Browsing by Author "Dee, Sylvia"

Now showing 1 - 11 of 11
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    A Proxy System Modeling Approach to Combining Tree-Ring and Sediment-Based Paleotempestological Records
    (Wiley, 2024) Wallace, Elizabeth J.; Dee, Sylvia; Bregy, Joshua; Emanuel, Kerry A.
    The short and biased observational record of tropical cyclones (TCs) limits scientific understanding of how these destructive storms respond to climate forcing. Paleohurricane records use natural archives (tree rings, coarse-grained sediment) to reconstruct TC properties (frequency and intensity of rainfall, wind) over the past few hundreds to thousands of years. However, different sensitivities and sampling biases in the various paleohurricane proxies restrict our ability to compile these records into regional or basin-scale TC estimates. Here we test how well pseudo tree-ring records of paleohurricanes capture TC rainfall and occurrence. Using a large set of statistically downscaled storms forced with the Max Planck Institute (MPI-ESM-P) model as boundary conditions for the past millennium, we generate a 1000-member ensemble of pseudo tree-ring records of latewood width from southern Mississippi using a Poisson process-based random draw. Pseudo records convert synthetic TC rainfall into latewood width using a previously published statistical calibration and seasonal sensitivity. We show that fourth quantile thresholds applied to pseudo latewood data successfully identify years with TC strikes. Comparing pseudo tree-ring records with pseudo sediment records from the Gulf Coast indicates promise in combining proxies sensitive to TC rainfall with proxies sensitive to storm overwash. Sediment records that are sensitive to lower intensity storms (≥Saffir Simpson Category 1) are more compatible with tree-ring records, suggesting a need for more of these low intensity threshold records in the Gulf to facilitate future multi-proxy efforts to reconstruct past TC properties.
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    Changes in Atlantic Tropical Cyclones and the Bermuda High: Clues from the Last Millennium to Inform the Future
    (Rice University, 2023) Pitchon, Emilia; Dee, Sylvia
    Current anthropogenic climate change is expected to increase hurricane intensity, with stronger winds, higher rainfall, and increased flooding, all of which pose a major threat to coastal communities. However, climate models vary in their predictions of how climate change will impact hurricane frequency and tracks, and 20th century data sources are limited given the brevity of the satellite era. To address this knowledge gap, we study the strength and position of the Bermuda High and how it has changed over the past millennium. The Bermuda High is a semipermanent high-pressure system over the Atlantic Ocean which impacts hurricane tracks and steering currents. To study its behavior, we evaluate two climate products with sea level pressure data spanning the past 1000 years: the Last Millennium Reanalysis and the Community Earth System Model. We compare various Bermuda High indices (BHI) as defined by previous studies, representing different measures of position and strength of the high-pressure system. Maps of sea level pressure anomalies and hurricane tracks are generated during years with high vs. low BHI values. This allows us to better understand the relationship between the Bermuda High and hurricane characteristics over the last 1000 years, providing important context for the future. This work is critical to better constrain hurricane risks under anthropogenic climate change and may help protect the people and environments at risk.
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    Characterizing the El Niño-Southern Oscillation and its North American Teleconnections over the Last Millennium
    (2024-08-05) Luo, Xinyue; Dee, Sylvia
    The El Niño-Southern Oscillation (ENSO) dominates interannual climate variability in the tropical Pacific, with sea surface temperatures (SSTs) oscillating between cooler (La Niña) and warmer (El Niño) conditions. ENSO affects midlatitude weather and climate through reorganizing winds and circulation, shifts known as teleconnections for their far-reaching impacts. ENSO-induced teleconnections were once considered stationary and predictable, but observations show they differ widely between individual events. Internal climate variability produces distinct ENSO events with diverse amplitudes, SST patterns, and ocean-atmosphere feedbacks, causing large uncertainties in ENSO rainfall prediction. Moreover, limited instrumental observations, based on fewer than 50 ENSO events in the 20th century, are insufficient to characterize ENSO complexity and its teleconnections. This thesis uses highly-resolved paleoclimate data spanning the Last Millennium (LM), including climate models and paleoclimate data assimilation (DA) techniques, to expand 20th-century data by thousands of years. We examine the changes in ENSO characteristics and North American hydroclimate teleconnections over the LM, investigating rainfall shifts due to background temperature changes and ENSO teleconnection variability on decadal-to-centennial timescales. We first evaluate ENSO diversity and North American teleconnections over the LM. DA products show intensified Eastern Pacific El Niño in the 20th century, but no change in Central Pacific El Niño frequency. Shifts in El Niño hydroclimate teleconnections over North America are variable in magnitude but consistent in sign over the LM. Then, we present a regional focus on Mississippi River basin hydroclimate extremes, revealing that strong El Niño-like warming over the tropical Pacific and North Atlantic SST patterns jointly contribute to wet extremes across the basin and its major tributaries. Finally, we assess the stationarity of North American hydroclimate responses to various climate modes, finding that teleconnections vary on multi-decadal and multi-centennial timescales throughout the LM, driven partly by predictable SST patterns. Natural variability in rainfall patterns characterized in this work will compound with global warming to shape 21st-century conditions. This research provides new insights from paleoclimate data, expanding statistical constraints on the stationarity and predictability of ENSO impacts, and serves as a last-millennium benchmark for climate prediction and water risk assessments under the 21st-century anthropogenic warming.
