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
Outline • Central Tropical Pacific climate • Coral and its paleoclimate proxies • Extracting coralline salinity record • Results & Discussions
(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.
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
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)
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
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
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 (‰)
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).
- 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.