Geophysical Monograph Series
Aspects of Climate Variability in the Pacific and the Western Americas
Vol. 55
Geophysical Monograph 55
THE
INFLUENCE
OF NORTH
ON
PACIFIC
STREAMFLOW
ATMOSPHERIC
IN
THE
CIRCULATION
WEST
Daniel R. Cayan
ScrippsInstitutionof Oceanography,A-024,
University of California-San Diego, La Jolla, CA 92093-0224
David
H. Peterson
U.S. GeologicalSurvey, MS-496, 345 Middlefield Road,
Menlo Park, CA 94025
Abstract.The annualcycle andnonseasonal
variabilityof streamflow
over westernNorth America andHawaii is studiedin termsof atmospheric
similar,but the E1 Niffo signalalsoincludesa tendencyfor greaterthan
normal stream flow in the southwestern United States. These indices are
its varianceat mostof the stations,there is alsoa distinctannualcycle in
theautocorrelation
of anomalies
thatis relatedto the interplaybetweenthe
annualcyclesof temperatureandprecipitation.Of particularimportance
to theselag effectsis the well-knownrole of waterstoredas snowpack,
whichcontrolsthedelaybetweenpeakprecipitation
andpeakflow andalso
introduces
persistence
intothenonseasonal
streamflow
anomalies,
withtime
scalesfrom 1 month to over 1 year.
The degreeto which stream
flow is relatedto winter atmospheric
significantlycorrelatedwith streamflowat oneto two seasons
in advance
of the December-Augustperiod, which may allow modestly skillful
forecasts.It is importantto notethatstreamflow variabilityin someareas,
suchas BritishColumbiaand California, doesnot respondconsistently
to
thesebroadscalePacificatmospheric
circulationindices,but is relatedto
regionalatmospheric
anomalyfeaturesover the easternNorth Pacific.
Spatially,streamflowanomaliesare fairly well correlatedover scales
of severalhundredkilometers.Inspection
of the spatialanomalies
of streamflow in this studysuggestan asymmetryin the spatialpatternof positive
versusnegativestreamflow anomaliesin the westernUnited States:dry
patternshave tendedto be larger and more spatiallycoherentthan wet
circulationoverthe North PacificandwesternNorth America is testedusing
patterns.
forcingelements.This studyusesseveraldecadesof monthlyaverage
stream
flow beginningas early as the late 1800's over a network of
38 stations.In additionto a strongannualcyclein meanstreamflowand
correlations
with time averaged,griddedsealevelpressure(SLP), which
beginsin 1899. Streamflowfluctuationsshow significantlarge-scale
correlations
for thewinter(DecemberthroughFebruary)meanSLP anomaly
patternsoverthe North Pacificwith maximumcorrelations
rangingfrom
0.3 to about0.6. For streamsalongthewestcoastcorridorthecirculation
patternassociated
with positivestreamflowanomaliesis low pressure
centeredoff the coastto the westor northwest,indicativeof increasedwinter
stormsandan anomalouswesterly-to-southwesterly
wind component.For
streamsin the interiorpositivestreamflow
anomalies
are associated
with
a positiveSLPanomalystationed
remotelyoverthecentralNorthPacific,
and with negativebut generallyweakerSLP anomalieslocally.
Oneimportant
influence
onstreamflow
variabilityis thestrength
of the
AleutianLow in winter. This is represented
by the familiarPacific-North
America(PNA) index and alsoby an indexdefinedhereinthe "CNP"
(CentralNorthPacific).Thisindex,beginning
in 1899, is takento be the
averageof the SLP anomalysouthof theAleutiansandthewesternGulf
of Alaska.Correlations
betweenPNA or CNP andregionalanomalies
reflect
streamflow
thealternations
in strength
andposition
of themeanNorthPacific
stormtrackenteringNorthAmericaaswell asshiftsin thetradewindsover
thesubtropical
NorthPacific.Regionswhosestream
flow is besttunedto
the PNA or CNP include coastalAlaska, the northwesternUnited States,
andHawaii;the lattertwo regionshavethe oppositesignanomalyas the
former.Thepatternof streamflow
variations
associated
withE1Niffo is
This paper is not subjectto U.S. copyright.Publishedin 1989by
the American Geophysical Union.
375
Introduction
Most of the surface water west of the Continental
Divide
in North
Americais suppliedby North Pacificair masses[Rasmusson,
1967;Klein
and Bloom, 1987]. The bulk of this precipitationoccursduringthe cool
season[HsuandWallace, 1976]fromextratropicalstormsthatare carried
in the westerlyflow. The seasonalmigrationof this stormtrack provides
a markedannualcycleand resultsin a distinctgeographical
pattern,with
precipitationgenerallydecreasingfrom north to southand from coastal
rangesto theinterior.Thisdistribution
is compounded
by topography,
which
can either enhancethe precipitationin a given region or diminish it,
depending
onitsorientation
relativeto theflow [e.g., Weaver,1962;Pittock,
1977].
Atmosphericpatternsinvolvedin westerncool seasonprecipitationare
oftenbroadin scalewith up anddownstream
connections,
sothatsystematic
deviationsin the atmospheric
flow overthe North Pacificare involvedin
North Americanprecipitationanomalies.In the centralNorth Pacificthe
time-meansurface
pressure
configuration,
calledtheAleutian-Gulfof Alaska
Low, is generallylower duringwinterswith frequentcyclonepassages
and
higherduringwinterswhenthesecyclonepassages
are lessfrequent,less
intense,or divertedto anotherlocation.Marked year-to-yearchangesin
thisactivitymakesthisregionthe seatof largestmonthlyandseasonal
scale
variabilityin the northernhemisphere[Namias, 1975]. The dynamicsof
the atmospheric
flow producean organizedspatialpattern:averagesof a
Geophysical Monograph Series
376
Aspects of Climate Variability in the Pacific and the Western Americas
Vol. 55
CAYAN AND PETERSON
Sea Level Pressure
Streamflow Anomalies
2
ß':.
":.'•
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" •":'!'
';"4
' •
.' '..,
'.';i'
,.
ß
ß:?!'
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' ':•:1•
•
¾"
:i'i:'::
'.:•..
':':.•
ß
.........
-
.':;i'
December
'"'i'. ,.January
' '. February March
'"":.:..
'.
•s
'"
-
15
i:!:!:i:!
Above
average IIIIII Below
average
Fig. 1. Streamflow
anomalyfor December-March
andwintermeanSLPfor pairsof winters(a) 1963-1964and1976-1977and
(b) 1955-1956and 1968-1969whenatmospheric
circulationpatternoverNorth Pacificwassimilar.For anomalies
in theupper
or lowerquartileof a givenstream'smonthlydistribution
seePeterson
et al. [1987a].1000mbis subtracted
fromSLP, contour
interval
is 5 mb.
few daysand more showthat stormactivityaroundthe hemisphere
is
arrangedin cellsthatcorrespond
to thetroughs(or negativeanomalies
of
pressureor geopotential)
of atmospheric
quasi-stationary
planetarywaves
whichusuallyhavedimension
of severalhundred
to thousands
of kilometers
[see,e.g., Madden,1979]. This wavycharacterof the atmospheric
flow
givesriseto remotecorrelations
of theanomalypatternscalled"teleconnections,"which are pronouncedfeaturesof time-averaged
circulation
[Namias, 1981; Wallace and Gutzler, 1981; Blackmonet al., 1984]. As
will be shownthislargepeakwinteratmospheric
structure
providesa loose
organizationto the streamflowanomaliesthat carriesthroughthe runoff
season.
Many authorshave demonstratedhow the climaticconditionsin North
America
are related
to the circulation
far afield
over the central North
Pacific. In particularthe precipitationalong the Pacific coastof North
America is stronglyaffectedby the orientationof the North Pacificstorm
track [e.g., Namias, 1978a;Yarnal andDiaz, 1986]. Meteorologists
have
Geophysical Monograph Series
Aspects of Climate Variability in the Pacific and the Western Americas
ATMOSPHERIC
struggled
to measure,describe,andforecastprecipitation;
thedifficultyhas
beenits spottyspatialmake-upandepisodicnaturethat are symptomsof
complexgoverningdynamics.Fortunately,a significantportion of the
precipitation
variabilitycanbe specified
fromtheanomalous
time-averaged
atmospheric
circulation,usingregression
techniques
[Klein, 1963; Walsh
et al., 1982; Weare and Hoeschele,1983; Cayanand Roads,1984; Klein
andBloom,1987].Spatialaveraging
of theprecipitation
appears
to improve
theresultantrelationswith thecirculationrangingoversynoptictime scales
to monthly and seasonalphenomena.Numerousstudieshave relatedthe
time-averaged
atmospheric
circulationto precipitation,
but very few have
examineditslinkageto stream
flow, possiblybecause
of complications
that
mightbeintroduced
by surfaceprocesses
suchasevapotranspiration.
These
processes
renderstream
flow to be onlya fractionof theprecipitation
input
to a watershed[see, e.g., Court, 1974]. This fraction varies with
temperature,
soil, andsoilcover,andwith meteorological
properties;values
typicallyrangefromgreaterthan70 percentfor highlatitudetundraregions
to lessthan3 percentfor deserts[Sellers,1965]. In Californiarunofftotaled
overthe stateis estimatedto be 35 percentof thetotalprecipitation
input
[Stateof California, 1979]. There are, however,incentivesfor studying
streamflow variability. Streamflow is a more usableform of water thanthe
precipitation,and streamflowrepresents
a naturalspatialand temporal
averagethat filters the noisierprecipitationfield. In additionstreamflow
samples
highelevation,severetopographic
regionswheredirectprecipitationmeasurements
are oftensparse,andthusis capableof representing
the
regionswheretheinputof wateris oftengreatest.An interestingissuethat
underliesthis study is the extentto which the atmosphericcirculation
"signal" can be distinguished
in the monthlystreamflowvariability.
Streamflows have fluctuatedconsiderablyover the instrumentalrecord
[e.g., Roden, 1967; Langbeinand Slack, 1982; Bartlein, 1982; Meko and
Stockton, 1984] and the socio-economic,physical, and biological
significance
of this year-to-yearvariabilityis great. The impactof this
variabilityis particularlystrongin thewesternUnitedStates,wherewater
resources
arelimited.While mostof theareaeastof the MississippiRiver
receives,on average, an excessof 30 inches(76 cm) of precipitation
annually, much of the West is limited to considerablyless. With the
exceptionof the coastalNorthwest,the surficialwater supplyis largely
derivedfrom high elevationprecipitation
thatis distributedin the form of
streamflow.A better graspof how interannualstreamflowvariability is
driven by the climate systemis a first steptowardpredictionand better
management
of the watersupply.Also, to the extentthatstreamchemistry
is flow related,an associated
conn•tion with the streamflowvariability
is thebudgetof chemicalspeciesin riversaffectedby interannualfluctuationsin flow driven by climate[Peterson,et al., 1987a; Petersonet al.,
1987b].Thesenaturalvariationsneedto beunderstood
in orderto quantify
man-caused
effects.
Visual examinationof anomalousstreamflowmaps [Holmes, 1987]
suggests
that similar streamflowanomalypatternsemergefrom similar
winteratmospheric
circulationovertheNorthPacific.As an example,the
dependence
of streamflowon circulationtypeis demonstrated
in Figure 1,
showingDecember-March streamflowanomalieswithin two pairs of
winters,eachpair sharinga distinctatmospheric
circulationpattern.The
remarkable
difference
between
thetwopairsof mapssuggests
a ratherstrong
effectof winter North Pacificatmospheric
circulationon streamflowin the
West. This studyis aimedat demonstrating
the link betweenclimateforcing and westernstreamflow.We shalladdressthe following two issues:
(1) What is the seasonalbehavior of streamflowanomaliesand how is it
relatedto atmospheric
elementssuchastemperature
andprecipitation?
and
(2) What is thelink betweentheNorthPacificwinteratmospheric
circulation and the streamflow
anomalies over western North America?
The text is arrangedas follows:datasetsemployedare described,the
seasonal
cycleof streamflowoverthedomainis discussed,
an analysisof
the linkage with the large scalewinter circulationis presented,foreshadowing
of anomalous
streamflowfrom seasonal
atmospheric
indicesis
discussed,
anda view of someregionalscalecorrelations
withinthe streamflow network
is shown.
