone wide of tobeam the parametric oversize of the oversized , d, This image compares December 2010 Dr. James E. Hansen of NASA describes AO thus: postrolling-Panamaxpost, -or,Panamax hull.the unstable hull. phenomenon whereby large roll angles are to the previous 10-year average, "Thecoupled degree with to significantwhich Arctic pitch air penetrates motions. into middle latitudes is related to showing that AO and the shape of the In APLthe AOChinaIn APLindex, litigation China which litigation, theis defined expert, the bywitnesses expert surface witnesses provedatmospheric thatproved in pressure certain that in sea certainpatterns. sea jet stream altered the temperature conditionsWhenParametricconditions the, post AO -rolling Panamaxindex, post isintroduces-Panamax positive, ships aresurface shipsextreme especially arepressure stresses especially prone is lowon to pronecontainershipsin parametric the topolar parametric region. and gradient in the Northern Hemisphere. rollingThistheir, rollinghelpsor, securing the the ,unstable or, middle thesystems unstable phenomenon latitude, and phenomenon jet it differsstream whereby dynamically to blowwhereby large strongly roll fromlarge angles andsynchronous roll consistently areangles rolling.are Arctic air was sent as far south as the coupledfromStormy coupledwest with conditions to significant east, with thus significant can pitchkeeping cause motions pitch cold steerage Armotions. ctic toair be .locked completely in the polar lost .region. Carolinas and Portugal. Red (blue) When the AO index is negative, there tends to be high pressure in the polar ParametricOf particularParametric rolling interest introducesrolling is that introduces normal,extreme 40ftextreme stresses waves stresses onof acontainerships certain on containerships wave-period and and indiThecates Ital areasFlorida that is exampleare warmer of one of the region, weaker zonal winds, and greater movement of frigid polar air into theircan securingtheir cause securing systemsa post- Panamaxsystems, and it differs, candontainership itdynamically differs dynamically to fromheel synchronousmore from than synchronous 40 rolling. rolling. (colder) than average. Note how the jet middle latitudes." containerships that have staggered into degrees. It isn’t necessary for freak waves, or monsterwellen, to cause steel stream’s strange shape can juxtapose Stormy Stormyconditions conditions can cause can steerage cause steerage to be completely to be completely lost. lost. port after being whipped by the ‘watery lashings to lose their hold on large loads. warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in wilderness’ of Herman Melville’s Of particularOf particular interest interestis that normal, is that normal,40ft waves 40ft ofwaves a certain of a certainwave-period wave -period turn plays a crucial role in steering European ETCs. During active ETC Thewith mythosItal lessThe Florida .thanItal Floridais 50 example miles is examplebetween of one of of theone of canthe Lcauseitigationcan a cause postfor the- Panamaxa APL post China-Panamax containership ended c ontainershipin 2003; to theheel proof moreto heel of than parametric more 40 than rolling40 phases, the jet stream tends to be reinforced and prevented from containershipsextendedcontainerships warm that and have thatcold staggered have fronts. staggered into intodegrees helpeddegrees. It isn’tlimit necessary .APL’s It isn’t liabilitynecessary for freak and for waves,allowed freak orwaves, for monsterwellen, a smaller or monsterwellen, settlement to cause. Since to steel cause steel shifting. This is an effect called eddy feedback. port afterport being after whipped being whipped by the ‘wateryby the ‘waterylashings then,lashings to parametric lose their to lose holdrolling their on holdhaslarge helpedon loads. large explain loads. what happened on the wilderness’wilder ofness’ Herman of Herman Melville’s Melville’s Maersk Carolina, P&O Nedlloyed Genoa and CMA CGM Otello and By maintaining the shape and consistency of the jet stream, eddy feedback mythosmythos. . LitigationmanyL itigationfor other the APLmaritime for the China APL mysteries ended China in ended .2003; in the 2003; proof the of proof parametric of parametric rolling rolling increases the propensity for clustering of ETCs. Such clustering helped limithelped APL’s limit liability APL’s liabilityand allowed and allowedfor a smaller for a settlementsmaller settlement. Since . Since patterns were seen during the destructive 1987, 1990, 2000 and 2014 then, parametricthen, parametric rolling hasrolling helped has explainhelped explainwhat happened what happened on the on the European ETC seasons. This is a month-to-month phenomenon. It does Maersk MaerskCarolina Carolina, P&O Nedlloyed, P&O Nedlloyed Genoa andGenoa CMA and CGM CMA Otello CGM and Otello and not persist on a yearly basis. many othermany maritime other maritime mysteries mysteries. .

