Amul&'proxymodel'testedNAO reconstruc&onforthelastmillennium · The"reconstruc&on"technique" It is based on a Principal Component Regression (PCR), that in essence represents a multivariate

A"mul&'proxy"model'tested"NAO"reconstruc&on"for"the"last"millennium""

Pablo&Ortega&(1,2),&Flavio&Lehner&(3,4),&Didier&Swingedouw&(5),&Valerie&MassonBDelmoDe&(1),&Christoph&Raible&(3,4),&Mathieu&Casado&(1)&and&Pascal&Yiou&(1)&&

& & & &(1)&LSCE/IPSL,&GifBsurBYveDe&(France)&

& & & &(2)&LOCEAN/IPSL,&Paris&(France)&

& & & &(3)&KUP,&University&of&Bern&(Switzerland)&

& & & &(4)&OCCR,&University&of&Bern&(Switzerland)&&

& & & &(5)&EPOC,&Universite&Bordeaux&I&(France)&

& &&

& & & &&

Green Greenland"

""

AMA2015 / Workshop on storm-tracks, 21-23/01/2015

The NAO is the dominant mode of atmospheric variability in the North Atlantic

Spatial'pattern'in'SLP'

Defini&on"of"the"NAO"

NAO'variability'since'1823'(Vinther'et'al'2003)'

-2

0

2

1840 1860 1880 1900 1920 1940 1960 1980 2000Time (years)

NA

O+

NA

O -

1&





-2

0

2

1840 1860 1880 1900 1920 1940 1960 1980 2000Time (years)

NA

O+

NA

O -

2&





-2

0

2

1840 1860 1880 1900 1920 1940 1960 1980 2000Time (years)

NA

O+

NA

O -

3&

Casado et al (2013)





-2

0

2

1840 1860 1880 1900 1920 1940 1960 1980 2000Time (years)

NA

O+

NA

O -

3&

NAO'impact'on'winter'precipitation'ERABInterim&(1990B2010)&

NAO'impact'on'winter'temperature'

Casado et al (2013)





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0

2

1840 1860 1880 1900 1920 1940 1960 1980 2000Time (years)

NA

O+

NA

O -

3&

NAO'impact'on'winter'precipitation'ERABInterim&(1990B2010)&

Role"of"the"NAO"in"the"last"millennium"

4&

NAO impact on winter precipitation

Precipitation sensitive proxy

Drought severity proxy

Casado et al (2013)


Trouet et al (2009)

Bi-proxy NAO reconstruction

4&

NAO impact on winter precipitation

Precipitation sensitive proxy

Drought severity proxy

Casado et al (2013)


Trouet et al (2009)

Northern Hemisphere temperature reconstructions for the last millennium

Frank et al (2010)

Medieval"Warm"Period"LiBle"Ice"Age"


5&


Trouet et al (2009)

Northern Hemisphere temperature reconstructions for the last millennium

Frank et al (2010)

Medieval"Warm"Period"LiBle"Ice"Age"


5&

NAO'impact'on'winter'temperature'

Casado et al (2013)


Trouet et al (2009)

Is it robust or does it arise from the limited selection of proxies?

¿? Bi-proxy NAO reconstruction

5&

NAO correlation with winter precipitation

Robustness"of"Trouet’s"NAO"reconstruc&on"

6&

Lehner et al (2012) Perfect"model"approach"



6&


l

l

Normalized PDF of 50-yr moving correlations in PMIP3 millennial runs



6&




Correlations for alternative pseudo-reconstructions


6&



Are there other proxy records available that could be used to produce a more robust NAO reconstruction?

Millennial"proxies"sensi&ve"to"the"NAO"

Trouet et al (2009)

1." Significant& (α<0.10)& correla[on&with& the&NAO& in& their&overlapping"period""!2."Documented"rela&onship"with&a&climate" variable" easily& extractable&from&models&&3."Annual"or"seasonal"resolu&on"

We identify 48 proxy records

34 from tree-rings 8 from ice cores 4 from lake sediments 2 from speleothems

7&

Millennial"proxies"sensi&ve"to"the"NAO"

Trouet et al (2009)

1." Significant& (α<0.10)& correla[on&with& the&NAO& in& their&overlapping"period""!2."Documented"rela&onship"with&a&climate" variable" easily& extractable&from&models&&3."Annual"or"seasonal"resolu&on"

We identify 48 proxy records

34 from tree-rings 8 from ice cores 4 from lake sediments 2 from speleothems

Thanks to their annual resolution we can produce a yearly resolved NAO reconstruction in which to test the impact of the solar and volcanic forcings

