Coral records of central tropical Pacific SST and hydrology during the MCA Kim Cobb Hussein Sayani Georgia Inst. of Technology Chris Charles Scripps Inst.

Coral records of central tropical PacificSST and hydrology during the MCA

Kim CobbHussein SayaniGeorgia Inst. of Technology

Chris CharlesScripps Inst. of Oceanography

Larry Edwards, Hai ChengUniversity of Minnesota

with thanks to ACS-PRF, NOAA, NCL, PARC, Cobb lab undergrads

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CLIMATOLOGY

EL NIÑO

SST, hydrology, and coral δ18O in the Line Islands

Coral δ18O decreases when warm (thermodynamics)Coral δ18O decreases when rainy (lower seawater δ18O; Cole et al., 1990)

Palmyra coral δ18O is a sensitive proxy for ENSO

Cobb et al., 2003

Overlapping corals provide extended, replicated records

But withlarge offsetsin meancoral δ18O

Cobb et al., in prep

Palmyra coral δ18O: where do we stand?

-significant decadal to centennial-scale changes in coral δ18O

-largest excursions in MCA (cooler, drier) &late 20th century (warmer, wetter)

BUT-we really want to separate SST and hydrology (esp. given evidence for large hydrological shifts in recent studies)

Palmyra coral δ18O and solar variability

Emile Geay et al., submitted

Coral Sr/Ca paleo-temperature to the rescue (sort of)

I. A 20th century success story

Nurhati et al., 1999

So this promises a means of quantifying low-frequency SST and seawater δ18O variability in 20th century and in fossil corals.

20th century Palmyra coral δ18O, Sr/Ca, and δ18Osw

SST has little trend (in range of instrumental SST trends for Palmyra);δ18Osw has significant trend towards freshening

Nurhati et al., submitted

Palmyra fossil coral δ18O, Sr/Ca, and δ18Osw

Summary of fossil coral data

SO either…

a)it was cooler and fresher in the MCA and LIA than 20th century(dynamically possible? loosen “ENSO-like” model for MCA?)

or

b) the coral Sr/Ca paleo-SST estimates are “too cold”

In pursuit of accurate paleo-temperaturesLesson #1: avoid diagenesis (use clean corals or micro-sample clean portions

of altered corals); SEM screening critical

Sayani et al., submitted

Preliminary SIMS data consistent with “bulk” data

In pursuit of accurate paleo-temperaturesLesson #2: replication, replication, replication

20th century need spread of Sr/Ca-SST calibration equationsfossil need more than 1 fossil coral Sr/Ca record to define mean

A heartening modern-fossil Sr/Ca comparison

Crespo et al., in prep

Conclusions and Implications

Strong evidence for late 20th century freshening in the centraltropical Pacific, consistent with GCMs response to anthropogenic forcing.

If we trust coral δ18O the most (which we should), relationship toexternal forcing is unclear. Perhaps we’re missing key SST and hydrological signals that might relate better.

Low-frequency SST and hydrological changes in the central tropical Pacific may be decoupled, in 20th century and lastmillennium (prominent role for mid-latitude influences?other good explanations exist we are reconstructing salinity)

Firm conclusions about MCA climate in the central tropical Pacific await verification, but preliminary results suggest that appreciable cooling occurred during ~950AD.