CIRCULATION
AND
STREAMFLOW
Vol. 55
IN THE
WEST
377
Data and Data Processing
Data employed in this study include monthly and seasonalmean
atmospheric
andstream
flow instrumental
records.The streamswereselected
to providereliablerecordsoverthelongestpossibleperiod.Monthlyaveragestreamflowfrom 38 stationsover westernNorth Americaand Hawaii
wereobtainedfromtheU.S. GeologicalSurveyandCanadaarchive[Department of Interior, 1975]. The 38 streamsinclude 29 from the western conterminousUnited States, 3 from British Columbia, 4 from Alaska, and 2
from Hawaii. Most of the streamflowrecordsbeginbetween1900 and the
1930's;thelongestseries,SpokaneRiver at Spokane,Washington,begins
in 1891. The four Alaskan streamshave no data until the late 1940's, with
the exceptionof Gold Creek at Juneau,whichhasa shortsegmentof early
datafrom 1917-1920. Somestreamr•ords havea several-yeargap; such
gapsoccurmostlyin 1900-1935. However, becausewe foundno obvious
inconsistencies
of theseearlier(pregap)datawiththelongerandmorerecent
data segments,the pregapdata were includedin the analysis.
The 38 streams are described with associated statistics in Table 1. The
largestriver in the setis FraserRiver (at Hope, British Columbia),which
hada long-term
annual
meanof 2720(m3 s- 1),andthesmallest
riversof
thesetwereArroyoSecoin southern
Californiaat 0.28 (m3 s-1), and
Kalihi River in Oahu,Hawaii,at 0.17 (m3 s-1). Most of the North
American rivers which drain into the North Pacific Ocean in this set are
describedby Roden[1967]. Nonseasonal
(anomalous)
variabilitychanges
considerablybetweentheserivers. In relative terms the streamwith the
smallestvariabilityis FraserRiver, a large northernriver fed by ample
snowmelt,whose annual mean flow ranged between 71 percent and
135 percentof its long-term mean. In contrastArroyo Seco, a small
ephemeralstreamin southernCalifornia occasionallypulsedby floods,
rangedfrom0 percentto 628 percentof its long-termmeanflow. In general thesestreamsexhibitedminimumannualdischarges
from 20 percentto
60 percentof their long-termmeanandmaximumannualdischarges
from
150 percentto 250 percentof their long-termmean. Estimatesof the averageannualflow per unit areafor the watersheds,
obtainedby dividingthe
meanannualdischargeby the total area of the drainagebasin, are also
presented
in Table 1 to directlycomparetherunoffwith theannualprecipitation. Theseannualflow per unit areavaluesare highestfor the northern
coastal
streams
withvaluesfrom200cmyr-1 to over350cmyr-1 and
lowestin theinteriorSouthwest
withvaluesaslow as 1 cmyr-1
In this studythe primarydatarepresenting
the atmospheric
circulation
is seasonal
meansealevelpressure(SLP) overtheNorth Pacificandwestern
North America. Although upper level geopotentialfields, such as the
700 millibar (mb) height, are often employedin diagnosticstudiesof the
atmosphericcirculation[e.g., Klein and Bloom, 1987; Namias, 1978a],
we usedSLP. Sealevel pressurehasa long r•ord length,beginningin
1899,whilecommonly
usedgeopotential
fieldsdonotbeginuntilafterWorld
War II in about1947. Sealevelpressure
hasbeenshownto be anadequate
indicatorof atmospheric
circulation,especiallyovertheextratopical
oceans
duringwinter[Davis, 1978;EmeryandHamilton,1985]andis reasonably
well correlatedwith precipitationover the westcoast[Cayanand Roads,
1984]. Numerousstudieshavedemonstrated
theusefulness
of monthlyand
seasonal
meansasa representation
of thecollectiveanomalous
synoptic
scale
eventsthat comprisea givenperiod [e.g., Namias, 1978a, Davis, 1978].
MonthlyaverageSLP for the period1899 through1974, documented
by
Trenberth and Paolino, [1980] was obtainedfrom the National Center for
Atmospheric
Research.It wasupdatedfrom dataarchivedat ScrippsInstitutionof Oceanography
obtainedfromtheNationalMeteorological
Center
from 1975 through1985. Questions
havebeenraisedaboutthe veracity
of partsof theSLP dataduringtheearlierperiod,butthesemainlyconcern
the regionover Asia andthe higherlatitudesnorthof 55 øN. [Trenberth
andPaolino,1980;Jones,1987]. Sealevelpressure
datawerenotavailable
for December1944 andwere missingfor scatteredgrid pointsacrossthe
North Pacificduringothermonths.Seasonal
meanswere constructed
by
averagingthethreeappropriate
months.Winteris definedasthe average
of December,January,and February, and so on for the other seasons.
Geophysical Monograph Series
378
ATMOSPHERIC CIRCULATION
Aspects of Climate Variability in the Pacific and the Western Americas
AND STREAMFLOW IN THE WEST
ß
o
ß
Vol. 55
Geophysical Monograph Series
Aspects of Climate Variability in the Pacific and the Western Americas
CAYAN
Vol. 55
AND
PETERSON
379
Geophysical Monograph Series
380
ATMOSPHERIC
Aspects of Climate Variability in the Pacific and the Western Americas
CIRCULATION
AND
STREAMFLOW
IN THE
Regionalaverageprecipitationdatafor the conterminous
UnitedStates
andAlaskariver basinswasobtainedfrom theNationalOceanographic
and
AtmosphericAdministration
(NOAA) divisionalmonthlyaveragedataset
[e.g., Karl and Knight, 1985; Cayanet al., 1986] for 1895 through1985
for divisionsin theconterminous
Unitedstatesandfrom 1931through1982
for Alaska[Diaz, 1980]. Representative
monthlyprecipitation
for thethree
Canadianstreamswasconstructed
by averagingselectedstations
with long
precipitationrecordsextractedfrom the NationalCenterfor Atmospheric
Researchmonthlymeansurfacedataset. No precipitationanalyseswere
carriedoutfor thetwo Hawaiianstreams
because
regionalaverageprecipitationandtemperature
datawerenotconveniently
availablefor theHawaiian
streambasins,and becauseof concernthat precipitationat singlestations
might be corruptedby local effectssuchas topography.
Becauseprecipitation
and streamflowobservations
oftendeviatefrom
a normaldistribution,theyare sometimes
transformedto minimizethese
non-normal
effects[e.g. KleinandBloom,1987].Also,because
instrumental
recordsmayhavepickedupa non-natural
component
because
of man-caused
effects,theyaresometimes
treatedby removingpartof theirlow frequency
content(e.g., by removingthe lineartrend [Namias,1978b]).To guard
againstdrawingimproperconclusions
that mighthavearisenfrom either
of theseproperties,the analyses
hereinwere carriedout on two versions
of the streamflowtime series.One versionwas first detrendedby removing eachmonth'slineartrendindividually,thenlog transformed,andthen
"anomalized"by removing
long-term
monthlymeansof theresultant
series.
The secondversionwastheoriginalmonthlydatawith onlymonthlymeans
subtractedbut no trendsremovedand with no transformations.Comparisonof the two setsof analysesyieldedsimilarresults,suggesting
thatthe
trendremovalandthe log transformation
were not necessary.
Inspection
of the monthlytrendsshowedthemto be relativelysmallfor mostof the
stationsincluded.The Sacramento
River at Verona showsa ratherlarge
increasing
trend,butthisis thought
to resultfromthefortuitous
timingwith
the recordstartingat thebeginningof a prolongeddry spellfrom the late
1920'sto themid-1930'sandtherecordendingjust aftera remarkablywet
periodin theearly 1980's.Furthermore,manyof the analyses
employthe
time-averaged
anomalies
of December-August
streamflow,
a 9-month
period
thathasconsiderably
lessskewness
thantheseriesfromanindividual
month.
For thesereasons
nearlyall of theanalyses
shownherearethosethatused
the original nontreateddata.
To studyanomalousbehaviorin this systemthe annualcycleof each
of the variableswasremovedby subtracting
the long-termmonthlymean.
For streamflow the entirelengthof recordwasusedto construct
the longterm mean. This periodrangedfrom 36 to 96 yearsfor the 38 streams.
A careful accountof precautionsnecessaryfor interpretingstreamflow
recordsis presented
by Roden[ 1967].For SLP andtheprecipitation,
the
40-yearperiodfrom 1946-1985wastakenas the climatological
base.To
investigate
thewintercirculationinfluenceon theresultingstreamflow,the
December-August
(winterthroughsummer)totalstreamflowis employed
for severalof the analyses.Thisperiodaccountsfor thebulk of the streamflow in the basinsconsidered
here, sincewesternstreamstypicallyundergo a low flow interludein fall. For most of the streamsthe average
December-Auguststreamflow was80 percentor moreof the annualmean
flow; overall it rangedbetween67 percent (Gold Creek, Alaska) and
96 percent(Cosumnes,Merced, and Kings Rivers). This seasonality
is
apparentin Figure2 showinghistories
of streamflow
throughthe 12months
for eachyearat UmpquaRiverin coastal
Oregon,JohnDay Riverin interior
Oregon,andWeberRiver in northernUtah. Thesethreerivers,representinglow, moderate,andhighelevationwatersheds,
arealsousedto illustrate
variouspropertiesof the streamflowin the followingsection.Average
WEST
I Umpqua
River,
Oregon
I
I John
Day
River,
Oregon
I
• E•200•-••----•...•
iWeber
River,
Utah
I ,•
Fig. 2. Time historyof monthlystreamflowfor Umpqua,JohnDay, and
Weber Rivers. Monthsandyearsas indicated.Mean annualand monthly
streamflowvaluesare presentedin Tables 1 and 2, respectively.
anomalies
in Figure3. The statistical
significance
of the cross-correlation
andcompositinganalysescarriedoutin this studydependson the number
of independentsamplesin the streamflowtime series.The numberof
independentsamplesis lessthan the numberof monthsof observations
becauseof autocorrelationin the streamflow.The time betweenindependent sampleswas estimatedby integratingthe autocorrelation
function
[Leith, 1973].Thisintegraltimescaleis onlya roughapproximation,
owing
to the limitedrecordlengthof the sampleandbecausethe autocorrelation
structure varies with season for most streams. In this case the autocorrelation
andtheresultantintegraltime scaleshownin Table 1 werecomputedfrom
the detrendedlog transformedstreamflowanomalies.For the 38 stations
consideredthis integraltime scalerangedfrom 1 to 21 months(Table 1).
For mostof the stationsthe integraltime scalewas 1 year or less.For 6
stations,it wasgreaterthan 12 months,andfor 17 stations,it was6 months
or less.With the abovecautionsin mind, we diminishthe effectivelength
of recordwhenapplyingtraditionaltestsas a gaugeof the significanceof
the correlationcoefficients.For example,given a recordfor 1930-1985
elevations of the three watersheds are 756 m, 1341 m, and 2771 m,
(56 winter seasons)having 18 monthsbetween independentsamples
(37 independent
samples),
thencorrelation
coefficients
with absolute
values
respectively.
Many of the analysesare carriedout on the streamflow,in the form of
in excessof 0.3 are significantat the 95 percentconfidencelevel. Thus
in the analysesthat follow, the 0.3 level is usedthroughoutas a coarse
cross-correlations
and composites.An idea of the anomalystructurefor
severalof the streamsis providedby the time seriesof the standardized thresholdof significantcorrelations.
Vol. 55
Geophysical Monograph Series
Aspects of Climate Variability in the Pacific and the Western Americas
CAYAN
41Ct•ena
•lver' I
41Kenai
R•ver,
Vol. 55
AND PETERSON
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'
E
ß
"
6.3cms
02
1, I"[rr,'l,'l"]r'[,'
• I I I , , , f[?][l[]['[,[l[l['l[l'l[Ip•'ill[I]"r'[[q[Im/"'["l"
'['
!','[r?["[',,'
'r[
4[- Merced
R[ver,
Ca,ifor.a
-ø2
t' '
11': ,['",
: ,",
I •,',
i .....
'•..d.
",,,'",",r.?rr?r'",'
....
,,....
,'['r.""l','r'
".
,,q",
'l",,'
?['r
iqf
,i10cms•
[.'I,
l
'
2•
California
. t ,.[11 I.IL.,I,
4rArrøyøSecø'
[ L i Ilk[,.,,,,
illif111,r,ll,qllllNiii
i.iq]qrlq..iql.lnlllql.q,#,ll,
llq,
11
'qlll'
'll!' Illlllllllll['.[llllfl'lllq-l-r#-l•qll#'lr'l'rll
,,ql'I .],rll[[.
