Tornado climatology of Canada revisited: tornado activity during different phases of ENSO

INTERNATIONAL JOURNAL OF CLIMATOLOGY

Int. J. Climatol. 21: 915–938 (2001)

TORNADO CLIMATOLOGY OF CANADA REVISITED: TORNADOACTIVITY DURING DIFFERENT PHASES OF ENSO

DAVID ETKINa,*, SOREN E. BRUNb, AMIR SHABBARa and PAUL JOEa

a Meteorological Ser�ice of Canada, 4905 Dufferin St., Downs�iew, Ontario, Canadab North Carolina Department of Transportation, G.I.S. Unit, 1020 Birch Ridge Rd, Raleigh, NC, USA

Recei�ed 14 April 2000Re�ised 21 January 2001

Accepted 26 January 2001

ABSTRACT

Tornadoes are a significant hazard in some parts of Canada, particularly in the southern Prairie provinces andsouthwestern Ontario, though they are not as common as in some parts of the US. Since the early 1980s, the regionalweather offices in Canada have been recording tornado event information on a routine basis, and thus data exists thatcan be used to update older analyses of tornado frequencies. On average, about 60 tornadoes are reported each year,though many doubtless occur that are not observed or recorded in the Environment Canada records. An analysis oftornado frequencies with El Nino–Southern Oscillation (ENSO) events suggest that the cooler La Nina events tendto suppress tornadic activity, while El Nino events tend to enhance it (though there are exceptions to this trend).Copyright © 2001 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.

KEY WORDS: Canada; climatology; ENSO; tornadoes

DOI: 10.1002/joc.654

1. INTRODUCTION

Tornadoes are capable of creating incredible amounts of damage and significant numbers of fatalities andare, from a meteorological and climatological perspective one of nature’s more challenging perils. Becauseof this, developing climatologies of tornado occurrences is of fundamental importance. However, due tobiases and misreporting of events that are part of any tornado database, developing a climatology isextremely challenging. The purpose of this paper is to present and analyse data on tornado occurrencebased on event information archived after the development of Canada’s regional severe weather officesaround 1980, after which time the tornado database can be assumed to be better and more homogeneousthan prior to 1980. In addition to developing a climatology of tornado frequencies, the impact of ElNino–Southern Oscillation (ENSO) events on tornado occurrence is explored.

This paper initially reviews the physics of tornado formation in Canada, then reviews historicalinformation on tornadoes, including their impacts. Subsequently, a thorough review of the literature onCanadian tornadoes is carried out, followed by an analysis of the effect of ENSO events on tornadofrequencies.

2. TORNADO FORMATION AND CONDITIONS THAT RESULT IN TORNADOES

The conditions that lead to tornadic storms are generally the same as those for the formation of anysevere storm. The relevant atmospheric components for severe storms are dynamics, thermodynamics and

* Correspondence to: Adaptation and Impacts Research Group, Environment Canada at the University of Toronto, Institute forEnvironmental Studies, 33 Willcocks St., Toronto, Ontario, M5S 3E8, Canada; e-mail: [email protected]

Copyright © 2001 Crown in the right of Canada.Published by John Wiley & Sons, Ltd.

D. ETKIN ET AL.916

a mechanism to release the atmospheric instability. However, we are far from understanding all thefactors leading to the actual formation of tornadoes from these storms (Joe and Leduc, 1993; Trapp,1999). Most of our early knowledge has been derived from studies of a limited number of very largetornadoes, numerical modelling studies and empirical studies (Bluestein and Golden, 1993; Church et al.,1993; Grazulis, 1993). The understanding of small downburst winds that previously confounded theinterpretation of damage patterns (Fujita, 1981; Fujita and Smith, 1993) and recent research fieldprogrammes and operational experience have added to our understanding in that a larger variety oftornadoes have been studied and the mechanism for their formation is not as clear as it was once thought(Wilson, 1986; Burgess et al., 1993; Joe et al., 1994; Rasmussen et al., 1994; Bluestein et al., 1998).

The synoptic scale factors are intertwined in that differential motions in the atmosphere create themoisture, wind and temperature fields or patterns that are conducive to the development of convection,and hence the thunderstorm (Fawbush and Miller, 1953; Turcotte and Vigneux, 1987; Doswell et al.,1993).

Dynamics refer to the horizontal and vertical structure of the wind field. A vertical wind profile withconsiderable shear is needed for long-lived severe storm formation. Shear refers to the speed differencebetween the surface and another level in the atmosphere (for example, 3-km altitude). Larger shears aregenerally associated with the formation of long-lived thunderstorms. When the shear is small, short-lived‘pulse’ thunderstorms are formed.

Observational (Patrick and Keck, 1987; Davies and Johns, 1993) and theoretical studies (Davies-Jones,1984) have made the case that the low-level directional change in the wind profile is a key requirement fortornado-producing storms. This is embodied in a parameter called storm-relative helicity, which is definedas the cross-product of the storm-motion vector and the low-level wind field from the surface to 3 km.Davies-Jones et al. (1990) presented the case for various values of helicity that correspond to variousintensities of tornadoes. While the empirical and some theoretical studies argue for helicity, others arguethat it is only the shear that is important (Weisman and Klemp, 1984) and the internally generated stormmotions create the necessary conditions for vertical vorticity leading to the formation of tornadoes.

Thermodynamics refer to the vertical structure of the temperature and humidity profile. For explosivegrowth of thunderstorms, the release of the latent heat of the low-level moisture is needed to acceleratethe vertical motions. Dry mid-level air is a common feature that enhances the energetics of this process.A common measure of this is the Lifted Index, which is the difference in the temperature of a surface airparcel lifted to 50 kPa and the wet bulb temperature at 50 kPa and the convective available potentialenergy (CAPE) which is the positive area of the process sounding curve of a parcel lifted from the surface.The larger the absolute value of the Lifted Index or the larger the CAPE, the more explosive thethunderstorm with stronger updrafts and higher cloud tops. A low-level cap or inversion is needed to trapthe moist air; otherwise the moisture is merely distributed over a great depth and little happens. Thestrength of the inversion is called the convective inhibition (CIN), which describes the negative area of thesurface parcel process curve. There must be sufficient lift to overcome the CIN before the storm will form.

A variety of mechanisms can cause the initial vertical motion that is required to break the cap orovercome the CIN. These mechanisms include lift due to cold or warm fronts, outflow boundaries due toexisting thunderstorms, sea or lake breezes, other types of convergence lines, surface heating, upper ormid-level divergence, amongst others. The action of warm and cold fronts can be readily forecast due totheir persistent, large-scale and, therefore, observable nature. However, the lift to initiate convectionprovided by thunderstorm outflow boundaries, convergence lines, sea and lake breezes is more difficult toforecast, because the features themselves are not easily observed. However, the recent development ofsensitive radar has permitted the forecaster to observe these boundary layer features when close to theradar (Wilson and Schreiber, 1986; King, 1996; Wilson et al., 1998; Lapczak et al., 1999). In conditionallyunstable air, these mechanisms are needed to get the vertical motion initiated or they are active and thesemechanisms take precedence in spontaneous convection situations.

Once the lift or updraft is generated, the horizontal vorticity that is created by the low-level wind shearis tilted into the vertical to generate the mesocyclone, which is a rotation of the 5–20-km scale and atmid-levels of the storm, 5–7 km above the ground (Lilly, 1982; Davies-Jones, 1984; Lilly, 1986). The link

Copyright © 2001 Crown in the right of Canada. Int. J. Climatol. 21: 915–938 (2001)Published by John Wiley & Sons, Ltd.

CANADIAN TORNADOES AND ENSO 917

of the mesocyclone to the tornado was first made by Donaldson (1970). Subsequent studies suggestedthat, once the mesocyclone forms, a vertical stretching occurs that makes the mesocyclone extend upwardsand downwards and somehow leads to the formation of the tornado (Burgess et al., 1976, 1982, 1993).

Theoretical objections to this conceptual model lie in that the mesocyclonic rotation is inherentlyassociated with the updraft and that a downdraft would lead to the demise of the mesocyclone and notan intensification of it leading to tornado formation (Davies-Jones, 1982a,b; Davies-Jones and Brooks,1993).

Another low-level mesocyclone exists that is distinct from the mid-level mesocyclone (Klemp andRotunno, 1983; Rotunno and Klemp, 1985; Davies-Jones and Brooks, 1993). The main source oflow-level rotation is baroclinic vorticity, which originates with the mid-level up and downdraft andtemperature structure. The horizontal baroclinic vorticity is tilted vertically by the downdraft; subsidencetransports this air closer to the ground and then it enters the updraft where it is vertically stretched bythe enhanced convergence. The downdraft firsts creates an anti-cyclonic circulation at low levels (�250m above ground level (AGL)) and then merging with the updraft creates a cyclonic circulation at evenlower levels (�100 m AGL) which can be more intense than the mid-level mesocyclone and may lead totornado formation. This low-level mesocyclone is distinct and displaced away from the mid-levelmesocyclone and is difficult to detect by Doppler radar since it is low to the ground and perhaps withoutstrong reflecting targets.

