Central tropical Pacific temperature and hydrology

1. 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

2. 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 Model AnthroForcing Natural Forcing (Vecchi et al., 2006)

3. 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

4. The Line Islands Fossil Coral Collection • Strategy • - rare long cores (40-100y) for variability • common short cores (10-20y) for mean • climate Palmyra 40 cores U/Th dated 28 cores undated Christmas 18 cores U/Th dated 63 cores undated Fanning 33 cores undated

5. 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

6. 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

7. 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’

8. Palmyra coral d18O reconstruction to date 2 corals 5 corals 3 corals 1 coral 2 corals 1 coral coming soon coming soon 2s error mean d18O What controls ENSO & tropical Pacific climate variability over the last millennium? Cobb et al., 2003

9. Palmyra coral (high- and band-passed) vs. NIÑO3.4 SST SST=75% of interannual coral d18O signal

10. 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: temperature 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

11. Sr/Ca temperature calibration DT=DSr/Ca /0.0638 analytical error = 0.5°C (Nurhati et al., in prep) d18O salinity calibration Dpsu=Dd18Osw /0.27 (Fairbanks et al., 1997)

12. -0.52‰ +0.65ºC Palmyra Sr/Ca shows +0.65°C warming since 1970 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)

13. +0.1‰ +0.56‰ -3.9°C -1.7°C -0.21‰ -0.24‰ 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

14. 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)

15. 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 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

16. 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.

17. 17th Century Splice 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

18. 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

19. 12th-13th centuries splice - 1180-1245AD: most regular ENSO period of reconstruction (5y period)

20. winds 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

21. Does ENSO respond to solar and volcanic forcing? model mean (with ensemble spread) 18: 447-456

22. 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?

23. 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