The"reconstruc&on"technique"

It is based on a Principal Component Regression (PCR), that in essence represents a multivariate regression using the Principal Components of the standardised proxies as predictors

8&


It is based on a Principal Component Regression (PCR), that in essence represents a multivariate regression using the Principal Components of the standardised proxies as predictors To identify which features are robust, an ensemble of 200 reconstructions is produced, by considering: "

1)  50 different subsets of years for calibration/validation Calibration: in a random draw of 117 years from the common period to all proxies and the NAO: 1823-1969

Validation: in the remaining 30 years

8&


It is based on a Principal Component Regression (PCR), that in essence represents a multivariate regression using the Principal Components of the standardised proxies as predictors To identify which features are robust, an ensemble of 200 reconstructions is produced, by considering: "

1)  50 different subsets of years for calibration/validation Calibration: in a random draw of 117 years from the common period to all proxies and the NAO: 1823-1969

Validation: in the remaining 30 years 2)  Four subsets of proxies depending on their type

All proxy types, ice cores excluded, tree-ring excluded, and tree rings plus ice cores

8&

Betaweight ensemble sum for each proxy

124 80-4-8-12-16

Calibration Constrained Reanalysis Constrained Model Constrained

Refining"the"proxy"selec&on"

Calibration Constrained

41 proxies

9&


124 80-4-8-12-16



124 80-4-8-12-16



9&


124 80-4-8-12-16

Calibration Constrained Reanalysis Constrained Model ConstrainedCalibration Constrained

Reanalysis Constrained

41 proxies 10 proxies


124 80-4-8-12-16



Calibration Constrained (NAOcc)

Reanalysis Constrained (NAOrc)

Model Constrained(NAOmc)

41 proxies 10 proxies 7 proxies

9&

Cor

rela

tion

Coe

ffici

ents

0

0.2

0.4

0.6

0.8

●*

**

●●

NAOccNAOrcNAOmc

median

quartile 3

quartile 1

max

min

Ensemble

Ensemble Mean

First significant correlation

Refining"the"selec&on"of"ensemble"members"

Correlation with NAOVinther

1823-1969

10&

Refining"the"selec&on"of"ensemble"members"C

orre

latio

n C

oeffi

cien

ts

0

0.2

0.4

0.6

0.8

●*

**

●●

NAOccNAOrcNAOmc

median

quartile 3

quartile 1

max

min

Ensemble

Ensemble Mean

First significant correlation

Correlation with NAOVinther

1823-1969 Calibration Validation

We only keep the realizations whose correlation is in the upper quartile The calibration, reanalysis and model constrained NAO gives better validation scores

Refining"the"selec&on"of"ensemble"members"C

orre

latio

n C

oeffi

cien

ts

0

0.2

0.4

0.6

-0.2

Jones et al (1999)

[1781-1822]

Luterbacher et al (2002)

[1659-1822]

Kuettel et al (2010)

[1751-1822] Glueck & Stockton (2001)

[1429-1822]

Luterbacher et al (2001)

[1500-1822]

Cook et al (2002)

[1400-1822]

Trouet et al (2009)

[1049-1822]

●

●

●

●

●

●

●

●

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●

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Correlation with other NAO reconstructions

Jones et al (1999)

[1781-1822]

We only keep the realizations whose correlation is in the upper quartile The calibration, reanalysis and model constrained NAO gives better validation scores

Luterbacheret al (2002) [1659-1822]

Kuettel et al (2010)

[1751-1822] Glueck &

Stockton (2001) [1429-1822]

Luterbacher et al (2001) [1500-1822]

Cook et al (2001)

[1400-1822]

Trouet et al (2009)

[1049-1822]

The"final"ensemble"NAO"reconstruc&on"

1200 1400 1600 1800 1850 1900 1950

NA

O p

hase

-2

0

2

4

-4

Cor

rela

tion

ë��

0

��

Time (years)1200 1400 1600 1800 1850 1900 1950

NAOTrouet

NAOVinther

Calibration, reanalysis and model constrained NAO ensemble

The reconstruction does not support a persistent positive NAO in medieval times, although its suggest predominantly positive phases during the 13th century

Evalua&on"in"a"perfect"model"approach"

