To reconstruct oceanographic variations in the subtropical South Pacific, 271-year long subseasonal time series of Sr/Ca and δ
18O were generated from a coral growing at Rarotonga (21.5°S, 159.5°W). In this case, coral Sr/Ca appears to be an excellent proxy for sea surface temperature (SST) and coral δ
18O is a function of both SST and seawater δ
18O composition (δ
18O
sw). Here, we focus on extracting the δ
18O
sw signal from these proxy records. A method is presented assuming that coral Sr/Ca is solely a function of SST and that coral δ
18O is a function of both SST and δ
18O
sw. This method separates the effects of δ
18O
sw from SST by breaking the instantaneous changes of coral δ
18O into separate contributions by instantaneous SST and δ
18O
sw changes, respectively. The results show that on average δ
18O
sw at Rarotonga explains ∼39% of the variance in δ
18O and that variations in SST explains the remaining ∼61% of δ
18O variance. Reconstructed δ
18O
sw shows systematic increases in summer months (December-February) consistent with the regional pattern of variations in precipitation and evaporation. The δ
18O
sw also shows a positive linear correlation with satellite-derived estimated salinity for the period 1980 to 1997 (r = 0.72). This linear correlation between reconstructed δ
18O
sw and salinity makes it possible to use the reconstructed δ
18O
sw to estimate the past interannual and decadal salinity changes in this region. Comparisons of coral δ
18O and δ
18O
sw at Rarotonga with the Pacific decadal oscillation index suggest that the decadal and interdecadal salinity and SST variability at Rarotonga appears to be related to basin-scale decadal variability in the Pacific.
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