1. Late 20 th 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 Extracting coralline salinity record Coral and its paleoclimate proxies Results & Discussions

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

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

5. Coral δ 18 O & Sr/Ca Depleted δ 18 O in the late 20 th 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 δ 18 O Period 1972-98 at 1mm incrm. ICP-AES RSD = (±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 (r 2 = 0.55 for raw, 0.62 for 5 months running average) 2. Instrumental Data Comparison 1978-89 overlap (r 2 = -0.23) I error bar SST

7. Separating SST and Salinity Unlike δ 18 O, Sr/Ca does not exhibit a trend in the late 20 th century. Use instaneous change of the function by the variables (SST and salinity) to weight the contribution of each variable (Ren et al. 2002). Δδ 18 O coral = Δδ 18 O sst + Δδ 18 O sw = ∂δ 18 O ·ΔSST + ∂δ 18 O · Δδ 18 O sw ∂SST ∂δ 18 O sw known calculated from Sr/Ca a constant

8. Separating SST and Salinity SST as well as salinity drive the depletion of δ 18 O in the late 20 th century, with salinity having a bigger contribution. Salinity has a decreasing trend, implying enhanced precipitation. δ 18 O coral (‰) δ 18 O sst (‰) δ 18 O sw (‰)

9. Discussions & Conclusions Modern coral δ 18 O and Sr/Ca records suggest a late 20 th 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 δ 18 O seawater component from subseasonal coral δ 18 O 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.