ë�� 7ZL\KVë5(6mc

7ZL\KVë5(6Lehner

ë�� 1

5VYTHSPaLK�7+-

5VYTHSPaLK�7+-

5VYTHSPaLK�7+-

)**ë*:4�ë�-.6(3:ëNS.0::ë,�ë907:3ë*4�(ë394096*ë,:4470ë,:4ë7

PMIP3 ENSEMBLE

ë�� 1*VYYLSH[PVU�JVLMMPJPLU[

�

7ZL\KVë5(6;YV\L[

Our mean ensemble reconstruction clearly outperforms the bi-proxy timeseries from Trouet et al (2009)

Impact"of"the"external"forcings"•  No clear response to solar irradiance is identified

14&

Impact"of"the"external"forcings"•  No clear response to solar irradiance is identified •  Positive NAO phases appear 2 years after major volcanic

eruptions

Lag -1 In phase Lag 1 Lag 2 Lag 3 Lag 4 Lag 5

Gao et al (2008) Sigl et al (2014)Crowley and Unterman (2013)

95%

99%

Common-0.5

0

0.5

1.0

-1.0

-2.0

-3.0

0

1.0

2.0

3.0

Com

posi

te N

AO

NA

O In

dex

NAO response to 11 volcanoes with large radiative forcing

Common volcanoes

Useful to better constrain the magnitude and timing of this response

Impact"of"the"external"forcings"•  No clear response to solar irradiance is identified •  Positive NAO phases appear 2 years after major volcanic

eruptions


Gao et al (2008) Sigl et al (2014)Crowley and Unterman (2013)

95%

99%

Common-0.5

0

0.5

1.0

-1.0

-2.0

-3.0

0

1.0

2.0

3.0

Com

posi

te N

AO

NA

O In

dex

NAO response to 11 volcanoes with large radiative forcing

Common volcanoes

Useful to better constrain the magnitude and timing of this response

Summary"and"conclusions"

•  We have produced a new multiproxy yearly-resolved NAO-reconstruction for the last millennium, compatible with models and previous NAO reconstructions. The model-constrained proxy selection systematically improves the agreement with the reconstructions.

•  Our reconstruction updates the previous results from Trouet et al (2009), and suggests that predominant, but not persistent, positive NAO conditions ocurred only during the second half of the Medieval Era.

•  Positive NAO phases occur systematically two years after the 11 largest volcanic eruptions, offering a potential for multi-annual prediction

Thanks for your attention!!!!

And"we"validate"it"with"a"perfect"model"approach"

Validation scores over the whole millennium remain significant and close to the calibration range

We"produce"our"final"reconstruc&on"

1200 1400 1600 1800 1850 1900 1950

NA

O p

hase

-2

0

2

4

-4

Cor

rela

tion

ë��

0

��

Time (years)1200 1400 1600 1800 1850 1900 1950

NAOTrouet

NAOVinther

Calibration, reanalysis and model constrained NAO ensemble

50 year moving correlations with NAOTrouet

The reconstruction does not support a persistent positive NAO in medieval times, although its suggest predominantly positive phases during the 13th century

Common Period [1823-2008]

NAO

Proxy

A proxy may correlate significantly in the common period…

Calibration Period[1823-1969]


NAO

Proxy


NAO

Proxy

A proxy may correlate significantly in the common period…

… but not in the Common Period

They&all&extend"con&nuously&from&the&11th&to&the&20th&century&&

They&have&a&minimum"resolu&on"of&10"years"during&the&observa[onal&period&&

They&correlate"significantly"(α<0.10)"with&the&NAO"in"their"overlapping"period""!

There&is&a&documented"rela&onship"between&each"proxy"and&a"climate"variable""poten[ally&sensi[ve&to&the&NAO&(temperature,&precipita[on,&drought,…)&&&Our&final&dataset&consist&of&59"different"proxies:&&

&34&from&treeBring&chronologies&&9&from&ice&cores&(accumula[on&and&δ18O)&&9&from&lake&sediments&&4&from&speleothems&&3&from&ocean&sediments&&&

&

Choice"of"proxies"for"the"new"reconstruc&on"


It! is!based!on!a!Principal!Component!Regression!(PCR),! the!same!methodology!used!in!the!reconstruc;on!of!Cook!et!al!(2002)!

In&essence&it&represents&a&mul&variate"regression"in&which:&&1)  To&reduce&overfidng,&a&principal"component"analysis"of&the&standardised&

proxies&is&first&performed&

2)  The&corresponding"PCs,&orthogonal&to&each&other&by&construc[on,&are&used&as&predictors&(provided!they!correlate!significantly!with!the!NAO)!


It! is!based!on!a!Principal!Component!Regression!(PCR),! the!same!methodology!used!in!the!reconstruc;on!of!Cook!et!al!(2002)!

