El Niño-Southern Oscillation during the past 50 years:

Atmospheric and oceanographic fluctuations across the tropical Pacific Ocean are partly a function of the phenomena known as the El Niño Southern Oscillation (ENSO), which is recognised as the strongest signal of interannual climate variation in the world. Pioneering studies, which utilise recent living corals from Tarawa (1oN, 172oE) and Maiana (1oN, 173oE), in the western equatorial Pacific Ocean, indicate that coral oxygen isotope (δ18O) records from this region can be used to reflect variations in ENSO-related climate patterns (Cole et al. 1993; Urban et al. 2000). The δ18O signal, however, is a combination of both changes in sea surface temperature (SST) and in the δ18O of seawater (δ18Osw), yet both these factors may change with ENSO variations. This study investigates the coral-derived climate signals of the past 50 years (~1960 to 2010) from Butaritari Atoll (~172o30’E, 3o30’N) located in the western central equatorial Pacific using paired Sr/Ca and δ18O proxies to separate the SST and δ18Osw signals. The Sr/Ca-derived SST results show a warming trend of 0.77 ± 0.17 oC and that decadal variability dominates over interannual. The Butaritari δ18Osw results provide additional climatic information about the balance of evaporation and precipitation, although no significant freshening trend is evident. Taken together, the Butaritari Sr/Ca-derived SST and δ18Osw results suggest the warming and/or freshening trends observed in coral δ18O reflect predominantly changes in SST for the past 50 years. The Butaritari SST trend is also consistent with Sr/Ca estimates from central equatorial Pacific islands and suggests warming at broader scale.