Geoscience Reference
In-Depth Information
south. and. the. central\northern. areas. tend. to. be. dry.. The. opposite. is. true.
during. the. negative. or. warm. phase.. PNA. values. were. obtained. from. the.
Joint.Institute.for.the.study.of.the.Atmosphere.and.Ocean.(JISAO).of.the.
University.of.Washington.(http://jisao.washington.edu/data/pna/).
.
4..
Paciic Decadal Oscillation (PDO).
The. Paciic. Decadal. Oscillation. is. a.
teleconnection.pattern.that.resembles.El.Nĩo.in.its.effect,.but.operates.on.
a.much.longer.time.scale.(15-25.years)..It.is.indexed.by.the.irst.principal.
component. of. North. Paciic. sea. surface. temperature. variability. (Mantua.
et.al..1997)..Of.the.circulation.modes.used.here,.its.causes.and.effects.are.
probably.the.least.well.understood.(Latif.and.Barnett.1994)..Warm-phase.
PDO. is. associated. with. above-average. precipitation. in. the. southwestern.
United. States. and. northern. Mexico,. and. below. average. precipitation. in.
northwestern. North. America.. Cool-phase. PDO. effects. are. essentially. the.
opposite..PDO.index.values.were.obtained.from.the.JISAO.of.the.University.
of.Washington.(http://jisao.washington.edu/pdo/).
METHODS
Methods.used.for.analyzing.the.gridded.and.LTER.site.data.were.similar..For.the.
gridded.precipitation.data,.a.spatial.subset.of.the.global.data.large.enough.to.encom-
pass.all.of.the.16.LTER.sites.used.in.this.analysis.was.extracted.from.the.complete.
CRU05. data. set.. These. data. were. further. subset. temporally. to. extract. only. those.
monthly.values.between.1961.and.1990..These.values.were.converted.to.anomalies.
using.the.mean.and.standard.deviation.values.for.each.grid.cell..The.gridded.values.
were.then.correlated.to.each.of.the.four.circulation.indices.using.Spearman's.Rank.
Order.Correlation.(ρ)..Rank.order.correlation.was.used.due.to.concerns.about.the.lin-
earity.of.the.relationship.between.precipitation.and.correlation.indices..Signiicance.
of.the.ρ-values.was.determined.using.a.permutation.approach..Note.that.the.purpose.
of. this. analysis. was. not. to. predict. precipitation. using. the. circulation. indices,. nor.
was.it.to.ind.the.optimal.timing.of.the.correlation.between.these.two.time.series..
Instead,.the.goal.was.to.understand.the.geographical.pattern.of.the.correlations.and.
thus. the. pattern. of. inluence. by. each. circulation. mode.. Therefore,. no. attempt. was.
made.to.lag.the.cross.correlation.of.the.time.series.in.order.to.improve.the.overall.
it.. All. correlations. were. between. values. of. corresponding. months.. The. results. of.
these.correlations.were.a.series.of.gridded.maps.with.each.grid.cell.containing.an.
ρ-value.(Figure 9.2)..Although.the.four.circulation.indices.operate.on.different.time.
scales,. it. cannot. be. assumed. that. they. are. independent. of. each. other.. Evaluating.
their. spatial. effects. therefore. requires. a. multivariate. approach. capable. of. evaluat-
ing.the.most.signiicantly.correlated.circulation.modes.for.each.grid.cell,.assuming.
that.more.than.one.mode.might.be.signiicant..Maximum.basic.probability.assign-
ment.(MBPA).was.used.for.this.classiication.(Denœux.2008)..MPBA.is.a.form.of.
Bayesian.probability.classiication.that.uses.the.Dempster-Shafer.weight-of-evidence.
approach. to. assign. to. the. most. important. classiication. variables. to. each. cell.. The.
resulting. map. shows. geographical. patterns. of. association. between. grid. cells. and.
circulation.modes.(Figure 9.3)..MPBA.not.only.classiies.data.according.to.a.set.of.
input.variables,.it.also.discriminates.between.signiicant.and.insigniicant.variables.