I'11[111'lq
'1i ,illI ,llI /
:
i
i
0 3 cms
4r-Salt
River, ii,,•.,.,i.,•
I..L,,.,•.',,.'•,,',,I
.I•.....
02•
III
4I-'
Wailua
River,
2[-.-Hawaii
2• Arizona • II. , III,I
-• ,
1890
, I , • , I , , , I • , , I , , ,l
'
i
1900
1910
1920
:
1930
:
1940
1950
:
1960
1970
1.4 cms-I
1980
1990
Fig.3. Timeseries
ofmonthly
streamflow
anomalies
forselected
stations.
Anomalies
arestandardized
bydividing
bystandard
deviation
foreachmonth.
Long-term
meanannual
streamflow
(m3 s-•) isnoted
foreachseries.
Streamflow"Climatology"
amongvariousbasinsprovidesinsightintothephysicalfactorsaffecting
their streamflowproperties.
Most of the streamsin thisnetworkexhibita strongseasonal
variation
in meanstreamflow,whichfollowsthe climateforcingelementssuchas
precipitation
andtemperature.
In fact,theannual
cycleissucha prominent
featureof hydrologyin thewestthatit permeates
the statistical
character
of anomaliesas well as the meanfields. Comparisonof the annualcycle
The meanannualcycleof streamflowandprecipitation
is represented
in Figure4 by the amplitude
andphaseof its firstharmonic(periodicat
12months),
patterned
aftera similaranalysis
of globalprecipitation
by Hsu
andWallace[1976].Very simply,thisanalysis
presents
eachbasin'sannual
harmonic(havinga periodof 12 months)of precipitation
andstreamflow
Geophysical Monograph Series
382
Aspects of Climate Variability in the Pacific and the Western Americas
ATMOSPHERIC
CIRCULATION
AND STREAMFLOW
Vol. 55
IN THE WEST
usingan arrow, whoselengthis proportionalto the amplitudeof the cycle
and whose direction indicatesits phase. The amplitudeand phase are
obtainedby fitting a 12-monthsinewaveto the long-termmonthlymeans
dividedby the annualaverageof the 12 monthlymeans.In this rendition
the amplitudeis the ratioof varianceof the fittedannualharmonicdivided
by the actualvarianceof themonthlymeansabouttheannualaverage.Thus
stationswith amplitudescloseto 1 havemuchof their climatologicalvariability in the annualcycle, while thosewith smallamplitudesrequireother
harmonicsfor a gooddescription.The directionof the arrowspointto the
monthwhen the annualharmonichas its maximum(positive)value. For
example,a stationwhoseannualharmonicis maximumin Januarypoints
up, as denotedby the dial on the figure. For the mean precipitation,
represented
by thedashedarrowsof Figure4, theannualharmonicisprominentalongthe westcoast,with amplitudesrangingbetween0.4 and 0.9.
The annualharmonicof precipitationis not very strongat severalbasins
intheinteriorwestern
UnitedStates,
particularly
thosein Arizona,Colorado,
Idaho,andMontanawithamplitudes
lessthan0.4. Thetimingof maximumprecipitation
throughtheyearvariesconsiderably
overtheregionas
shownby the directionof the dashedarrows[seealsoPyke, 1972; Hsu
andWallace,1976]. Maximumprecipitation
occursin summerin thethree
northern Alaskan basins, in fall in southernAlaska and central British
Columbia,in early-to-latewinteralongthewestcoast,andvariesbetween
spring,summerandfall in theinteriorwesternUnitedStates.Thisanalysis
doesnotinclude
precipitation
forthetwoHawaiianstream
basins,
butLyons
[1982] reportedthat mostHawaiianrainfall stationshavea modestannual
cycleof precipitation
thatpeaksin northernhemisphere
winter.Turning
to thestreamflow,
represented
by thesolidarrowsin Figure4, theannual
harmonicrangesfrom 0.2 to almost1.0 over mostof the basins.Several
streams
havean annualharmonicthatis somewhat
strongerthanthatof
theprecipitation,
probablyowingto theinfluence
of temperature
through
thesnowstorage
andmeltingprocess.
Thephaselagbetween
meanstreamflowandprecipitation
isreadilyseeninthisfigure,withstreamflow
lagging
by 0 to severalmonths.Thisdelayis greatest
in northern
andhighelevationbasinswith an appreciable
snowpack.
Virtuallyall of thesestreams
exhibit a maximum in the annual harmonic between midwinter and late
summer.
Estimated
Annual
Cycle:
"'::•!:•.,•:.
..............
:'::'•'"'"'i•
......
:j:.Li•.'.'.....,
.:.•;:,:....
For comparison
with theharmonicanalysistheobserved
phaselag can
be seenfrommonthlymeanprecipitation
andstreamflow
in the Umpqua,
JohnDay, andWeberRiversin Figure5. Meanprecipitation
for bothcoastal
Precipitation
and
Streamflow
I
and interior Oregonis maximumin December,but while the streamflow
lagsby only about1 monthalongthe coastat UmpquaRiver, it lagsby
about4 monthsat JohnDay River. The WeberRiver regionhasa broad
precipitation
maximumin winter, but the coldtemperatures
in this high
elevation
watershed
produce
a sharply
delayed
streamflow
peakin May-June
(seealsoFigure 2). Similarplotsshowthat basinsin the northand in the
rotenorhavepeakstreamflowwhichlagsone-to-several
monthsaftermaximumprecipitation,
apparently
because
of thesnowpack.
On theotherhand
basinsalongthe southerncoastandthosein Hawaii andsouthernArizona
are too warm for storageof precipitationin the form of snow,and they
exhibit a nearly immediateresponse(0-1 month)of the streamflowto
precipitation.
Thesevariationsin thelag betweenprecipitation
andrunoff
is well knownto hydrologists
andalsodepends
on otherphysicalcharac-
"."/.v,•?....•,
4["'"•'•/
Oahu
Hawaii
':'(;'.Peak
inAnnual
Cycle
k ••.•;i"
,, •
ø.
•- Strea
flo
o
i:...:.......
I
I
January
/
•
38
I
",
-•i;:::•'•:•'i;.
i6
'-
Canada
................
•::/'-"•'
L 10•'-'"'"'
11'k United
States
October-•--Apri'
/
',/V
14 9,' 12•
l/i:i:!/ J13
•"•'"'
July J
161, 19=
33
'.•...
........
..•4 35/
Fig. 4. Normalizedamplitudeand phaseof the annualcycle of monthly
precipitation(dottedarrows)and streamflow(solidarrows). Normalized
amplitudeindicatedby lengthof thearrows,givenby scale.Phaseis given
by directionof arrow(anarrowpointingup indicates
maximumin January,
onepointingto therightindicates
maximumin April, etc.). Stationnumbers
refer to numberingof stationsin Table 1.
teristicsof a watershed.Analysesof the basinsincludedhere confirm that
the delayincreases
asthe temperature
decreases,
whichis in manycases
affectedby increased
elevation.In additionto pervadingthe annualcycle
theexpected
delaybetween
precipitation
andstream
flowis apparent
in the
anomalystructureof streamflow,as demonstrated
below throughthe
autocorrelation.
Besidesthe meanthe higherorderstreamflowstatistics
alsoexhibita
discernible
annualvariation,as illustratedby the standarddeviationand
the 1-monthautocorrelation
at Umpqua,JohnDay, and Weber Rivers
(Figure5), andlistedfor severalother"representative"
stations
(Table2).
For mostbasinsthe variance(standarddeviation)of thesetwo streamsis
highest
in months
havinggreatest
meanstreamflow.
To compare
thevariancebetweenstations
andwithintheyearthecoefficient
of variation,defined
astheratioof thestandard
deviation
tothemean,wasalsocomputed.
This
coefficientis smallestfor large northernstreams,suchas FraserRiver,
showing
valuesof 0.2 and0.4 andishighestfor smallstreams
suchasSalt
River, showingvaluesof 0.7 to 2.0. Severalstreams
havea maximumcoefficientof variation
justbeforeorjustafterpeakflow,implyingthatyear-toyearfluctuations
in timingof theriseor fall in flowis animportant
factor
in determining
the relativeamountof stream
flow variance.In additionto
showing
monthlystatistics,
Table2 alsoshowssomeanalogous
statistics
calculatedfrom annualstreamflowvalues.Becausehigherfrequency
monthly
variations
arediminished
whentheannualmeaniscomputed,
the
coefficient
of variationof theannualmeans
is smallerthanthatreflecting
monthlydata,rangingfrom0.13 at FraserRiverto 1.32atArroyoSeco.
In Californiatheseannualvaluesaresomewhat
higherthanvaluesderived
Geophysical Monograph Series
Aspects of Climate Variability in the Pacific and the Western Americas
CAYAN
for cool seasonprecipitation,which typically range from 0.3 to 0.5
[Granger,1977].The skewness
of monthlystreamflowis usuallypositive,
asit is for precipitation.Its patternoverthe year is morevariablethanthe
lower order moments,asthe skewness
canbe dominatedby relativelyfew
extremehighstreamflowevents.Skewness
is relativelyhighduringmonths
thathavegenerallyconsistent
streamflow
exceptfor occasional
extremeflood
events. Geographically,drier areas to the south that are irregularly
punctuated
by floodshavehighskewness,
suchasat SanPedroRiver (mean
annualrunoffperunitarea1.2 cmyr-l) in southern
Arizona.Thehigh
skewnessin Southweststreamsmainly resultsfrom late summerthunderstormsandtropicalstormactivityandearly winterfrontalsystems
or cutoff lows (D. Meko, R. WebbandJ. Betancourt,personalcommunications)
that occurinfrequentlyenoughto yield heavyrunoff duringmonthswhen
it is normallylight. Skewnessis low for streamswith a high baseflow that
are not markedby floods.This is typicalof wetterareasto thenorthwhich
havehigh meanprecipitationandrelativelysmalleranomalous
variability,
suchasSkykomish
River(mean
annual
runoffperunitarea252cmyr- l)
in the Pacific Northwest.
The relatively high persistenceof streamflow distinguishesit from
precipitation
andmakesit a potentially
valuableindexof shortperiodclimate
variability. The persistence
of streamflowanomalies,represented
by the
Umpqua
River,
Oregon
I
I
I
I
I
I
I
I
I
1905-1985
1.0
I
I
I
I
0.8
0.6
0.4
0.2
[Oregon
] ] [ [ [ 1929-1985
[ [ [ 1.0
5.0
"Jøh•
nD]ay,Riv[er,
.
4.0
",
',
3.o.
lT
-
/
,•½
[/,.I/ l\ ',
2.O
- 0.8
'
/
.
\
0.4 •
_.
o
1.o
0.2
o
o
WeberRiver,Utah
i
I
i
_
I
1904-1984
i
.'.''''t0.8
' - Streamflow
]t0'6
. r
[--- Precipitation
II
I"A••o.4
-I I I I I i [/0
............
I
I
I
I
I
I
0.2
Jan Mar May July Sept Nov Jan
Fig. 5. Monthly long-term mean "streamflow" (solid line), standard
deviationof monthly"streamflow"(bars),1-monthlag autocorrelation
of
streamflowanomalies
(o),andmonthlylong-termmeanprecipitation
(dotted
line) for Umpqua, JohnDay, and Weber Rivers. Streamflowhas been
dividedby the area of the watershedto make meanand standarddeviation
unitscomparableto precipitation.One-monthlag autocorrelation
plotted
underleadingmonth,e.g., the one labeled"January" is for Januaryto
February, etc.
