Geoscience Reference
In-Depth Information
There. is. much. popular. confusion. between. the. El. Nĩo. phenomenon. and. its.
effects..El.Nĩo.refers.to.ocean.warming.in.the.top.100-200.meters.in.a.narrow.band.
between.South.America.and.the.international.date.line,.typically.within.5°.latitude.
of.the.equator..The.effects.of.El.Nĩo,.by.contrast,.are.global.in.reach..At.time.scales.
between.the.regular.seasonal.cycle.and.several.years,.El.Nĩo.is.the.single.largest.
contributor.to.climate.variability.on.earth..The.warm.area.may.look.small.on.a.map.
of.the.Paciic.Ocean,.but.can.easily.be.larger.than.the.United.States..The.shape,.mag-
nitude,.extent,.duration,.and.longitudinal.position.of.the.warm-water.patch.can.vary.
from.one.episode.to.the.next—factors.that.can.signiicantly.inluence.the.impacts.of.
the.phenomenon.on.the.West.(Hoerling.and.Kumar.2002)..Typical.events.last.6-18.
months.and.recur.irregularly.at.2-.to.7-year.intervals..La.Nĩa.refers.to.unusually.
cool.temperatures.in.this.same.area.
El. Nĩo. exhibits. characteristics. of. an. oscillation. in. the. sense. that. during. one.
phase.of.the.cycle,.forces.are.at.work.that.lead.to.the.demise.of.that.phase.and.often.
even.the.eventual.growth.of.the.opposite.phase,.like.a.very.complicated.pendulum,.
albeit.one.subject.to.irregular.forcing.by.short-term.weather.events..The.atmospheric.
pressure.difference.between.stations.at.Tahiti.and.Darwin.(Australia).is.negatively.
correlated. with. ocean. temperatures. in. the. El. Nĩo/La. Nĩa. area—a. phenomenon.
known.as.the.Southern.Oscillation:.it.is.the.atmospheric.counterpart.of.the.oceanic.
El. Nĩo.. The. magnitude. of. this. correlation,. usually. strong,. has. varied. somewhat.
through.time.(McCabe.and.Dettinger.1999),.so.the.atmosphere.and.the.ocean.each.
carry. somewhat. different. information.. In. recognition. of. the. coupled. oceanic. and.
atmospheric.nature.of.this.vacillation,.this.phenomenon.is.called.ENSO.(“El.Nĩo/
Southern.Oscillation”).
In.the.Western.United.States,.the.effects.of.El.Nĩo.and.La.Nĩa.are.experienced.
during. the. cold. half. of. the. year,. from. approximately. October. through. March;. sum-
mer.signals.are.very.weak..The.climatic.effects.of.ENSO.are.also.found.in.stream-
low. (Andrews. et. al.. 2004;. Barnett. et. al.. 2004). where. they. are. greatly. accentuated.
with.respect.to.precipitation.(Cayan.et.al..1999).in.the.western.states..Because.annual.
tree.growth.in.the.Southwest.is.strongly.dependent.on.prior.winter.precipitation,.these.
ENSO.effects.are.clearly.seen.in.tree.ring.widths.(Swetnam.and.Betancourt.1990).
The.frequency.of.El.Nĩo.has.varied.through.time..During.the.period.1947-1976,.
El.Nĩo.occurred.relatively.infrequently.and.La.Nĩa.was.common..A.sudden.and.
still. unexplained. change. (the. “1976-1977. shift”). in. the. Paciic. ushered. in. an. era.
of.much.more.common.El.Nĩo.and.a.virtual.dearth.of.La.Nĩa..This.appeared.to.
many.observers.to.have.switched.again.in.the.late.1990s,.although.present.evidence.
remains.somewhat.ambiguous.
In.higher.latitudes,.this.slow.variation.of.about.50.years'.duration.is.expressed.
in. a. pattern. of. ocean. temperatures,. atmospheric. pressures,. jet. stream. positions,.
and.ocean.currents.seen.from.the.tropics.to.the.high.latitudes.in.the.Paciic,.irst.
described. by. Mantua. et. al.. (1997). and. Mantua. and. Hare. (2002). as. the. Paciic.
Decadal. Oscillation. (PDO). and. elaborated. by. others.. They. related. the. PDO. to.
strong.differences.in.salmon.abundance.between.Alaska.and.the.Paciic.Northwest..
There.is.much.debate.about.the.origin.of.the.PDO,.whether.it.truly.is.an.oscilla-
tion,.and.even.whether.it.really.exists.except.as.a.response.to.ENSO.(Zhang.et.al..
1996;.Newman.et.al..2003).with.strong.elements.of.chaotic.behavior.(Overland.et.
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