In&essence&it&represents&a&mul&variate"regression"in&which:&&1)  To&reduce&overfidng,&a&principal"component"analysis"of&the&standardised&

proxies&is&first&performed&

2)  The&corresponding"PCs,&orthogonal&to&each&other&by&construc[on,&are&used&as&predictors&(provided!they!correlate!significantly!with!the!NAO)!

-3-2-1 0 1 2 3

NAO

Anom

aly

NAO indices from instrumental sources

1840 1860 1880 1920 1940 1900 1960 1980 2000Time (years)

Azores-ReykjavikGibraltar-Reykjavik

Lisbon-Reykjavik1st PC of SLP

Ref: 1900-2001

How"do"we"characterize"its"variability?"NAO$from$instrumental$sources$

1st leading PC of SLP in the North Atlantic Sector

-3-2-1 0 1 2 3

NAO

Anom

aly


1840 1860 1880 1920 1940 1900 1960 1980 2000Time (years)



Ref: 1900-2001

How"do"we"characterize"its"variability?"

Different alternative definitions have been proposed, all showing a great degree of coherence among them (correlations around 0.9)

Three stations with long in-situ SLP measurements are close to the Southern node (Azores, Lisbon and Gibraltar)

NAO$from$instrumental$sources$

-3-2-1 0 1 2 3

NAO

Anom

aly


1840 1860 1880 1920 1940 1900 1960 1980 2000Time (years)



Ref: 1900-2001

How"do"we"characterize"its"variability?"

Different alternative definitions have been proposed, all showing a great degree of coherence among them (correlations around 0.9)

Three stations with long in-situ SLP measurements are close to the Southern node (Azores, Lisbon and Gibraltar)

We use the longest NAO record (Vinther et al 2003), going back to 1823

NAO$from$instrumental$sources$

New"proxy"selec&on"In!each!of!the!64!individual!reconstruc;on!only!proxies!that!correlate!!with!the!NAO!now!also!within-the-calibra2on-period-are!used!


NAO

Proxy

New"proxy"selec&on"In!each!of!the!64!individual!reconstruc;on!only!proxies!that!correlate!!with!the!NAO!now!also!within-the-calibra2on-period-are!used!

Calibration Period[1823-1969]


NAO

Proxy

Ice coresLake SedimentsOcean SedimentsSpeleothemsTree rings

1

2

3

4

5

6

7

89

10

11

12

13

14

15

16

17

18

19

20

21

22

23

2425

2627

30

3134

3538

39

42

43

4647

48 49

44

45

5051

5253

54

55

56

57

5859

40

41

36

373233

2829

0

6

12

18

24

30

36

42

48

Num

ber of times proxies are used

Location of preselected proxiesConstrained by calibration

Constrained by models

Two"alterna&ve"final"proxy"selec&ons…"

What"do"we"know"from"climate"models?"

Winter'NAO'variability'in'the'ensemble'of'PMIP3'runs'

Given& that& no& coherent& features& emerge& within&simula[ons,& is& there& any&way& in&which& they& can&be&used&to&validate&proxy&reconstruc[ons?&

No&persistent&posi[ve&&NAO&is&reproduced&

Green&stars:&loca[on&of&the&gridBpoints& used& for& the& southern&node&of&sta[onBbased&indices&Yellow& stars:& the& same& for& the&northern&node&

Note:&not&all&models&reproduce&the&centers&of&high&and&low&pressure&of&the&NAO&at&the&same&place.&This&implies&that&the&sta[onBbased&indices&will&not&always&correspond&well&with&the&leading&mode&of&SLP.&

Leading&EOFs&of&the&winter&SLP&in&the&North&Atlan[c&sector&(corresponding&to&the&PCBbased&NAO&indices)&

BCCJCSM1! CCSM4! CSIROJMk3L! FGOALSJG1!

HadCM3!GISSJE2JR25!GISSJE2JR24!FGOALSJS2!

IPSLJCM5A! MIROCJESM! MPIJESM!

Four&main&centers&of&ac[on&appear:&(Northern&Eurasia,&North&Africa,&Greenland&and&Central&North&America)&


HadCM3!GISSJE2JR25!GISSJE2JR24! IPSLJCM5A!

MIROCJESM! MPIJESM!

ERAJInterim!(1990J2010)!Casado&et&al&2013&

Which"are"the"robust"climate"fingerprints?" Temperature"

The&fingerprints&are&more&diverse&in&terms&of&precipita[on&Two&main&centers&remain&(Northern&Europe&and&Mediterranean&Area)&


HadCM3!GISSJE2JR25!GISSJE2JR24! IPSLJCM5A!