Vol. 55
AND
PETERSON
383
autocorrelation, exhibits a marked annual variation at most basins and
displaysseveralclimaticinfluences.An understanding
of the autocorrelation behaviorcan be derivedfrom a nonstationarystreamflowmodel, such
as by MossandBryson[1974], wherethe flow in a river duringa given
periodis the resultof two kindsof components:
(1) directrunoff andbase
flow from within-periodprecipitationoccurringin the basinand (2) water
releasedfrom storageof pre-periodprecipitation
in thebasin.Sincepr•ipitationanomalieshavelittle persistence
on monthlyand longertime scales
(autocorrelations
of monthlypr•ipitationat 1- and2-monthlagshavevalues
less than 0.25 for nearly all pairs of monthsat severalclimatological
divisionsthat were examined),mostof the month-to-month
persistence
of
streamflow anomaliesresultsfrom releasesinto the river from storagethat
havetime scalesof a monthor longer.The MossandBryson[ 1974] model
considersthesereleasesto be in the form of groundwater thatpercolated
into a stream,but in the basinsconsideredhere, a majorportionof storage
mustarisefromsnowandice. Thepersistence
strengthens
bothasthesteadinessof releasesfrom storageand as the importanceof storagereleases
relativeto the directprecipitation.In severalbasinsthe precipitationminimum during the late springthroughsummercoincideswith and follows
the period of maximumstreamflow.Consequently,if late springstreamflow is higherthannormal,thenthe subsequent
flow in the summermonths
still tendsto remainhigherbecausethis flow is virtually all drawn from
storedwater.Similarly,negativeanomalies
tendto persistfrom lower-thannormalspringflows. Thusthe components
of this systemcanbe thought
of as "signal" and "noise" in a predictionscheme,whoseskill is indicated by theautocorrelation
of anomalous
streamflow. The unsteady,atypical
releasesfrom storage,suchas (1) an unusuallyearly springmelt due to
anomalously
hightemperatures
and(2) thedirectprecipitation
components
of stream
flow in thetargetmonth,actasnoise.Steadyreleases
fromstorage,
suchasthe normalseasonal
meltingof the wintersnowpack,is the signal,
andthe autocorrelation
is high duringmonthswhen(on average)this signal is high relativeto the unsteadycomponent.
The seasonalstructureof the anomalypersistence
can be clearly seen
from the 1-monthlag autocorrelation
andmeanmonthlyflow at Umpqua,
JohnDay, andWeberRiversin Figure5. All threerivershavea minimum
in autocorrelation
duringthe periodwhenmeanflow is rising. The autocorrelationis sensitiveto the characteristics
of eachbasin,seenby the shift
in timing from winter to late springaspeakprecipitationandpeak runoff
shifts.UmpquaRiver, the coastallow elevationbasinwith little snowpack
andthuslittle storage,hasa January-Februarydip in 1-monthlag autocorrelationwhen nonpersistent,
high varianceprecipitationinput to the
streamis greatest.This is confirmedby the cross-correhttions
between
precipitationand streamflowduringwinter months(not shown)whichare
generallygreaterthan0.7 at Umpqua.Furtherinland,the higherelevation
JohnDay watershedshowstheeffectof snowmelt,with an April-May peak
in runoff and a pronouncedautocorrelation
minimumin February-March
when precipitationis still high and streamflowis rising sharply.CrosscorrelationsbetweenJohn Day streamflowand both precipitationand
temperature(not shown)are bothpositiveduringthis periodof the year.
Finally, Weber River, fed by high elevationRocky Mountainsnowmelt,
haspeakstreamflowin Juneanda sharpdip in autocorrelation
in April-May,
when streamflowis rising rapidly. Cross-correlations
of Weber River
streamflowand temperature(not shown)are positiveand relatively high
(0.3 to 0.5) during late spring, again supportingthe interpretationthat
streamflowis significantly
affectedby eithertemperature
hastening
or delaying the snowmeltthen. All three basinsshow high 1-monthlag autocorrelations
duringtheperiodof yearafterprecipitation
hasfallenoff. This
reflectsthe "uncontaminated"
influenceof the storageof waterin thebasin.
The apparentinfluenceof climatesuggested
by the progressionof the lag
structureat thesethree stationsis reinforcedfrom inspectionof the autocorrelationsof the entire set of streams,which revealsa surprisingly
consistent
organizationaccordingto elevation(temperature)andseasonal
mean precipitation.
Geophysical Monograph Series
384
ATMOSPHERIC
Aspects of Climate Variability in the Pacific and the Western Americas
CIRCULATION
AND
STREAMFLOW
Vol. 55
IN THE WEST
ß
ß
.
Geophysical Monograph Series
Aspects of Climate Variability in the Pacific and the Western Americas
CAYAN
AND PETERSON
Vol. 55
385
Geophysical Monograph Series
386
ATMOSPHERIC
Aspects of Climate Variability in the Pacific and the Western Americas
CIRCULATION
AND
STREAMFLOW
IN THE WEST
Relationof Streamflowto North Pacific Atmospheric
Circulation
Vol. 55
Patterns
North Pacific CirculationVersusStreamflow
It mightbeexpected
thatthestrongest
linksto atmospheric
circulation
wouldoccurduringwinterwhenNorthPacificstormsaremostactiveand
anomalous
atmospheric
variations
aregreatest.
Thedeepening
of theNorth
Pacific Low and the increase in variance of associatedvariables is indicated
by thestandard
deviation
of seasonal
SLPin thecentralNorthPacific,whose
maximum value south of the Aleutian Islands increases from 2 mb in sum-
mer to 6 mb in winter [Namias, 1975]. The effect of winter atmospheric
circulationon streamflowis shownin Figures6 and 7 by mapsshowing
the cross-correlation
betweenthe anomalyof December-August
(9-month
cumulation)
streamflow
at a givenbasinandwinterSLP acrosstheNorth
Pacific.Sincethespatialvariation
in thestandard
deviation
of SLPissmall,
the contourson thesemapscan be qualitativelyinterpretedas anomalous
geostrophic
windswith cyclonicor counterclockwise
flow aroundnegative centersand anticyclonic
or clockwiseflow aroundpositivecenters
[Stidd, 1954]. Very generally,regionsof cyclonicflow anomalies
are
associated
with anomalously
highprecipitation
andanticyclonic
flow with
low precipitation.
Inspection
of SLPcross-correlations
andcomposite
maps
from the otherseasons
showsthat winter SLP hasthe strongeststatistical
connectionto December-August
streamflowfor virtually all basinsconsidered,butthattheSLP anomalyin fall (preceding)and,for somebasins,
spring(during)alsomakesignificantcontributions
(for brevity,correla-
the northeastPacific.The enhanced
southerlyto southwesterly
cyclonicflow
associated
with the anomalouslow to the west or northwestrepresentsan
aggregrateof weathersystems
thatproducestrongeradvectionof moisture
and vorticity as well as increasedupslopeverticalmotionalongthe west
coastmountaintopography,whichis generallyalignedfrom southto north.
The oppositepatternof anomalous
highpressureoff-shoreindicatesa lack
of storminess,
reducedmoisturetransport,andsuppressed
upwardvertical
motion. The north-southsequenceof maps from Alaska to southern
Californiaclearly indicatesthe sensitivityof the coastalstreamflowto the
placementof the low; the anomalous
correlationcenter"migrates" southward along the coastas the basinlocationis taken from north to south.
Atmosphericpatternsfavoringhighstreamflow in the interiorbasinsof
theWest (shownin thetopfourmapsof Figure7) differ sharplyfromthose
seenin Figure 6 for the coastalbasins.Severalof the interiorbasinshave
highestpositivecorrelations
with positiveSLP anomalies
locatedremotely
WinterSeaLevelPressure
vs.
• Positive
December-August
Streamflow
Correlation
• Negative
Kenai River at
Cooper
Landing,
tions for theseother seasonsare not shownhere). For severalbasinsthe
SLP anomalypatternsdisplayed
by fall are similarto winter,albeitsomewhatweaker,suggesting
thatthe samephysicalmechanisms
operate.For
near-coastal low-to-moderate elevation basins in the southern half of the
domainthereare significantcorrelationpatternswith springSLP. These
are similar to onesfoundfor winter, and also similarto patternslinking
atmospheric
circulation
withprecipitation
shownby otherresearchers
[e.g.
Klein andBloom,1987].Thisimpliesthat,for thesesouthern
lowerelevationbasins,springprecipitation
is an importantinfluenceon stream
flow,
andthatit is produced
by similaratmospheric
circulation
anomalypatterns
asin winter. However,correlations
with springSLP are weakfor several
basinsin thenorthandin higherelevations.Sincein springthereis nonnegligibleprecipitation
throughout
westernNorthAmerica,andprecipitation is still relatedto the circulation[e.g. Klein andBloom, 1987], it is
likely thatothermechanisms
competeto diminishthe relationof streamflow with the atmospheric
circulation.One competing
mechanism
is the
temperature
field, whichwouldlikely increaseor decreasestreamflow
independent
of precipitation,
sincetemperature
andprecipitation
areonly
%o
SproatRivernear
Alberni,BritishColumbia
•r•ltshceRnl•,%;l•foa;n
ia _
(24
weaklycorrelated
in theNorthwest
duringspring(J.Namias,
personal
communication);
fromthiswe mightinferthatthecirculation
patterns
associated
withtemperature
anomalies
arenearlyindependent
fromtheonesassociatedwithprecipitation
anomalies.
In support
of thisinterpretation
is theobservationthat the cross-correlation
of streamflowwith precipitationis
weakenedandthecross-correlation
of streamflowwith temperature
is posi-
tiveduringspringmonths
for severalof thebasins.Analyses
arenotshown
for brevity.A multivariateanalysisof the circulation,precipitation,
temperature,andstreamflowis requiredto betterisolatethesemechanisms.
Thereis a remarkablyconsistent
patternin thenorth-to-south
sequence
of coastalbasinSLP cross-correlation
maps(Figure6). An anomalous
low
pressure
centerto thewestor northwest
in winterprovides
heavierprecipi••søaY•eSn•n
•'•rr
nia
tationandhigherstreamflow
onthecoast,whileananomalous
highpressure
centerthereresultsin reducedprecipitationand stream
flow. This pattern
is consistent
with 700-mbheightversusprecipitation
relationfor periods
Fig. 6. Cross-correlationbetween winter North Pacific SLP and
of 1 dayto 1 monthalongthewestcoastduringwinternotedby Klein and
December-August
streamflowanomalies
at five coastalstreams(location
collaborators[Klein, 1963; Klein et al., 1965; Klein and Bloom, 1987].
at dots) from Alaska to SouthernCalifornia. Correlationsare contoured
The synopticinterpretation
of thehighstreamflowphaseof thiscirculation
at 0, +0.3 and +0.5, with shadingto indicatemagnitude.
patternis thatit indicatesincreased
storminess
thatis carriedonshorefrom
Geophysical Monograph Series
Aspects of Climate Variability in the Pacific and the Western Americas
CAYAN
to the westand somewhatlower positivecorrelationswith negativeSLP
anomalieslocally.The westernpositivecenteris a remoteteleconnection
ratherthana directregional(low) forcingpattern.This type of patternis
displayedby the northernthreebasinsin Figure7 (Susitna,Alaska;Clark
Fork, Montana;andWeber,Utah). Onlythesouthernmost
basin,SaltRiver,
Arizona, showsa winter SLP cross-correlation
patternsimilarto thoseof
the coastalbasins(Figure6). Correlationwith a positivepressureanomaly
in the centralNorth Pacificimpliesbotha southerlydisplacedstormtrack
acrossthe easternNorth Pacific and into the West, and probablymore
activity from stormsfrom NorthernCanadaand Alaskathat drop into the
West, east of the Cascades.For Clark Fork and Weber River basins, there
are local negativecorrelationswith SLP, but they are relativelysmall in
spatialscaleand in magnitude.This resultis consistent
with Klein et al. 's
[ 1965] observation
thatin partsof the interiorwesternUnitedStates,daily
Vol. 55
AND
PETERSON
387
precipitation
in winteris notfavoredby southwesterly
flow asin thecoastal
region,but ratherby flow with an anomalous
northeasterly
to southeasterly
component.Further, he points out that precipitationin this region is
associated
with a localtroughproducingpositivevorticity,andnot a more
remoteupstream
regimecharacterized
by vorticityadvection,
asis precipitation in coastalregions.Klein et al. [1965], andWeare andHoeschele[1983]
found, in contrastto the west coast, that in the Rockies and in the Great
Basina smallerfractionof the precipitationvariancecouldbe accounted
for by upper level geopotentialheightanomalies.The streamflow-SLP
resultsare consistent
with their findings,suggesting
that local influences
suchas topography,surfaceheating,and friction contributesignificantly
to theprecipitation
variability[CayanandRoads,1984].A strikinginfluence
of topographycan apparentlybe seenby comparingthe SLP relationsof
the two Alaskanstreams,seenin thetwo top mapsof Figure6 andFigure
7, respectively.
Despitetherathercloseproximityof thetwo basins(Kenai
River drains the northern Kenai Peninsula in the northern Gulf of Alaska
Winter
SeaLevelPressure
vs.
8 Positive
December-August
Streamflow
Correlation
•3Negative
-,....:..:
•...:-.:.,:...:.,..::•::./:.:.•:...:..:.:
:.•,.:
• :..,... :...::.:::...:::..,..-,.v:..::
.:.:....•.:
..