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The Science of European Extratropical Implications for Insurers Cyclones As sophisticated insurance markets continue to thrive in Western Europe andClimatology insurance penetration expands in Central and Eastern Europe, mid- latitude weather patterns remain unpredictable. The major factors driving European ETCs are the Arctic Oscillation and WhileNorth countless Atlantic Oscillationprojects attempt, two climate to accurately indices predictfor the tropicalNorthern storm Hemisphere. intensification, only modest improvements are made; anticipating non- tropicalArctic storm Oscillation path and strength (AO) is even more difficult. Clashing weather systems often lead to extreme weather that can only be forecasted up to The Arctic is home to a semi-permanent low pressure circulation two weeks in advance. known as the ‘Polar Vortex’, which rose to prominence in the first quarter Thus,of 2014. every The year Vortex between is in Octoberconstant and opposition April, Europe to (and braces therefore itself represents against opposing pressure to) the weather patterns of the northern middle latitudes what could be a powerful winter storm season. It is not uncommon for (i.e. Northern Europe, Northern Asia and northern North America). ETCs to lash the continent with winds over 100mph, sending trees, scaffolding and buildings to their demise. AO measures the variation in the strength, intensity and size of the jet to alter the shape of the fast-flowing Givenstream the asdestruction it expands caused and contractsby European ETCs, it is important to inform air current. AO is measured by opposing sea-pressure anomalies, ranging and prepare our industry of the financial and social impact of these from the Arctic Circle approximately to the 38th parallel. weather patterns.

It Duringis clear the that ‘negative NAO, AO, phase’ MJO, ofQBO AO, and sea the-level 300mb pressure high -ilevels high wind in the The Arctic Oscillation helps define the anomaliesArctic and show low indistinct the northern effects middleon extratropical latitudes. stormThe ‘positive path, intensity, phase’ of jet stream, which can send cold, polar AO has the exact opposite pattern. clustering and frequency. A combination of each climate index phase and air far south with its large oscillations. signal corresponds to a greater or lesser percentage of ETC landfall along Dr. James E. Hansen of NASA describes AO thus: This image compares December 2010 certain sections of the European coastline, with the ability to continue into to the previous 10-year average, the"The continent degree. to which Arctic air penetrates into middle latitudes is related to showing that AO and the shape of the the AO index, which is defined by surface atmospheric pressure patterns. jet stream altered the temperature AssumingWhen the climateAO index change is positive, scenarios, surface society pressure should is low bein the equipped polar region. for . Climate change will not act as a ‘Doomsday gradient in the Northern Hemisphere. theThis worst helps case the scenariosmiddle latitude jet stream to blow strongly and consistently Device’ i.e. Dr. Strangelove, but altered climate interactions are indubitably Arctic air was sent as far south as the from west to east, thus keeping cold Arctic air locked in the polar region. changing global weather patterns. Carolinas and Portugal. Red (blue) When the AO index is negative, there tends to be high pressure in the polar indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into Our research shows how much work is being done by governments (colder) than average. Note how the jet middle latitudes." and private institutions to develop more comprehensive regional stream’s strange shape can juxtapose climate models that incorporate and weigh all the pertinent climate warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in indices, climate signals and risk exposures. with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC extended warm and cold fronts. Asphases research, the continues, jet stream so tendsconsumers to be andreinforced insurers and will preventedbe more informed, from adjustingshifting their. This insurance is an effect and called reinsurance eddy feedback.buying habits to accommodate new scientific findings. Based on the available data, it would be wise to By maintaining the shape and consistency of the jet stream, eddy feedback anticipate more intense and more frequent European ETCs. increases the propensity for clustering of ETCs. Such clustering patterns were seen during the destructive 1987, 1990, 2000 and 2014 European ETC seasons. This is a month-to-month phenomenon. It does not persist on a yearly basis.

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The Science of European Extratropical Glossary Cyclones Arctic Oscillation- An index of the dominant sea-level pressure variations betweenClimatology the North Pole and 450N.

AzoresThe majorHigh- factorsA large driving subtropical European semi -ETCpermanents are the center Arctic of Oscillation high pressure and centeredNorth Atlanticover the Oscillation Azores. Also, two known climate as indices the Bermuda for the High.Northern Hemisphere.