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    Convergent soft tissue structure drives patterns of skull shape evolution
    (2024-09-24) West, JoJo; Evans, Kory; Dee, Sylvia; Egan, Scott; Saltz, Julia
    Key innovations are thought to promote the diversification of lineages by allowing them to exploit their environment in a previously unexplored way. The more popular interpretation, defined by a trait that provides access to novel resources, excludes innovations which merely increase the rate of diversification or levels of phenotypic diversity. Restricted gill openings (RGOs) are a modification of the teleost branchiostegal membrane which results in smaller gill openings, which may free up the evolution of bones and tissues that were previously constrained by gill size and shape. Here we use 3D geometric morphometrics and phylogenetic comparative methods to investigate the influence of branchiostegal membrane state on skull shape evolution in Actinopterygii. We find higher rates of evolution in lineages with RGO than other gill membrane states, and varying levels of disparity across states.
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    The dynamics of warming during the last deglaciation in high-elevation regions of Eastern Equatorial Africa
    (Elsevier, 2022) Garelick, Sloane; Russell, James; Richards, Adin; Smith, Jamila; Kelly, Meredith; Anderson, Nathan; Jackson, Margaret S.; Doughty, Alice; Nakileza, Bob; Ivory, Sarah; Dee, Sylvia; Marshall, Charlie
    Tropical mountain environments, such as the Rwenzori Mountains in equatorial Africa, are thought to be particularly sensitive to climate change. Ongoing warming in the Rwenzori is impacting local environments and communities through glacial retreat, fires, and flooding. Paleoclimate reconstructions from elsewhere in Africa suggest considerable warming accompanied glacier retreat during the last glacial termination, from ∼21 thousand years before present (ka) through the early to mid-Holocene. Quantifying these changes has been difficult but could help to assess future impacts in the Rwenzori. Here, we present a ∼21 thousand-year (kyr) temperature reconstruction based on the relative abundance of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from Lake Mahoma (2,990 m above sea level; m asl) in the Rwenzori Mountains, Uganda. Our record, paired with existing Rwenzori glacial moraine 10Be exposure ages, suggests that deglacial warming and glacial retreat began by ∼20 ka and accelerated at ∼18–18.5 ka. The timing of the onset of rapid warming matches the timing of the post-glacial rise in radiative forcing from atmospheric greenhouse gases (GHGs) from Antarctic ice cores (Brook et al., 1996; Marcott et al., 2014; Monnin et al., 2004; Schilt et al., 2010). Our temperature reconstruction registers ∼4.9 °C warming from the Last Glacial Maximum (LGM) to the late Holocene. This increase is larger than the average ∼2-4 °C warming observed in records from lower elevation sites in tropical East Africa, but similar to that observed at other high-elevation sites in this region. The increased warming at higher elevations thus confirms that the temperature lapse rate steepened during the LGM over this region. Our results also indicate ∼3 °C of warming during the mid-Holocene relative to the late Holocene. This suggests that the freezing-level height rose above Rwenzori summit elevations at that time, likely causing complete deglaciation of the Rwenzori Mountains from ∼5 to 7 ka. The mid-Holocene is thus a potential analog for the glacial and environmental changes that these mountains are likely to experience in the coming decades. Overall, the timing and magnitude of temperature change observed in our record has important implications for climate model projections of future warming in tropical Africa.
