GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 8, PAGES 1147-1150, APRIL 15, 2000
Atlantic hurricanes and NW Pacific '.typhoons:
ENSO spatial
impacts on occurrence and landfall
M. A. Saunders
• R.E. Chandler2 C J. Merchant• andF. P. Roberts•
•Benfield
GreigHazard
Research
Centre,
Department
ofSpace
andClimate
Physics,
University
College
London,
HolmburySt Mary, Dorking,SurreyRH5 6NT, U.K.
2Department
of Statistical
Science,
University
College
London,
Gower
Street,
London
WC1E6BT,U.K.
Abstract.
Hurricanes
are the United
States' costliest natural
disaster.
Typhoonsrankasthemostexpensiveanddeadlynatural
catastropheaffecting much of southeastAsia. A significant
contributorto the year-to-yearvariability in intensetropical
cyclonenumbersin the north Atlantic and northwestPacific is
ENSO - the strongestinterannualclimatesignalon the planet.
We establishfor the first time: (1) the spatial(0.5 degreegrid)
impacts of ENSO on the basin-wide occurrenceand landfall
strikeincidenceof hurricanesandtyphoons;(2) the spatial(7.5
degreegrid or US statelevel) statisticalsignificancebehindthe
differentincidenceratesin warmandcoldENSO episodes;
and
(3) the effectof strengthening
ENSO on regionalstrikeratesand
significances
(hurricanesonly). Our data comprise98 years
(1900-97) for the Atlantic and 33 years(1965-97) for the NW
Pacific.At the US statelevel, we find severalregionswherethe
differencein landfallingincidencerate betweenwarm and cold
ENSO regimesis significantat the 90% level or higher. Our
findings offer promiseof useful long-rangepredictabilityto
seasonal
forecastsof landfallingtropicalcyclones.
Introduction
Tropical cyclonesrank aboveearthquakesand floods as the
majorgeophysicalcauseof propertydamagein the United States.
The annualhurricanedamagebill in the continentalUS for 19261995 is estimatedas US $5.0 billion (1997 $) [Pielke and
Landsea,1998]; for the period 1990-1998the annualfigure is US
$5.2 billion (1997 $). In muchof Japan,SouthKorea, Taiwan,
the Philippines, and other southeast Asian coastal regions,
tropical cyclonesare the most costly and deadly of all natural
disasters.The southeastAsian damagebill and mortality rate
fromtropicalcyclonesaverages
US $3.1 billion(1997 $) peryear
and 740 deathsper year for 1990-1998 [basedon information
from Munich Re]. Intensetropical cyclones(maximum 1-min
sustained
windsof at least33 ms-• or 64 knots)- termed
interannualclimatefluctuationon the planet[eg Philander,1990]
- wouldappeara promisingcandidate.
(See[eg Trenberthet al.,
1998, and Latif et al., 1998] for recent reviews of ENSO's
teleconnections
and predictability).It is well acceptedthat the
ENSO warmphase(El Nifio) leadsto reducedhurricanenumbers
in the north Atlantic as a whole [Gray, 1984; 1993], to fewer
hurricanestrikeson the continentalUS as a whole [Gray, 1984;
O'Brien et al., 1996; Bove et al., 1998], and to a lower US
hurricanedamagebill [Pielke and Landsea,1999]. Gray [1984]
reportsa factor of three reductionin total US landfallingintense
hurricaneimpacts from 0.74 per year during La Nifia (ENSO
coldphase)yearsto 0.25 per year duringE1Nifio years.Recently
Bore et al. [1998] analysedthe probability of hurricaneand
intense
hurricane
strikes
for the whole
continental
US as a
functionof the concurrentENSO phase.They find the probability
of at least one intense hurricane
strike is 63% and 23% for La
Nifia and E1 Nifio years respectively. These differences are
statisticallysignificant.The spatialimpactsof ENSO on the local
frequencyand strike incidenceof NW Pacific typhoonshas, to
our knowledge, not been reported in the refereed literature.
However, Chan [1985, 1990] shows that the frequency of
tropical cyclonesin the north Pacific between 140ø and 160øE
increasesduringE1Nifio years,while tropicalcycloneoccurrence
in the SouthChina Sea increasesin La Nifia years(see also Gray
[1993] andLander [1994]).
