HR: 17:15h
AN: PP34A-06    [Abstracts]
TI: Coral Records of Late 20th Century Warming and Freshening in the Central Tropical Pacific
AU: * Nurhati, I S
EM: intan@gatech.edu
AF: Georgia Inst. of Technology, School of Earth and Atmospheric Sciences, 311 Ferst Dr., Atlanta, GA 30302, United States
AU: Cobb, K M
EM: kcobb@eas.gatech.edu
AF: Georgia Inst. of Technology, School of Earth and Atmospheric Sciences, 311 Ferst Dr., Atlanta, GA 30302, United States
AU: Charles, C D
EM: ccharles@ucsd.edu
AF: Scripps Inst. of Oceanography, Univ. of California San Diego, 9500 Gilman Dr., La Jolla, CA 92093, United States
AB: Tropical Pacific climate variability strongly impacts global temperature and rainfall patterns. The evolution of tropical Pacific climate under anthropogenic greenhouse warming is unclear, observational and modeling studies support both a strengthening (Cane et al.,1997) and a weakening (Vecchi et al.,2006) of the tropical Pacific zonal sea-surface temperature (SST) gradient. Corals from the central tropical Pacific (CTP) have provided monthly-resolved climate proxy records that contain late 20th century trends toward more negative oxygen isotopic (d18O) values, suggesting that regional warming and/or freshening has occurred over the last several decades (Urban et al.,2000; Cobb et al.,2001). It is important to identify the physical mechanisms that underlie these trends in order to better predict low-frequency changes in tropical Pacific climate over the next decades. One potential mechanism involves a reduction in the tropical Pacific zonal SST gradient, which would warm the CTP by reducing upwelling while increasing rainfall (thereby decreasing salinity). On the other hand, warming might have occurred absent any change in the zonal SST gradient, and the freshening may signal changes in the location and/or intensity of the Inter-Tropical Convergence Zone (ITCZ). Here we apply d18O and Sr/Ca proxies to modern corals from Palmyra, Fanning and Christmas Islands (2-6°N,157-162°W) in order to reconstruct SST and sea-surface salinity (SSS) over the last several decades. While coral d18O changes in SST and/or the d18O of seawater (d18Osw, which is linearly related to salinity in this region), coral Sr/Ca is mainly controlled by SST. When used together, these two proxies allow us to quantify recent climate trends in the CTP with respect to SST and SSS, particularly because Palmyra, Fanning and Christmas span strong gradients in SST and SSS. The islands are aligned in a NW-SE fashion, such that Christmas (2°N) is bathed by the South Equatorial Current and dominated by upwelling variability; while Palmyra (6°N) lies in the core of the North Equatorial Counter Current (NECC) and is heavily influenced by ITCZ variability. We measure coral Sr/Ca ratios with an analytical error of better than ±0.3percent or ~±0.4°C(1σ). We measure coral d18O values with an analytical error of ±0.05percent(1σ). After converting the coral Sr/Ca measurements into SST anomalies, we construct d18Osw timeseries by removing the Sr/Ca-derived SST contribution from the coral d18O. The Sr/Ca-based SST reconstructions show that Palmyra and Christmas have warmed by 0.6 and 1.5°C over the period 1970-1998, respectively. The stronger warming trend at Christmas is consistent with a reduction in equatorial upwelling over this period. The d18Osw timeseries suggest that Palmyra and Christmas have experienced seawater freshening of 0.94psu and 0.36psu, respectively over the period. The stronger freshening trend at Palmyra may suggest enhanced and/or a southward migration of the ITCZ, and/or a reduction in the salinity of waters advected from the West Pacific Warm Pool by the NECC. Preliminary SST and SSS reconstructions from Fanning, which lies between Palmyra and Christmas, confirm these late 20th century warming and freshening trends. Taken together, the coral-based evidence for simultaneous warming and freshening in the CTP is consistent with a weakening of the tropical Pacific zonal SST gradient in the late 20th century. References: Cane et al.(1997).Science,275:957-960. Cobb et al.(2001).Geophys.Res.Lett.,28(11):2209-2212. Urban et al.(2000).Nature,407:989-993. Vecchi et al.(2006).Nature,441:73-76.
DE: 4513 Decadal ocean variability (1616, 1635, 3305, 4215)
DE: 4522 ENSO (4922)
DE: 4902 Anthropogenic effects (1803, 4802)
DE: 4916 Corals (4220)
DE: 4954 Sea surface temperature
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting