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  • 1
  • 2
    Publication Date: 2019-08-01
    Print ISSN: 2572-4517
    Electronic ISSN: 2572-4525
    Topics: Geosciences
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  • 3
    Publication Date: 2020-11-24
    Description: Scientific understanding of low-frequency tropical Pacific variability, especially responses to perturbations in radiative forcing, suffers from short observational records, sparse proxy networks, and bias in model simulations. Here, we combine the strengths of proxies and models through coral-based paleoclimate data assimilation. We combine coral archives (δ18O, Sr/Ca) with the dynamics, spatial teleconnections, and intervariable relationships of the CMIP5/PMIP3 Past1000 experiments using the Last Millennium Reanalysis data assimilation framework. This analysis creates skillful reconstructions of tropical Pacific temperatures over the observational era. However, during the period of intense volcanism in the early 19th century, southwestern Pacific corals produce El Niño Southern Oscillation (ENSO) reconstructions that are of opposite sign from those from eastern Pacific corals and tree ring records. We systematically evaluate the source of this discrepancy using 1) single-proxy experiments, 2) varied proxy system models (PSMs), and 3) diverse covariance patterns from the Past1000 simulations. We find that individual proxy records and coral PSMs do not significantly contribute to the discrepancy. However, following major eruptions, the southwestern Pacific corals locally record more persistent cold anomalies than found in the Past1000 experiments and canonical ENSO teleconnections to the southwest Pacific strongly control the reconstruction response. Furthermore, using covariance patterns independent of ENSO yield reconstructions consistent with coral archives across the Pacific. These results show that model bias can strongly affect how proxy information is processed in paleoclimate data assimilation. As we illustrate here, model bias influences the magnitude and persistence of the response of the tropical Pacific to volcanic eruptions.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
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  • 4
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    Massachusetts Institute of Technology and Woods Hole Oceanographic Institution
    Publication Date: 2022-05-25
    Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2009
    Description: Estimates of natural climate variability during the past millennium provide a frame of reference in which to assess the significance of recent changes. This thesis investigates new methods of reconstructing low-latitude sea surface temperature (SST) and hydrography, and combines these methods with traditional techniques to improve the present understanding of western North Atlantic climate variability. A new strontium/calcium (Sr/Ca) - SST calibration is derived for Atlantic Montastrea corals. This calibration shows that Montastrea Sr/Ca is a promising SST proxy if the effect of coral growth is considered. Further analyses of coral growth using Computed Axial Tomography (CAT) imaging indicate growth in Siderastrea corals varies inversely with SST on interannual timescales. A 440-year reconstruction of low-latitude western North Atlantic SST based on this relationship suggests the largest cooling of the last few centuries occurred from ~1650-1730 A.D., and was ~1ºC cooler than today. Sporadic multidecadal variability in this record is inconsistent with evidence for a persistent 65-80 year North Atlantic SST oscillation. Volcanic and anthropogenic radiative forcing are identified as important sources of externally-forced SST variability, with the latter accounting for most of the 20th century warming trend. An 1800-year reconstruction of SST and hydrography near the Gulf Stream also suggests SSTs remained within about 1ºC of modern values. This cooling is small relative to other regional proxy records and may reflect the influence of internal oceanic and atmospheric circulation. Simulations with an atmospheric general circulation model (AGCM) indicate that the magnitude of cooling estimated by proxy records is consistent with tropical hydrologic proxy records.
    Description: Funding for this research was provided by a National Science Foundation Graduate Student Fellowship, National Science Foundation grants OCE-0402728, OCE-0623364, ATM-033746, the WHOI Ocean and Climate Change Institute, the WHOI Ocean Ventures Fund, the WHOI Ocean Life Institute, the MIT Student Assistance Fund, award number USA-0002, made by King Abdullah University of Science and Technology (KAUST), and the Inter-American Institute for Global Change Research.
    Keywords: Ocean-atmosphere interaction ; Climatic changes
    Repository Name: Woods Hole Open Access Server
    Type: Thesis
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  • 5
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 36 (2009): L11703, doi:10.1029/2009GL038677.