However, it is still unclear how the tornado forms from the low-level mesocyclone (Trapp, 1999). Onemechanism is the vertical stretching of the mid-level mesocyclone, but theoretical arguments indicate thatatmospheric conditions do not support this. Another concept is that tornadogenesis originates as aconcentration of vertical vorticity (the low-level mesocyclone) by the convergent low-level wind fieldsfrom the downdraft to the updraft (Davies-Jones, 1982a,b; Walko, 1993). A third mechanism is thathorizontal shear instability at wind shift lines may cause the sheets of vertical vorticity at the boundaryto be stretched in the vertical by updrafts to form the boundary layer- or gustnado-type tornado (Brandes,1977; Golden and Purcell, 1978; Bluestein and Golden, 1993). Possibly, these vortices might develop intothe mid-level mesocyclone (Wakimoto and Wilson, 1989). However, these explanations do not explain theDoppler observations that have identified tornado vortex signatures (TVSs) that first appear aloft thatlead to the formation of tornadoes within 20–30 min. They tend to be strong and weak, but still potentenough to cause damage. In Canada and elsewhere, the operational Doppler radar observations suggestthat all of these mechanisms are in play.

While the actual mechanism for tornado formation are localized, some broad statements describing theclimatology of severe conditions may be made. The overall geographical pattern of tornado incidence inCanada can be understood within a continental context. The factors that form the climatological synopticconditions for severe weather in Canada are northern extensions of those in the United States. In verygeneral terms, the moisture conditions that originate from a low-level southerly flow from the Gulf ofMexico that affect the central US also affect Canada about 1 or 2 months later.

Some insight to the tornado incidence pattern may be explained by examining prevalent stormformation mechanisms across the country. Adiabatic drying of descending air over the Rocky Mountainsleads to prevalent dry conditions in Alberta, which is known for its predominance of hailstorms. Studiesat the Alberta Research Council’s Hail Project (Chisholm and English, 1973; Krauss and Marwitz, 1982)indicated that severe storms often develop in the foothills of the Rocky Mountains. Solar heating of theeast-facing foothill slopes generates vertical motion to initiate thunderstorms. In the early stages of thestorm, they are often stationary indicating growth in a low-shear environment in the sheltered lee of themountains. As they become larger and taller, the storms then begin to propagate to the east and northeastas the storms move with the flow over the mountains. The cloud bases in Alberta tend to be quite highwith substantial sub-cloud evaporation. While there have been some significant tornadoes in Alberta thatare initiated by other mechanisms (Wallace, 1987; Charlton et al., 1998), the general situation is that thereis a weakly sheared and low moisture environment at low levels due to the synoptic scale environment.


D. ETKIN ET AL.918

In Saskatchewan and Manitoba, moisture from the south originating from the Gulf of Mexico and flowover the Canadian Rocky mountains create the mid-level dry conditions that create the ‘loaded gun oronion shaped’ convective sounding. This also creates the high-shear wind profile conditions conducive tothe formation of severe weather. A dry line, similar to that in Oklahoma, is also often observed as apreferred area of lift for the formation of storms (Knott, per. comm.). So the severe storm formation andtornado mechanisms are similar to that in Oklahoma. Many storms are large and single cell in nature.

In Ontario, northeast of the Great Lakes is an infrequent zone of storm and tornadic activity, whichis the result of the moderating effect of the Great Lakes. In the southwest of Ontario, beginning atWindsor, lies the area of highest tornadic activity in Canada. This is essentially an extension of the broadridge of tornado incidence originating in Oklahoma and stretching through the narrow neck of landbetween Sarnia and Windsor and beyond. The storms in southwest Ontario tend to be low based andproduce heavy precipitation and tend to be embedded, this likely being due to the local moisture sourcesof the Great Lakes. Many Ontario tornadoes are not readily observed because they are obscured by theprecipitation. The classic supercell storm with a single isolated cell is infrequent though not a rarity.Recent studies of storms in Ontario show that lake breeze boundary generated convection may be theprevalent mechanism for storm formation and lead to a preferred southwest-to-northeast axis of tornadicactivity (Sills, 1998). This activity stretches into the province of Quebec.

While the broad description of the storm environments may explain the large-scale climatologicalpattern in a very general sense, severe storm and tornado formation are highly dependent on the localenvironment. Recent studies show that many of the tornadoes, in particular F0–F2, originate onboundaries (Wilson et al., 1998). These boundaries can originate from many mechanisms and may explainthe localized minima or maxima in the tornado incidence patterns. Boundaries in drainage flows inmountains valleys may explain the development of thunderstorms in the Prince George area of BritishColumbia. Clear air boundaries due to drainage flows, thunderstorm outflows and reverse Prairie flowhave been seen on Doppler radar in the Edmonton area of Alberta and may explain the local maximathere. In Ontario, lake breeze boundaries may be the dominant mechanism for convective initiationexplaining the Ontario tornado alley.

3. HISTORICAL TORNADOES (SOURCE: NOTES BY NEWARK, COMPILED IN THE 1980s)

The first report of a tornado in Canada occurred on 1 July 1792. A violent tornado passed over thesouthwestern end of the Township of Thorold, from present day Fonthill to Port Robinson in Ontario.It levelled all houses in its path and uprooted trees along a narrow track. The early settlers tookadvantage of this to build a road along the trail of fallen trees, which they named Hurricane Road.

The first eyewitness report of a tornado in western Canada occurred on 11 July 1826 at the Red RiverSettlement (present day Winnipeg, Manitoba). A waterspout (tornado) was reported by Francis Heron,the Hudson’s Bay Company clerk. It tore up trees, damaged the roof of the Protestant church and tossedpeople into the air.

The first reported tornado death, on 7 August 1844, occurred near Galt, Ontario (now part ofCambridge). A tornado tore roofs from barns, destroyed fences and clogged the roads with fallen trees.Mrs McIntyre, the wife of a schoolmaster, was crushed ‘one foot into the ground’ and killed by a fallingelm tree.

On 5 July 1850, multiple deaths due to a major tornado were reported for the first time. Slashing a pathat least 70 km in length and up to 1500 m wide across present day Durham Regional municipality inOntario, this tornado took the lives of three, or possibly four, people as well as killing many cattle, sheep,horses and geese. Another four people were injured along its track, which stretched from Lake Simcoesoutheastwards to Lake Scugog and then towards Lake Ontario.

The largest known tornado outbreak happened on 11 June 1968. Multiple tornado events on the sameday are quite common. A well-developed cold front or squall line during the tornado season is capable



of producing several tornadoes. On this particular day, 12 occurred in southern Ontario. This is far fromthe maximum possible number, however. In England, for example, more than 100 were recorded on 23November 1981; while in the US 148 occurred during a 16-h period on 3–4 April 1974.

The worst Canadian tornado occurred at Regina on 30 June 1912. Twenty-eight people were killed,hundreds injured and property damage amounted to $4 million (1912 dollars). In comparison, the US (the‘tornado capital of the world’) has experienced 94 tornadoes that resulted in more than 28 deaths perevent up to 1993 (Grazulis, 1993), the worst of which occurred on 18 March 1925 and killed 695 peoplein the states of Missouri, Illinois and Indiana. A tornado on 14 April 1969 in Bangladesh, however, killed922 and injured 16 511 people.

The first tornado warning in Canada was issued on 14 July 1950 by the Forecast office in Regina,Saskatchewan. It was prompted by the report of a tornado sighting by a Trans Canada flight west ofJohnstone Lake, Saskatchewan.

Though perhaps of more literary than historical interest, on Monday, 20 May 1996, a tornado rippedthrough a drive-in theatre in Niagara (source: The Tribune, Wednesday, 22 May 1996) that was, ironically,showing the movie Twister, in which a tornado destroys a drive-in theatre.

4. TORNADO IMPACTS

Much of the damage that occurs from thunderstorms that create tornadoes occurs from hail and flooding,and it is impossible to separate out the individual costs. The 11 worst tornadoes in Canada since recordsbegan to be kept, by order of death toll are:

(i) Regina ‘Cyclone’, Saskatchewan; 30 June 1912. Twenty-eight dead and hundreds injured. Damageof $4 million.

(ii) Edmonton, Alberta; 31 July 1987. Twenty-seven dead and 300 injured. Thousands homeless.Damage of $300 million (see Charlton et al., 1998 for a comprehensive review of this storm).

(iii) Windsor to Tecumseh, Ontario; 17 June 1946. Seventeen dead and hundreds injured. Damageconservatively estimated at $1.5 million.

(iv) Hopeville to Barrie, Ontario; 31 May 1985. Twelve dead and 155 injured. More than 1000 buildingsdamaged. Damage of $100 million. A description of this event follows in the case study.

