Geoscience Reference
In-Depth Information
little.inluence.on.seasonal.means.as.far.south.as.western.North.America,.extremes.
of. the. AO. index. are. associated. with. outbreaks. of. Arctic. air. that. greatly. enhance.
the. likelihood. of. snow. in. lowland. areas. of. Washington,. Oregon,. and. California.
(Thompson.and.Wallace.2001).
Researchers. have. identiied. a. variety. of. other. proposed. oscillations,. including.
the. Atlantic. Multidecadal. Oscillation. (AMO),. which. appears. to. inluence. rainfall.
patterns.around.the.Atlantic.basin,.Atlantic.hurricane.activity,.and.summer.climate.
in.Europe.and.much.of.North.America.(e.g.,.Enield.et.al..2001;.Knight.et.al..2006)..
There. are. some. indications. that. the. AMO. may. be. related. to. climatic. and. hydro-
logic. variability. in. the. Upper. Colorado. River. Basin. (McCabe. et. al.. 2004;. USGS.
2004;.Gray.et.al..2003,.2004a),.even.though.the.mechanism.is.unclear..Gray.et.al..
(2004a).reconstructed.the.AMO.back.to.1567.using.tree-ring.data.including.those.
from.North.America..However,.as.with.PDO,.there.are.questions.about.the.validity.
and.independence.of.the.AMO,.the.true.time.scale.(deriving.a.60-.to.80-year.period.
from. 120. years. of. data),. and. the. relationship. between. the. AMO. and. global. mean.
temperatures.(e.g.,.Mann.and.Emanuel.2006).
The.climate.research.community.has.recently.begun.to.make.a.concerted.effort.to.
understand.the.cause.of.decadal-to-centennial.scale.climate.variability..In.the.West,.
modern.(e.g.,.Cayan.et.al..1998).and.paleoclimate.(e.g.,.Cook.et.al..2004;.Fye.et.al..
2003;.Gray.et.al..2004a,.2004b;.Hughes.and.Brown.1992;.Stahle.et.al..2000,.2001;.
Woodhouse.2003;.Woodhouse.and.Overpeck.1998;.Woodhouse.et.al..2005).records.
have.irmly.established.that.long-term.drought.(5-20.years.and.more).is.an.inherent.
part.of.climate.variability.in.this.dry.region..Recent.model.studies.to.understand.the.
source.of.the.1930s.Dust.Bowl.(Schubert.et.al..2004a,.2004b;.Seager.et.al..2005),.
the. 1950s. Southwest. drought,. and. the. intense. 2000-2004. drought. (Hoerling. and.
Kumar.2003).have.shown.that.a.major.part.of.the.answer.lies.in.the.world's.oceans,.
in.particular.the.Western.Paciic.and.the.Indian.Oceans.
d
eTeCTion
and
a
TTribuTion
A.critical.question.in.examining.and.understanding.local,.regional,.or.global.vari-
ability. is,. what. caused. it?. Was. the. source. ENSO,. AO,. AMO,. greenhouse. gases,.
aerosols,.or.something.else?.Climate.scientists.use.a.theoretical.framework.(Hegerl.
et. al.. 2007). to. perform. detection. (statistically. identifying. a. trend). and. attribution.
(identifying. causes).. A. typical. detection. and. attribution. exercise. its. an. observed.
spatiotemporal. pattern. Y. to. an. expected. spatiotemporal. pattern. X:. Y. =. αX. +. ε,.
where.α.is.the.scaling.factor.and.ε.is.the.residual..The.expected.pattern,.as.well.as.
estimates.of.unforced.internal.variability,.comes.from.a.model.simulation.with.the.
posited. forcing.. For. example,. typical. detection. and. attribution. exercises. examine.
greenhouse.gases,.solar,.and.volcanic.forcings..If.there.is.no.statistical.relationship.
between.X.and.Y,.the.scaling.factor.is.indistinguishable.from.zero,.but.if.the.statisti-
cal.conidence.range.includes.1.and.excludes.zero,.the.pattern.of.change.Y.is.said.to.
be.attributed.to.the.forcing.in.question..Usually.a.few.different.expected.patterns,.
generated.by.different.climate.models,.are.used.to.provide.an.additional.indication.
of.the.conidence.of.the.results..(For.a.more.thorough.derivation,.see.Hegerl.et.al..
2007,.Appendix.9A).