    Description: Proxy reconstructions and model simulations suggest that steeper interhemispheric sea surface temperature (SST) gradients lead to southerly Intertropical Convergence Zone (ITCZ) migrations during periods of North Atlantic cooling, the most recent of which was the Little Ice Age (LIA; ∼100–450 yBP). Evidence suggesting low-latitude Atlantic cooling during the LIA was relatively small (〈1°C) raises the possibility that the ITCZ may have responded to a hemispheric SST gradient originating in the extratropics. We use an atmospheric general circulation model (AGCM) to investigate the relative influence of low-latitude and extratropical SSTs on the meridional position of the ITCZ. Our results suggest that the ITCZ responds primarily to local, low-latitude SST anomalies and that small cool anomalies (〈0.5°C) can reproduce the LIA precipitation pattern suggested by paleoclimate proxies. Conversely, even large extratropical cooling does not significantly impact low-latitude hydrology in the absence of ocean-atmosphere interaction.
    Description: This work was supported by NSF grants OCE 0623364 and ATM 033746 as well as the student research fund of MIT’s Department of Earth, Atmospheric and Planetary Science.
    Keywords: Climate ; ITCZ ; Little Ice Age
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 6
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 23 (2008): PA3102, doi:10.1029/2007PA001572.
    Description: We analyzed strontium/calcium ratios (Sr/Ca) in four colonies of the Atlantic coral genus Montastrea with growth rates ranging from 2.3 to 12.6 mm a−1. Derived Sr/Ca–sea surface temperature (SST) calibrations exhibit significant differences among the four colonies that cannot be explained by variations in SST or seawater Sr/Ca. For a single coral Sr/Ca ratio of 8.8 mmol mol−1, the four calibrations predict SSTs ranging from 24.0° to 30.9°C. We find that differences in the Sr/Ca–SST relationships are correlated systematically with the average annual extension rate (ext) of each colony such that Sr/Ca (mmol mol−1) = 11.82 (±0.13) – 0.058 (±0.004) × ext (mm a−1) – 0.092 (±0.005) × SST (°C). This observation is consistent with previous reports of a link between coral Sr/Ca and growth rate. Verification of our growth-dependent Sr/Ca–SST calibration using a coral excluded from the calibration reconstructs the mean and seasonal amplitude of the actual recorded SST to within 0.3°C. Applying a traditional, nongrowth-dependent Sr/Ca–SST calibration derived from a modern Montastrea to the Sr/Ca ratios of a conspecific coral that grew during the early Little Ice Age (LIA) (400 years B.P.) suggests that Caribbean SSTs were 〉5°C cooler than today. Conversely, application of our growth-dependent Sr/Ca–SST calibration to Sr/Ca ratios derived from the LIA coral indicates that SSTs during the 5-year period analyzed were within error (±1.4°C) of modern values.
    Description: This work was funded by National Science Foundation (NSF) grant OCE- 0402728, the WHOI Ocean and Climate Change Institute, and an NSF Graduate Student Fellowship.
    Keywords: Coral ; Strontium/calcium ; Growth rate
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 7
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 26 (2011): PA2206, doi:10.1029/2010PA002038.
    Description: Western subtropical North Atlantic oceanic and atmospheric circulations connect tropical and subpolar climates. Variations in these circulations can generate regional climate anomalies that are not reflected in Northern Hemisphere averages. Assessing the significance of anthropogenic climate change at regional scales requires proxy records that allow recent trends to be interpreted in the context of long-term regional variability. We present reconstructions of Gulf Stream sea surface temperature (SST) and hydrographic variability during the past two millennia based on the magnesium/calcium ratio and oxygen isotopic composition of planktic foraminifera preserved in two western subtropical North Atlantic sediment cores. Reconstructed SST suggests low-frequency variability of ∼1°C during an interval that includes the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). A warm interval near 1250 A.D. is distinct from regional and hemispheric temperature, possibly reflecting regional variations in ocean-atmosphere heat flux associated with changes in atmospheric circulation (e.g., the North Atlantic Oscillation) or the Atlantic Meridional Overturning Circulation. Seawater δ 18O, which is marked by a fresher MCA and a more saline LIA, covaries with meridional migrations of the Atlantic Intertropical Convergence Zone. The northward advection of tropical salinity anomalies by mean surface currents provides a plausible mechanism linking Carolina Slope and tropical Atlantic hydrology.
    Description: This study was supported by the Woods Hole Oceanographic Institution’s Ocean and Climate Change Institute (OCCI) and by the National Science Foundation.