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    La Niña-driven flooding in the Indo-Pacific warm pool during the past millennium
    (Elsevier, 2019) Rodysill, Jessica R.; Russell, James M.; Vuille, Mathias; Dee, Sylvia; Lunghino, Brent; Bijaksana, Satria
    Extreme precipitation events are one of the most consequential components of climate change for society. The El Niño-Southern Oscillation (ENSO) is the dominant mode of precipitation variability in the tropics and causes severe flooding and drought in many socioeconomically vulnerable regions. It remains unclear how tropical rainfall extremes and ENSO are changing in response to anthropogenic forcing, demanding that we investigate the relationships between precipitation, ENSO, and external forcing in the past. Lake sediment records have provided benchmark records of extreme flood events from the eastern tropical Pacific, where paleofloods have been interpreted to reflect El Niño events during the last millennium. However, the connections between flooding and ENSO variability in this region are uncertain, and the eastern Pacific can only capture precipitation events driven by El Niño, not La Niña. Thus, it is unclear how the ENSO system and tropical rainfall extremes have changed in the recent past. Here, we reconstruct flood events during the past millennium using a lake sediment record from East Java, Indonesia, which can provide insight into flooding driven by La Niña. We detect flood frequency variations in the western tropical Pacific that are highly coherent with records from the eastern part of the basin over the past millennium. Our findings demonstrate that heavy rainfall and flooding occurs more frequently on both sides of the tropical Pacific during periods of warmer Northern Hemisphere mean temperatures, implying that ENSO-driven rainfall extremes could intensify in the near future.
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    Last Millennium ENSO Diversity and North American Teleconnections: New Insights From Paleoclimate Data Assimilation
    (Wiley, 2022) Luo, Xinyue; Dee, Sylvia; Stevenson, Samantha; Okumura, Yuko; Steiger, Nathan; Parsons, Luke
    El Niño-Southern Oscillation (ENSO) variability affects year-to-year changes in North American hydroclimate. Extra-tropical teleconnections are not always consistent between El Niño events due to stochastic atmospheric variability and diverse sea surface temperature anomalies, making it difficult to quantify teleconnections using only instrumentally-based records. Here we use two paleoclimate data assimilation (DA) products spanning the Last Millennium (LM) to compare changes in amplitudes and frequencies of diverse El Niño events during the pre-industrial period and 20th century, and to assess the stationarity of their North American hydroclimate impacts on multi-decadal to centennial timescales. Using several definitions for Central Pacific (CP) and Eastern Pacific (EP) El Niño, we find a marked increase in 20th century EP El Niño intensity, but no significant changes in CP or EP El Niño frequencies in response to anthropogenic forcing. The associated hydroclimate anomalies indicate (a) dry conditions across the eastern-central and northwestern U.S. during CP El Niño and wetter conditions in the same regions during EP El Niño; (b) wet conditions over the southwestern U.S. for both El Niño types. The magnitude of regional hydroclimate teleconnections also shows large natural variability on multi-decadal to centennial timescales. However, when the entire LM is considered, mean hydroclimate anomalies in North America during CP or EP El Niño are consistent in terms of sign (wet vs. dry). Results are sensitive to proxy data and model priors used in DA products. Inconsistencies between El Niño classification methods underscore the need for improved ENSO diversity classification when assessing precipitation teleconnections.
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    A Pliocene Precipitation Isotope Proxy-Model Comparison Assessing the Hydrological Fingerprints of Sea Surface Temperature Gradients
    (Wiley, 2022) Knapp, Scott; Burls, Natalie J.; Dee, Sylvia; Feng, Ran; Feakins, Sarah J.; Bhattacharya, Tripti
    The Pliocene offers insights into future climate, with near-modern atmospheric pCO2 and global mean surface temperature estimated to be 3–4°C above pre-industrial. However, the hydrological response differs between future global warming and early Pliocene climate model simulations. This discrepancy results from the use of reduced meridional and zonal sea surface temperature (SST) gradients, based on foraminiferal Mg/Ca and Alkenone proxy evidence, to force the early Pliocene simulation. Subsequent, SST reconstructions based on the organic proxy TEX86, have found warmer temperatures in the warm pool, bringing the magnitude of the gradient reductions into dispute. We design an independent test of Pliocene SST scenarios and their hydrological cycle “fingerprints.” We use an isotope-enabled General Circulation Model, iCAM5, to model the distribution of water isotopes in precipitation in response to four climatological SST and sea-ice fields representing modern, abrupt 4 × CO2, late Pliocene and early Pliocene climates. We conduct a proxy-model comparison with all the available precipitation isotope proxy data, and we identify target regions that carry precipitation isotopic fingerprints of SST gradients as priorities for additional proxy reconstructions. We identify two regions with distinct precipitation isotope (D/H) fingerprints resulting from reduced SST gradients: the Maritime Continent (D-enriched due to reduced convective rainfall) and the Sahel (wetter, more deep convection, D-depleted). The proxy-model comparison using available plant wax reconstructions, mostly from Africa, is promising but inconclusive. Additional proxy reconstructions are needed in both target regions and in much of the world for significant tests of SST scenarios and dynamical linkages to the hydrological cycle.
