This dissertation describes studies on the Pacific climate and ocean circulation changes inferred from instrumental and coral records, with a focus on the decadal time scale. The studies primarily concentrate on (1) developing a 200-year record of sea surface temperature (SST) variations in the west Pacific warm pool (WPWP) using a coral Sr/Ca record from Gizo coral in New Georgia (8°6'S, 156°52'E), and evaluating the role of Sr/Ca as a valid paleothermometer; (2) exploring decadal scale climatic variations in the Pacific and their response to global warming using combined paleoclimatic records; (3) investigating impacts of Pacific decadal climate change and recent warming on drought in the western United Sates (US); and (4) investigating modes of the South Pacific Central Water (SPCW) circulation and its role in the climate system using spatial and time series analysis.
Our results show a moderate correlation between Sr/Ca and SST (r=-0.4) for a Gizo coral, and a decoupling of the relationship between Sr/Ca and SST during the last two decades. Comparison with a coral δ18O record from nearby Espiritu Santo suggests that changing water mass properties, possibly associated with the halocline that develops in the region of the South Pacific Convergence Zone (SPCZ), may contribute to decoupling of Sr/Ca from ambient SSTs. To investigate whether this response may be more widespread, we compare our Sr/Ca record with four corals from nearby locations. Results suggest that large errors occur in WPWP records, while two records from subtropical sites (New Caledonia and Rarotonga), with a lower range of seasonal variability in salinity, generally show good fidelity. The inferred water-mass related decoupling could be due to the direct effect of salinity on Sr/Ca, or indirect effects resulting from isolation of the corals in a shallow halocline that may not be well mixed with the whole-ocean reservoir. Until the reasons are better understood, we recommend that considerable caution be used in interpretation of coral Sr/Ca, especially in regions of low or variable salinity, or for boundary conditions far removed from the present.
We successfully developed a Pacific decadal variation index by combining this core with four published coral records, and a tree ring reconstruction back 270 years. This joint proxy indicates that the present state of the PDO is nearly "neutral", with no significant changes of amplitude, trend, or frequency over the entire record. The stability implies that different feedbacks identified for this mode are not only sufficiently strong to neutralize (so far) the effects of global warming, but that any mechanism proposed to explain the observed stability must be able to account for the changing energy input into the tropical Pacific system from global warming.
We then compared our joint PDO index with a Drought Area Index derived from precipitation-sensitive tree ring records in the western United States. Our results have shown that the hydrological situation of the western United States has been well correlated with the Pacific climate at a decadal scale from 1820, but the correlation started to decrease in the middle of the 20 th century, which indicates that recent, prolonged drought events in the Western US are probably affected by human-induced warming. This conclusion foreshadows, in conjunction with previous work from model studies, an imminent crisis in water supply for the western US considering the current continuing warming trend.
After completing the PDO study we realized that we have directed more attention to the North Pacific than the ocean basin where the corals are from. We therefore undertook a spatial and temporal examination of South Pacific SSTs over the instrumental period to see whether there were any modes unique to this water mass. Our results show four modes in the 1870-2000 record of South Pacific SSTs - an ENSO mode, a mode similar to the Pacific Decadal Oscillation, and two modes that appear to have received much less attention. One of these new SST modes is unexpectedly related to the SPCZ and the occasional appearance of an eastern South Pacific Intertropical Convergence Zone (ITCZ). The second unexpected mode has an SST anomaly similar to El Nino in the eastern equatorial Pacific but with an SST pattern in the western South Pacific that is opposite in sign to that occurring with the (normal) ENSO first mode. This result suggests that farfield correlations with just SST in the eastern equatorial region may sometimes yield ambiguous results because of the mixing of South Pacific modes 1 & 4 in this region. Comparisons between the reconstucted SPCW circulation and paleoclimate records indicated that a strong east-west "seesaw" pattern in the South Pacific may be relate to a particularly extreme time in the 1730s. (Abstract shortened by UMI.)