,.:.:.:...,:..:.......:.....•...,::,::.. .............•
.....
Susitn•"F•ive,r
-24'•'%• •il"
and SusitnaRiver drainsthe TalkeetnaMountainsjust a few hundredkm
to thenortheast),
thelinksto thecirculation
areastonishingly
different.Increasedstream
flow in KenaiRiver resultsfrom a deepened
AleutianLow
andanomalous
southerly
winds,whileincreased
streamflow
in SusitnaRiver
is associatedwith high SLP centeredwell to the southbut little evidence
for a localor regionalcirculationmechanism.ChenaRiver, whichis even
farthernorthandinlandof theAlaskaRange,displays
a correlation
to SLP
(not shown)whichresemblesthatwith Susitna.In fact, the recordof streamflow at ChenaRiver hasa weak negativecorrelationwith that at Kenai.
This contrastappearsto be a symptomof Alaska'scomplexclimate,with
the fall andwintermaritimestormsthatproducesubstantial
precipitation
alongthe immediatecoast,not as apparentin the interior,whichis dominatedby summerconvective
precipitation
(S. A. Bowling,personalcommunication).
Glacialmeltcontributing
to theflow mayalsocomplicate
the
relation of SLP to stream flow in some of the watersheds
An evenlarger spatialscaleperspectiveof the influenceof circulation
on hydrologicalvariabilitycanbe seenfrom the HawaiianstreamflowversusSLP correlations,(bottompanel, Figure 7) in contrastwith thosefor
:::-34:--
North American streams. The Wailua River, located on the east side of
•0•
0.
Weber River
nearOakley,Utah
Kauai, HawaiianIslands,lies in a precipitationregimethat is dominated
by the interplaybetweentopographyand the wind [Solot, 1950]. High
streamflowin WailuaRiveris stronglyassociated
with positivewinterSLP
anomaliesto its north in the central North Pacific and negativeSLP
anomalies
to its southwest.
Accordingto Lyons[ 1982],thisSLP configurationyieldsa mostprominentmechanism
for precipitation
in theHawaiian
Islands:strongernortheasterly
tradewinds,whichproduceheavyprecipitationon the easternsideof Kauai, dueto orographiclifting of the moist
subtropicalair mass.In contrasthigh streamflowin SouthernCalifornia
and Arizona is associatedwith negativeanomalousSLP centeredto the
Northwestor westthatindicates
strongsouthwesterly
flow, discussed
above.
Comparisonof the Wailua cross-correlation
with the one favoringhigh
streamflowin Clark Fork andWeberRiver in the upperpanelsrevealsa
remotelyconnected
similarity;thesamewintercirculation
regimesthatresult
in high or low streamflow in Hawaii tend to result in similar conditions
over the northwestern United States. This teleconnection is further illustrated
below.
Weakand StrongWinterNorth Pacific CirculationVersusStreamflow
Cross-correlations
discussedabove suggestthat strongnegativeSLP
anomaliesin the centralNorth Pacificproducelow streamflowin much
of the Northwestandin the Hawaiianstreams.Conversely,positiveSLP
Fig. 7. Cross-correlation
betweenwinter SLP over the North Pacificand
December-Auguststreamflow anomaliesat four interior westernNorth
America streamsfrom Alaskato Arizona, plusonestreamin the Hawaiian
Islands.Correlationsare contouredat 0, + 0.3 and + 0.5, with shadingto
indicatemagnitude.
anomalies over the central North Pacific are associated with a circulation
that favorshigh stream
flow in theseregions.
The well-known PNA index [Horel and Wallace, 1981; Wallace and
Gutzler, 1981] is stronglyrelatedto anomalousSLP in the centralNorth
Pacific,but alsoincludesatmospheric
valuesfartherafield over the sub-
Geophysical Monograph Series
388
ATMOSPHERIC
Aspects of Climate Variability in the Pacific and the Western Americas
CIRCULATION
AND
STREAMFLOW
Vol. 55
IN THE WEST
tropicalNorth Pacific, westernCanada,and southeastern
United States.
For this studya PNA index [Horel and Wallace, 1981] was constructed
fromseasonal
700 mbheightvaluesoverthe 1947-1986period.Thisversion
of the PNA is given by
TABLE 3. Central North Pacific (CNP) Index: Strongand Weak
Winter
Cases
[Sea-levelpressure(SLP) anomalyaveragedover
170ø E.-150 ø W., 350-55 ø N.]
PNA = H(170ø W., 20ø N.) - H(165ø W., 45ø N.)
+ H(115ø W., 58ø N.) - H(90ø W., 30ø N.)
(1)
Strong
Winter*
Anotherphenomenon
relatedto atmospheric
circulation
overtheNorth
Pacificis a prevalence
of anomalously
deepcentralNorthPacificlowsin
northernhemisphere
winterduringthe "maturephase"[Rasmusson
and
Carpenter,
1982]of theE1Niffo/Southern
Oscillation
(ENSO)[Bjerknes,
1969;Namias,1985].In the availablehistoricalrecordtheseeventshave
occurred
irregularly
at anintervalof 3-10 years[QuinnandNeal, 1986].
E1 Niffo/SouthernOscillationepisodeshave accompanied
impressive
extratropical
atmospheric
andoceanic
anomalies,
astherecent1982-1983
eventremindsus [Quiroz,1983].The centralNorthPacificteleconnection
is one of the strongest
extratropical
responses
to ENSO [Dicksonand
Livezey1984;Namias,1985].Forthepresent
studytheSouthern
OscillationIndex(SO1),provided
bytheNOAAClimateAnalysis
Center,isused
Weak
SLP
Winter*
SLP
..............................
1903 ................
1.3
..............................
1907 ................
1.3
..............................
1909 ................
0.9
..............................
1910 ................
1.4
..............................
1911 ................
1.6
1915 ................
1.1
..............................
1916 ................
1.6
..............................
1922 ................
1.2
1912'*
1926'*
.............
.............
-0.5
-1.4
.............................
1927 ................
-0.7
.............................
1929 ................
-0.9
.............................
asan indexfor ENSO. It hasbeennoted[Douglaset al., 1982;Esbensen,
1931'*
-1.5
1984]thatthePNA often,butnotalways,occursduringENSOepisodes;
..............................
1932 ................
1.5
..............................
1933 ................
0.6
.............
it can also occur separately.
To furtherexploretherelationof streamflowto atmospheric
circulation
1934 ................
-1.0
over the central North Pacific, an SLP index, abbreviated "CNP,"
1936 ................
-1.6
was
.............................
.............................
.............................
constructedby averagingthe SLP over the region 350-55ø N. and
..............................
170ø E.-150 ø W. This index is similar to the PNA index, but CNP is
1939 ................
-0.5
.............................
availablefor a longerperiodthanPNA (CNP is availablesince1899,while
PNA beginsin 1947).Thereare25 "strong"and25 "weak" CNP winters,
definedasstandardized
indexvaluesgreaterthanor equalto -0.5 andless
thanor equalto +0.5, respectively
(Table3). To characterize
themagnitude
of strongand weak CNP casesthe averageCNP for the 25 strongCNP
1940'*
.............
-2.3
.............................
1941'*
.............
-1.8
.............................
1942'*
.............
-1.0
1944 ................
-0.7
casesis less than -7 mb, while it exceeds + 7 mb for the 25 weak cases.
1946 ................
-0.6
In bothof thesecomposites
the strongest
anomalycenteris foundat about
50ø N., 165ø W. It is interesting
thattheseextremewinterstendto cluster
in runs,asdiscussed
herelaterin regardto low frequencyvariability(also
seeEbbesmeyeret al. [thisvolume]for remarkablelow frequencyeffects
on the Puget Soundestuary).
The interrelation
amongSO1,PNA, andCNP is shownbytheirseasonal
cross-correlations
(Table4). The significantcorrelations
appearto be SO1
andPNA duringwinterandspring,SO1andCNP duringwinter, andPNA
andCNP duringwinter, spring,andfall. It is alsointeresting
to notethat
SO1 and PNA correlationsare of the samemagnitudeas SO1 and CNP
correlations
for everyseason
exceptspring.Duringwinterandspring,PNA
is significantly
(90 percent)correlated(•, = -0.41 and -0.31, respectively)
withSO1over1947-1986.Duringwinter,butnotspring,CNP is correlated
at aboutthe samemagnitude(•, - 0.40 and0.16, respectively)
with SO1
albeitwith more statisticalsignificancesincethe 1931-1986 samplesize
is larger. PNA andCNP are stronglycorrelated,especiallyduringwinter,
spring,andfall (•, = -.90, -0.75, and -0.69, respectively).
Thisis somewhat surprisingsincePNA includesthreecentersin additionto the central
North Pacific region of the CNP. These high correlationsindicatethe
reliability of the teleconnection
from the centralNorth Pacific;therefore
the greatimportanceof the role of the centralNorth Pacificin formingthe
PNA. Teleconnection
mapsof Namias[1981], and Wallaceand Gutzler
[1981] providegreaterdetailsof thePNA structure.We concludethatCNP,
formed from the more historicallyextensiveSLP field, is an adequate
surrogatefor the PNA during winter and fall.
To testtherelationof streamflow
variabilityto theseatmospheric
indices
maps of cross-correlations
between winter CNP, PNA, and SO1 and
December-Auguststreamflowanomaliesare shownin Figure 8. There is
interdependence
amongthesethreeindices,asindicatedby similarspatial
patternsin theirrespectiveanomalypatterns,theirtemporalvariations,and
the resultantcorrelationswith streamflow.As suggested
by the individual
..............................
1948 ................
0.8
..............................
1949 ................
1.5
..............................
1950 ................
1.3
..............................
1952 ................
1.0
1937 ................
..............................
1953 ................
.............................
1943 ................
-1.1
2.2
0.6
.............................
.............................
.............................
..............................
1955 ................
1.0
..............................
1956 ................
1.4
1957 ................
0.6
..............................
1958'*
.............
-0.7
.............................
1961 ................
-1.3
.............................
1963 ................
-1.4
.............................
1964'*
-0.7
.............................
.............
..............................
1966 ................
0.9
..............................
1969 ................
1.5
1970'*
.............
-1.5
.............................
..............................
1971 ................
0.7
..............................
1972 ................
1.4
1977'*
.............
1978 ................
-2.2
-1.2
..............................
-0.7
1981 ................
-1.7
..............................
.............
1986 ................
.............................
1979 ................
1980 ................
1983'*
.............................
1.1
.............................
.............................
1982 ................
-1.8
.............................
-1.9
.............................
0.7
*Winter is definedsuchthatWinter 1912is averageof (December1911,
January1912, February 1912).
**Northernhemisphere
wintercorresponding
to E1Niffo eventdefined
by Quinn et al. [1986] of magnitudegreaterthanor equalto 2.
Geophysical Monograph Series
Aspects of Climate Variability in the Pacific and the Western Americas
CAYAN
TABLE 4. Cross-Correlation Between Seasonal SOI, PNA, and CNP
Circulation
Indices
SOI
SOI
PNA
and
and
and
PNA
CNP
CNP
Winter ..............................................
-0.41
0.40
-0.90
Spring..............................................
-0.31
0.16
-0.75
Summer ............................................
-0.10
0.13
-0.21
Fall ..................................................
-0.23
0.20
-0.69
Number
39
54
of Pairs .................................
39
Two-tailedtestvalueat 95% significance...10.311 [0.271 10.311
Vol. 55
AND
PETERSON
389
level asthosefor the periodfollowing.This suggests
thattheseteleconnectionsandtheir attendantphysicalmechanisms
are stablefeatures.A second
virtueof theCNP is its simplicityof representing
themostimportantwinter
circulation mode in the central North Pacific, the PNA [Davis 1978;
BarnstonandLivezey, 1987; Namiaset. al., 1988] with regionalaverage
SLP anomalies.
The "mature" winter phase[Rasmusson
and Carpenter, 1982] of the
ENSO is associated
with low pressurein the centralNorth Pacificin winter.
