West Pacific warm pool, South Pacific convergence zone and El Niño : southern oscillation variability in the common Era
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Observations and reconstructions of climate variability are necessary to place predictions of future global climate change into temporal context (Goddard et al., 2012). I focus here on the western tropical Pacific (Solomon Islands; ~9.5°S, ~160°E), a region directly influenced by: the South Pacific Convergence Zone (SPCZ), the West Pacific Warm Pool, the Pacific Walker Circulation, and the Hadley Circulation. The West Pacific Warm Pool (WPWP) is an integral source of heat and moisture to the atmosphere. It is part of both Walker and Hadley circulations, and the El Niño-Southern Oscillation (ENSO), which is the largest source of interannual climate variability on Earth. Modern observations of changes in the mean state of the WPWP, Walker circulation, and ENSO are limited in temporal scale and overprinted with the signal of anthropogenic climate change; hence, proxy-based observations in the pre-instrumental period are needed to place any future changes in these systems, and attribution of these changes, into context. I present evidence for large (~1.5 m), abrupt, and periodic changes in total annual rainfall amount on decadal to multidecadal timescales since 1423±5 CE (Year Common Era) in the Solomon Islands utilizing a fast growing (>2 mm yr⁻¹) stalagmite sample and an empirical relationship developed between stalagmite δ¹⁸O and local rainfall amount. The ~600-yr Solomon Islands stalagmite δ¹⁸O record indicates that decadal oscillations in rainfall are a persistent characteristic of SPCZ-related climate variability, and available evidence points to these changes arising from internal forcing. Such changes have important implications to water resource management in this region, in addition to contextualizing models and future predictions of rainfall amount changes and latent heat release into the atmosphere via anthropogenic climate change. I also present a reconstruction pre-instrumental variability utilizing a monthly δ¹⁸O time series generated series from the long lived, slow-growing coral species Diploastrea heliopora spanning 1716-2008 CE. The record demonstrates strong coherence with modern instrumental variability, and in addition, demonstrates variance in centennial-multicentury, decadal-multidecadal and interannual bands. Quantitative comparison of the interannual component of the record to instrumental ENSO indices, and application of the resulting metrics to previously undocumented, potentially extreme ENSO events occurring during the early eighteenth and nineteenth centuries. These extreme events are absent from existing multiproxy reconstructions due to temporal limitations of extant proxies that have the greatest ENSO sensitivity. The intensity of these events may rival or exceed the largest in the instrumental record. The record also contains significant but transient decadal to multidecadal variability. The coral record exhibits a trend toward isotopic depletion, implying significant changes in SST and/or SSS over the entirety of the record. I also document the unique behavior of the SPCZ and the Intertropical Convergence Zone (ITCZ) over the past six centuries, present evidence of influence by decadal-scale advection processes on Solomon Islands SSS, and explore basin-scale changes in the tropical Pacific conditions potentially related to global average temperature change over the past three centuries.