(v) Pine Lake, Alberta; 14 July 2000. Twelve dead and hundreds injured. Six hundred and thirty-fivetrailers and recreational vehicles (RVs) severely damaged or destroyed.

(vi) Lancaster Township, Ontario to St-Zotique to Valleyfield, Quebec; 16 August 1888. Nine (possibly11) dead and 14 injured. Extensive property damage.

(vii) Windsor, Ontario; 3 April 1974. Nine dead and 30 injured. Damage of $500 thousand.(viii) Sudbury, Ontario; 20 August 1970. Six dead, 200 injured. Damage of $10 million or more.

(ix) St. Rose (Montreal) Quebec; 14 June 1892. Six dead and 26 injured.(x) Buctouche, New Brunswick; 6 August 1879. Five to seven dead and 10 injured. Damage of $100

thousand and 25 families homeless.(xi) Portage La Prairie, Manitoba; 22 June 1922. Five dead and scores injured. Damage of $2 million.

There have been six tornado disasters from 1985 to 1997 (Table I), that have cost the insurance industry$339 million ($1995 CAN) (IBC, 1997) from almost 120 000 claims.

Tornadoes also occasionally impact the hydro-electric industry, when they cause tower failures, asshown in Table II which shows damage caused to Ontario Hydro by severe weather. Power lines areparticularly susceptible to tornadoes in Ontario and the Prairie Provinces, since they mostly runnorth–south, while the tornadoes tend to move west–east.

In Canada, there have been at least 231 confirmed deaths (Table III) from 111 tornadoes (sources:Newark, 1983, 1988; Hage, 1994; AES Regional Weather Centres).


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Table I. Tornado disasters from the insurance perspective (1984–1999)a

CostYear Location(millions 1995 $)

1985 83.9Barrie, Ontario1987 148.4Edmonton, Alberta1988 Medicine Hat, Alberta 50.01991 Sarnia, Ontario 18.71991 25.4Maskinonge, Quebec1994 Aylmer, Quebec 12.8

a Other tornadoes have occurred costing lesser amounts, but were not defined as a ‘disaster’ by theIBC.

Table II. Damage to Ontario Hydro from severe weather, 1988–1998

RepairDate of event CommentsDescription of event Cause of(YY-MM-DD) costs inevent

$000

96-04-20 Tower failure Tornado Three towers failed north of2800Grand Valley

95-11-11 Pole failure 20 poles failed north of KingstonWeather 250(unknown)

Three towers replaced north of Barrie95-05-22 500 kV tower failure Tornado 35093-07-05 500 kV tower failure Two towers replaced north of SudburyTornado 700

16 towers replaced north of Kingston92-11-13 230 kV tower failure Tornado 300091-07-07 230 kV tower failure Seven towers replaced near SarniaTornado 1400

Single circuit 500 kV towers failed88-08-02 500 kV tower failure Weather 615(unknown) north of Sudbury

85-05-31 230 kV tower failure Tornado 300 Double circuit tower failures. Barrie85-05-31 500 kV tower failure Single circuit tower failures. BarrieTornado 40085-05-31 500 kV tower failure Double circuit outage Grand ValleyTornado 1800

Table III. Tornado deaths by province (1879–1999, all known events)

Province No. of deathsNo. of events

Saskatchewan 8534Ontario 26 84Alberta 16 65Quebec 289Manitoba 6 9New Brunswick 2 6

These deaths tend to occur episodically, with the years 1912 (Regina cyclone) and 1985 (Edmontontornado) showing the greatest spikes.

The National Building Code of Canada (NBCC, 1995) reported that during the 1985 Barrie tornadoes,buildings in which over 90% of the occupants were injured or killed, did not satisfy two key criteria forbuilding construction. First, the floors were not anchored to the foundations (the entire house was liftedand moved from its foundation with the occupants inside— in one case because washers were not put onthe anchoring bolts). Second, roofs were not properly anchored to the building walls. The floors on whichpeople were standing actually became airborne, causing casualties on impact with the ground (Allen,1986). The latter caused roof lift-off resulting in wall collapse. Almost all of the deaths and injuriesoccurred in cases where the NBC was not followed correctly. Structures built according to the NBC fared



well overall. Wind speeds in the 160–200 km/h range correspond to the maximum resistance of buildingsdesigned according to the NBC. The NBCC (1995) recommends building in such a way as to protectagainst tornado damage for areas in Canada ‘prone to tornadoes’, though it is not clear what is meantby the phrase ‘prone to tornadoes’.

Carter et al. (1989) analysed risk factors related to deaths and injuries in the Barrie tornado. There were12 deaths, 48 serious injuries and 233 minor injuries (4, 16 and 80%, respectively). The deaths resultedfrom head and chest trauma. Eleven of the 12 deaths occurred before the injured could reach the localhospital. Ten of the 12 deaths resulted from becoming airborne, the remaining two by being crushed.Head and neck injuries accounted for about 49% of the seriously injured, with fractures andconcussion/brain injuries the most common diagnoses. Most of these injuries resulted from being struck(60%) while 25% became airborne. The head and neck injuries were also the most common in the minorinjury group, followed by the arm and the back–spine area. Most of these injuries resulted from beingstruck by moving objects, often flying glass.

Tornado warnings are an important part of reducing society’s vulnerability. Unfortunately, providingtimely tornado warnings that are delivered to, heard by and acted upon by the population at risk is achallenging task to say the least. In the 31 May 1985 Ontario event, 91% of affected people only had aone minute warning or less (Carter et al., 1989). Due to power outages from the tornado, radio and TVwere of no use, and those who most needed to be aware of it did not hear the weather warning put outby Environment Canada. In the Aylmer tornado of 4 August 1994, 3% of those affected heard theEnvironment Canada weather warning (White et al., 1995). White estimated that 74% of the residents hadno knowledge of appropriate response to a tornado. These two studies suggest that tornado warnings arenot effective in terms of optimizing people’s response to tornadoes, in Canada at least.

The potential exists in Canada for a tornado disaster much worse than has happened thus far; forexample:

� The 1987 Edmonton tornado was an F4, but only at F2 strength when it went through the trailer parkwhere most of the fatalities occurred; 15 of the 27 deaths occurred in a trailer park (Black Friday,Edmonton Sun/Jasper Printing Group, 1987). Had it been an F4 in that vulnerable region, many morewould have been killed and injured.

� If the 1985 Barrie F4 tornado had been a few kilometres further north, it would have passed overapproximately 1000–1500 homes, six schools with 70–75 portable classrooms, a senior citizens’ centre,and a shopping mall. In the 1985 event, four of eight unanchored portable classrooms in the tornadopath became airborne and disappeared. Approximately one-third of the 605 homes in the originalevent were left completely uninhabitable, with fatalities of about one person per 50 damaged homes.If one-third of the homes-at-risk were damaged, this translates into six– ten deaths. Deaths could bemuch higher at the schools. Assuming an average of 20 students per portable, up to 1500 studentswould be at significant risk. If one-half of the portables became airborne, as occurred in the 1985event, then over 700 students would be at very high risk.

5. PREVIOUS STUDIES

The first study of tornadoes in Canada was produced by Lowe and McKay (1962) who compiled a recordof 200 tornadoes that occurred in western Canada between 1890 and 1958. Their study indicated thattornadoes struck southeastern Saskatchewan and southwestern Manitoba more often than other parts ofthe Prairies, with 76% of events occurring in June and July.

Newark (1983) produced a national climatology based on events from 1950 to 1979, which included afactor designed to correct for biases due to population density. He assumed that all tornadoes would bereported where the population density exceeded 1 person/km2.


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Table IV. Tornado frequencies in urban Alberta andSaskatchewan from 1910 to 1960 (Hage, 1987)

No. tornadoes/year/10 000 km2City

Edmonton 8.6Calgary 3.5Regina 18

15SaskatoonMoose Jaw 33Lethbridge 13Medicine Hat 36

Hage (1987, 1994) studied tornadoes in Alberta and Saskatchewan1 from the period 1879 to 1984,estimating that there were over 700 tornadic events and over 1000 other severe windstorms that werenon-tornadic in nature. He found that 90% of the tornadoes occurred between 1 June and 15 August.Through an analysis of building damage, he found a downward trend in the frequency of destructivewindstorms between 1915 and 1940. Spatial patterns were highly variable, with some regions havingfrequencies an order of magnitude larger than others. The greatest frequencies were in ‘southeastern andsouth-central Saskatchewan, and to a lesser extent in a band through central Alberta from nearEdmonton to west of Medicine Hat’. He concluded that calculations of tornado frequencies were onlyreliable in urban areas; his urban frequencies are shown in Table IV.