The SO1versusstreamflowanomalycorrelationpatternis similar to that
of CNP andPNA, but with two differences.In thisanalysisthe SO1correlation alongthe Alaskancoastis weaker than that with CNP and PNA,
althoughYarnalandDiaz [1986]reportedsignificant
correlations
of coastal
Alaskanprecipitationwith ENSO activity. The weaknessof this relation
probablyarisesfrom the variabilityin the longitudeof the North Pacific
low, whichcanbe seenfrom the broadloci of anomaloustroughsaccom-
panyingstrongE1Niffo duringwinter(Peterson
et al., [1986],Figure8).
streamflowversusSLP results(Figure 7), when PNA or CNP indices
indicateanomalouslow pressurein the centralNorth Pacific, there is a
tendencyfor lightstream
flow overthenorthwest
coastintothe interiorfrom
Idahosouthwardinto Utah, light streamflowin the two Hawaiianstreams,
and high streamflowfor stationsalongthe Alaskancoast.The opposite
streamflowanomalyscenariois associated
with anomalouslyhighpressure
in the central North Pacific. However there is almost no correlation between
PNA or CNP and streamflow in California, in the Southwest, in British
Columbia, and interior Alaska.
The relationof streamflowwith strongandweakcentralNorth Pacific
winterlowscanbereconciled
usingatmospheric
teleconnections
[Namias,
1981; Wallace and Gutzler, 1981]. Teleconnections
(cross-correlations)
showthatanomalously
low 700-mbheightin thecentralNorthPacific(centerednear 50ø N., 170ø W.) duringwintertendsto be accompanied
by
positiveheightanomalies
overthewestcoast,as well as positiveheight
anomalies
in thesubtropical
NorthPacific;a patternof anomalies
of oppositesignis connected
withpositive
anomalies
atthiscenter.Because
monthly
scaleatmosphere
featuresarealmostbarotropic(anomalyfeaturesnotverticallytilted)in winterovertheextratropical
oceans[WallaceandGutzler,
1981],theinterpretation
of SLPanomalies
is nearlyequivalenttothe700-mb
anomalies.Inspectionof SLP composites
(not shown)for strongandweak
CNP patternsconfirmsthattheSLPteleconnection
patternis very similar.
Theseteleconnections
canbe translatedin termsof the large scalecirculation,thecarryingcurrentfor NorthPacificstorms.
WhenthecentralPacific
low is well developed,NorthPacificwinterstormstendto be carriednorthward toward northernBritish Columbiaand Alaska, makingthat region
wet while the northwesternUnited Statesis dry. Conversely,whenthe low
is weak, theGulf of Alaskalow to theeastis usuallyactive,andthe opposite
precipitation
andstream
flow anomalypatterntendsto occur.The meannegative SLP anomaliesin the centralNorthPacific,corresponding
to enhanced
storminesssouthof the AleutianIslands,are associated
with a high pressureanomalyridge downstreamover the Northwest,with the stormtrack
routed north into the Alaskan coast instead of into the Pacific coast further
south (see Klein [1957] and Reitan [1974] for storm tracks). In the subtropical North Pacific the flow is anomalouswesterly (west-to-east)in
association
with the gradientbetweenthe deepcentralPacific low to the
north and anomaloushigh pressureto the south. This flow anomaly
represents
reducedtradewindsandan associated
diminutionof precipitation and streamflowon the easterncoastalslopesof the HawaiianIslands.
The relativeweaknessof the southernpositionof the teleconnection
center
downstreamoverthe westcoastmeansthatthetendencyfor highpressure
(lack of storminess)is not very reliable; hencethere is not a strongCNP
relationshipover Californiaand the Southwest.
CNP appearsto be a usefulindexfor more thanone reason.It is encouragingthatfor winterthe correlations
of CNP with streamflowareabout
the samein magnitudeandmakea similarspatialpatternasthoseof PNA
with streamflow.This reflectsthe fact that the pre-1947 (before PNA)
relationsof streamflowwith CNP (not shown)hold up at aboutthe same
A morewesterlypositionof thistroughfavorsthewinter stormtrackmoving intotheAlaskancoast,while a moreeasterlypositionis associated
with
stormsenteringthe westcoastfarthersouth.Furtherinsightinto the types
of northernhemisphere
circulationpatternsthatappearwith ENSO is provided by Fu et. al., [1986], and Livezey and Mo [1987], who suggestthat
the configurationof tropicalPacificheatingmay helpto determinethe pattern of extratropicalresponse.
Interestingly,SO1is significantlyrelatedto precipitation
andstreamflow
over the southwesternUnited States,while PNA and CNP are not. Stream-
flow anomalies
in theSouthwest
from southernCaliforniathroughArizona
and southernColoradotendto be positive(wet), while thosein the Pacific
Northwestare negative(dry) duringthewarm easterntropicalPacificphase
of ENSO. Thereis an importantdistinctionbetweenthe resultswith streamflow andtheresultsof a previousinvestigation
of therelationbetweenENSO
andNorth Americanprecipitationby RopelewskiandHalpert [1986] who
founda suggestion
thatthe E1Niffo phaseof ENSO wasassociated
with
lighterthanaverageprecipitation
in thePacificNorthwest,butthistendency
was not strongenoughfor them to considerit to be reliable. Part of the
reasonfor thisweakness
maybe the waveringof timingof thelight precipitationbetweenfall and spring.However, in basinswith higherelevations
streamflowis not as sensitiveto thesetiming changesas is the precipitation, sincesnowmeltfrom the cumulativeprecipitationis a significantpart
of the streamflow.In the Southwestheavyrunoff appearsto resultfrom
activelow mid-latitudestormsthattap subtropicalPacificmoisture,which
is often transportedby the subtropicaljetstream.This heightenedactivity
occursoverseveralmonthsfromfall throughspring,asdiscussed
by Douglas
and Englehart[1981], Ropelewskiand Halpert [1986], and Andradeand
Sellers,[1988]. Increasedfall-through-spring
precipitation
in the Southwest
duringthe E1Niffo phaseof the SouthernOscillationcontributes
to the
observedabovenormalstreamflow.A simplifiedview of stormtracksand
the associated
broad scalestreamflowanomalypatternduringthe mature
phaseof ENSO is shownin the bottompanel of Figure 9.
Not all basinsin this network are well related to PNA, CNP, or SO1.
As shownpreviouslyin Figure 6, the SLP patternmostreliably correlated
with anomalousprecipitationand streamflowin California is an SLP
anomalycenteredto the northwestat about(40ø N., 130ø W.). The winter
700-mb heightteleconnection
patterncenteredat this origin [seeNamias,
1981] showsa similar patternto the California SLP correlationmap. In
comparisonwith the central North Pacific teleconnectionthe California
patternis moreregionallyconfined,andweaklyrelatedto anomaliesin the
AleutianLow region. A similarcomparisonis foundfor the atmospheric
circulationpatternandthe regionalteleconnection
pattern[Namias, 1981]
associated
with streamflowanomaliesalongcoastalBritishColumbia.The
moreregionalresponses
helpto explainthe lack of a statisticalconnection
betweenstreamflowin theseareasand strongor weak atmosphericcirculationin thecentralNorth Pacific.Saiddifferently,CaliforniaandBritish
Columbiaare closeto the nodeof the atmosphericlong wave patternthat
emanates from the central North Pacific so that small variations in the
Geophysical Monograph Series
390
ATMOSPHERIC
Aspects of Climate Variability in the Pacific and the Western Americas
CIRCULATION
AND
STREAMFLOW
.,..••
IN THE
WEST
,,.••
•._.•k.,,
=
• -•,
"%
/%
,,.••
,..•.•.%• %,
• •_
/%
•._ • ••,/'
•,•%
-•. •
•
• •
•,.,"¾'•
• -
q•.•-o.3
'7•
•._ •
••0
' Hawaii •
....0.3
_......
.
Correlation
of
%,, Atmospheric
Indices
/% withDec-Aug
/'
Streamflow
Anomalies
Hawaii 0 ¸ •3 Hawaii
0 0........
3 0 0.0
0.3
'
0 ••3
Correlation
,•;Po•
•'• ....... •
Vol. 55
•: •::::::55•:5
•sign
reversed)•'•
Fig. 8. CorrelationbetweenDecember-AuguststreamflowanomaliesandwinterCNP (left), PNA (middle)and SOI (right).
SignonPNA wasreversed(negative
valuesrepresent
deepAleutianLowphase)for easiercomparison
withotherindices.Contours
at 0, +0.3, +0.5.
positionof the remotecirculationanomalycentercanyieldeitherpositive
or negativestreamflowfluctuations.
An a posterioriperspective
of the effectof theatmospheric
circulation
on streamflow is providedby comparingthe annualstreamflowat eachof
four selectedrivers with the appropriatearea averagewinter SLP index
Weak Central North Pacific Low
•
Strong
Central
North
index of winter SLP stationed to the northwest
of California
centered at
(40ø N., 130ø W.), the regionof maximumcorrelationin Figure6, This
indexis foundeduponcorrelations
with Californiaprecipitation[e.g. Klein
and Bloom, 1987] as well as resultsfrom this study.The Kenai (coastal
Alaska), Clark Fork (Idaho), and Wailua (Hawaii) rivers are eachpaired
to the winter CNP, while the Cosumnes(California SierraNevada)is related
to winter SLP northwestof the California coast.To emphasizethe lower
:
Hawaii
_Pacific
Low•NA)
Hawaii
ElNifo
S
Anomalies
:i:i:i:i:
High
I
IIIILow
I
(Figure 10). Three of the streamschosenhere are takenfrom the regions
that were most sensitiveto the central North Pacific low. In addition, an
Hawaii
,//
4:[•.•
Subtloplcal
jet
frequencyportion of the variability the annual time serieshave been
smoothed
with a 1-2-1weightedrunningmeanf'fiter.The variations
in annual
streamflow,whichare of order20-40 percentof the climatologicalmean,
are correlatedto theseSLP indices(winter seasononly) at the 0.4 to 0.6
level. Many of themajorfeaturesin thestreamflowvariabilityare strongly
relatedto the winter SLP fluctuations.For example,the deficitflow in the
NorthwestandHawaiiduring1939-1946wasa periodof unusually
frequent
deep winter lows in the central North Pacific (6 of these8 years have
"strong" low wintersin Table2). The relativelyhigh flow in the Northwestbetween1948-1957 was a period of weak winter lows in the central
North Pacific(7 of these10 yearshave "weak" low wintersin Table 2).
Duringthisperiodthe PacificNorthwestalsohadgenerallyhighprecipitation, andsnowdepth[WaltersandMeier, thisvolume;Ebbesmeyer
et. al.,
thisvolume].For KenaiRiver alongthe Alaskacoastthe latterperiodwas
one of relativelylow streamflow,but thereis no streamflowavailablefor
the earlier period. Heavy runoff in the California Sierra Nevadaduring
1978-1983wasa periodfeaturingnegativeSLP in theeasternNorthPacific
alongthe Californiacoast,andscrutinyof time seriesof Southwest
streams
plottedin Figure3 showsthatthisperiodstandsout as oneof the heaviest
streamflowepisodes
in theinstrumental
record(seealsoKay andDiaz [1985]
for a discussion
of effectsof this regime on Great Salt Lake).
AtmosphericIndicesLeadingStreamflowat SeasonalTimeLags
Fig. 9. Schematicof winteratmospheric
flow andassociated
streamflow
anomaliesduringweak centralNorth Pacific low, strongcentralNorth
Pacific low, and duringnorthernhemispherewinter "mature" phaseof
E1 Niffo.
It is of interestto determinehow well streamflowcanbe predictedfrom
theselarge scalecirculationindices.As a simplefirst cut at this issue
correlationsof seasonalSOI, PNA, and CNP versusDecember-August
Geophysical Monograph Series
Aspects of Climate Variability in the Pacific and the Western Americas
Vol. 55
CAYAN
AND
PETERSON
391
Atmospheric
Indices
andAnnual
Streamflow
11o
]
[
•
[
[
[
[
100 Kenai
River,
Alaska
90-CNP
(s,gn
reversed)
CNP
24O
•',
160
•
120
•
•
[
]
[
[
•
,,-/',
,'-
'"'
,'
•
•
,•
-t,,.J\
[
-•0
1-2
v
J-4
_
t',•,'\
,.
•i
[
0-059_[
4
- '
,l
",.k,(/
•
•' / •
,,,.. Y%
,, w • •,•,j
[
/ \i /'•/".._q
,' •','
%/'i
,,,,..,,',,/
./•
• /
• 8O
•
[
,,
, ½,./,\ /•,,•/,,,',••
'• '•
/• / •
'""
'"
..-. 200
/'•
/' '\
',',,,
/ ',,,;; ',' \,", •"
'" "'
' ',,v
/ ,,,,,
ClarkForkMontana,"',
28O
[
,/'..
..... ',, / \/\
- "/'..'"o /,",
- "! ;...,
,-,'"/' /
".,
70
60
50
32O
[
P=ø'55-1-
•,
•
,,,..,,
'-
•0 C•
1.8
1.6
,,,
1.4
,,,
_ .