ColdArctic-core- AOscillation cyclone associated (AO) with a cold pool of air residing at the storm’s center high in the troposphere. The Arctic is home to a semi-permanent low pressure circulation Convectiveknown as- A the storm ‘Polar characterized Vortex’, which by the rose presence to prominence of lighting in theand first thunder. quarter of 2014. The Vortex is in constant opposition to (and therefore represents Disturbance- An unstable creates a variation from normal wind conditions. opposing pressure to) the weather patterns of the northern middle latitudes (i.e. Northern Europe, Northern Asia and northern North America). Eddy feedback- The feedback system keeps the jet stream from shifting.

ExtratropicalAO measures Cyclone the variation- Middle latitudein the strength, cyclones thatintensity have distinctand size fronts of the jet andstream horizontal as itgradients expands in and temperature contracts and to alterdew point.the shape of the fast-flowing air current. AO is measured by opposing sea-pressure anomalies, ranging th Icelandicfrom the Low Arctic- Semi Circle-permanent approximately low pressure to the 38 system parallel. centered between Iceland and southern Greenland. During the ‘negative phase’ of AO, sea-level pressure is high in the The Arctic Oscillation helps define the JetArctic Stream and- Fast low flowing, in the northernnarrow air middle currents latitudes. found in Thethe atmosphere. ‘positive phase’ The of jet stream, which can send cold, polar shapeAO andhas locationthe exact of theopposite jet stream pattern. influence local weather patterns. air far south with its large oscillations. This image compares December 2010 MaddenDr. James-Julian E. OscillationHansen of NASA-Major describespart of intraseasonal AO thus: (30-90 days) variability in the tropical atmosphere. Travels east from Indian Ocean. to the previous 10-year average, "The degree to which Arctic air penetrates into middle latitudes is related to showing that AO and the shape of the Norththe AtlanticAO index, Oscill whichation is defined- Climactic by surface phenomenon atmospheric that measures pressure thepatterns. jet stream altered the temperature atmosphericWhen the differenceAO index isbetween positive, Icelandic surface lowpressure and Azores is low high.in the polar region. gradient in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently Arctic air was sent as far south as the Occludedfrom west Front to east,- Formed thus duringkeeping cyclogenesis, cold Arctic air when locked a cold in the front polar overtakes region. Carolinas and Portugal. Red (blue) a warmWhen front. the AOThey index usually is negative, form around there mature tends tolow be pressure high pressure areas. in the polar indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into Parametric Rolling- The phenomenon where large roll angles are coupled (colder) than average. Note how the jet middle latitudes." with significant pitch motions. These can be prompted by normal size waves. stream’s strange shape can juxtapose AO has a dominant role in determining the shape of the jet stream, which in warm and cold weather anomalies, Polar Vortex- The semi-permanent and large-scale cyclone located near the turn plays a crucial role in steering European ETCs. During active ETC with less than 50 miles between planet’s geographic North Pole. extended warm and cold fronts. phases, the jet stream tends to be reinforced and prevented from Seashifting Level Pre. Thisssure is an- Atmospheric effect called pressure eddy feedback. at sea level.

SinusoidalBy maintaining- A curve the that shape shows and a consistency repetitive oscillation, of the jet stream,named aftereddy the feedback sine function.increases the propensity for clustering of ETCs. Such clustering patterns were seen during the destructive 1987, 1990, 2000 and 2014 TropicalEuropean Cyclone ETC- seasonsTropical latitude. This is cyclone a month fueled-to-month by high phenomenon. sea surface It does temperaturesnot persist andon a vertical yearly basis.temperature gradients. Warm -core-A cyclone whose central core has a higher potential energy than the general circulation around it.

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The Science of European Extratropical Contacts Cyclones To discuss any of the contents of this paper, please contact: Climatology James Rohman Global Catastrophe Management, New York The major factors driving European ETCs are the Arctic Oscillation and North AtlanticT: 1 (212) Oscillation 365 2438, two climate indices for the Northern Hemisphere. E: [email protected] Arctic Oscillation (AO)

ToThe discuss Arctic the is implications home to a ofsemi this- permanentpaper, and yourlow pressurereinsurance circulation needs in response,known asplease the ‘contact:Polar V ortex’, which rose to prominence in the first quarter of 2014. The Vortex is in constant opposition to (and therefore represents opposingGeoff pressure Peach to) the weather patterns of the northern middle latitudes (i.e. NorthernTransRe Europe, London Northern Asia and northern North America).