    Keywords: North Atlantic ; Regional paleoclimate ; LIA ; MCA ; NAO
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 8
    Publication Date: 2022-05-25
    Description: © The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Paleoceanography 30 (2015): 226–252, doi:10.1002/2014PA002717.
    Description: Most annually resolved climate reconstructions of the Common Era are based on terrestrial data, making it a challenge to independently assess how recent climate changes have affected the oceans. Here as part of the Past Global Changes Ocean2K project, we present four regionally calibrated and validated reconstructions of sea surface temperatures in the tropics, based on 57 published and publicly archived marine paleoclimate data sets derived exclusively from tropical coral archives. Validation exercises suggest that our reconstructions are interpretable for much of the past 400 years, depending on the availability of paleoclimate data within, and the reconstruction validation statistics for, each target region. Analysis of the trends in the data suggests that the Indian, western Pacific, and western Atlantic Ocean regions were cooling until modern warming began around the 1830s. The early 1800s were an exceptionally cool period in the Indo-Pacific region, likely due to multiple large tropical volcanic eruptions occurring in the early nineteenth century. Decadal-scale variability is a quasi-persistent feature of all basins. Twentieth century warming associated with greenhouse gas emissions is apparent in the Indian, West Pacific, and western Atlantic Oceans, but we find no evidence that either natural or anthropogenic forcings have altered El Niño–Southern Oscillation-related variance in tropical sea surface temperatures. Our marine-based regional paleoclimate reconstructions serve as benchmarks against which terrestrial reconstructions as well as climate model simulations can be compared and as a basis for studying the processes by which the tropical oceans mediate climate variability and change.
    Description: J.E.T. and K.J.A. acknowledge Woods Hole Oceanographic Institution for internal support. K.J.A. acknowledges the Frank and Lisina Hoch Endowed Fund at the Woods Hole Oceanographic Institution for support. N.J.A. is supported by an Australian Research Council QEII fellowship (DP110101161), and this research contributes to ARC Discovery Grant DP140102059. M.N.E. is supported by NSF/ATM0902794 and NSF/ATM0902715. J.Z. was supported by an Indian Ocean Marine Research Centre fellowship and an Honorary Research Fellowship by the University of the Witwatersrand. H.C.W. is supported by the Deutsche Forschungsgemeinschaft through DFG-Research Center/Cluster of Excellence “The Ocean in the Earth System” at the University of Bremen (MARUM Fellowship). C.G. acknowledges MARUM–Center for Marine Environmental Sciences for internal support. K.H.K. is supported by NOAA grant NA11OAR4310171.
    Keywords: Climate reconstruction ; Corals ; Paleoceanography ; Last millennium climate
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 9
    Publication Date: 2022-05-26
    Description: This paper is not subject to U.S. copyright. The definitive version was published in Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010): 299-310, doi:10.1016/j.palaeo.2010.08.009.
    Description: A new 2400-year paleoclimate reconstruction from Chesapeake Bay (CB) (eastern US) was compared to other paleoclimate records in the North Atlantic region to evaluate climate variability during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA). Using Mg/Ca ratios from ostracodes and oxygen isotopes from benthic foraminifera as proxies for temperature and precipitation-driven estuarine hydrography, results show that warmest temperatures in CB reached 16–17 °C between 600 and 950 CE (Common Era), centuries before the classic European Medieval Warm Period (950–1100 CE) and peak warming in the Nordic Seas (1000–1400 CE). A series of centennial warm/cool cycles began about 1000 CE with temperature minima of ~ 8 to 9 °C about 1150, 1350, and 1650–1800 CE, and intervening warm periods (14–15 °C) centered at 1200, 1400, 1500 and 1600 CE. Precipitation variability in the eastern US included multiple dry intervals from 600 to 1200 CE, which contrasts with wet medieval conditions in the Caribbean. The eastern US experienced a wet LIA between 1650 and 1800 CE when the Caribbean was relatively dry. Comparison of the CB record with other records shows that the MCA and LIA were characterized by regionally asynchronous warming and complex spatial patterns of precipitation, possibly related to ocean–atmosphere processes.
    Keywords: Medieval Climate Anomaly ; Little Ice Age ; Chesapeake Bay ; Holocene climate
    Repository Name: Woods Hole Open Access Server
    Type: Article
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