Coral Records of 20^th Century Tropical Pacific Climate

          My research investigates how global warming may impact tropical Pacific climate whose variability strongly regulates global temperature and precipitation patterns.  A better understanding of tropical Pacific climate variability and secular trend under anthropogenic greenhouse forcing is crucial in formulating adaptation strategies for climate change.  I reconstruct the histories of sea-surface temperature (SST) and salinity (SSS) using corals from the northern Line Islands chain in the central tropical Pacific (Palmyra, Fanning and Christmas Islands; 2N-6N, 157W-162W).  Corals from these islands are sensitive to changes in SST and SSS associated with  the interannual El Nino/Southern Oscillation (ENSO) and decadal Pacific climate variability [Cobb et al., 2003].

Location of study sites with climatological SST of the tropical Pacific

          Coral oxygen isotopic (d¹⁸O) records from the central tropical Pacific have shown late 20th century trends towards depleted coral d¹⁸O, suggesting that warming and/or freshening have occurred in the region [Evans et al., 1999; Urban et al., 2000; Cobb et al., 2001].  It is important to quantify the role of SST and SSS changes in order to better understand the mechanism underlying this trend.  I analyze coral Sr/Ca ratio as SST proxy using ICP-OES, and coral d¹⁸O values (as a dual SST and SSS proxy) using mass-spectrometer.  Subsequently, I derive the history of SSS via the values of d¹⁸O of seawater (d¹⁸Osw) by subtracting the SST contribution on coral d¹⁸O values.  I also attempted applying coral Cd/Ca and Ba/Ca ratios as upwelling proxies to our corals (in collaboration with Drs. Andrea Grottoli and Katie Matthews), however, we found that the interpretation of these trace metals in our sites is not straightforward. 

          The late 20th century portion of this work shows warming and freshening trends in the central tropical Pacific, which suggest a weakening of the Pacific zonal SST gradient [Nurhati et al., 2009].  We observe warming that is stronger towards the equator, consistent with a weakening of the equatorial upwelling inferred from instrumental data [McPhaden and Zhang, 1999].  Stronger freshening trends towards Palmyra suggests a strengthening and/or equatorward shift of the Inter-Tropical Convergence Zone that may support the spin up of hydrological cycle under global warming as projected by a majority of IPCC climate models [Held and Soden, 2000].  I am currently working on the rest of 20th century SST and SSS reconstructions [Nurhati et al., 2009].

           On my personal note, I am deeply interested in understanding how the tropical Pacific climate is evolving under the current state of warming.  The answer will greatly influence countries in and around the Pacific basin, including mine (Indonesia). I was so fascinated by the strength of the 1997-98 ENSO; the drought and the resulting fire hotspots in our tropical rainforests during El Nino, and the big rain that submerged the first floor of my school during La Nina.  I had never seen anything like that, and strangely had never heard the term ENSO (well, I was just born when the previous big El Nino hit).  More importantly, the strong El Nino and La Nina events were a double-punch mess that unfortunately could not come at any better political and economic situations.  Should global warming alter tropical Pacific climate, I hope to contribute to our understanding of climate trend in the region whose societies are very vulnerable to changes in climate patterns.  I find it motivating to keep in mind the ultimate goal of what I am working on during long hours of lab works and challenging fieldworks.