We extendthe above previouswork by quantifyingfor the
first time: (1) the spatial impactsof ENSO on the basin-wide
occurrenceand local landfalling strike rates of hurricanesand
typhoons.This is achievedthroughuseof a 0.5ø x 0.5ø in latitude
and longitudebasin-widegrid which permitscoastaland islands
regionsto be resolved;(2) the formal significancebehind the
different incidenceratesin warm and cold ENSO episodes(on a
7.5ø x 7.5ø grid or US statelevel scale);and (3) the effect and
significanceof strengtheningENSO on regional landfalling
intensetropicalcycloneincidence.
We use the NOAA/NESDIS/NCDC
best track global
historicaltropicalcyclonedatabase.To obtainfrequencieson a
0.5ø grid we linearly interpolatethe six-hourly records onto
hourlypositions,andcomputefor equalareacirclesof radius140
km centered on each grid point, the number of
hurricanes/typhoons
thatpassthroughthe circle.If an eventexits
a circle and reenters(a rare occurrence)it is countedonce.The
140 km distanceis chosenas a representative
radiusof damage
loss [Neumann and Pryslak, 1981]. For the Atlantic we use
hurricanesin the Atlantic and typhoonsin the NW Pacific - are
responsiblefor 98% of US damage[Pielke and Landsea,1998]
andthe vastmajority of southeastAsian damage.While satellites
and numericalweathermodelsprovide warningsof impending
landfallup to a week ahead,effortsare increasinglybeinggiven
to the seasonalprobabilisticforecastingof theselandfalls.Such
long-rangeforecastscan benefit a range of industry including
insurance,agricultureandtourism.
When considering potential long-range (out to 6 months)
hurricanerecordsfrom 1900-1997, and for the northwestPacific
predictorsof landfallingtropicalcyclones,ENSO- the strongest
from 1965-1997. Landfalling recordsare completein both. For
ENSO we useNifio 3.4 monthlyindexvalues(5øN-5øS,120øWCopyright2000 by the AmericanGeophysicalUnion.
170øW) computedfrom the U.K. Met. Office's Historical Sea
SurfaceTemperaturedata set version6 (MOHSST6) [Parker et
Papernumber1999GL010948.
0094-8276/00/1999GL010948505.00
al., 1995]. MOHSST6 values are a bulk temperatureretrieval
1147
1148
SAUNDERS ET AL.' ENSO SPATIAL IMPACTS ON HURRICANE
AND TYPHOON INCIDENCE
wherep is the probabilityof 1 or more hurricanesat a given
locationin a givenmonth,thexs arethepredictorvalues,andthe
]5sarethecoefficients
to beestimated,
via MaximumLikelihood,
Statistical Analysis and Modelling
from historicaldata.For largesamples,likelihoodratiotests[Cox
We first compute,for each0.5ø grid cell, the annualincidence and Hinkley, 1974] can be usedto comparemodels.Suchtests
rate (IR) of hurricanes/typhoons
in both E1 Nifio and La Nifia
years.The ENSO signis determinedusingmonthlyNifio 3.4 data
betweenlm and 10m depthand,for the tropics,containthe best
availablehistoricalqualitycontrolsandbiascorrections.
and by whetherthe Nifio 3.4 anomalyis greaterthan or lessthan
0.0øC.By takingthe ratio of incidenceratesunderthe two ENSO
ENSO Impact on Atlantic Hurricane Incidence 1900-1•
100'W
90'W
80'W
70'W
60'W
50'W
40W
conditions we obtain the incidence rate ratio, a statistic
commonlyusedin investigations
of the causesof diseases[e.g.,
Rothmannand Greenland, 1998]. This ratio is shown colourcoded for the north Atlantic and the NW Pacific in Figure 1
(upper
numeric
panel
marked
'LaNifia/E1Nifio').
40'N
}
We usetwo modelsto determinethe statisticalsignificanceof
any differencein hurricane/typhoonfrequenciesbetweenwarm
andcoldENSOepisodes.
