Central tropical Pacific temperature and hydrologyover the last millennium: testing the ENSO-like paradigm

for decadal to centennial variability

Kim CobbIntan Suci NurhatiGeorgia Inst. of Technology

Chris CharlesScripps Inst. of Oceanography

Larry Edwards, Hai ChengUniversity of Minnesota

with thanks to NCL, PARC, Cobb lab undergrads

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

(Cane et al., 1997)

(Vecchi et al., 2006)

Late 20th Century Tropical Pacific trends

The Thermostat Hypothesis

Observed SLP Trend

Model

Natural Forcing

AnthroForcing

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

Beached fossil corals range from gravel-sized to 2m-diameter(~100-150yrs).

rare longer cores: ENSO & decadal variability

common short cores: mean climate

Palmyra40 cores U/Th dated28 cores undated

Christmas18 cores U/Th dated63 cores undated

Fanning 33 cores undated

The Line Islands Fossil Coral Collection

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

Study Sites: Palmyra & Christmas

Warm & wet during El Niño negative 18O

Strong salinity gradient due to the ITCZ

Strong SST gradient due toocean currents

ENSO and coral 18O in the Central Tropical Pacific (CTP)

During El Niño events, positive SST and precipitation anomalies both contribute to negative coral 18O anomalies in the CTPc

Interpretation of coral 18O 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

SST and rainfall anomalies during the 1982 El Nino

CTP climate during the last millennium

warmer, wetter

cooler, drier

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 18O reconstruction to date

Cobb et al., 2003

comingsoon

comingsoon

2 errormean 18O

5 corals 3 corals2 corals1 coral

1 coral

2 corals

What controls ENSO & tropical Pacific climate variability over the last millennium?

SS

T A

nom

oly

(°C

)

3

2

1

0

-1

-2

18 O

(‰

)

-0.6

-0.4

-0.2

0.0

0.2

NIÑO3.4 SSTPalmyra coral

Year (A.D.)

1900 1920 1940 1960 1980 2000

SS

T A

nom

oly

(°C

) 1

0

-1

-2

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

R = -0.66

R = -0.84

18 O

(‰

)

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

SST=75% of interannual coral 18O signal

late 20th century trendsin 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:temperatureincreased rainfalldecreased upwelling(clear implications for past low-frequency coral 18O signals)

some geochemical tools:Sr/Ca reflects temperatureCd/Ca reflects upwelling changes

(Fairbanks et al.,

1997)

(Nurhati et al., in prep)

Sr/Ca temperaturecalibration

18O salinitycalibration

T=Sr/Ca /0.0638

analytical error = 0.5°C

psu=18Osw /0.27

(Nurhati et al., in

prep)

-0.52‰

+0.65ºC

-0.26‰ or 0.94psu

Palmyra Sr/Ca shows +0.65°C warming since1970

accounts for ~25% ofcoral 18O trend

large contribution (75%) from fresher water (lower 18Osw)

+0.1‰+0.56‰

-1.7°C-3.9°C

-0.24‰-0.21‰

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. 18O, Field et al., in prep)

Conclusions• 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 centennialtimescales- 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

splice

- absolute 18O 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 18O 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) = 0.90R(Palmyra, NIÑO3.4) = 0.84

12th-13th centuries splice

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

Clement et al., 1996

winds

Cane-Zebiak model w/ uniform heating

annual mean SST, begin uniform heating

SST increases in West, but SST in Eastis buffered by upwelling of cool waters,zonal SST gradient increases

Bjerknes feedback: trades strengthen,zonal SST gradient increases

The ‘Ocean Thermostat’: heating La Niña-like cooling El Niño-like

Can solar (heating)and volcanic (cooling)forcing shape tropicalPacific climate overthe last millennium?

18: 447-456

model mean (with ensemble spread)

Does ENSO respond to solar and volcanic forcing?

Date (A.D.)

1900 1920 1940 1960 1980 2000

18

O (

‰)

-5.2

-5.0

-4.8

-4.6

-4.4

-4.2

-4.0

-3.8

Age (kybp)

6.206.226.24

18

O (

‰)

-5.2

-5.0

-4.8

-4.6

-4.4

-4.2

-4.0

-3.8

Date (A.D.)

1520 1540 1560

Palmyra (6ºN) modern coral

Christmas (2ºN) modern coral

460-yr-oldChristmas fossil coral

6.2 kypbFanning (4ºN)

fossil coral

New sequences from Christmas and Fanning Islands

2 errormean 18O

Fanning (4ºN)?

6ky: cooler, drier conditions?; reduced ENSO activity0.5ky: statistically indistinguishable mean, ENSO variability wrt modern

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

Tudhope et al., 2001Woodruffe et al., 2003McGregor et al., 2004Correge et al., 2000Cobb et al., 2003, in progress

Late Holocene trends in ENSO variance

Equally importantare reconstructionsof tropical Pacificmean climate throughthe late Holocene….

Are corals up to the challenge?

Reduce error bar by usingmany small corals?

Are we up to the challenge?

Moy et al., 2002

Clement et al., 1999