Publications

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2019
Sanchez, SC, Amaya DJ, Miller AJ, Xie SP, Charles CD.  2019.  The Pacific Meridional Mode over the last millennium. Climate Dynamics. 53:3547-3560.   10.1007/s00382-019-04740-1   AbstractWebsite

The Pacific Meridional Mode, a coupled ocean-atmospheric interaction responsible for propagating subtropical anomalies to the tropics via thermodynamic mechanisms, features prominently in discussions of the response of climate variability to climate change. However, it is presently unclear how and why the variance in PMM might change, or even if greenhouse gas forcing might lead to heightened activity. Here, PMM variance over the last millennium is assessed in the Community Earth System Model Last Millennium Ensemble (LME). The model reproduces the main spatial characteristics of the PMM in the modern ocean in agreement with observations. With this basis, we assess the magnitude of the PMM variance over the past millennium, subject to forcing from a variety of sources. Internal (unforced) variability dominates the PMM variance in the LME, but prolonged periods of strong or weak PMM variance are found to be associated with characteristic spatial patterns, consistent across ensemble members and forcing experiments. The pattern of strong PMM variance features a cooler north Pacific, weaker Walker circulation, and a southward-shifted ITCZ. Comparison with a slab ocean model suggests that equatorial ocean dynamics are necessary to sustain the statistically significant multidecadal variability. With respect to the last millennium, present greenhouse forcing does not promote exceptional PMM variance. However, the PMM variability projected in the RCP8.5 scenario exceeds the thresholds expressed with the forcings applied over the Last Millennium. Aside from multidecadal variability, the model simulations also bear on ENSO variability and the sensitivity of climate variability to external forcing.

2013
Roach, LD, Charles CD, Field DB, Guilderson TP.  2013.  Foraminiferal radiocarbon record of northeast Pacific decadal subsurface variability. Journal of Geophysical Research-Oceans. 118:4317-4333.   10.1002/jgrc.20274   AbstractWebsite

The decadal dynamics of the subsurface North Pacific Ocean are largely inaccessible beyond sparse instrumental observations spanning the last 20 years. Here we present a approximate to 200 year long record of benthic foraminiferal radiocarbon (C-14), extracted at biennial resolution from the annually laminated sediments at the Santa Barbara Basin (SBB) depocenter (approximate to 600 m). The close match between core top benthic foraminiferal C-14 values and the C-14 of seawater dissolved inorganic carbon (DIC) suggests that benthic foraminifera faithfully capture the bottom water radiocarbon concentrations, as opposed to that of the deeper (>0.5 cm) sediment porewater zone. The full time series of benthic foraminiferal C-14 displays significant variability on decadal timescales, with excursions on the order of 40. These excursions are overprinted by a unidirectional trend over the late 20th century that likely reflects the sedimentary incorporation of bomb radiocarbon (via remineralized particulate organic carbon). We isolate this trend by means of a one-dimensional oxidation model, which considers the possible contribution of remineralized particles to the total ambient carbon pool. This oxidation model also considers the possible influence of carbon with a variety of sources (ages). Though variable oxidation of preaged carbon could exert a strong influence on benthic foraminiferal radiocarbon variability, the totality of evidence points to the vertical density structure along the Southern California Margin (SCM) as the primary driver of the SBB benthic foraminiferal C-14 record. For example, intervals characterized by significantly lower C-14 values correspond to periods of enhanced upwelling and subsurface equatorward flow along the SCM.

Carilli, JE, Charles CD, Garren M, McField M, Norris RD.  2013.  Baseline shifts in coral skeletal oxygen isotopic composition: a signature of symbiont shuffling? Coral Reefs. 32:559-571.   10.1007/s00338-012-1004-y   AbstractWebsite

Decades-long records of the stable isotopic composition of coral skeletal cores were analyzed from four sites on the Mesoamerican Reef. Two of the sites exhibited baseline shifts in oxygen isotopic composition after known coral bleaching events. Changes in pH at the calcification site caused by a change in the associated symbiont community are invoked to explain the observed shift in the isotopic composition. To test the hypothesis that changes in symbiont clade could affect skeletal chemistry, additional coral samples were collected from Belize for paired Symbiodinium identification and skeletal stable isotopic analysis. We found some evidence that skeletal stable isotopic composition may be affected by symbiont clade and suggest this is an important topic for future investigation. If different Symbiodinium clades leave consistent signatures in skeletal geochemical composition, the signature will provide a method to quantify past symbiont shuffling events, important for understanding how corals are likely to respond to climate change.