MODEL I is basedonincidence
rate
ratios and is the numericallysimplermodel. We apply it to
landfallingeventsonly. MODEL 2 employslogisticregression
[Coxand Snell, 1989;McCullaghand Nelder, 1989].We applyit
to landfalling
andtonon-landfalling
events.
Bothmodels
give 20'N
similarresultsfor landfallingevents.
MODEL 1 usesthe incidencerate ratios (IRRs) of eventsfor
E1 Nifio (EL) and La Nifia (LN) conditionsappliedto different
landfall regions. If the IRR differs significantly from 1.0, it
indicatesan associationbetweenthe phaseof ENSO and the
incidenceof landfallingintensetropicalcyclones.The statistical
ENSO Impact on NW Pacific Typhoon Incidence 1965-1997
100'E 110'E 120'E 130'E 140'E 150'E 160'E 170'E 180'
significance
testarisesthroughthe logarithmof the IRR being
50'N
approximatelyGaussiandistributedwith a standarddeviationof:
•
+•
40'N
Net
• N•
[seee.g. Rothmannand Greenland,1998], whereN• andNr• are
thetotalnumber
oflandfalling
events
inourrecord
inELandLN
30'N
years respectively. In terms of sample size, the normal
approximationfor log IRR is reasonableprovidingbothNELand
NL•vare greaterthan about 10. This criterionis met by 95% of our
regional landfalling subsets- the exceptions being a few
categories of hurricane impacts on the Lesser Antilles, and
typhoonstrikeson SouthKorea.
20'N
Inapplying
MODEL
I weuse
significance
levels
of90%and 10'N
ß
20'N
.J
o
'
10'N
95% in a two-tailedtest, i.e., a test for differencefrom unity in
either direction.Despite there being a 10% probabilitythat an
isolated
result
atthe90%levelcould
occur
bychance,
weinclude
0',
these results becauseIRRs at nearby locationsare spatially
0.11 0.25 0.43 0.67 1.00 1.50 2.33 4.00 9.00
La Nifia/EI Ni•
dependent.
Sincethe four mostactivemonthsfor intensetropicalcyclones
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 La Nifia/TOTAL
in both basinsare July, August,Septemberand October(JASO
months),we first showresultsusingJASO landfallingeventsand
the JASO-averagedNifio 3.4 anomaly. In the NW Pacific, we Figure 1. ENSO spatial impacts on (top) north Atlantic
alsoshowresultsbasedon the Augustto October(ASO months) hurricaneincidence(1900-1997) and (bottom)northwestPacific
averagedENSO conditionswith the annualnumberof landfalling typhoon incidence (1965-1997). The grid spatial resolutionof
0.5ø permitscoastal,islandand regionalimpactsto be resolved.
events. In the Atlantic, where our data time series is three times
The
colour bar displaystwo scales:the ratio of the numberof
the length of our NW Pacific time series, we additionally
intensetropicalcyclonenumbersoccurringin La Nifia monthsto
computethree separateIRRs for EL, LN and Neutral ENSO
the numberpresentin E1 Nifio months(upper scale), and the
conditions.The Nifio 3.4 anomalythresholdsusedfor theseare: ratioof the saidsameto thetotal'numberof intense
tropical
EN > 0.3øC, 0.3øC > Neutral > -0.3øC, LN < -0.3øC. The latter
investigationprobes the impact of strengtheningENSO on
changinghurricaneincidence,and clarifieshow EN IR andLN IR
values differ from Neutral IR values.
MODEL 2 employslogisticregression
andtakesthe form:
In P = ]•o'•' ]•ixi
1- p
i=•
cyclones(lower scale). These ratios are computedafter first
normalisingto ensureequal numbersof El Nifio and La Nifia
months.Grid cellscontainingfewerthan5 eventsare not plotted.
White lines enclose regions where the difference in intense
tropicalcyclonenumbersbetweenENSO warm and ENSO cold
episodesis statistically significant at the 95% level when
averagedover an area of 7.5ø x 7.5ø grid size centredon the
point(MODEL 2).
