SST Forced Atmospheric Variability in an SST Forced Atmospheric Variability in an AGCMAGCM

Arun KumarArun Kumar

Qin ZhangQin Zhang

Peitao PengPeitao Peng

Bhaskar JhaBhaskar Jha

Climate Prediction CenterClimate Prediction Center

NCEPNCEP

OutlineOutline

• MotivationMotivation

• Data and MethodologyData and Methodology

• ResultsResults

• Summary and ConclusionsSummary and Conclusions

MotivationMotivation

Horel and Wallace, 1981: Horel and Wallace, 1981: Planetary Scale Atmospheric Phenomenon Associated with the SOPlanetary Scale Atmospheric Phenomenon Associated with the SO

Correlation between DJF 700mb Z and SST index

MotivationMotivation

• “…“…What, then, are the prospects of utilizing information What, then, are the prospects of utilizing information on equatorial SST anomalies …to improve the quality of on equatorial SST anomalies …to improve the quality of long-range forecasts for middle latitudes?…” long-range forecasts for middle latitudes?…”

-- If the strength of correlations … is limited by the high noise -- If the strength of correlations … is limited by the high noise level inherent in seasonal averages… then the prospects of level inherent in seasonal averages… then the prospects of [seasonal predictions] are not encouraging[seasonal predictions] are not encouraging

-- On the other hand, if these patterns constitute blurred -- On the other hand, if these patterns constitute blurred images resulting from our inadvertent superposition of an images resulting from our inadvertent superposition of an ensemble of shaper patterns, … , then there is hope that ensemble of shaper patterns, … , then there is hope that (seasonal prediction of)… midlatitude climate anomalies with (seasonal prediction of)… midlatitude climate anomalies with higher degree of detail and accuracy than is now (will be) higher degree of detail and accuracy than is now (will be) possible.possible.

MotivationMotivation

MotivationMotivation

• Question:Question: How much does the atmospheric response in the How much does the atmospheric response in the extratropical latitudes depend on details of the ENSO SST extratropical latitudes depend on details of the ENSO SST anomalies, or to SST anomalies in different ocean basins?anomalies, or to SST anomalies in different ocean basins?

Data and MethodologyData and Methodology

• For each DJF seasonal mean from 1980-2000, we have For each DJF seasonal mean from 1980-2000, we have access to an 80-member ensemble of AGCM simulations access to an 80-member ensemble of AGCM simulations forced with the observed SSTsforced with the observed SSTs

• Ensemble mean for each DJF provides a good estimate of Ensemble mean for each DJF provides a good estimate of atmospheric response to that year’s SST forcingatmospheric response to that year’s SST forcing

• For this data set, we analyze how the ensemble mean For this data set, we analyze how the ensemble mean 200-200-mbmb height response varies with SSTs height response varies with SSTs

Data and MethodologyData and Methodology

ICs Target

Aug Sep Oct Nov DJF

Sep Oct Nov DJF

Oct Nov DJF

Nov DJF

ICs Target

Aug Sep Oct Nov DJF

Sep Oct Nov DJF

Oct Nov DJF

Nov DJF

80-member 80-member EnsembleEnsemble

From 2002 From 2002 and 2003 ICsand 2003 ICs

80-member 80-member EnsembleEnsemble

From 2002 From 2002 and 2003 ICsand 2003 ICs

• Data is from “Seasonal Forecast Model” archives from 2002-Data is from “Seasonal Forecast Model” archives from 2002-20032003

– 10-member ensemble from different atmospheric initial 10-member ensemble from different atmospheric initial conditions each monthconditions each month

– Lagged ensemble from different ICsLagged ensemble from different ICs

Data and MethodologyData and Methodology

Difference in 200-mb Difference in 200-mb eddy height eddy height climatology from climatology from December and December and September ICsSeptember ICs

200-mb eddy height 200-mb eddy height climatology for climatology for December ICsDecember ICs

Data and MethodologyData and Methodology

Difference in 200-mb Difference in 200-mb height variance from height variance from December and December and September ICsSeptember ICs

200-mb height 200-mb height variance for variance for December ICsDecember ICs

ResultsResults

Variance of Variance of Ensemble MeansEnsemble Means

External to Internal External to Internal Variance RatioVariance Ratio

ResultsResults

EOF1 53%EOF1 53%

ResultsResults

Remaining External Remaining External VarianceVariance

Fractional External Fractional External Variance Related to Variance Related to

Mode1Mode1

ResultsResults

EOF2 19%EOF2 19%

ResultsResults

EOF3 12%EOF3 12%

ResultsResults

Fraction of Variance Explained by Fraction of Variance Explained by Modes 1-3Modes 1-3

ResultsResults

Z = a*ΔSST + b*ΔSST2

if

ΔSST+ Z+ & ΔSST- Z-

then

a= (Z+ - Z-) / (2* ΔSSTavg)

and

b= (Z+ + Z-) / (2* ΔSSTavg)

(Monahan & Dai 2004)

(83+98) – (89+99)(83+98) – (89+99)

ResultsResults

Ensemble mean Ensemble mean (shaded); EOF1 (shaded); EOF1

(contour)(contour)

Ensemble mean – Ensemble mean – EOF1EOF1

DJF 1998DJF 1998

ResultsResults

Ensemble mean Ensemble mean (shaded); EOF1 (shaded); EOF1

(contour)(contour)

Ensemble mean – Ensemble mean – EOF1EOF1

DJF 1999DJF 1999

ResultsResults

-Strong Warm-Strong Warm

ColdCold

EOF1EOF1

- Warm- Warm

Strong ColdStrong Cold

ResultsResults

Composite Composite based on based on

1980, 81, 82 1980, 81, 82 & 86& 86

ResultsResults

Anomaly CorrelationAnomaly Correlation

Ensemble MeanEnsemble Mean EOF1EOF1

EOF1 + EOF2EOF1 + EOF2 EOF1:EOF3EOF1:EOF3

ResultsResults

AC(EOF1+EOF2) – AC(EOF1+EOF2) – AC(EOF1)AC(EOF1)

AC(EOF1:EOF2) – AC(EOF1:EOF2) – AC(EOF1+EOF2)AC(EOF1+EOF2)

ResultsResults

AC (EOF1)AC (EOF1)

AC

(E

OF

1:E

OF

3)

AC

(E

OF

1:E

OF

3)

Summary and ConclusionsSummary and Conclusions

• A large fraction of extratropical variability is indeed related A large fraction of extratropical variability is indeed related to “…high noise level inherent in seasonal averages… and to “…high noise level inherent in seasonal averages… and the prospects of [seasonal predictions] are limited.”the prospects of [seasonal predictions] are limited.”

• There are other modes of atmospheric response that are There are other modes of atmospheric response that are related to non-ENSO SSTs (e.g., EOF2), but this could be related to non-ENSO SSTs (e.g., EOF2), but this could be specific to the analysis period.specific to the analysis period.

• This (and previous) analysis has shown higher order This (and previous) analysis has shown higher order response to ENSO extremes, but it is hard to show any response to ENSO extremes, but it is hard to show any definite influence averaged over all SST years. This is either definite influence averaged over all SST years. This is either because of the rarity of such events, or because of incorrect because of the rarity of such events, or because of incorrect simulation by the AGCMsimulation by the AGCM

• Should be repeated with other AGCMsShould be repeated with other AGCMs