2010
Zaunbrecher, LK, Cobb KM, Beck JW, Charles CD, Druffel ERM, Fairbanks RG, Griffin S, Sayani HR.  2010.  Coral records of central tropical Pacific radiocarbon variability during the last millennium. Paleoceanography. 25   10.1029/2009pa001788   AbstractWebsite

The relationship between decadal to centennial changes in ocean circulation and climate is difficult to discern using the sparse and discontinuous instrumental record of climate and, as such, represents a large uncertainty in coupled ocean-atmosphere general circulation models. We present new modern and fossil coral radiocarbon (Delta(14)C) records from Palmyra (6 degrees N, 162 degrees W) and Christmas (2 degrees N, 157 degrees W) islands to constrain central tropical Pacific ocean circulation changes during the last millennium. Seasonally to annually resolved coral Delta(14)C measurements from the 10th, 12th-17th, and 20th centuries do not contain significant interannual to decadal-scale variations, despite large changes in coral delta(18)O on these timescales. A centennial-scale increase in coral radiocarbon from the Medieval Climate Anomaly (similar to 900-1200 AD) to the Little Ice Age (similar to 1500-1800) can be largely explained by changes in the atmospheric Delta(14)C, as determined with a box model of Palmyra mixed layer Delta(14)C. However, large 12th century depletions in Palmyra coral Delta(14)C may reflect as much as a 100% increase in upwelling rates and/or a significant decrease in the Delta(14)C of higher-latitude source waters reaching the equatorial Pacific during this time. SEM photos reveal evidence for minor dissolution and addition of secondary aragonite in the fossil corals, but our results suggest that coral Delta(14)C is only compromised after moderate to severe diagenesis for these relatively young fossil corals.

2009
Nurhati, IS, Cobb KM, Charles CD, Dunbar RB.  2009.  Late 20th century warming and freshening in the central tropical Pacific. Geophysical Research Letters. 36   10.1029/2009gl040270   AbstractWebsite

Global climate models and analyses of instrumental datasets provide a wide range of scenarios for future tropical Pacific climate change, limiting the accuracy of regional climate projections. Coral records provide continuous reconstructions of tropical Pacific climate trends that are difficult to quantify using the short, sparse instrumental datasets available from the tropical Pacific. Here, we present coral-based reconstructions of late 20th century sea-surface temperature and salinity trends from several islands in the central tropical Pacific. The coral data reveal warming trends that increase towards the equator, implying a decrease in equatorial upwelling in the last decades. Seawater freshening trends on the southern edge of the Inter-Tropical Convergence Zone suggest a strengthening and/or an equatorward shift of the convergence zone. Together, the new coral records support a late 20th century trend towards "El Nino-like" conditions in the tropical Pacific, in line with the majority of coupled global climate model projections. Citation: Nurhati, I. S., K. M. Cobb, C. D. Charles, and R. B. Dunbar (2009), Late 20th century warming and freshening in the central tropical Pacific, Geophys. Res. Lett., 36, L21606, doi:10.1029/2009GL040270.

2006
Field, DB, Baumgartner TR, Charles CD, Ferreira-Bartrina V, Ohman MD.  2006.  Planktonic foraminifera of the California Current reflect 20th-century warming. Science. 311:63-66.   10.1126/science.1116220   AbstractWebsite

It is currently unclear whether observed pelagic ecosystem responses to ocean warming, such as a mid-1970s change in the eastern North Pacific, depart from typical ocean variability. We report variations in planktonic foraminifera from varved sediments off southern California spanning the past 1400 years. Increasing abundances of tropical/subtropical species throughout the 20th century reflect a warming trend superimposed on decadal-scale fluctuations. Decreasing abundances of temperate/subpolar species in the late 20th century indicate a deep, penetrative warming not observed in previous centuries. These results imply that 20th-century warming, apparently anthropogenic, has already affected lower trophic levels of the California Current.