SAUNDERS
ET AL.' ENSO SPATIAL
IMPACTS
involve computingthe differencein log-likelihoodbetweentwo
competingmodels,doubling,and comparingwith a percentage
point of the appropriatechi-squareddistribution.If, for example,
the computedvalueexceedsthe 95% pointof thedistribution,
we
rejectthe simplermodelin favourof the more complexone,at
the 5% significancelevel.
In the presentcontext,a 'large' sampleis one in which a
reasonable number of hurricane occurrences are observed. This
ON HURRICANE
AND TYPHOON
INCIDENCE
1149
StrengtheningENSO (using a ñ0.3øC SST threshold,rather
than a 0.0øC threshold)increasesthe Atlantic landfallingENSO
incidencerate ratio (IRR (LN/EN)) in all regions.The size of the
increasevaries with region but is typically twice as large for
hurricanesas for tropicalstorms(not shown).It is alsotwice as
large when computedusing the 'JASO landfallingeventsand
JASO Nifio 3.4' datathan with the 'annuallandfallingeventsand
ASO Nifio 3.4' data set.For the latterdata,the averagehurricane
IRR (LN/EN) increasesby 69% when using a ñ0.3øC SST
dictatesa lower bound to the areal size used in applying the
model.When analysingthe effectof ENSO at anypointin space, threshold rather than a 0.0øC threshold. The effects of
we find it necessaryto considera regionof 7.5ø x 7.5ø in latitude strengtheningENSO noted above are statisticallysignificant.
andlongitudecentredon thatpoint,to ensurethatlikelihoodratio They indicate that: (a) the strongerthe ENSO the larger the
tests can be used with this model. Seasonality in
hurricane/typhoon
occurrence
is represented
by usinga setof 12
indicatorvariables(one for eachmonth).We refer to this as the
Table 1. The magnitudeandsignificanceof ENSO's impactson
'seasonalonly' predictionmodel.
regionalAtlantic(Table1(a)) andregionalNW Pacific(Table
To studythe effect of ENSO uponhurricaneoccurrencean
1(b)) landfallingintensetropicalcycloneincidence.Resultsare
extra predictor,coded 0 for a cold episodemonth and 1 for a
fromMODEL 1. Table 1(a) showsvaluesfor July-Augustwarm episode, is added to the 'seasonal-only' model. The
September-October
(JASO) landfallingevents1900-1997based
associated
regressioncoefficient,/3•NSO,
has the interpretation on the JASO Nifio 3.4 indexwith two SST thresholds.
Table 1(b)
that exp[/3•NSO]
is the proportionalincreasein the odds for
showsvaluesfor all landfallingevents1965-1997basedon the
hurricaneoccurrenceduringa warm episodemonthrelativeto a
August-September-October
(ASO) Nifio 3.4 index.The
landfallingincidencerates(IR) peryearfor E1Nifio (EN),
cold episode month (the odds for occurrenceare defined as
Neutral, and La Nifia (LN) conditionsare shown,togetherwith
p/(1-p) wherep is the probability of occurrence;see Dobson
thelandfallingIncidenceRateRatio (IRR) - theratioof incidence
[1990] for further details on coding and interpretation of
ratesin LN to EN periods.Shadingindicateswhetherthe latteris
predictors in regression-typemodels). The regions of >_95%
significantat eitherthe90% (light fill) or 95% level (darkfill)
significanceobtainedwith MODEL 2 are markedin Figure 1 by
levels.Shadingin the 'EN IR' columnindicatesthatthesevalues
the white lines. They are all based on likelihood ratio tests
comparingthe 'seasonalonly' modelto the 'seasonalplusENSO'
model. A naive analysis,which ignoresseasonality,produces
noticeablydifferentresults(the effectof ENSO is maskedby the
strongerseasonalvariation): this emphasizesthe need for the
methodologyusedhere if ENSO effectsare to be studiedat subannual timescales
Atlantic
Hurricanes
Figure 1 (top) showsthat in the Atlantic,ENSO cold episodes
are associatedwith higher hurricaneincidenceratesover nearly
the entirebasin,the only exceptionbeingthe northeastquadrant.
