Ryuji Asami

Interannual and decadal variability of the western Pacific sea surface condition for the years 1787–2000: Reconstruction based on stable isotope record from a Guam coral

[1]xa0We present a monthly resolved, 213-year stable isotope time series from a coral from Guam (13°N, 145°E), which is located on the northern edge of the western Pacific warm pool. Oxygen isotopic composition of the coral skeleton (δ18Ocoral) shows seasonal, interannual, and decadal variability, which documents significant oceanographic changes related to thermal and hydrologic variations in this region. The δ18Ocoral anomaly reflects sea surface temperature (SST) anomaly and sea surface salinity (SSS) anomaly with significant r values of −0.69 and 0.49, respectively, which are strongly linked to oceanographic changes that occur during El Nino–Southern Oscillation (ENSO) warm and cool phases. We identified 46 ENSO warm (El Nino) and 53 cool phases (La Nina) in the coral record, which are consistent with those phases reconstructed by Nino 3.4 SST anomaly. Spectral analyses of the δ18Ocoral anomaly record for the years 1790–1999 identified significant peaks around ∼3 to ∼7 years. These results indicate that the Guam coral has recorded ENSO periodicity. The δ18Ocoral anomaly shows decadal variability of ∼15- to ∼45-year periodicity with significant shifts (<0.2‰) from warmer to cooler condition and vice versa. An accumulative decrease in δ18Ocoral time series may imply ∼0.75°C warming of SST and ∼0.23‰ freshening of seawater δ18O, corresponding to a decrease of SSS by ∼0.85, in the northwestern tropical Pacific over the last 2 centuries.

Carbon and oxygen isotopic composition of a Guam coral and their relationships to environmental variables in the western Pacific

We examine the high-resolution (~32 samples/year) carbon and oxygen isotopic composition (y13Ccoral and y 18Ocoral) in a coral core (Porites lobata) from Double Reef, Guam over the years 1980–2000. The yCcoral shows clear seasonal variations with mean seasonal amplitude of 1.89x, which roughly corresponds with seasonal variations in solar radiation. The seasonal amplitude of y18Ocoral variations are small (0.23–0.57x), but they are significantly correlated with sea surface temperature (SST) and salinity (SSS). The yOcoral and SST are more strongly correlated during El Niño/Southern Oscillation (ENSO) warm phases (r= 0.81, pb0.01) than during non-ENSO phases (r= 0.65, pb0.01) and ENSO cool phases (r= 0.48, pb0.01). These different relationships are due to differences in winter SST and in seawater yO (y18Osw) during ENSO warm phases (b27 8C and higher values of yOsw) compared with cool phases (N28 8C and lower values of y Osw) at Guam. These differences in oceanic parameters result from movements of the Western Pacific Warm Pool (WPWP) during the different phases of ENSO. Anomalies in y18Osw, inferred from the y 18Ocoral and instrumental SST, are consistent with SSS anomalies for the years 1980–2000. These yOsw anomalies may reflect changes in SSS and evaporation–precipitation due to movements of the WPWP. This detailed analysis of a coral from Guam suggests that it may contain an excellent archive of past ENSO events. D 2004 Elsevier B.V. All rights reserved.