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    Reconstructing Holocene temperatures in time and space using paleoclimate data assimilation
    (Copernicus Publications, 2022) Erb, Michael P.; McKay, Nicholas P.; Steiger, Nathan; Dee, Sylvia; Hancock, Chris; Ivanovic, Ruza F.; Gregoire, Lauren J.; Valdes, Paul
    Paleoclimatic records provide valuable information about Holocene climate, revealing aspects of climate variability for a multitude of sites around the world. However, such data also possess limitations. Proxy networks are spatially uneven, seasonally biased, uncertain in time, and present a variety of challenges when used in concert to illustrate the complex variations of past climate. Paleoclimatic data assimilation provides one approach to reconstructing past climate that can account for the diverse nature of proxy records while maintaining the physics-based covariance structures simulated by climate models. Here, we use paleoclimate data assimilation to create a spatially complete reconstruction of temperature over the past 12 000 years using proxy data from the Temperature 12k database and output from transient climate model simulations. Following the last glacial period, the reconstruction shows Holocene temperatures warming to a peak near 6400 years ago followed by a slow cooling toward the present day, supporting a mid-Holocene which is at least as warm as the preindustrial. Sensitivity tests show that if proxies have an overlooked summer bias, some apparent mid-Holocene warmth could actually represent summer trends rather than annual mean trends. Regardless, the potential effects of proxy seasonal biases are insufficient to align the reconstructed global mean temperature with the warming trends seen in transient model simulations.
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    Tracking Shallow Convective Mixing and Its Influence on Low-Level Clouds With Stable Water Isotopes in Vapor
    (Wiley, 2022) Hu, Jun; Bailey, Adriana; Nusbaumer, Jesse; Dee, Sylvia; Sasser,Christiana; Worden, John
    Low-cloud feedbacks contribute large uncertainties to climate projections and estimated climate sensitivity. A key physical process modulating low-cloud feedbacks is shallow convective mixing between the boundary layer and the free troposphere. However, there are challenges in acquiring observational constraints of shallow convective mixing with global coverage. To this end, we propose a novel approach to constraining convective mixing using stable water vapor isotope profiles from satellite retrievals. We demonstrate that the vertical gradient of water vapor δD between the boundary layer and free troposphere can be used to track shallow convective mixing. Analyzing isotopes in water vapor alongside low-cloud properties from satellite retrievals, we find that low-cloud fraction appears largely insensitive to convective mixing in shallow cumulus regions. Our results also suggest that strong shallow convective mixing is associated with the moistening of the free troposphere. The new estimate of shallow convective mixing and its relationship with low-cloud properties offers a potential constraint on simulations of low-cloud feedbacks and estimates of climate sensitivity.
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    Tropical Pacific Modulation of the Asian Summer Monsoon Over the Last Millennium in Paleoclimate Data Assimilation Reconstructions
    (Wiley, 2023) Hu, Jun; Dee, Sylvia; Parajuli, Grant; Thirumalai, Kaustubh
    Large uncertainties exist in climate model projections of the Asian summer monsoon (ASM). The El Niño-Southern Oscillation (ENSO) is an important modulator of the ASM, but the ENSO-ASM teleconnection is not stationary. Furthermore, teleconnections between ENSO and the East Asian versus South Asian subcomponents of the ASM exhibit distinct characteristics. Therefore, understanding the variability of the ENSO-ASM teleconnection is critical for anticipating future variations in ASM intensity. To this end, we here use paleoclimate records to extend temporal coverage beyond the instrumental era by millennia. Recently, data assimilation techniques have been applied for the last millennium, which facilitates physically consistent, globally gridded climate reconstructions informed by paleoclimate observations. We use these novel data assimilation products to investigate variations in the ENSO-ASM relationship over the last 1,000 years. We find that correlations between ENSO and ASM indices are mostly negative in the last millennium, suggesting that strong ASM years are often associated with La Niña events. During periods of weak correlations between ENSO and the East Asian summer monsoon, we observe an El Niño-like sea surface temperature (SST) pattern in the Pacific. Additionally, SST patterns associated with periods of weak correlations between ENSO and South Asian summer monsoon rainfall are not consistent among data assimilation products. This underscores the importance of developing more precipitation-sensitive paleoclimate proxies in the Indian subcontinental realm over the last millennium. Our study serves as a baseline for future appraisals of paleoclimate assimilation products and an example of informing our understanding of decadal-scale ENSO-ASM teleconnection variability using paleoclimate data sets.