Usingthe basiccriterionof a zeroNifio 3.4 anomalythresholdto
distinguishwarm and cold ENSO episodes,we find highestIRR
(LN/EN) values of -4 occurringin a band stretchingfrom the
Caribbeanthroughthe centralGulf of Mexico to Texas. When
averagingIRR valuesover a 7.5ø x7.5ø grid we find, as indicated,
that the IRRs in this regionare significantto >95% as they are
alsoin a bandreachingfrom the LesserAntillesto the northeast
of the Bahamas.
differ from the 'Neutral IR' values at the level indicated. The
coastalregionsin Table 1(a) aredefinedasfollows:'USNortheast':CapeHatterasto Maine, 'US Southeast':
Jacksonville
(Florida)to CapeHatteras,'Florida': Jacksonville
to Pensacola,
'Gulf Coast': Pensacolato Brownsville(Texas),ZesserAntilles':
Trinidadto Anguilla,and 'GreaterAntilles': PuertoRico to Cuba,
includingJamaica.The countriesin Table 1(b) areself-evident
exceptthat 'Philippines'refersto the regioncentredon Manilia
(14øN- 16øN),and 'Vietnam'refersto theregion10øN- 20øN
only.The effectof strengthening
ENSO on landfallingIRR is not
examinedin Table l(b) dueto the shortness
(33 years)of theNW
Pacific timeseries.
Table l(a). ENSO RegionalImpactson Atlantic Landfalling
Hurricanes: JASO Events 1900-1997
JASO Nino 3.4
No SST Threshold
Coastal
Region
EN IR LN IR
JASO Nino 3.4
:[-0.3ø C SST Threshold
IRR
ENIR Neut.IR LN IR
IRR
(52 yrs) (46 yrs) (LN/EN) (31 yrs) (39 yrs) (28 yrs) (LN/EN)
US Northeast
0.19
0.33
1.74
0.16
0.28
0.32
2.00
US Southeast
0.35
0.48
1.37
0.32
0.33
0.61
1.91
Florida 0.44 0.67 1.52 '"11•i•i:::•.•:i.•.•i•iiiiiiiiiii•!
0.56 0.82................
'"••i:
•"'
Gulf
Coast 0.42 0.78'•"•'•'-•-•'•-*..--'••
0.42 0.54 0.86
.....................................
Results for regional landfalling hurricane incidence from
MODEL 1 are displayedin Table l(a). Six areasare considered:
Lesser
Antilles0.15 0.26
US northeast, US southeast, Florida, Gulf of Mexico, Lesser
Great.
Antilles
0.48 0.80•'•'•-'•
'•"ø•'""•':•''••'"'•••]"•'•'":•:""
..... "••i 0.69 1.00...........
"•
""•'•""•"'
Antilles and the GreaterAntilles. The IRR (LN/EN) valuesin all
regionsexceed 1.0, with typical valuesbeing 1.6 (JASO events
only, JASONifio 3.4 - no threshold),and2.8 (JASOeventsonly,
JASO Nifio 3.4 - ñ0.3øC threshold).Largestvaluesof- 3 to 4
-:•:.•....-(s•:•:•:•:•:::•:•:::
0.18
0.36
Tablel(b). ENSORegional
ImpactsonNW Pacific
LandfallingTyphoons:
1965-1997
occur for the Lesser and Greater Antilles with the ñ0.3øC Nifio
Coastal
3.4 threshold.In terms of statisticalsignificance,33% (50%) of
the 12 IRR values in Table l(a) are significantto levels >95%
(>90%). The regionsexhibitingthe highest(lowest)significance
for IRR (LN/EN) are the GreaterAntilles(US southeast).
Lack of
a significantresult in a given area does not mean there is no
effect, merely that any effect is small enough to be
indistinguishablefrom random variations on the basis of the
Region
available data.
1.73 0.10
All Events
ASO Nino 3.4
No SST Threshold
EN IR
LN IR
IRR
JASO Events
JASO Nino 3.4
No SST Threshold
EN IR
LN IR
IRR
(18yrs) (15yrs) (LN/EN) (19yrs) (14yrs) (LN/EN)
4.50
3.67
0.82
4.16
3.29
South Korea
Japan
0.44
0.53
1.20
0.42
0.57
0.79
1.35
Taiwan
2.11
1.60
0.76
1.84
1.21
0.66
Philippines
1.56 2.13 1.37 0.53 1.21......