MIROCJESM! MPIJESM!

ERAJInterim!(1990J2010)!Casado&et&al&2013&

Which"are"the"robust"climate"fingerprints?" Precipita&on"

Jones"et"al"(1999):""Meteorological&data&from&european&sta[ons&Back!to!1780!

1865&

1865&

Luterbacher"et"al"(2002):"Early&nstrumental+documentary!!Back!to!1659!

Can"we"validate"with"other"reliable"sources?"Three!other!SLP!datasets!exist!based!on!observa;onal!and!documentary!evidence!

KueBel"et"al"(2009):""Instrumental&SLP&+&wind&from&ship&logs&Back!to!1750!!

"

We"calculate"the"corresponding"Gibraltar'Iceland"NAO"index"for"the"valida&on"

The"alterna&ve"NAO"&meseries"

KueDel&et&al&2009:&&in&1750&reconstruc[on&skill&really&low&for&Iceland&(leh&plot)&

Jones&et&al&1999:&&Sta[ons&from&the&Atlan[c&do&not&start&un[l&XIXth&century&(closest&that&does&is&Edinburgh)&

1821&

1865&

1865&

1770&

Luterbacher&et&al&2002:&Clearly&biased&to&Europe&

The&three&seems&to&have&similar&limita[ons.& All& this& considered,&it& seems& that& we& shouldnt& be&too& restric[ve& imposing& proxy&coherence&with&all&these&indices&(Maybe& the& best& is& to& select&those& correla[ng& at& least& with&one&of&the&5&NAO&[meseries)&

Jones"et"al"(1999):""Meteorological&data&from&european&sta[ons&Back!to!1780!

1865&

1865&

Luterbacher"et"al"(2002):"Early&nstrumental+documentary!!Back!to!1659!

Can"we"validate"with"other"reliable"sources?"Three!other!SLP!datasets!exist!based!on!observa;onal!and!documentary!evidence!

KueBel"et"al"(2009):""Instrumental&SLP&+&wind&from&ship&logs&Back!to!1750!!

1865&

1865&

Can"we"validate"with"longer"&meseries?"Four!main!NAO!reconstruc;ons!are!also!available!

Luterbacher&et&al&(2002)&

Glueck&and&&Stockton&(2001)&

Cook&et&al.&(2002)&

Trouet&et&al.&(2009)&

Sta&on'based"data& (SLP,& temperature,&precip)&Documentary&evidence&&13"proxy&reconstruc[ons&

22"tree'rings"(21&precip,&1&temperature)&1"icecore"(d18O&and&accumula[on)&

Mul&'proxy"analysis&&Centered&on&the&Atlan&c"Basin"

A&tree'ring–based&drought&proxy&from&Morocco"A&speleothem&precipita&on&proxy&from&Scotland"

Some"words"on"the"reconstruc&on"technique"

We"follow"a"Principal"Component"Reconstruc&on"(PCR),"as"described"below:"&1)  Selected&proxies&are&standardised&with&respect&to&a&calibra[on&period&

2)  A&principal"component"analysis"of&the&series&is&performed&for&this&same&period&

3)  Only&those&PCs&that&correlate"significantly"(α=0.10)&with&the&NAO&are&retained&

4)  A&mul&variate"linear"regression"model"is&calculated&for&the&NAO,&using&the&selected&PCs"as&final&predictors&

5)  To&extend"the"reconstruc&on"as&far&as&the&proxies&go,&the&matrix"of"selected"proxies&is&projected"into"the"EOF"paBerns"(loadings)&corresponding&to&those&PCs&

6)  Finally,&the&same"mul&variate"model"is&applied,&now&for&the&“extended'PCs”&

7)  To&correct&for&the&loss"in"variance"inherent&to&the&regression,&the&final"NAO"reconstruc[on&is&re'standardised"

Impact"of"the"volcanoes"NAO response to 18 major volcanoes

3

2

1

0

-1

-2

-3

0.2

0

0.4

0.6

NA

O In

dex

Com

posi

te N

AO


95%

99%Years&of&volcanoes:&&

1168,"1176,"1190,"1231,"1258,"1269,"1278,"1286,"1452,"1460,"1600,"1641,"1809,"1815,"1831,"1835,"1884,"1903"

Amman et al. (2007)

Documents

Amul&'proxymodel'testedNAO reconstruc&onforthelastmillennium · The"reconstruc&on"technique" It is based on a Principal Component Regression (PCR), that in essence represents a multivariate