Late 20 th Century Freshening in the Central Tropical Pacific Inferred from Coral Records

1. Late 20th Century Freshening in the Central Tropical Pacific Inferred from Coral Records Intan Suci Nurhati Advisor: Dr. Kim Cobb Graduate Student Symposium 2006 School of Earth and Atmospheric Sciences Georgia Institute of Technology

2. Outline • Central Tropical Pacific climate • Coral and its paleoclimate proxies • Extracting coralline salinity record • Results & Discussions

3. (Central) Tropical Pacific Climate • Affected interannually by El Niño, which has a large spatial impact. Walker Circulation Cobb et al. (2003) • Sparse observational data in the Pacific requires paleodata to extend the evolution of El Niño.

4. Coral as a Paleoclimate Archive Geochemical proxy: δ18O as SST proxy ratio of 18O and 16O incorporated into coral skeleton f ( sst , salinity ) Limitations: Vital effects Colony topography, etcSolution: Use several samples to cross-validate. Annual bands on coral at about 1-1.5cm/year

5. Coral δ18O & Sr/Ca • Depleted δ18O in the late 20th century Q? Higher SST and/or lower salinity (freshening) Imp: Hydrological cycle • Use a SST-only proxy (Sr/Ca) to separate SST and salinity effects on δ18O • Period 1972-98 at 1mm incrm. • ICP-AESRSD = (±0.3%,1σ) ~ 0.44ºC Cobb et al. (2001), Urban et al. (2000)

6. Calibrating Sr/Ca Data 1. Inter-coral Heads Comparison • 1971-89 overlap (r2 = 0.55 for raw, 0.62 for 5 months running average) I error bar 2. Instrumental Data Comparison • 1978-89 overlap (r2 = -0.23) SST

7. Separating SST and Salinity • Unlike δ18O, Sr/Ca does not exhibit a trend in the late 20th century. • Use instaneous change of the function by the variables (SST and salinity) to weight the contribution of each variable (Ren et al. 2002). Δδ18Ocoral = Δδ18Osst + Δδ18Osw = ∂δ18O ·ΔSST + ∂δ18O ·Δδ18Osw ∂SST ∂δ18Osw known calculated from Sr/Ca a constant

8. Separating SST and Salinity • SST as well as salinity drive the depletion of δ18O in the late 20th century, with salinity having a bigger contribution. • Salinity has a decreasing trend, implying enhanced precipitation. δ18Ocoral (‰) δ18Osst (‰) δ18Osw (‰)

9. Discussions & Conclusions • Modern coral δ18O and Sr/Ca records suggest a late 20th century freshening in the central tropical Pacific. • Agrees with analysis of rain gauge measurement from the central tropical Pacific during 1971-90 (Morrissey and Graham, 1996) that suggest enhanced hydrological cycle. • A modeling study suggests a weakening of the east-west Walker circulation largely due to anthropogenic forcing (Vecchi et al. 2006).

10. - Thank you - References: Cobb, K.M., D.E. Hunter, C.D. Charles. A central tropical Pacific coral demonstrates Pacific, Indian, and Atlantic decadal climate connections. Geophys. Res. Lett. 28(11):  2209-2212, 2001. Cobb, K.M., C.D. Charles, R.L Edwards, H. Cheng, and M. Kastner. El Niño-Southern Oscillation and tropical Pacific climate during the last millennium. Nature 424:  271-276, 2003. Morrisey, M.L., and N.E. Graham. Recent trends in rain gauge precipitation measurements from the tropical Pacific: Evidence for an enhanced hydrological cycle. Bulletin of the American Meteorological Society 77(6): 1207-1219, 1996. Ren, L., B.K. Linsley, G.M. Wellington, D.P. Schrag, and O. Hoegh-Guldberg. Deconvolving the δ18O seawater component from subseasonal coral δ18O and Sr/Ca at Rarotonga in the southwestern subtropical Pacific for the period 1726 to 1997. Geochimica et Cosmochimica Acta 67(9): 1609-1621, 2002. Urban, F.E., J. E. Cole, and J.T. Overpeck. Influence of mean climate change on climate variability from a 155-year tropical Pacific coral record. Nature 407, 989-993, 2000. Vecchi, G.A., B.J. Soden, A.T. Wittenberg, I.M. Held., A. Leetmaa, and M.J. Harrison. Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature 441: 73-76, 2006.