Coral records of central tropical Pacific SST and hydrology during the MCA
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Transcript of Coral records of central tropical Pacific SST and hydrology during the MCA
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
150°E 180° 150°W 120°W
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SS
T (
°C)
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)
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)
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
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
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
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.