Paul (1995a,b) found that of 118 tornadoes found in Saskatchewan during the period 1950–1991, 48were strong (F2 or greater— two F4, eight F3 and 38 F2). By comparison, North Dakota had 114 (Pauldoes suggest that there might be some methodological differences in data collection, though thecomparison has validity), supporting the conventional wisdom that the US gets more tornadoes thanCanada. Up to 13 of these events occurred in the city of Regina, though an ‘extremely conservative’estimate would be three events. This leads to frequencies of between 3.5 and 15 tornadoes/year/10 000km2. Frequencies in larger blocks, mainly including rural regions were much smaller; the largest being0.56 tornadoes/year/10 000 km2 for the entire period of record, or 0.97 for the 1971–1991 period. Hefound that 35% of the strong tornadoes occurred in June, 31% in July and 28% in August. Average pathlengths where they could be estimated were 16 km, with the longest track being 40 km. Path widthsranged from 20 m to 1.3 km.

Murray (1990) estimated city tornado incidences for a number of cities across Canada (Table V). Theseestimates have been criticized as being too high, as many of the events assumed to be tornadoes may havebeen downbursts. Of course, from a damage perspective this distinction is not important, as a buildingcares little whether a tornado or a microburst destroys it. Frequencies in eastern Canada may be highrelative to western Canada since Newark concentrated his research there.

Paruk and Blackwell (1994) developed a severe thunderstorm climatology for Alberta that included acorrection for population density, based upon a linear regression of severe thunderstorm frequency andpopulation density for Alberta counties. Their figure 13 shows frequencies ranging from 1 to 13tornadoes/year/10 000 km2. The largest incidences occur in south central Alberta.

Etkin (1995) analysed the historical tornado record in an attempt to determine Western Canada’s futuretornado climatology. Comparing the frequency of tornado events before and after 1980, the datasuggested that tornado frequencies increase with positive mean monthly temperature anomalies. Theinference was made that if Canada’s climate warms, then a corresponding increase in tornado frequencieswill be exhibited. This is consistent with some other research; for example, Price and Rind (1993)suggested that a doubling in atmospheric CO2 with a 4.2°C warming would result in a 72% increase incloud-to-ground lightning strikes. Since lightning is a result of thunderstorm formation, this may logicallybe extended to suggest that more thunderstorms, possibly even more severe tornado producingthunderheads, may result from global warming.



Table V. City incidences according to Murray (1990)

No. tornadoes/year/10 000 km2City

Montreal 1.3Toronto 6.7

29.0WoodstockLondon 7.8Windsor 15.0Sarnia 34.0

5.2WinnipegRegina 16.0Saskatoon 9.3Calgary 0.96

1.5Edmonton

6. DATABASE

The tornado database is based upon the original data collected by M. Newark of the AES (Newark, 1983)up to about 1990, combined with events listed by the annual severe weather reports from the regionalweather offices of Environment Canada for the post-1980 time period. Newark attempted to rate theprobability that an event was indeed a tornado using the categories of confirmed (34% of the finaldatabase), probable (20%), and possible (22%). A total of 23% of the events were not rated, most likelybecause they were simply listed as a tornado in the regional severe weather report with no comment asto its probability. It is likely that unrated events would best fit into the categories of ‘confirmed’ or‘probable’. Quality control is currently done by the severe weather co-ordinators at each weather centre,and will vary depending upon the resources available to them. Most tornadic events are not subject tofield investigations, resulting in limited data (e.g. path lengths and width data are not normally available).

7. BIASES IN THE DATA

Many authors have discussed the biases intrinsic to the various US and Canadian tornado databases.Among these, population and topographic biases have received the most attention (Abbey and Fujita,1979; Newark, 1983; Raddatz and Hanesiak, 1991; Grazulis et al., 1993; Paruk and Blackwell, 1994).Tornadoes that are not reported do not end up in tornado databases, and therefore there is a well-knownbias in tornadoes reported, such that regions with greater population densities report more events. Othershave discussed economic factors (Schroeder and Agee, 1971) and F-scale reporting biases (Tecson et al.,1979). Weaker tornadoes that cause lesser amounts of damage are less likely to be reported than strongertornadoes, which results in a bias towards stronger events, though strong tornadoes over unpopulatedterrain would cause little damage, and therefore would generally be underclassified (since the tornadointensity scale is damage based). Canada, though slightly larger than the US, has only about one-tenth itspopulation. Embedded in this country, though, are some very dense urban regions. The effect of varyingpopulation densities in Canada is possibly the most important bias, though this cannot be proven.

Etkin and Leduc (1994) show that non-tornadic severe weather events in Ontario can have a populationbias as large as 10:1. Paruk and Blackwell (1994) show a population bias for tornadoes of up to an orderof magnitude (see their figure 8). Hage (1987) calculated large incidences for some cities in the Prairies,from which one can infer large biases. Newark (1983) noted that tornadoes in unpopulated areas are veryseldomly observed. McNulty (1981) also found that tornado reports are strongly related to populationdensity and that the problem of accurate tornado reporting is greatly magnified in the sparsely inhabitedregions. Schaefer and Galway (1982) estimate that two of three tornadoes are observed in the US, thoughthey reference two other studies suggesting ratios of 1:3 and 4:10. Snider (1977) notes that about


D. ETKIN ET AL.924

two-thirds of rural tornadoes are not reported, and that statistics from urban areas are more reliable thanthose from rural areas. Bieringer et al. (1996) shows that tornado occurrence appears to be directlycorrelated with NWS radar locations and/or population centres. Biddle (1996) notes that linear patternsof historical tornado tracks on path map seem to concentrate along highway routes and that areas of lowpopulation density correspond to areas of low tornado incidence.

Prior to around 1980, tornado sightings were not routinely archived by the AES of Canada. Events upto that time were researched using various historical sources (e.g. newspapers), and suffer from regionaldisparities, particularly with respect to the efforts put into it by researchers who tended to concentrate onregions near where they lived. For this reason, plots of tornado events and frequencies using post-1980data better reflect regional disparities, and more accurately reflect the actual number of tornadic events(though still subject to biases).

Some events reported as tornadoes are undoubtedly non-tornadic severe wind events (such asdownbursts). This would be particularly true in the early and mid-1980s. As the downburst phenomenonbecame better understood, fewer events were reported as tornadoes and more as microbursts. Often thereis insufficient information for a meteorologist to make a good assessment of the type of severe weatherevent, since few site assessments are made in Canada.

Tornadoes are rated by the F-scale in Canada. This is a damage-based scale that ranges from F0 (weak)to F5 (devastating). No F5 event has ever been reported in Canada, though there have been many in theUS, even relatively close to the Canadian Border. Strong tornadoes that do not hit any structures will berated weaker than they actually are, and therefore the statistics on F-scale are biased towards lower valuesby an unknown amount.

8. DESCRIPTION/ANALYSIS OF DATA

Though some of the events listed in the database are undoubtedly non-tornadic in nature (e.g.downbursts), the following analysis includes all events, whether confirmed or not. This imposes a slightbias towards a greater number of tornadoes than actually occurs. Of course, the bias of unreported eventsis in the opposite direction, and likely to be of greater magnitude. The most common tornadoes observedin Canada are F0 (Table VI), with the frequencies rapidly decreasing to zero for F5. By comparison, theUS frequencies from 1990 to 1998 (calculated from 1200 events) are shown (Harold Brooks, per. comm.).

The greatest number of tornadoes are reported in the province of Ontario, followed by Saskatchewan,Alberta and Manitoba (Table VII).

On average, 60 tornadoes are reported each year in Canada (Table VIII), though the numbers reportedvary considerably from year to year. Though recent years tend to have more tornadoes reported, probablydue to a better observing system, the percentage of tornadoes in each category is very stable (Table IX).This was somewhat surprising to the authors, who had thought that the inclusion of downbursts, which

Table VI. Frequency of tornadoes by intensitya

Canadian%cCanadianF Scale US%(1990–1998)events

32.6355F0 60.6F1 161 14.8 27.3F2 8.882 7.5F3 2.724 2.2

0.70.33F4F5 0 0.0 0.1

42.6463Unknown

100Total 1088

a There is no significant difference in these statistics between western and eastern Canada.



Table VII. Frequency of tornadoes by province, 1980–1997

Province No. of events % Events

Nfld 0.11PEI 0.22NS 2 0.2BC 0.89

1.1NB 12Que 58 5.3Man 13.6148Alta 241 22.2Sask 268 24.7Ont 31.8345

Total 100.01086

is likely a greater problem in the early and mid-1980s than after that time period, would result in a higherfrequency of F0 and F1 events. Either misreporting of downbursts as tornadoes is not significant, or thatbias is compensated for by lower magnitude tornadoes being reported in more recent years.

Tornadoes occur most frequently in June and July (Table X), and more rarely in the other months,though there has not been any month without at least one tornado reported. The patterns are slightlydifferent in eastern and western Canada (Etkin, 1995), with a longer season in the east. There is a strongdiurnal component to tornado occurrence, as solar heating is a trigger for severe thunderstorms.Approximately 70% of tornado observations occur between 14:00 and 22:00 h, and 40% between 16:00and 20:00 h.

