Late 20th Century Freshening in the Central Tropical Pacific

Inferred from Coral Records

Intan Suci NurhatiAdvisor: Dr. Kim Cobb

Graduate Student Symposium 2006School of Earth and Atmospheric Sciences

Georgia Institute of Technology

Outline

Central Tropical Pacific climate

Extracting coralline salinity record

Coral and its paleoclimate proxies

Results & Discussions

(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

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-AES

RSD = (±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)

2. Instrumental Data Comparison 1978-89 overlap (r2 = -0.23)

I error bar

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.

δ18O

cora

l (‰

)δ1

8O

sst

(‰)

δ18O

sw (‰

)

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.