1.2
1.0
h ,,
..'....
/
,, . .
, •
0.8
V
40
Cos•mn•s
River,
California
-CA
SUP
Index
(sign
reversed)
30
20
,-,
[
i
890
[
1900
..,, -
,,
[
[
'[
Z
1910
•,,I
.....Winter
sea
level
pressure
Annualstreamflow
•
/•
.
z
• "•
i vj
1920
?,,
1930
I
•
,'i [ •,I• '"[ .,,,
L'
[ i • r'
•
•
,
1940
•
/
'
195•
,
,
1960
1970
,
p=u
,
i
•4
• 2
-2
[ 1-4
1980
Fig. 10. AnnualstreamflowandanomalousSLP indicesshowingrelationshipto winter CNP for Kenai, Clark Fork, andWailua
Riversandto SLP anomalies
averagedaround(40ø N., 130ø W.) for Cosumnes
River. Smoothing
by 1-2-1 weightedrunning
mean filter.
streamflowis shownin Table 5. To quantifythe effectsof seasonallag
relationships,
precedingfall (fl-1) PNA and CNP andprecedingsummer
(sm-1) andfall SOI were consideredaspredictorsin additionto winter and
spring(duringthe December-Augustperiod) valuesof all three indices.
The activecentralPacificperiodis primarily a cool seasonphenomenon
[BarnstonandLivezey, 1987], so summerPNA andCNP predictorswere
not employed,but SOI is characterizedby a time scaleof about 1 year
(Rasmussonand Carpenter[1982]), so summerSOI was included.Also,
lower frequencyatmospheric
behaviormightconstitute
a betterlongrange
predictor,so the averageof theseindicesfor two seasonsas well as the
value for a singleseasonwas consideredas a singlepredictor.
The resultsareencouraging
for at leasttwo reasons:
(a) predictors
preceding the December-Auguststreamflowperiodaccountfor nearlyas much
varianceasthoseduringwinter (thebeginningof the streamflowperiod),
and(b) theapparentskilllevels,whilemodest(amountof varianceaccounted
for is 5-25 percent),rivalsthe skill achievedby state-of-the-art
long range
predictionmodels[Namias, 1980b; Nicholls, 1980]. Predictorsfrom all
seasonsindicatesignificantskill for severalstreams.Springpredictors
are generallylower than thosefrom winter. Table 5 showssignificant
correlationsof SOI with streamflowfrom asearly asthe previoussummer.
In fact, summerSOI correlationswith December-Auguststreamfloware
aboutthe sameasthosefrom winterSOI. Theseresultsagainpointoutthe
reliableconnectionof hydrologyin the West to the centralNorth Pacific
atmosphericactivity that occurs over regionally favored areas. The
mechanism
leadingto theskill at theselagsis apparentlypersistence
of the
atmosphericcirculation,whichdeploysthe North Pacificwinter stormsas
describedearlier;the basinsthatexhibitthe strongest
correlationswith the
precursoryindices are virtually the same ones that exhibit significant
correlationswith the winter circulation.The centralPacific Low anomaly
is probablyamongthe mostpredictable
northernhemisphere
wintercirculationfeatures[e.g., Namias, 1985; Dicksonand Livezey, 1984].
BarnstonandLivezey [1987; Table4b] havepointedoutthatthereis some
tendencyof PNA to persistfrom fall to winter, and ENSO is known to
have a high seasonalautocorrelation.More details of long-range
predictabilityusingENSO asa possiblepredictorare discussed
by Madden
[1981], Namias and Cayan [1984], and Nicholls [1985]. The changein
correlationassociatedwith averagingover two seasonsappearsto yield
modestlyhigher correlationsfor fall plus winter CNP, particularlyfor
streamsin the northwesternUnited States;the changein correlationsfor
summerplusfall SOI predictorswasinsignificant.From this samplingthe
use of summerSOI or fall SOI individuallyas predictorsis evidentlyas
skillfulasusingthe averageof the two. Somewhatsurprisingis the result
thattheearlierseason
predictors,summerSOI andfall PNA, achievednearly
aswell astheindicesfromtheseason
closerto thestreamflowperiod,which
wouldallowfor longerforecastleadsif it holdsuponanindependent
sample.
Spatial Scalesof High versusLow Streamflow
The cross-correlationsof streamflow anomalies with atmospheric
circulationrevealconsiderable
spatialcoherenceandsuggestpossibleoutof-phaseremoteconnections
of the streamflowanomalies.To quantifythe
spatial responseof anomalousstreamflowto its climatic forcings the
December-Auguststreamflowanomaliesat each individual stationwere
orderedfrom lowestto highestandyearsof the lowestandhighestextreme
Geophysical Monograph Series
392
ATMOSPHERIC
Aspects of Climate Variability in the Pacific and the Western Americas
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AND
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CAYAN
quartiles
wereidentified.
Depending
ontherecordlengthof a givenstation,
the numberof casesenteringinto the low or highquartilerangedfrom 9
to 20. Composite
December-August
meananomalies
wereconstructed
for
all stations
fortheyearsof the(a) low and(b) highquartileof a givenstation.
Frommapsof thecomposite
thespatialdistribution
of anomalies
associated
withlow or highstreamflow
at thegivenstationcanbe seen.Also, these
two composites
illustratewhetherthe spatialpatternof low streamflow
anomalies
at thegivenstationis differentfromhighstreamflow
anomalies
there.Thesemapsare contoured
at 0.5 standard
deviationintervalsand
shadedto indicatesignificantlow or high compositeanomaliesat the
95 percentconfidence
level, assuming
normallydistributed,
independent
data.In thiscasesignificance
wasestimated
usinga t-testof thedifference
betweenthe meanstreamflowover the yearsin the quartileandthe mean
overtheentiresamplefor thatstream(i.e., a testof thenull hypothesis).
This testwastailoredto eachstationindividuallydueto the varyingnumber
of data for different
stations.
Vol. 55
AND
PETERSON
393
Streamsin the Northwestare representedby Wilson River in coastal
OregonandSpokaneRiver in easternWashington(Figure 11). The compositeSLP patternfor high anomalousstreamflowat SpokaneRiver (not
shown)is similarto the patternassociated
with weak atmospheric
circulation in the North Pacific, as seenfrom the CNP, PNA, and SOI. Nearly
the oppositepatternis associated
with strongcirculationover the North
Pacific.High streamflowat SpokaneRiver is accompanied
by high streamflow at WailuaRiver in Kauai,andlow streamflowalongthesouthern
Alaska
coast(represented
by Gold Creek) and in streamsin the Southwestfrom
SouthernCaliforniathroughArizona. Accompanying
low streamflowin
Spokane
River,thelow flow in theNorthwestextendssouthward
intonorthern California and Utah, and includesboth Hawaiian streams.Although
thereis a tendencyfor low streamflowin Spokane
Riverto haveassociated
highstreamflowin theSouthwest
andin coastalAlaska,composite
anomalies in theseregionsdo not meetthe 95 percentsignificancelevel as they
did in association
with thehighflow quartileat SpokaneRiver. For Wilson
River thepatternsthatissuefrom theheavyandlight streamflowanomaly
compositeare nearlymirror images.High andlow streamflowin Wilson
River extends eastward to Clark Fork in Montana and southward to Smith
•
•. ,,/""
•
"••_.,.,-• . •"•'/o -•.,••,•
"'"•
:ii•
0'5Hawaii
5 • ••5
.:•'•5•:.
x.
,
5
'x,,•/"• Streamflow
/o
Anomalies
Hawaii
High
Oo.
I
Low
111111o"
River in coastalnorthernCalifornia,andhasopposinglow andhigh streamflow anomaliesin Arizona. In general, the spatialinterrelationsof the
streamflowvariability reflect the patternsestablishedin the precipitation
field [see,e.g. Sellers, 1968; McGuirk, 1982].
An interesting
differenceemergesin the streamflowanomalycomposite
for California and stationslocatedhundreadsof kilometersto the east, which
is illustrated
by Sacramento
Riverin theCaliforniaSierraNevadaandGrin
Riverin Utah(Figure12). The composites
derivedfromthesestations
show
a tendencyfor negativestreamflowanomaliesto be more extensivethan
thosefor positivestreamflowanomalies.From the negativeDecemberAugustcomposites
for Sacramento
River and for GreenRiver the region
of 95 percentsignificantanomaliesspansthe domainfrom Californiato
Utah, all of which exc•d 0.5 standarddeviations.In contrastresultsfor
highcomposites
showthatpositiveanomaliesof bothbasinsare moreconfined. Significantpositivestreamflowanomaliessurroundingpositive
anomaliesat Green River are limited to the region eastwardof Nevada,
whilethosesurrounding
positiveanomalies
at Sacramento
Riverextendfrom
Nevada westward.It is noteworthythat interior basinsin the Northwest
mayshowa similartendency,aslow stream
flow anomalies
at Spokane
River
(Figure 11) apparentlyhavea larger southwardextentthanhigh streamflow anomaliesthere. At Wilson River, however, no such asymmetries
appear.
Does the asymmetryin the streamflowanomalypatternsresult from
fundamentaldifferencesbetween wet and dry atmosphericcirculation
'::•:::•.
,I o.s•
regimes?The streamflowanomalycomposites
imply that atmospheric
patternsproducingheavyprecipitationand high runoff in Californiaare
oftendistinctfromonesproducing
heavyprecipitation
in UtahandColorado.
•0.5•
:•::..
Wilson
River,
Oregon
High
."::•.•g•
E0w
II1'
Fig. 11. Compositeof December-Auguststream
flow anomaliesat all
streamsfor thehigh(left) andlow (right)quartileDecember-August
cases
at Spokane(above)and Wilson (below)Rivers. Contoursof intervalsof
0.5; regionsexceeding95 percentsignificancelevel are shaded.
On the otherhandcirculationsresponsible
for extremelydry conditionsin
Californiaapparentlyoftenaffecta broaderregionthat extendswell into
the interior. In fact, there doesappearto be prominentcontrastsin the
respectivemean circulationpatterns, as revealed by compositeSLP
anomaliesaccompanying
thepositiveandnegativeflow anomalies
at Green
andSacramento
Riversin Figure 13. Positivestreamflowanomalycirculationsfor the Sacramentoand GreenRivers are disparate:high streamflow
in SacramentoRiver is associated
with negativeSLP anomaliescentered
to the northwest (the characteristiccoastal basin pattern discussed
previously),andhighstreamflowin GreenRiver is associated
with a broad
regionof positiveSLP anomaliesin the centralNorth Pacificcenteredin
theGulf of Alaska(typicalof theinteriorpatternalreadydiscussed).
Meanwhile,meancirculations
corresponding
to low streamflow
in thetwo regions
are more similar:bothare associated
with negativeSLP anomaliessouth
of the AleutianIslandsandpositiveSLP anomaliesin the subtropics
and
to theeastalongthe westcoast,althoughpositiveanomaliesin the eastern
Pacificare more stronglydevelopedin the Sacramento
River composite.
Geophysical Monograph Series
394
Aspects of Climate Variability in the Pacific and the Western Americas
ATMOSPHERIC
CIRCULATION
AND
STREAMFLOW
Vol. 55
IN THE WEST
Summaryand Conclusions
,'•.
,,/"'-
'•1,•,/'
•
N. /'-,
Streamflow
•'--•_•.• .•./'•,•o Anomalies
• ••0 •--- •awaii
' ,
in relation to climate
variables.In particulareffectsof anomalous
largescaleatmospheric
circulation in winter on the resultantstreamfloware investigated.Streamflow
wasrepresented
by 38 streamsthatcovera largeareafromAlaskato southwesternUnited Statesand alsoHawaii. Althoughthis is a sparsearray it
appearsthatthe ability of streamflowto integratethe spatialandtemporal
distribution
of precipitation
withina basinmakesthisnetworka usefulindex
of shortperiodclimatefluctuations
overthisbroadregion.Thisis particularly importantin remotelocationswith severeterrainandhighelevations.
Most of thesestreamsexhibit a strongannualcycle. Monthly mean
streamflowlagsmeanprecipitationby zero to severalmonths,depending
on the climateand probablythe physicalcharacteristics
of a givenbasin.
The resultingannualcycleis not only apparentin the climatological
mean
streamflowbut alsopervadesmostof its otherstatistics.Comparedwith
precipitation,
streamflow
is stronglyautocorrelated
withanomalytimescales
rangingbetween1 andabout20 monthsoverthe38 stations.