The original Canadian tornado database consisted of tornadic events by latitude and longitude for theperiod 1918–1997. Records prior to 1980 were removed and the subset of the data for the period1980–1997 was used for the analysis. Canada’s regional severe weather offices were initiated around 1980and, therefore, the tornado database is assumed to be much more homogeneous after that time.

The 1980–1997 tornado database, a base map of Canada and a map delineating all 0.5°×0.5° grid cellsin Canada were imported into ArcView(tm). The number of tornadic events and the amount of land areawere computed for each 0.5°×0.5° grid cell in Canada. The tornado frequency (number ofevents/year/km2 land area) was computed for each grid cell. Tornado frequencies were then normalized by10 000 km2 to produce the number of events per year per 10 000 km2 land area for each grid cell. Thefrequencies were normalized in order to maintain consistency with previous analyses (e.g. Newark, 1983).

The tornado frequencies were then mapped to the centroids of each 0.5°×0.5° grid cell to create apoint map of tornado frequencies across Canada. Since most 0.5°×0.5° grid cells in Canada are roughly2000 km2 depending on latitude, a kriging algorithm was used to produce tornado frequencies at a finerresolution. A kriging algorithm with a spherical semi-variogram (range=4.3; sill=0.68; nugget=0.4)and a search radius of 100 km was used to interpolate a contour map of tornado frequencies down to aresolution of approximately 400 km2.

The spatial pattern of tornado events in Canada is shown in Figures 1–3. Figure 1 shows the locationsof all touchdowns during the period 1980–1997. The events are clustered in the well-known tornadoregions of southern Ontario and the southern Prairies. Figure 2 is a plot of tornado frequencies by0.5°×0.5° grid squares. These gridded values were smoothed and contoured as described above, and areshown in Figure 3(a) and (b).

These areas correlate well with population density, as can be seen when these figures are compared toFigure 4, with the exception of the maritime provinces on the east and west coasts. In the PrairieProvinces, the main centres of occurrence are located around the major Canadian cities of Edmonton(Alberta), Saskatoon and Regina (Sask), and near and south of Winnipeg (Manitoba). Lobes of higher


D. ETKIN ET AL.926

Table VIII. Trends of observed tornadoes since 1980

Year No. of observedtornadoes

1997 461996 591995 40

9519941993 57

391992691991521990861989

1988 671987 721986 711985 421984 761983 611982 551981 39

Mean 60.4S.D. 16.2

Table IX. Relative frequency of tornadoes during two time periods

1988–1997 (10 years)1980–1987 (8 years)F-scale

Number % Number %

F0 152 31.9 203 33.215.574 95F1 15.5

45 9.4 5.131F22.022.512F3

F4 3 0.6 0 0.0F5 0 0 0 0

44.327140.0191Unknown

Table X. Frequency of tornado observations by month

Month % Frequency

0.3JanuaryFebruary 0.2

0.2March2.8April

11.3MayJune 32.5July 30.3August 14.4

6.3September1.6October0.1November0.0December



Figure 1. Locations of reported tornado touchdowns, 1980–1997

Figure 2. Tornado frequency per 0.5°×0.5° grid (events/year/10 000 km2)

frequencies running south from Winnipeg, then westward to Regina and Sakatoon, westward again toEdmonton then southward along the foothills of the Rocky Mountains appear to be suspiciously close tomajor highways (it should be noted though, that the foothills of the Rockies are well known to be an areawhere severe thunderstorms are generated). Relatively heavy traffic on these (and other) routes may leadto more frequent reporting (as noted by Biddle, 1996). In Ontario, the main maxima are along thesoutheast coast of Lake Huron, near the Windsor area and around Toronto. Windsor and Toronto aremajor population centres, and may experience more frequent reporting than rural regions. In addition, theOntario Weather Centre is located in Toronto, and this concentration of meteorologists may enhancesevere weather reporting. Windsor is well known as getting more frequent tornadoes than other areas inOntario, as it lies at the northeast portion of the US tornado alley. The patterns in the area surroundedby the Great Lakes are a function of complex topography and interaction of sea breezes that occur. Someedge effects may be occurring that artificially enhance frequencies along the coasts since land areas can


D. ETKIN ET AL.928

Figure 3. (a) and (b) Tornado frequency after smoothing (events/year/10 000 km2)



be small. Frequencies are calculated by dividing by the land area in the grid, and sampling error in arelatively small dataset can result in anomalously large values. An attempt was made to minimize thiseffect by eliminating all cells with less than 5% land area. The maximum around Toronto is puzzling, andcertainly does not reflect the real pattern. Toronto is well know as being in an area less subject to severethunderstorms than to its northwest, due to downslope conditions when the flow is westerly. In fact, if thegrid cell containing Toronto is examined closely, almost all of the events are located north, northwest andwest of the city. The lobe running southwest from Toronto lies along a major highway (c401), andanother lying northward towards North Bay also lies near another travel route (c400). The maximumat Sarnia (see Table V) at the extreme southern part of Lake Huron can best be seen in Figure 3. Thismaximum, also supported by anecdotal information, may be due to local wind convergence in that area,when a northwesterly flow gets funnelled by the lake. It should also be noted that there is ameteorological observing station at Sarnia.

Separating out the effects of climatology from population, travel routes and other confounding factorsis an extremely difficult task, not possible with the dataset as it currently exists.

Figure 4. Population density. Reproduced by permission of Statistics Canada


D. ETKIN ET AL.930

Table XI. Years following the onset of strong to moderate El Nino events. (a) During these events the 5-monthrunning mean of the SOI remained in the lower 25% of the distribution for 5 months or longer. (b) During theseevents, the 5-month running mean of the SOI remained in the upper 25% of the distribution for 5 months or longer

1911–1919 1920–1929 1990–19971930–1939 1940–1949 1950–1959 1960–1969 1970–1979 1980–1989

(a)1919 1926 19921930 1942 1952 1966 1973 1983

1927 1931 1940 19931954 1970 198719941958 197719951959

(b)1918 1925 1939 19961951 1965 1971 1989

1929 1956 197219741976

9. THE EFFECTS OF ENSO ON TORNADIC ACTIVITY IN CANADA

The ENSO phenomenon occurs irregularly at intervals of roughly 2–7 years (Quinn et al., 1987). Ittypically lasts 12–18 months, and accounts for the largest interannual variability in the climate system.The manifestations of the changes in the tropical atmospheric circulation are felt throughout the globalatmosphere via teleconnections. Horel and Wallace (1981) showed that ENSO affects the North Americanclimate via the Pacific North American (PNA) teleconnection pattern. Yarnel and Diaz (1986)demonstrated that the negative phase of the PNA pattern accompanies the cold phase of ENSO. Thislongwave pattern typically occurs in association with specific ENSO events, though it can occur as a resultof the internal dynamics of the climate system. The impact of ENSO on the North American climate hasbeen well documented (Ropelewski and Halpert, 1986; Shabbar and Khandekar, 1996; Shabbar et al.,1997).

Tornadoes, which occur mainly as microscale features, frequently bring destruction, and wind gusts canexceed 500 km/h. Tornadoes, by their very nature, are elusive. However, if during certain years theiroccurrence departs significantly from normal, and if the underlying cause of the change can be identifiedthen this information may lead to a generalized prediction scheme. Although ENSO events cannot beresponsible for individual tornado outbreaks, ENSO alters the atmospheric circulation so as to producean environment in which the creation of these violent storms is either enhanced or suppressed dependingon the phase of ENSO. These changes may involve subtle or dramatic changes in the position andintensity of jet streams and moisture flux in the atmosphere. The objective here is to compare statisticallytornadic activity in different ENSO phases. Only those tornadoes with F2 or greater will be examined inthis context.

9.1. (a) Data

The circulation anomalies in the extratropics can be related to the Southern Oscillation Index (SOI)(Deser and Wallace, 1987; Shabbar et al., 1997). Strong to moderate ENSO years have been defined asthose years in which the 5-month running mean SOI remains in the lower 25% (El Nino) or upper 25%(La Nina) of the distribution for 5 months or longer (Table XI(a–b)). This definition is consistent withthe canonical El Nino event as described by Rasmusson and Carpenter (1982). The remaining years areclassified as neutral years. Both annual and seasonal totals of tornado counts are created from theCanadian tornado database. Spring is defined as Mar–Apr–May, the rest of the tornado season islabelled as summer, which mainly comprises the months of Jun–Jul–Aug–Sep. Relative to the summer



season, there are considerably fewer tornadoes in spring (Table X). Most of the emphasis will, therefore,be placed on the summer season.