AO measuresT: +44-20 the-7204 variation-8607 in the strength, intensity and size of the jet E: [email protected] stream as it expands and contracts to alter the shape of the fast-flowing air current. AO is measured by opposing sea-pressure anomalies, ranging th from theLorne Arctic Zalkowitz Circle approximately to the 38 parallel. TransRe Zurich During the ‘negative phase’ of AO, sea-level pressure is high in the The Arctic Oscillation helps define the Arctic T:and +41 low 44 in227 the 6104 northern middle latitudes. The ‘positive phase’ of jet stream, which can send cold, polar AO hasE: the [email protected] exact opposite pattern. air far south with its large oscillations. This image compares December 2010 Dr. James E. Hansen of NASA describes AO thus: Bertrand Levy to the previous 10-year average, "The degreeTransRe to whichParis Arctic air penetrates into middle latitudes is related to showing that AO and the shape of the the AO index, which is defined by surface atmospheric pressure patterns. jet stream altered the temperature When T:the +33 AO - index 1 4006 is 1201positive, surface pressure is low in the polar region. E: [email protected] gradient in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently Arctic air was sent as far south as the from west to east, thus keeping cold Arctic air locked in the polar region. Carolinas and Portugal. Red (blue) When Rtheüdiger AO index Skaletz is negative, there tends to be high pressure in the polar indicates areas that are warmer region,TransRe weaker zonalMunich winds, and greater movement of frigid polar air into (colder) than average. Note how the jet middle latitudes." stream’s strange shape can juxtapose T: +33 - 1 4006 1201 warm and cold weather anomalies, AO hasE: a [email protected] dominant role in determining the shape of the jet stream, which in with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC extended warm and cold fronts. phases, the jet stream tends to be reinforced and prevented from Krzysztof Koperski shifting. This is an effect called eddy feedback. TransRe Warsaw By maintaining the shape and consistency of the jet stream, eddy feedback T: +48 22 630 63 71 increasesE: [email protected] the propensity for clustering of ETCs. Such clustering patterns were seen during the destructive 1987, 1990, 2000 and 2014 European ETC seasons. This is a month-to-month phenomenon. It does not persist on a yearly basis.

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The Science of European Extratropical Citations Cyclones Accuweather Climatology Associated Press. Cover image shows winter storm Stephanie lashing Douro’sThe major river factorsmouth drivingin Porto, European Portugal ETC on Monday,s are the February Arctic Oscillation 10th, 2014. and North Atlantic Oscillation, two climate indices for the Northern Hemisphere. Cameron Beccario, creator of http://earth.nullschool.net/. Arctic Oscillation (AO) Designed by Inc Design The Arctic is home to a semi-permanent low pressure circulation Ianknown Bell and as theMartin ‘Polar Visbeck, Vortex Lamont’, which- Dohertyrose to prominence Earth Observatory in the first at quarter Columbiaof 2014. University’s The Vortex Earthis in constant Institute. opposition to (and therefore represents http://www.ldeo.columbia.edu/res/pi/NAO/opposing pressure to) the weather patterns of the northern middle latitudes (i.e. Northern Europe, Northern Asia and northern North America). Lin, Brunet and Derome., ‘An observed connection between the North AtlanticAO measures Oscillation the and variation the Madden in the-Julian strength, Oscillation’, intensity Journal and sizeof Climate of the. jet stream as it expands and contracts to alter the shape of the fast-flowing NASA air current. AO is measured by opposing sea-pressure anomalies, ranging th NOAA’sfrom the Climate Arctic Circle Prediction approximately Center to the 38 parallel.

OutsideDuring Magazine the ‘negative phase’ of AO, sea-level pressure is high in the The Arctic Oscillation helps define the Arctic and low in the northern middle latitudes. The ‘positive phase’ of jet stream, which can send cold, polar ProfessorAO has theMike exact Wallace, opposite Professor pattern. Emeritus, Department of Atmospheric air far south with its large oscillations. Sciences, University of Washington. This image compares December 2010 Dr. James E. Hansen of NASA describes AO thus: to the previous 10-year average, Risk Prediction Initiative at Bermuda Institute of Ocean Sciences, ‘European"The degree Windstorms to which andArctic the air North penetrates Atlantic into Oscillation: middle latitudes Impacts, is related to showing that AO and the shape of the Characteristicsthe AO index, and which Predictability’ is defined by surface atmospheric pressure patterns. jet stream altered the temperature When the AO index is positive, surface pressure is low in the polar region. gradient in the Northern Hemisphere. UKThis Met helps Office the, University middle latitude of Reading jet stream and toUniversity blow strongly of Exeter. and consistentlyLicensed Arctic air was sent as far south as the underfrom Creative west to east,Common thuss keeping CC BY cold4.0 International Arctic air locked Licens in ethe polar region. Carolinas and Portugal. Red (blue) When the AO index is negative, there tends to be high pressure in the polar indicates areas that are warmer region, weaker zonal winds, and greater movement of frigid polar air into (colder) than average. Note how the jet middle latitudes." stream’s strange shape can juxtapose warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC extended warm and cold fronts. phases, the jet stream tends to be reinforced and prevented from shifting. This is an effect called eddy feedback.

