Geoscience Reference
In-Depth Information
al..2000)..Indeed,.paleoclimatic.evidence.suggests.that.the.low-frequency.behavior.
of. the. PDO. during. the. twentieth. century. is. not. a. fundamental. property. of. PDO:.
during. most. of. the. nineteenth. century,. variations. had. a. much. shorter. time. scale.
(Gedalof.et.al..2002).
Climate. models. have. performed. poorly. in. simulating. Paciic. interdecadal. vari-
ability,.particularly.transitions.from.one.phase.to.the.other,.and.in.simulating.the.pos-
sible.effects.of.Paciic.variability.on.temperature.or.precipitation.in.western.North.
America,. despite. tantalizing. evidence. that. there. are. such. effects.. For. now,. ENSO.
has.predictive.value,.but.the.PDO.remains.primarily.diagnostic..Brown.and.Comrie.
(2004).saw.the.same.western.“dipole”.at.longer.time.scales.in.the.PDO.signal.as.seen.
on.the.shorter.time.scales.of.ENSO.(e.g.,.Redmond.and.Koch.1991);.McCabe.and.
Dettinger.(2002).see.this.dipole.on.both.time.scales.in.the.historical.western.United.
States.snow.course.data.
Another. kind. of. teleconnection. operates. in. the. Paciic. at. intermediate. time.
scales.of.approximately.40-70.days..Pairs.of.mostly.cloudy.and.mostly.clear.regions.
slowly.drift.eastward.from.the.Indian.Ocean.toward.the.western.Paciic,.a.phenom-
enon.known.as.the.Madden-Julian.Oscillation.(MJO),.or.sometimes.Intra-Seasonal.
Oscillation. (ISO).. Interactions. with. the. eastward. lowing. jet. stream. coming. off.
Asia.can.lead.to.multiday.precipitation.episodes.on.the.West.Coast.5-10.days.later.
(Mitchell. and. Blier. 1997;. Mo. 1999;. Mo. and. Higgins. 1998).. These. are. important.
because. much. of. the. annual. precipitation. occurs. in. the. largest. 3-5. storms. of. the.
20-25.that.typically.strike.the.coast.of.California.in.winter,.and.indeed.there.is.a.
clear. connection. between. MJO. activity. and. looding. in. the. Northwest. (Bond. and.
Vecchi.2003).
These.phenomena.of.Paciic.climate.variability.exert.a.strong.inluence.on.west-
ern.climate.and.complicate.interpretation.of.long-term.changes..Attempts.to.extend.
these.records.rely.on.proxy.paleoclimatic.data,.such.as.tree.rings.(Biondi.et.al..2001;.
D'Arrigo. et. al.. 2001,. 2005;. Gedalof. and. Smith. 2001;. MacDonald. and. Case. 2005;.
Wiles. et. al.. 1998),. corals. (Gedalof. et. al.. 2002),. and. nitrogen. isotopes. in. salmon.
(Finney.et.al..2002)..The.relevance.of.variations.on.these.scales.is.that.slow.natural.
changes.appearing.in.the.short.sample.afforded.by.historical.records.(50-150.years).
can.masquerade.(perhaps).as.“climate.change”.if.our.temporal.perspective.for.inter-
pretation.is.too.short..The.PDO.shifted.abruptly.in.1976,.at.about.the.time.when.the.
temperatures.shown.in.Figure 1.2.began.to.rise..Our.best.global.climate.observations.
are. unfortunately. from. this. very. same. post-World. War. II. period. and. thus. nearly.
coincide.with.this.approximately.50-year-long.period.of.the.PDO.(1947.to.the.lat-
ter. 1990s).. One. might. interpret. this. in. two. ways:. (1). some. of. the. recent. warming.
in.western.North.America.is.a.natural.effect.due.to.the.mid-1970s.phase.change.of.
ENSO.or.PDO,.or.(2).because.the.atmosphere.expresses.the.effects.of.climate.warm-
ing.through.rearrangements.in.circulation.patterns,.this.pattern.may.in.part.represent.
such.a.change..However,.western.warming.continued.into.2007,.despite.the.possible.
change.in.phase.of.PDO.
Another.pattern.of.global.climate.variability.that.inluences.the.western.United.
States.is.the.Arctic.Oscillation.(AO;.Thompson.and.Wallace.1998,.2000,.2001;.Wu.
and.Straus.2004)..It.is.the.leading.pattern.of.variability.of.sea.level.pressure.over.the.
Northern.Hemisphere,.and. varies.on.time. scales.of.days.to.years.. Although.it.has.