In this section, observed tornado occurrences in Canada are analysed statistically in the context of ElNino, La Nina and neutral years. Separate analyses are carried out for eastern Canada (Ontario eastward)and western Canada (Manitoba westward). Trenberth and Hoar (1997) noted that the character andfrequency of ENSO events have changed in the last 23 years between 1976 and 1998. In this context, wewill examine the nature of tornado frequency in Canada during the same period. By calculatingnon-parametric correlations, we will seek an association between the SOI and the number of tornadoes inCanada.

9.2. (b) Frequency analysis

Frequency of tornadoes in Canada in spring, summer and annually during El Nino, La Nina andneutral years are shown in Figure 5(a) for western Canada and in Figure 5(b) for eastern Canada. Forcomparison purposes, the bars on the right, in each panel, indicate tornado climatology over the 80-yearperiod (1918–1997). For eastern Canada during the spring season, no significant difference in tornadicactivity is evident relative to neutral years during La Nina; however, strong increase in tornadic activity(relative to neutral years) is found during El Nino springs. During summer, substantial decreases intornadic activity are found during both the warm and the cold phases of ENSO. For western Canada, nosignificant difference between the neutral spring and El Nino spring is evident. During La Nina spring,there is a decrease in the activity. In summer, El Nino years bring an increase in tornado occurrences, butLa Nina brings a decrease in tornadic activity. Thus, in western Canada, relative to neutral years, tornadooccurrence events are enhanced in summer, and during La Nina summer tornadic activity is suppressed.This apparent linear relationship will be further analysed later.

9.3. (c) Statistical comparison of means

The statistical difference between the means of the two phases of ENSO and neutral years is determinedby comparing the characteristics of their distribution. Tornadoes can occur in clusters over a relativelysmall period of time. For the most part, however, a single tornado can be regarded as an independentevent. The number of tornadoes during an ENSO year and neutral year is considered a discrete randomvariable, and thus follows a Poisson distribution. The means of the Poisson distribution have a binomialdistribution Lehmann (1986). The Z statistics can then be calculated as follows (Wilks, 1995),

Z=��

p−0.5−� r � n

n+m�

�r� n

n+m�� m

n+m��

��

(1)

where p is the number of tornado events in n ENSO extreme years, and q is the number of events thatoccurred in m neutral years, and r=p+q.

Using Equation (1), the confidence level at which the means of El Nino and neutral years and La Ninaand neutral years are different from each other can be determined for eastern and western Canada forspring, summer and the entire tornado season. Table XII shows the Z statistics and the confidence levelsat which the means of various ENSO extreme years and neutral years are statistically different. During LaNina years, the number of tornado occurrences are suppressed relative to the neutral years in both easternand western Canada in summer and annually at the 1% level. The number of tornadoes in eastern Canadaduring spring is not statistically different from neutral years. In the El Nino extreme years, moretornadoes occur in summer and annually in western Canada, and in spring in eastern Canada at the 5%level. The number of tornadoes in eastern Canada, however, is smaller than in neutral years during


D. ETKIN ET AL.932

Figure 5. (a) Number of tornadoes in El Nino, La Nina and neutral years in western Canada. Climatology is shown by the barson the right. (b) Number of tornadoes in El Nino, La Nina and neutral years in eastern Canada. Climatology is shown by the bars

on the right

summer in eastern Canada. Statistical testing for difference in the means between ENSO extreme andneutral years supports the results of the frequency analysis above.

To elucidate further the statistical relationship found in the differences in the mean during the twophases of ENSO in western Canada during summer, the probability of tornadoes with respect to ENSOphenomena is assessed empirically. The analysis is done for both western and eastern Canada during the



summer season. Figure 6(a) shows that when compared to the summers of El Nino years, La Ninasummers exhibit a lower occurrence of tornadoes. The probability of 15 or more tornadoes during a LaNina summer is 35%, as compared to 50% during an El Nino summer. For 30 tornadoes or more,however, the probability is nearly the same for the two phases of ENSO. In eastern Canada (Figure 6(b)),the probability of tornadoes during a La Nina summer is consistently lower than during an El Ninosummer. In this part of the country, the probability of 15 or more tornadoes is about 15% during thesummer of La Nina year and 25% during an El Nino summer.

9.4. (d) Association between SOI and western Canadian tornadic acti�ity

Through the PNA teleconnection pattern, ENSO manifests its largest variability over North Americaby producing a positive height anomaly centre over western Canada during its warm phase. During thecold phase of ENSO, a negative anomaly centre is found in the same location, suggesting a linearrelationship (Hoerling et al., 1997). To what degree is the ENSO variability reflected in tornadooccurrences in western Canada? Both the frequency analysis and comparison of means analyses suggestthat El Nino years bring more tornadoes than in neutral years and that La Nina years are accompaniedby fewer tornadoes than in neutral years in summer. Since the number of tornadoes does not follow anormal distribution, a non-parametric measure is used to explore the relationship. Spring SOI is relatedto the annual occurrences of tornadoes. While the Pearson correlation coefficient reflects the strength ofthe linear relationship between the SOI and tornado occurrences in western Canada, the Spearman rankedcorrelation that reflects the strength of monotonic relationships is used. The spring SOI and the annualnumber of tornadoes in western Canada are ranked and the Spearman rank-order correlation co-efficientis calculated. Although the plot between the two parameters shows considerable scatter (Figure 7), andthe Spearman correlation r= −0.25 is modest, the correlation is statistically significant at the10% level. The analysis suggests a monotonic relationship between the tropical SOI and tornado count inwestern Canada. In so far as skilful prediction of the SOI can be made several seasons in advance(Keppenne and Ghil, 1992), some idea of the count of number of tornadoes in western Canada can beassessed.

9.5. (e) Recent changes in ENSO and tornadoes in western Canada

Since 1976, there have been more El Ninos and fewer La Ninas occurring, including the two biggest(1982–1983, 1997–1998) and the longest (1990–1995). Trenberth and Hoar (1997) hypothesized thatglobal warming may have influenced this changed behaviour in ENSO. It also seems coincidental that the

Table XII. Confidence level (CL) at which the mean of tornado count in the ENSO extreme years arestatistically different from the mean of tornado count in neutral yearsa

El Nino La Nina

Z stats CL More (+)/Less(−) Z statsCL More (+)/Less(−)

Eastern Canada5% + 1.92 — 1.14Spring5% − 1.88 1% − 3.78Summer

3.89−1%Entire tornado season 0.92—

Western CanadaSpring — 0.20 1% − 2.37

+5% 3.16−1%1.68SummerEntire tornado season +5% 1.69 1% − 3.68

a Dash indicates non-significance. Positive (negative) sign indicates the confidence of more (less) tornadoes than in neutralyears. Numbers represent the Z test statistics.


D. ETKIN ET AL.934

Figure 6. (a) Inverse cumulative frequency distributions of western Canadian F2 or greater tornadoes during summer. Solid lineindicates warm phase of ENSO, dashed line indicates cold phase of ENSO. (b) Inverse cumulative frequency distributions of easternCanadian F2 or greater tornadoes during summer. Solid line indicates warm phase of ENSO, dashed line indicates cold phase of

ENSO



Figure 7. Scatter plot of ranked spring SOI with ranked annual tornado count in western Canada. The Spearman correlationcoefficient is −0.25, significant at the 10% level

number of tornadoes in western Canada has increased dramatically in the same last 25 years (Figure 3).Whether this increase is due to the burgeoning population, and hence to ability to observe moretornadoes, as well as better detection methods, or whether the increased frequency and strength of ElNinos are influencing the behaviour of tornado counts, may be difficult to separate. However, it wouldappear that the results presented above, relating El Nino years to more occurrences of tornadoes, wouldsupport the increasing frequency in both El Ninos and the occurrences of tornadoes in western Canada.

9.6. ( f ) Summary

The results in this section show that the cool temperatures that usually accompany La Ninas suppresstornadic activity in summer in Canada. Unlike the Tennessee and the Ohio Valleys, where tornadoes aremore prevalent during spring in La Nina years, spring in eastern Canada experiences no significantchange in tornadic activity. Directional wind shear between 850 and 500 hPa is an important parameterfor tornado genesis. The absence of strong wind shear during La Nina years in Canada (not shown) thusinhibits tornado development. Frequency analysis, comparison of means and the association of the SOIwith tornadoes suggest a linear type of response between ENSO and the occurrences of tornadoes inwestern Canada, i.e. more tornadoes during El Nino years and fewer during La Nina years. Cumulativefrequency distribution during the two phases of ENSO confirms the above results. It should be noted thatthere are exceptions to this general rule. For example during the 1989 La Nina year, considerably moretornadoes were recorded in western Canada. This highlights the need for a larger La Nina sample, thancurrently available, in order to establish a robust relationship between ENSO and tornadoes in Canada.The recent warming trend in Canada (Zhang et al., 2000) may play a prominent role in enhancingconvection and thus increase the likelihood of tornadoes in Canada. This aspect of tornado frequency wasnot examined in this study and remains an area of future research.