"••••
•"'
Vietnam
1.00
1.40
1.40
0.89
1.00
1.12
1150
SAUNDERS ET AL.: ENSO SPATIAL IMPACTS ON HURRICANE AND TYPHOON INCIDENCE
impact on landfalling frequencyand (b) the calculationof the
maximum IRR (LN/EN) for each sub-regionis sensitiveto a
combinationof the monthsselectedfor landfallingeventsandthe
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Figure 1 (bottom)showsthat in the NW Pacific,ENSO warm
periodsare associatedwith highertyphoonfrequenciesover the
majorityof the basin.The exceptions
arethe SouthChinaSeaand
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3.4 anomalyto distinguishwarm and cold ENSO episodes,we
observesmallestIRR (LN/EN) valuesof-•0.25 in a bandreaching
from 10øN, 170øEto 20øN, 135øE.When averagedover a 7.5ø x
7.5ø grid, thesesmallestIRR valuesare significantto >95%, as
they alsoare locally in the SouthChina Sea,overwesterncentral
Japan,and over the westerncentralPhilippines.A few localised
hot spotsof highsignificance
alsoexistelsewhere.
Resultsfor regionallandfallingtyphoonsfrom MODEL 1 are
displayedin Table l(b). Five countriesare considered:Japan,
SouthKorea, Taiwan, Philippinesand Vietnam. In generalthe
ENSO impactson landfallingtyphoonsare lessthanfor Atlantic
landfallinghurricanes.Also, use of the 'JASO event and JASO
Nifio 3.4' criterion does not change the magnitudeof IRR
(LN/EN) values as in the Atlantic. The only areas exhibiting
landfalling incidence rates which are significantly different
between warm and cold ENSO episodesare the Philippines
region centeredon Manilla (14øN - 16øN)for typhoons,andthe
'Vietnam' region 10øN- 20øNfor tropicalstorms(not shown).
Conclusions
and Further
Work
We quantify for the first time the spatial impacts and
significance of ENSO on Atlantic and NW Pacific intense
tropicalcycloneoccurrenceand landfall.We find severalregions,
notably in the Atlantic, where differences in the landfalling
incidence rate between warm and cold ENSO regimes are
statisticallysignificantat the 90% or 95% level. Theseresultsdo
not provecausalitybetweenENSO regimesand changingstorm
landfallingfrequency.Suchclaimsshouldbe supportedby sound
evidenceof a physicalmechanism- for example- hurricane
suppresionin the Atlantic via enhancedtroposphericwind shear
[e.g. •1onesand Thorncroft, 1998]. Furtherwork is alsorequired
to determine the maximum IRR (LN/EN) for each sub-region.
This will dependupon the monthsselectedfor the landfalling
eventsand for Nifio 3.4, the optimummonthsprobablyvarying
with region.
Most importantly our findings offer promiseof long-range
predictabilityto landfallingintensetropicalcyclones.At present,
25% of the observedNifio 3.4 varianceis predictable9 monthsin
advance[Latifet al., 1998]. As ENSO's predictabilityimproves
with future developments
in coupledclimatemodelsso will the
skill of seasonalforecastsof landfallinghurricanesandtyphoons.
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temperatures,
d. Geophys.
Res.,103, 14,291-14,324,1998.
M. A. Saunders,C. J. Merchantand F. P. Roberts,Benfield Greig
Hazard ResearchCentre,Departmentof Spaceand ClimatePhysics,
UniversityCollegeLondon,HolmburySt. Mary, DorkingSurrey,RH5
6NT, U.K. (e-mail:mas•mssl.ucl.ac.uk).
R. E. Chandler,Department
of Statistical
Science,UniversityCollege
London, Gower Street, London WC1E 6BT, U.K. (e-mail:
richard•stats.ucl.ac.uk).
Acknowledgements.ChrisMerchantis fundedby theUK TSUNAM!
Initiative for this work. Frank Roberts gratefully thanksthe Benfield
Greig Group for sponsorship.
We thankJustinMansleyfor helpful (ReceivedJuly27, 1999;revisedJanuary13,2000;
assistance.
accepted
January21, 2000.)