[1] The South Pacific Convergence Zone (SPCZ) is the largest and most persistent spur of the Intertropical Convergence Zone. At the southeastern edge of the SPCZ near 170°W and 15°-20°S a surface ocean salinity frontal zone exists that separates fresher Western Pacific Warm Pool water from saltier and cooler waters in the east. This salinity front is known to shift east and west with the phase of the El Niño Southern Oscillation. We have generated subannually resolved and replicated coral oxygen isotopic time series from Fiji (17°S, 179°E) and Rarotonga (21.5°S, 160°W) that have recorded interannual displacements of the salinity front over the last 380 years and also indicate that at lower frequencies the decadal mean position of the salinity front, and eastern extent of the SPCZ, has shifted east-west through 10°to 20°of longitude three times during this interval. The most recent and largest shift began in the mid 1800s as the salinity front progressively moved eastward and salinity decreased at both sites. Our results suggest that sea surface salinity at these sites is now at the lowest levels recorded and is evidence for an unprecedented expansion of the SPCZ since the mid 1800s. The expansion of the SPCZ implies a gradual change in the South Pacific to more La Niña-like long-term mean conditions. This observation is consistent with the ocean thermostat mechanism for the Pacific coupled ocean-atmosphere system, whereby exogenous heating of the atmosphere would result in greater warming in the western Pacific and a greater east-west surface temperature gradient.
In the South Pacific, interdecadal‐decadal oceanic and atmospheric variability, referred to as the Interdecadal Pacific Oscillation (IPO), is most pronounced in the South Pacific Convergence Zone (SPCZ) salinity front region. Here we have used annual average oxygen isotope (δ18O) time series from five coral cores collected from Fiji and Tonga in this region to construct a Fiji‐Tonga Interdecadal‐Decadal Pacific Oscillation (F‐T IDPO) index of low‐frequency (>9 and <55 years) climate variability back to 1650 A.D. We first demonstrate the consistency between this F‐T IDPO index and a mean sea level (MSL) pressure‐based SPCZ position index (SPI) (1891–2000), thus verifying the ability of coral δ18O to record past interdecadal‐decadal climatic variations in this region back to 1891. The F‐T IDPO index is then shown to be synchronous with the IPO index (1856–2000), suggesting that this coral‐based index effectively represents the interdecadal‐decadal scale climate variance back to 1650. The regularity of the F‐T IDPO index indicates that interdecadal‐decadal variability in the SPCZ region has been relatively constant over the past 350 years with a mean frequency of ∼20 years (variance peaks near 11 and 35 years). There is a consistent antiphase correlation of the F‐T IDPO index and the interdecadal‐decadal components in equatorial Pacific coral δ18O series from Maiana and Palmyra. This observation indicates that the eastward expansion (westward contraction) of the eastern salinity front of the Western Pacific Warm Pool (WPWP) occurs simultaneously (±<1 year) with the westward (eastward) shift of the SPCZ salinity front during positive IPO (negative IPO) phases. This is the same relationship observed during the phases of the El Niño Southern Oscillation.
[1] To date, coral-based paleoclimate research in the Pacific has primarily utilized core samples from the genus Porites and has been most successful reconstructing past variability on interannual timescales, particularly the El Niño Southern Oscillation (ENSO). The Indo-Pacific coral genus Diploastrea, however, owing to its slower extension rate, denser structure, and longer lifespan, can potentially preserve geochemical proxy records 2-3 times longer than Porites cores of the same length. Before its potential can be realized, Diploastrea must first be calibrated and its climate signal assessed. We present oxygen isotope (d 18 O) and Sr/Ca results from two Diploastrea cores collected in Fiji (16°49 0 S, 179°14 0 E) that allow for simultaneous evaluation of this coral's paleoclimatic utility and the reproducibility of each tracer at this site. Comparison to a Porites record from the same location allows for further evaluation of Diploastrea as a paleoclimatic archive. We demonstrate that Diploastrea's septal and columellar material yield similar d 18 O-SST relationships and that despite some sacrifice of the seasonal d 18 O amplitude, bulk sampling of either region is satisfactory for resolving interannual and lower frequency modes of climatic variability. Therefore paleoclimate reconstructions employing either a septal or columellar sampling regime of this genus may be useful at filling in spatial and temporal sampling gaps which currently hinder the reconstruction of long-term changes in major climate fields in the western Pacific.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.