10. CONCLUSIONS

The post-1980 tornado data show that they are reported most frequently in parts of the southern Prairiesand southwestern Ontario. The spatial patterns must be interpreted with caution, since various reporting


D. ETKIN ET AL.936

biases in the data (such as the bias from population) make these patterns quite different from tornadooccurrence. The various climatological patterns shown in this analysis are similar to previous studies,showing a maximum of occurrence in the summer. Occasionally, devastating tornadoes do occur, thoughon average few people in Canada die from them. No F5 events have ever been recorded, but a numberof F4 events have been, and there is no reason why an F5 tornado could not happen.

The cool temperatures that usually accompany La Ninas seem to suppress summer tornadic activity inCanada. Unlike the Tennessee and the Ohio Valleys, where tornadoes are more prevalent during spring,in La Nina years, spring in eastern Canada experiences no significant change in tornadic activity.Directional wind shear between 850 and 500 hPa is an important parameter for tornado genesis. Theabsence of strong wind shear during La Nina years in Canada (not shown) thus inhibits tornadodevelopment. Frequency analysis, comparison of means and the association of the SOI with tornadoessuggest a near-linear type of response between ENSO and the occurrences of tornadoes in westernCanada, i.e. a tendency towards more tornadoes during El Nino years and fewer during La Nina years.It should be noted that there are exceptions to this general rule. For example during the 1989 La Ninayear, considerably more tornadoes were recorded in western Canada. This highlights the need for a largerLa Nina sample, than currently available, in order to establish a robust relationship between ENSO andtornadoes in Canada.

NOTES

1. The lack of a similar study in Manitoba may be the reason for many fewer deaths being reported in that province.

REFERENCES

Abbey RF, Fujita TT. 1979. The DAPPLE method for computing tornado hazard probabilities: Refinements and theoreticalconsiderations [preprints, 11th Conference on Severe Local Storms]. American Meteorological Society, Boston, MA, 2–5 October;241–248.

Allen DE. 1986. Tornado damage in the Barrie/Orangeville area, Ontario, May 1985. Building Research Note No. 240, NationalResearch Council Canada.

Biddle MD. 1996. Re-examining tornado risk ‘coping styles’ and other tornado warning issues under the modernized NationalWeather Service. In 18th Conference on Se�ere Local Storms. American Meteorological Society; 605–608.

Bieringer P, Ray P, Niu X, Whissel B. 1996. An improved estimate of tornado occurrence in the United States. In 18th Conferenceon Se�ere Local Storms. American Meteorological Society; 631–635.

Bluestein HB, Rasmussen EN, Davies-Jones R, Wakimoto R, Weisman ML. 1998. VORTEX workshop summary, 2–3 December1997, Pacific Grove, CA. Bulletin of the American Meteorological Society 79: 1397–1400.

Bluestein H, Golden JH. 1993. A review of tornado observations. In The Tornado: Its Structure, Dynamics, Prediction and Hazards,Geophysical Monograph, 79. American Geophysics Union, USA; 319–352.

Brandes E. 1977. Gust front evolution and tornado genesis as viewed by Doppler radar. Journal of Applied Meteorology 16:333–338.

Burgess DW, Wood VT, Brown RA. 1976. Mesocyclone evolution statistics [preprints, 12th Conference on Radar Meteorology].Boston, MA; 422–424.

Burgess DW, Wood VT, Brown RA. 1982. Mesocyclone Evolution Statistics, 12th Severe Storms Conference. AmericanMeteorological Society.

Burgess DW, Donaldson RJJR, Desrochers P. 1993. The Tornado: Its Structure, Dynamics, Prediction and Hazards, GeophysicalMonograph, 9. American Geophysics Union: USA; 203–222.

Carter AO, Millson ME, Allen DE. 1989. Epidemiological study of deaths and injuries due to tornadoes. Amercian Journal ofEpidemiology 130(6): 1209–1218.

Charlton BC, Kachman BM, Wojtiw L. 1998. The Edmonton Tornado and Hailstorm: A decade of research. CMOS Bulletin 26:1–56.

Chisholm AJ, English M. 1973. Meteorological Monographs 14(36), 98 pp. American Meteorological Society: Boston.Church C, Burgess D, Doswell D, Davies-Jones R (eds). 1993. The Tornado: Its Structure, Dynamics, Prediction and Hazards,

Geophysical Monograph, 79. American Geophysics Union: USA.Davies J, Johns R. 1993. Some wind and instability parameters associated with strong and violent tornadoes, 1, wind shear and

helicity. In The Tornado: Its Structure, Dynamics, Prediction and Hazards, Geophysical Monograph, 79. American GeophysicsUnion: USA; 33573–33582.

Davies-Jones R. 1982a. Observational and theoretical aspects of tornadogenesis. In Topic in Atmospheric and OceanographicSciences: Intense Atmospheric Vortices, Bengtsson L, Lighthill J (eds). Springer-Verlag: New York; 175–189.

Davies-Jones R. 1982b. A new look at the vorticity equation with application to tornadogenesis [preprints, 12th Conference onSevere Local Storms]. American Meteorological Society, Boston, MA; 249–252.

Davies-Jones R. 1984. Streamwise vorticity: The origin of updraft rotation in supercell storms. Journal of Atmospheric Science 41:2991–3006.



Davies-Jones R, Brooks H. 1993. Mesocyclogenesis from a theoretical perspective, Tornado III Symposium, American GeophysicsUnion; 105–114.

Davies-Jones R, Burgess DW, Foster MP. 1990. Test of helicity as a tornado forecast parameter [preprints, 16th Conference onSevere Local Storms]. American Meteorological Society; 588–592.

Deser C, Wallace JM. 1987. El Nino events and their relation to the Southern Oscillation: 1925–1986. Journal of GeophysicsResearch 92: 14189–14197.

Donaldson RJ Jr. 1970. Vortex signature recognition by Doppler radar. Journal of Applied Meteorology 9: 661–670.Doswell CA, Weiss SJ, Johns RH. 1993. Tornado forecasting. In The Tornado: Its Structure, Dynamics, Prediction and Hazards,

Geophysical Monograph, 79. American Geophysics Union: USA; 557–572.Etkin D. 1995. Beyond the year 2000, more tornadoes in western Canada? Implications from the historical record. Journal of Natural

Hazards 12: 19–27.Etkin D, Leduc M. 1994. A non-tornado severe storm climatology of southwestern ontario adjusted for population bias: Some

surprising results. Canadian Meteorology and Oceanic Society Bulletin 22(3): 4–8.Fawbush EJ, Miller RC. 1953. Forecasting tornadoes, Air University Quarterly Review, Maxwell AFB, Alabama; 108–117.Fujita TT. 1981. Tornadoes and downbursts in the context of generalized planetary scales. Journal of Atmospheric Science 38:

1511–1534.Fujita TT, Smith B. 1993. Aerial survey and photography of tornado and microburst damage. In The Tornado: Its Structure,

Dynamics, Prediction and Hazards, Geophysical Monograph, 79. American Geophysics Union: USA; 479–494.Golden J, Purcell D. 1978. Life cycle of the Union City, Oklahoma, tornado and comparison with waterspouts. Monthly Weather

Re�iew 106: 3–11.Grazulis TP. 1993. Significant Tornadoes 1680–1991. Environmental Films: St. Johnsbury, VT.Grazulis TP, Schaefer JT, Abbey RF. 1993. Advances in tornado climatology, hazards, and risk assessment since Tornado

Symposium II. In The Tornado: Its Structure, Dynamics, Prediction and Hazards, Geophysical Monograph 79, Church C, BurgessD, Doswell C, Davies-Jones R (eds). American Geophysical Union; 409–426.

Hage KD. 1987. A Comparative Study of Tornadoes and Other Destructive Windstorms in Alberta and Saskatchewan. Proceedingsof a Symposium, Edmonton, Alta, Sept. 9–11. In The Impact of Climate Variability and Change on the Canadian Prairies, MagillBL, Geddes F (eds). Alberta Environment; 351–377.

Hage KD. 1994. Alberta Tornadoes, Other Destructi�e Windstorms and Lighting Fatalities: 1879–1984. Spruce Grove: Alta.Hoerling MP, Kumar A, Zhong M. 1997. El Nino, La Nina and the nonlinearity of their teleconnections. Journal of Climate 10:

1769–1786.Horel JD, Wallace JM. 1981. Planetary-scale atmospheric phenomena associated with the Southern Oscillation. Monthly Weather

Re�iew 109: 813–829.IBC. 1997. Facts of the General Insurance Industry in Canada. Insurance Bureau of Canada.Joe P, Leduc M. 1993. Radar signatures and severe weather forecasting. In The Tornado: Its Structure, Dynamics, Prediction and

Hazards, Geophysical Monograph, 79. American Geophysics Union: USA; 233–240.Joe P, Crozier C, Donaldson N, Etkin D, Clodman S, Abraham J, Siok S, Biron H, Leduc M, Chadwick P, Knott S, Archibald

J, Vickers G, Blackwell S, Drouillard R, Whitman A, Brooks H, Kouwen N, Verret R, Fournier G, Kochtubajda B. 1994. Recentprogress in the operational forecasting of summer severe weather. Atmosphere–Ocean 33: 249–302.