By maintaining the shape and consistency of the jet stream, eddy feedback increases the propensity for clustering of ETCs. Such clustering patterns were seen during the destructive 1987, 1990, 2000 and 2014 European ETC seasons. This is a month-to-month phenomenon. It does not persist on a yearly basis.

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The Science of European Extratropical

Cyclones Copyright and Disclaimers Climatology The material and conclusions contained in this document are for information Thepurposes major onlyfactors and driving the authors European offer ETC no guarantees are the Arctic for the Oscillation completeness and of its Northcontents. Atlantic The Oscillationstatements, twoin this climate document indices may for providethe Northern current Hemisphere. expectations Arcticof future Oscillationevents based on (AO) certain assumptions. These statements involve known and unknown risks, uncertainties and other factors which are not Theexhaustive Arctic .is The home authors to a ofsemi this- permanentdocument undertake low pressure no obligation circulation to publicly knownrevise or as update the ‘P olarany statements,Vortex’, which whether rose toas prominence a result of new in the information, first quarter offuture 2014. events The V orortex otherwise is in constant and in noopposition event shall to (and Transatlantic therefore Reinsurancerepresents opposingCompany pressure or any of to) its theaffiliates weather or emplopatternsyees of bethe liable northern for anymiddle damage latitudes and (i.e.financial Northern loss Europe,arising in Northern connection Asia with and the northern use of Norththe information America). relating to this document. AO measures the variation in the strength, intensity and size of the jet streamAbout TransReas it expands and contracts to alter the shape of the fast-flowing air current. AO is measured by opposing sea-pressure anomalies, ranging TransRe is the brand name for Transatlantic Holdings, Inc. and its subsidiaries. from the Arctic Circle approximately to the 38th parallel. TransRe, wholly owned by Alleghany Corporation (NYSE-Y), is a reinsurance Duringorganization the ‘negative headquartered phase’ in Newof AO, York sea with-level operations pressure worldwide. is high inSince the The Arctic Oscillation helps define the Arctic1978, TransReand low hasin the been northern offering middleits clients latitudes. the capacity, The expertise‘positive and phase’ of jet stream, which can send cold, polar AOcreativity has the necessary exact opposite to structure pattern. programs across the full spectrum of property air far south with its large oscillations. and casualty risks. This image compares December 2010 Dr. James E. Hansen of NASA describes AO thus: Visit www.transre.com for additional information. to the previous 10-year average, "The degree to which Arctic air penetrates into middle latitudes is related to showing that AO and the shape of the the© 201 AO4 i ndex,Transatlantic which is Holdings, defined by Inc surface. All rights atmospheric reserved. pressure patterns. jet stream altered the temperature When the AO index is positive, surface pressure is low in the polar region. gradient in the Northern Hemisphere. This helps the middle latitude jet stream to blow strongly and consistently Arctic air was sent as far south as the fromTransRe west to east, thus keeping cold Arctic air locked in the polar region. Carolinas and Portugal. Red (blue) WhenOne Liberty the AO Plaza index is negative, there tends to be high pressure in the polar indicates areas that are warmer region,165 Broadway weaker zonal winds, and greater movement of frigid polar air into (colder) than average. Note how the jet middleNew York, latitudes." New York 10006 stream’s strange shape can juxtapose warm and cold weather anomalies, AO has a dominant role in determining the shape of the jet stream, which in with less than 50 miles between turn plays a crucial role in steering European ETCs. During active ETC extended warm and cold fronts. phases, the jet stream tends to be reinforced and prevented from shifting . This is an effect called eddy feedback.

By maintaining the shape and consistency of the jet stream, eddy feedback increases the propensity for clustering of ETCs. Such clustering

patterns were seen during the destructive 1987, 1990, 2000 and 2014 European ETC seasons. This is a month-to-month phenomenon. It does not persist on a yearly basis.

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