2002
Perks, HM, Charles CD, Keeling RF.  2002.  Precessionally forced productivity variations across the equatorial Pacific. Paleoceanography. 17   10.1029/2000pa000603   AbstractWebsite

[1] Measurements of combustion oxygen demand (COD) in two sediment cores provide a record of paleoproductivity driven by surface-ocean dynamics in the equatorial eastern and western Pacific for the past 400,000 years. The COD time series are well correlated with each other over this time span and show pronounced precessionally forced peaks of higher productivity during globally colder periods. The phase of this signal in the two cores is identical, to within chronological uncertainties, suggesting a common insolation forcing mechanism for the upper ocean across the equatorial Pacific. COD is also in phase with the precessionally forced component of global ice volume, as indicated by oxygen isotopes, and with atmospheric methane in the Vostok ice core. These relationships imply that the COD relative paleoproductivity index provides an important diagnostic measure of the mechanisms of tropical ocean dynamics and climate change.

2001
Hodell, DA, Kanfoush SL, Shemesh A, Crosta X, Charles CD, Guilderson TP.  2001.  Abrupt cooling of Antarctic surface waters and sea ice expansion in the South Atlantic sector of the Southern Ocean at 5000 cal yr B.P. Quaternary Research. 56:191-198.   10.1006/qres.2001.2252   AbstractWebsite

Antarctic surface waters were warm and ice free between 10,000 and 5000 cal yr B.P., as judged from ice-rafted debris and microfossils in a piston core at 53 degreesS in the South Atlantic. This evidence shows that about 5000 cal yr B.P., sea surface temperatures cooled, sea ice advanced, and the delivery of ice-rafted detritus (IRD) to the subantarctic South Atlantic increased abruptly. These changes mark the end of the Hypsithermal and onset of Neoglacial conditions. They coincide with an early Neoglacial advance of mountain glaciers in South America and New Zealand between 5400 and 4900 cal yr B.P., rapid middle Holocene climate changes inferred from the Taylor Dome Ice Core (Antarctica), cooling and increased IRD in the North Atlantic, and the end of the African humid period. The near synchrony and abruptness of all these climate changes suggest links among the tropics and both poles that involved nonlinear response to gradual changes in Northern Hemisphere insolation. Sea ice expansion in the Southern Ocean may have provided positive feedback that hastened the end of the Hypsithermal and African humid periods in the middle Holocene. (C) 2001 University of Washington.

2000
Moore, MD, Charles CD, Rubenstone JL, Fairbanks RG.  2000.  U/Th-dated sclerosponges from the Indonesian Seaway record subsurface adjustments to west Pacific winds. Paleoceanography. 15:404-416.   10.1029/1999pa000396   AbstractWebsite

Stable isotope records from sclerosponges collected at 10-20 m depth in the Indonesian Seaway and Solomon Islands are particularly well suited for reconstructing century-scale trends in ambient temperature variability and the oceanic uptake of fossil fuel carbon. Basal U/Th dates demonstrate that the sclerosponges analyzed are 85-100 years old. Isotopic records from the Indonesian specimens suggest a strong subsurface cooling over the past 20 years that is not manifested in either surface instrumental or shallower coral proxy records. However, analysis of observed subsurface temperatures in Indonesia, observed winds in the west Pacific, and simulated subsurface temperatures from a steady state general circulation model hindcast forced by observed winds combine to suggest that thermocline adjustments could account for at least part of the recent cooling inferred from the Indonesian sclerosponges. If so, the sclerosponge data suggests that, on average, the west Pacific thermocline has shoaled significantly over at least the past 2 decades.

1997
Fairbanks, RG, Evans MN, Rubenstone JL, Mortlock RA, Broad K, Moore MD, Charles CD.  1997.  Evaluating climate indices and their geochemical proxies measured in corals. Coral Reefs. 16:S93-S100.   10.1007/s003380050245   AbstractWebsite

Standard ocean/climate indices such as the Nino-3 sea surface temperature (SST) index, based on sparse instrumental data, and atmospheric indices such as the Southern Oscillation Index (SOI), may now be substituted and/or extended by coral-based indices. Several elements or their isotopes are incorporated in coral aragonitic skeletons at predictable concentrations, some of which are temperature or salinity dependent. The availability of century-old corals, at key oceanographic sites, permits the establishment of a network of proxy climate indices.