Keppenne CL, Ghil M. 1992. Adaptive filtering and prediction of the Southern Oscillation Index. Journal of Geophysics Research97: 20449–20454.

King P. 1996. A long lasting squall line induced by interacting lake breezes. In 18th Conference on Severe Local Storms. February19–23, 1996. San Francisco CA; 764–769.

Klemp J, Rotunno R. 1983. A study of the tornadic regions within a supercell thunderstorm. Journal of Atmospheric Science 40:359–377.

Krauss TW, Marwitz JD. 1982. Precipitation processes within an Alberta supercell hailstorm. Journal of Atmospheric Science 41:1025–1034.

Lapczak S, Aldcroft E, Stanley-Jones M, Scott J, Joe P, Van Rijn P, Falla M, Gagne A, Ford P, Reynolds K, Hudak D. 1999. TheCanadian National Radar Project [preprints, 29th AMS Radar Conference]. Montreal, Quebec; 327–330.

Lehmann EL. 1986. Testing Statistical Hypotheses (2nd edn). John Wiley and Sons: New York.Lilly DK. 1982. The development and maintenance of rotation in convective storms. In Topics in Atmospheric and Oceanographic

Sciences: Intense Atmospheric Vortices, Bengtsson L, Lighthill MJ (eds). Springer-Verlag: New York; 149–160.Lilly DK. 1986. The structure, energetics and propagation of rotating convective storms, II, Helicity and storm stabilization. Journal

of Atmospheric Science 43: 126–140.Lowe AB, McKay GA. 1962. The tornadoes of western Canada. Meteorological Branch, Internal Report. Department of Transport,

Queens Printer, Ottawa.McNulty RP. 1981. Tornadoes west of the divide: A climatology. National Weather Digest 6(2): 26–30.Murray JGA. 1990. Tornado rural–urbans for eleven Canadian cities—1920–1989. Atmospheric Environment Service, Downsview,

Ontario.NBCC. 1995. Structural Commentaries on the National Building Code of Canada, Commentary ‘B’. National Research Council of

Canada: Ottawa.Newark MJ. 1988. Research Notes, Tornado Files, Environment Canada Library, Downsview, Ontario. (Note, these files were

compiled during the decade of the 1980s, and a specific year cannot be assigned to them).Newark MJ. 1983. Tornadoes in Canada for the period 1950 to 1979. Report No. CLI-2-83, Atmospheric Environment Service,

Downsview, Ontario.Newark MJ. 1988. The tornado hazard in Canada. In Natural and Man-Made Hazards, El-Sabh M, Murty T (eds). D. Reidel

Publishing Co.: Dordrecht, Netherlands; 743–748.Paruk BJ, Blackwell SR. 1994. A severe thunderstorm climatology for Alberta. National Weather Digest 119(1): 27–33.


D. ETKIN ET AL.938

Patrick D, Keck AJ. 1987. The importance of the lower level windshear profile in tornado/non-tornado discrimination. InProceedings of Symposium on Mesoscale Analysis and Forecasting, ESA SP-282. European Space Agency; 393–397.

Paul AH. 1995a. The Saskatchewan Tornado Project. University of Regina, Department of Geography Internal Report.Paul AH. 1995b. Strong Tornadoes in Saskatchewan and North Dakota. In Annual Meeting of Great Plains Rocky Mountain

Di�ision, Association of American Geographers, Rapid City, SD, 5–7 October.Price C, Rind D. 1993. Lightning fires in a 2×CO2 world. In Proceedings of the 12th Conference on Fire and Forest Meteorology,

26–28 October, Jekyll Island, GA. American Meteorological Society; 77–84.Quinn WH, Neal VT, Antunez de Mayolo SE. 1987. El Nino occurrences over the past four and a half centuries. Journal of

Geophysics Research 92: 14449–14461.Raddatz RL, Hanesiak JM. 1991. Climatology of tornado days 1960–1989 for Manitoba and Saskatchewan. Climatological Bulletin

25(1): 47–59.Rasmusson EM, Carpenter TH. 1982. Variation in tropical sea surface temperatures and surface wind fields associated with the

Southern Oscillation/El Nino. Monthly Weather Re�iew 110: 354.Rasmussen EN, Straka JM, Davies-Jones R, Doswell CA III, Carr FH, Eilts MD, MacGorman DR. 1994. Verifications of the

Origins of Rotation in Tornadoes Experiment: VORTEX. Bulletin of the American Meteorological Society 75: 995–1006.Ropelewski CF, Halpert MS. 1986. North American precipitation and temperature patterns associated with the El Nino–Southern

Oscillation (ENSO). Monthly Weather Re�iew 114: 2352–2362.Rotunno R, Klemp JR. 1985. On the rotation and propagation of simulated supercell thunderstorms. Journal of Atmospheric Science

42: 271–292.Schaefer JT, Galway JG. 1982. Population biases in the tornado climatology [preprints, 12th Conference on Severe Local Storms].

American Meteorological Society, Kansas City, MO, 12–15 January; 51–62.Schroeder TA, Agee EM. 1971. A regression model for Tornado distribution in a synoptically homogeneous region. In 7th

Conference on Se�ere Local Storms. American Meteorological Society: Kansas City.Shabbar A, Khandekar M. 1996. The impact of El Nino–Southern Oscillation on the temperature field over Canada.

Atmosphere-Ocean 34: 401–416.Shabbar A, Bonsal B, Khandekar M. 1997. Canadian precipitation patterns associated with the Southern Oscillation. Journal of

Climate 10: 3016–3027.Sills D. 1998. Lake and land breezes in Southwestern Ontario: Observations, analyses and numerical modelling. PhD thesis, York

University, Toronto.Snider CR. 1977. A look at Michigan tornado statistics. Monthly Weather Re�iew 105: 1341–1342.Tecson JJ, Fujita TT, Abbey RF. 1979. Statistics of US tornadoes based on the DAPPLE (Damage Area Per Path Length) tornado

tape [preprints, 11th Conference on Severe Local Storms]. American Meteorological Society, Boston, MA, 2–5 October; 227–234.Trapp JF. 1999. Observations of non-tornadic low-level mesocyclones and attendant tornadogenesis failure during VORTEX.

Monthly Weather Re�iew 127: 1693–1705.Trenberth KE, Hoar TJ. 1997. El Nino and climate change. Geophysics Resource Letters 24: 3057–3060.Turcotte V, Vigneux D. 1987. Severe thunderstorms and hail forecasting using derived parameters from standard RAOBS data. In

2nd CMOS Workshop on Operational Meteorology ; 142–153.Wakimoto RM, Wilson J. 1989. Non-supercell tornadoes. Monthly Weather Re�iew 117: 1113–1140.Walko R. 1993. Tornado spin-up beneath a convective cell: Require basic structure of the near-field boundary layer winds. In The

Tornado: Its Structure, Dynamics, Prediction and Hazards, Geophysical Monograph, 79. American Geophysics Union: USA;33573–33582.

Wallace AG. 1987. Track of the Edmonton Tornado, July 31 1987. Preliminary Report, Western Regional Technical Note.Government of Canada, EC Alberta Weather Centre, Edmonton.

Weisman ML, Klemp JB. 1984. The structure and classification of numerically simulated convective storms in directionally varyingwind shears. Monthly Weather Re�iew 112: 2479–2498.

White K, McKie C, Laverdiere J. 1995. Human response to warning: The August 4th, 1994 Alymer, Quebec Tornado. Report forthe City of Aylmer, EPC, EC and Regional Municipality of Ottawa-Carleton.

Wilks DS. 1995. Statistical Methods in Atmospheric Sciences. Academic Press.Wilson J. 1986. Tornadogenesis by nonprecipitation induced wind shear lines. Monthly Weather Re�iew 114: 270–284.Wilson JW, Schreiber WE. 1986. Initiation of convective storms at radar-observed boundary-layer convergence lines. Monthly

Weather Re�iew 114: 2516–2536.Wilson J, Crook A, Mueller CK, Sun J, Dixon M. 1998. Nowcasting thunderstorms: A status report. Bulletin of the American

Meteorological Society 79(10): 2079–2100.Yarnel B, Diaz HF. 1986. Relationships between extremes of the Southern Oscillation and the winter climate of the Anglo-American

Pacific coast. Journal of Climatology 6: 197–218.Zhang X, Vincent LA, Hogg WD, Niitsoo A. 2000. Temperature and precipitation trends in Canada during the 20th century.

Atmosphere-Ocean 38(3): 395–429.


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Tornado climatology of Canada revisited: tornado activity during different phases of ENSO