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Central tropical Pacific temperature and hydrology over the last millennium: testing the ENSO-like paradigm for decadal to centennial variability. Kim Cobb Intan Suci Nurhati Georgia Inst. of Technology. Chris Charles Scripps Inst. of Oceanography Larry Edwards, Hai Cheng

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Central tropical Pacific temperature and hydrology

over the last millennium: testing the ENSO-like paradigm

for decadal to centennial variability

Kim Cobb

Intan Suci Nurhati

Georgia Inst. of Technology

Chris Charles

Scripps Inst. of Oceanography

Larry Edwards, Hai Cheng

University of Minnesota

with thanks to NCL, PARC, Cobb lab undergrads


Late 20th Century Tropical Pacific trends

Has the tropical Pacific zonal SST gradient strengthened or weakened in response to anthropogenic forcing?

The Thermostat Hypothesis

(Cane et al., 1997)

Observed SLP Trend



Natural Forcing

(Vecchi et al., 2006)


Fossil corals collected during fieldtrips in 1998, 2000, and 2005

Beached fossil corals range

from gravel-sized to 2m-diameter


rare longer cores: ENSO & decadal


common short cores: mean climate


The Line Islands

Fossil Coral Collection

  • Strategy
  • - rare long cores (40-100y) for variability
  • common short cores (10-20y) for mean
  • climate


40 cores U/Th dated

28 cores undated


18 cores U/Th dated

63 cores undated

Fanning 33 cores undated

study sites palmyra christmas
Study Sites: Palmyra & Christmas

Warm & wet

during El Niño

 negative d18O

Strong salinity

gradient due to

the ITCZ

Strong SST

gradient due to

ocean currents


ENSO and coral d18O in the Central Tropical Pacific (CTP)

SST and rainfall anomalies during the 1982 El Nino

During El Niño events, positive SST and precipitation anomalies both contribute to negative coral d18O anomalies in the CTPc

Interpretation of coral d18O on lower frequencies relies on assumption that warm SST drives higher precipitation in the CTP, and vice versa

Coral Sr/Ca can be used to test this relationship


CTP climate during the last millennium





  • Conclusions:
  • late 20th century trend towards warmer, wetter conditions unprecedented
  • most severe El Niños of millennium during 17th century
  • cooler, drier conditions during ‘Medieval Climate Anomaly’

Palmyra coral d18O reconstruction to date

2 corals

5 corals

3 corals

1 coral

2 corals

1 coral





2s error

mean d18O

What controls ENSO & tropical Pacific climate

variability over the last millennium?

Cobb et al., 2003


Palmyra coral (high- and band-passed) vs. NIÑO3.4 SST

SST=75% of interannual coral d18O signal


late 20th century trends

in the tropical Pacific

coral trend post-1950 = -0.5‰

if 75% SST, warming = +1.5ºC

and 0.5psu freshening

if 50% SST, warming = +1ºC

and 1psu freshening

trend likely reflects combination of:


increased rainfall

decreased upwelling

(clear implications for past

low-frequency coral d18O signals)

some geochemical tools:

Sr/Ca reflects temperature

Cd/Ca reflects upwelling changes


Sr/Ca temperature


DT=DSr/Ca /0.0638

analytical error = 0.5°C

(Nurhati et al., in prep)

d18O salinity


Dpsu=Dd18Osw /0.27

(Fairbanks et al., 1997)




Palmyra Sr/Ca shows

+0.65°C warming since


accounts for ~25% of

coral d18O trend

large contribution (75%) from fresher water (lower d18Osw)

-0.26‰ or 0.94psu

(Nurhati et al., in prep)








slight cooling during LIA; significant cooling during MCA

cooling accompanied by wetter conditions??

(errors allow for cooling accompanied by no change in hydrology)

Sr/Ca data imply that coral d18O is almost entirely temperature


Tropical Pacific link to US West drought?

Cook et al., 2004

Evidence of La Niña-like conditions from ~900-1250AD:

Mono Lake lowstands (Stine et al., 1994)

high Warm Pool temperatures (Sr/Ca MD81, Stott et al., 2004)

decreased Peru runoff (lithic counts, Rein et al., 2004)

cool Santa Barbara basin temperatures (G. bull.d18O, Field et al., in prep)



  • corals provide quantitative reconstructions of both temperature and salinity
    • - error bars from extensive replication; no inherent biases for low-freq
    • separate T & S geochemically; replication essential
  • contributions of T and S very different on interannual, decadal, and centennial
  • timescales
  • - anthropogenic era dominated by warming (25%) freshening (75%)
  • - MCA dominated by cooling (~100%) of ~4°C
    • - LIA dominated by cooling (~100%) of ~1.5°C

Mann et al., 2005 updated with new data

Large model ensemble spread still encompasses coral data,

but large volcanic eruption in 1258AD a more direct test of ‘thermostat’ response.


17th Century Splice


- absolute d18O values agree (give consistent picture of mean climate)

- large, well-reproduced El Niño events in mid-17th century

- low-amplitude, less well-reproduced decadal variability


Modern Line Islands coral d18O records

Christmas Island (Evans et al., 1999)

Palmyra Island (Cobb et al., 2001)

Fanning Island (Dunbar et al., in prep; Nurhati et al., in prep)

Southern Line Islands (upcoming cruise, fall 2006, needed for

resolving 20th century tropical Pacific climate trends)

R(Christmas, NIÑO3.4) = 0.90

R(Palmyra, NIÑO3.4) = 0.84


12th-13th centuries splice

- 1180-1245AD: most regular ENSO period of reconstruction (5y period)



The ‘Ocean Thermostat’: heating  La Niña-like

cooling  El Niño-like

Cane-Zebiak model w/ uniform heating

Can solar (heating)

and volcanic (cooling)

forcing shape tropical

Pacific climate over

the last millennium?

annual mean SST, begin uniform heating

SST increases in West, but SST in East

is buffered by upwelling of cool waters,

zonal SST gradient increases

Bjerknes feedback: trades strengthen,

zonal SST gradient increases

Clement et al., 1996


Does ENSO respond to solar and volcanic forcing?

model mean

(with ensemble


18: 447-456


New sequences from Christmas and Fanning Islands

Fanning (4ºN)?

2s error

mean d18O

6ky: cooler, drier conditions?; reduced ENSO activity

0.5ky: statistically indistinguishable mean, ENSO variability wrt modern

GOAL: A unified tropical Pacific climate reconstruction from Line Islands fossil corals?


Late Holocene trends in ENSO variance

Equally important

are reconstructions

of tropical Pacific

mean climate through

the late Holocene….

Are corals up to the challenge?

Reduce error bar by using

many small corals?

Are we up to the challenge?

Clement et al., 1999

Moy et al., 2002

Tudhope et al., 2001

Woodruffe et al., 2003

McGregor et al., 2004

Correge et al., 2000

Cobb et al., 2003,

in progress