Coral records of central tropical Pacific SST and hydrology during the MCA

1. Coral records of central tropical Pacific SST and hydrology during the MCA Kim Cobb Hussein Sayani Georgia Inst. of Technology Chris Charles Scripps Inst. of Oceanography Larry Edwards, Hai Cheng University of Minnesota with thanks to ACS-PRF, NOAA, NCL, PARC, Cobb lab undergrads

2. SST, hydrology, and coral δ18O in the Line Islands SST (°C) CLIMATOLOGY b EL NIÑO 150°E 180° 150°W 120°W Coral δ18O decreases when warm (thermodynamics) Coral δ18O decreases when rainy (lower seawater δ18O; Cole et al., 1990)

3. Palmyra coral δ18O is a sensitive proxy for ENSO Cobb et al., 2003

4. Overlapping corals provide extended, replicated records But with large offsets in mean coral δ18O Cobb et al., in prep

5. 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)

6. Palmyra coral δ18O and solar variability Emile Geay et al., submitted

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

8. 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

9. Palmyra fossil coral δ18O, Sr/Ca, and δ18Osw

10. Summary of fossil coral data • SO either… • 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”

11. In pursuit of accurate paleo-temperatures Lesson #1: avoid diagenesis (use clean corals or micro-sample clean portions of altered corals); SEM screening critical Sayani et al., submitted

12. Preliminary SIMS data consistent with “bulk” data

13. In pursuit of accurate paleo-temperatures Lesson #2: replication, replication, replication 20th century  need spread of Sr/Ca-SST calibration equations fossil  need more than 1 fossil coral Sr/Ca record to define mean

14. A heartening modern-fossil Sr/Ca comparison Crespo et al., in prep

15. Conclusions and Implications Strong evidence for late 20th century freshening in the central tropical Pacific, consistent with GCMs response to anthropogenic forcing. If we trust coral δ18O the most (which we should), relationship to external 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 last millennium (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.