Althoughsnow
datawerenotavailable,cursoryexamination
of thegeneralsettingof each
particularwatershedconfirmsthe well knownpropertythatthestreamflow
persistence
is governedby the capacityof a watershedto storeprecipitation as snow.If the watershedis cold enough(usuallythismeanshaving
highenoughelevation),snowpack
canprovidea memoryof winterconditionsthroughtherunoffseason
untilfall rainsbegin.Duringtherisinglimb
of the annualstreamhydrographat the onsetof the precipitationseason,
the relativerole of storedwater in supportingstreamflowdeclinesand,
accordingly,
theautocorrelation
is low. Later,throughvariations
in thetiming of the snowmelt,it appearsthat fluctuations
in temperature
diminish
the autocorrelation.
Finally, duringthe fallinglimb of the annualstream
hydrograph
whenprecipitation
hasdiminished,the relativerole of stored
waterin support
of streamflow
increases
and,therefore,theautocorrelation
strengthens.
- Hawaii
Green
River,
•-•..........
•?--•
•-•-•
Utah
Thisstudydescribes
aspects
of temporalandspatialvariabilityof monthly
streamflow in western North America and Hawaii
0.5
"::•:•::..
o.5 I
•'•'
)'n?111111
%*
I•
Hawaii%
River,
Sacramento
I•
Ha•ii
High
California
,
•Streamflow
• :•1.0,;•
Fig. 12. Compositeof December-Auguststreamflowanomaliesat all
streamsfor thehigh (left) andlow (right)quartileDecember-August
cases
at Green (above)and Sacramento(below) Rivers. Contoursof intervalsof
0.5; regionsexceeding95 percentsignificancelevel are shaded.
•0% __
Composite
WinterSeaLevelPressure
Anomalies
(mb)
December-August
streamflow
%0J
Green
River,
Utah
I
";'i!!¾
¾":;
:11:':!?"::;i?.:
:"':
::':
:":
;¾0
'¾":;'!'!::•.
• ..:.¾'::
!?'
W /
J
\
09q,
Low
quartile••'-•
, •
Low
quartile
)
High
quartile
' / •'u
/
/
High
quartile
_
o•
• Negative
I
Fig. 13. Composite
winterSLP anomaly(mb)for high(left) andlow (right)quartileof December-August
casesat Greenand
SacramentoRivers; contoursat 1 mb intervals;regionswith absolutevaluesgreaterthan 1 mb are shaded.
Geophysical Monograph Series
Aspects of Climate Variability in the Pacific and the Western Americas
CAYAN
Effectsof large scaleatmospheric
circulationanomalieson streamflow
are importantenoughto be clearlyseendespitecomplications
that may be
introduced
by physicalcharacteristics
of individualwatersheds.
Topographic
controls on the circulation can complicatethe pattern of streamflow
variability, however, as evidencedby the remarkablelack of correlation
betweeninteriorandcoastalAlaskanstreams.Correlationpatternsof streamflow with the SLP field were clearestin winter when the atmospheric
circulationis most variable and tendsto exhibit largestscaleanomalies;
significantconnections
to the SLP field were foundat all of the basins.In
contrastto the contemporaneous
relationslinking precipitationto the
atmospheric
circulation,thereis a noticeablelag wherebyspring-summer
streamflowis relatedto winter SLP in severalof the higherelevationor
northernbasins.This likely resultsfrom meltingof the snowpackin these
watersheds.
BecausewinterSLP-streamflowcorrelationpatternsresemble
thoserelatingthemeancirculationto precipitation[e.g., Klein andBloom,
1987], we interpretthe connectionbetweenwinter circulationand streamflow to reflecttheprecipitationsuppliedto thewatershed
via theanomalous
stormactivity.There are hintsthatthis simplerelationlinkingcirculation
andprecipitation
to streamflowbreaksdownin spring,though,whensnowmelt is affectedby temperature.
For mostbasinsin the West, SLP grid pointshavingstrongestcorrelationswith streamflow were locatedimmediatelyalongthe west coastor
to west over the ocean,confirmingthe influenceof atmosphericactivity
overtheNorthPacific.The patternof winteratmospheric
circulationrelated
to streamflowfluctuations
fell intotwo ratherdistinctpatterns:onefor basins
near-the-coast
and the otherfor basinsin the interior. Both high and low
elevation basins near the Pacific coast exhibit connections that tend to be
regionalin scale,with high streamflowrelatedto negativeSLP anomalies
to the northwestor the west bringingan active stormtrack, anomalous
southerlyto westerlyflow, and an amplesupplyof moistair. In contrast
correlations of stream flow basins in the interior west with North Pacific
SLP producedteleconnections;
e.g., the largestcentersare remoterather
thanlocal,wherebywatersheds
frominteriorAlaskato Utahhaveincreased
streamflowin association
with anomalously
highSLP overthecentralNorth
Pacific.Because
thisteleconnection
reachessouthintothe subtropical
high,
Hawaiianstreamsare alsopositivelycorrelatedwithpositiveSLP anomalies
in thecentralNorthPacificdueto enhanced
wintertradewindprecipitation.
Two largescaleindices,thewell-knownPNA indexandthe CNP, a longer
historicalsurrogatefor PNA madefroma simpleareaaverageof the SLP
anomaliessouthof the AleutianIslands,representa major ingredientof
theseteleconnections.
Sinceatmospheric
activityover the centralNorth
Pacifictendsto be excitedduringENSO events,it is not surprisingthat
streamflow is alsocorrelatedwith the SOI. The spatialpatternsof streamflow anomaliesassociated
with SOI resemblethosefoundfor precipitation
by otherinvestigators
(e.g., DouglasandEnglehart[ 1981],YarnalandDiaz
[1986], Ropelewskiand Halpert [1986], and K. Redmondand R. Koch,
personalcommunication).There is someencouragingevidencethat these
relations can be used to forecast streamflow anomalies one to two seasons
in advance,probablybecause
theassociated
atmospheric
patternsmayhave
largeamplitudes
andevolveslowly.TheSOI seems
to havepredictivevalue
at leasttwo seasons
in advanceof theDecember-Auguststreamflowperiod (from the previoussummer)andthe PNA exhibitsapparentskill from
the precedingfall.
It is importantto notethatall strongcentralNorth Pacificlow casesdo
not havethe sameprecipitation/streamflow
patternsover westernNorth
America.Similarly,all ENSOcasesdonotproduce
thesameeffects[Namias
andCayan, 1984]. There were markeddifferencesbetween1976-1977 and
1982-1983,for example,eventhoughbothof thesewinterswereE1Niffo's
andhadunusuallystrongNorth Pacificlows. This is an illustrationof how
a singleareaaverageof the meancirculationis too simplisticto capture
variationsfromonecaseto thenextor betweenregions.A betterspecification of precipitationand streamflowcouldbe madeby accountingfor the
longitudinal
positionof thelow, whichisrelatedto themeridional
component
of the geostrophicflow anomaly. Some regionsin the West are not
Vol. 55
AND PETERSON
395
statisticallywell related to the central North Pacific circulationmode.
Regionally,we found no significantcorrelationswith either PNA or SOI
for streamflowin interiorAlaska, BritishColumbia,California, and Nevada.
Theseregionslie betweenthe strongcentersof activityof thestrongcentral
North Pacific teleconnectionpattern, and streamflowanomaliesin these
regionsare stronglycorrelatedwith otherregionalatmospheric
patterns.
The large scaleof atmospheric
circulationanomaliesis reflectedin the
spatialpatternof stream
flow anomalies.While stream
flow is high/lowin
southernAlaskaandnorthernBritishColumbia,it tendsto be low/highin
streamsin CaliforniaeastwardthroughColorado.Similarly, whenstreams
have high/low flow in the Pacific Northwest,thosein Arizona are often
low/high. Thesepatternsseemconsistent
with resultsof previousstudies
[e.g., Langbeinand Slack, 1982' Meko and Stockton,1984; Lins, 1985]
of streamflowfrom differentstationarraysthatcoveredpartof thedomain
considered
here.Thereappearsto be an asymmetryin the east-westextent
of dry, as opposedto wet, streamflowregimesin the California-throughColoradoregion.Comparison
of low andhighquartilesof streamflowcases
from the available record shows that while low stream flow in the South-
westoftenincludesa broadregionfrom Californiaeastwardto Colorado,
thehighflow casesfor Californiaweremoreconfinedto the far west.High
flows in Coloradoand Utah are usuallyclusteredin the southernRockies
andnot as stronglyto the west.Therewasa hint thatthe spatialscaleof
low stream
flow in the interiornorthwestalsotendsto be largerthanthat
for highflow. Theseanomalypatterndifferencesappearto resultfrom an
asymetryin thecirculations
causinglow versushighprecipitation.
Thisnonlinearbehavior,if real, is noteworthy
because
of thedifferentscalesimplied
for droughtversuswet conditionsin the Southwest.
Perhapsthe greatestsocialand economicimpactsof thesefluctuations
resultfromperiodsof prolongedhighor low streamflow,andit is important
to view modernday variationsin the contextof the fluctuationsover the
entire availablerecord. During the earlier part of the recordcan be seen
a persistent
episodeof theunusuallyhighstreamflowthroughout
the Southwest during the early 1900's (e.g., see Weber River anomalyplot in
Figure 3), while prolongedlow flows showup in all streamsin the northwesternUnitedStatesduringthe mid-1920'sthroughthe early 1940's. The
CNP indexprovidesa simplemeansof analysisandinterpretation
of regional
climatevariabilityinfluenced
b•ytheNorthPacificatmospheric
teleconnection and is well relatedto low and high streamflowepisodesat sensitive
regionsin thisdomain.Largespatialvariationscanalsobe interpretedfrom
thepointof view of the North Pacificcirculation.Interestingly,the out-ofphasetendencybetweencoastalAlaskanstreamsand thosein the Pacific
Northwestare similarto behaviorof the massbalancesof glaciersin these
sameregions[Waltersand Meier, this volume]. Other physicaland biologicalsystemsin the regionshowthe influenceof thesedecadalclimatic
variations[e.g., Ebbessmeyeret. al., this volume;Venrick et. al., 1987].
Also, althoughthisstudyhasfocusedon westemNorthAmericaandHawaii,
previouswork [e.g., RopelewskiandHalpert, 1986]hasdemonstrated
that
large scaleatmosphericmodessuchas ENSO are connectedto precipitation fartherdownstreameastof the Rockies,and it seemslikely that the
remainderof theNorthAmericastreamflowlink to NorthPacificatmospheric forcingwill emergein a similaranalysisover a larger streamnetwork.
Acknowledgments.
Discussions
with Nick Graham,Art Douglas,Roy
Walters,TonyBarnston,SueAnn Bowling,andChetRopelewskiwere very
useful,aswerecomments
by Bill Klein, Henry Diaz, JeromeNamias,Dave
Meko, andtwo anonymous
reviewers.Carefuleditingandsuggestions
by
JurateLandwehrimprovedthemanuscript
significantly.
We alsothankTom
Rossof the U.S. GeologicalSurveyfor streamflowanomalymaps.Much
credit goesto Ray Herndonfor dataprocessing,Emelia Baintoand Tam
Vu for calculations,computerplottingand editing, and to Marguerette
Schultzand JeanneDiLeo-Stevensfor excellentartwork. Mary Ray and
MarthaNicholspreparedillustrations
andtables,providedexperteditorial
help, and skillfully processedseveralversionsof the manuscript.
Geophysical Monograph Series
396
ATMOSPHERIC
Aspects of Climate Variability in the Pacific and the Western Americas
CIRCULATION
AND
STREAMFLOW
IN THE WEST
DRC greatlyappreciates
fundingforthisworkfromtheNational
Climate
ProgramOffice throughthe ExperimentalClimate ForecastCenter
program,Grant NA86-AA-D-CP104; NationalScienceFoundation,
Grant ATM86-12853; the United States Geological Survey, Grant
14-08-0001-G1483;theUniversityof CaliforniaWaterResources
Center,
ProjectNumberW-720; andthroughtheUniversityof CaliforniaINCOR
Program
between
Scripps
Institution
of Oceanography,
Lawrence
Livermore
NationalLaboratory,andLos AlamosNationalLaboratory.The authors
alsothanktheU.S. GeologicalSurvey,NationalScienceFoundation,
the
National
ClimateProgram
Office,andtheMonterey
BayAquarium
forfundingthePACLIM Workshop
Series,whereideasfor thisworkoriginated.
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