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ItemUncertainties in Projections of Tropical Precipitation and Atmospheric Circulation and Their Remote Impacts(Georgia Institute of Technology, 2023-12-10) Lu, KezhouMy doctoral work focuses on understanding anthropogenic responses of precipitation and atmospheric circulation by employing both statistical methods and climate models of varying levels of complexity. My research has two main goals: (1) to understand the forced response of tropical air-sea interactions across different time scales and their subtropical impact, and (2) to investigate the reasons underlying the uncertainties in climate models when simulating tropical and extra-tropical climate. My dissertation research comprises four individual projects. For my first project, I have explored the mechanism of how the Walker circulation (WC) responds to CO2 forcing across different time scales. The WC, a significant tropical atmospheric circulation spanning both horizontally and vertically, plays a crucial role in the tropical climate and is closely related to phenomena such as the Madden–Julian Oscillation and El Nino-Southern Oscillation. The prevailing consensus suggests that the long-term weakening of the WC is primarily driven by the sea surface temperature (SST) warming caused by increased greenhouse gases, while the fast response of the WC appears largely independent of changes in SST. However, my findings indicate that the air-sea interactions play a substantial role. By analyzing data output from Coupled Model Intercomparison Project Phase 5 (CMIP5) under abrupt4xCO2 scenarios, models with a stronger air-sea coupling in the equatorial Pacific are discovered to simulate an initial strengthening of the WC following the external forcing, which contrasts with the long-term response. Conversely, models characterized by weaker air-sea coupling simulate a monotonically weakening of the WC. My results suggest that the inter-model discrepancy in the WC changes is associated with then uncertainty in the fast component. My second project focuses on understanding the summer North Pacific subtropical high (NPSH). As part of the planetary wave system, the NPSH integrates both tropical and extra-tropical impacts on the monsoons and typhoons over East Asia and hydroclimate over California. Given its considerable socioeconomic significance, reliable future projections of the NPSH are crucial for preparing adaptation plans. However, state-of-the-art climate models exhibit diverging responses of the NPSH to anthropogenic CO2 forcing. This project has revealed that model variability in the future projection of the summer NPSH originates from both inter-model SST-driven and non-inter-model SST-driven uncertainties in the tropical precipitation. Specifically, I investigate the connection between the tropical precipitation and the NPSH by modifying the diabatic heating in both a baroclinic stationary wave model and a comprehensive climate model, i.e. the Community Atmospheric Model version 5 (CAM5). Drawing upon the knowledge acquired from the previous two chapters, my third project has explored how the tropical air-sea interaction and the summer NPSH are influenced by anthropogenic forcing, as well as their interplay. I have discovered that the inter-model spread in projecting the fast changes of the WC directly contributes to the inter-model spread in tropical SST responses. By analyzing CMIP5 and CMIP6 data under abrupt4xCO2 scenario, models with a stronger tropical equatorial Pacific air-sea coupling simulate a strengthening of the WC and a La Nina-like central Pacific cooling. And this La Nina-like SST anomaly induces anomalous tropical precipitation and further modulates the NPSH via the Matsuno-Gill wave response. During the collaboration with my advisor on the fourth project, we have found considerable inter-basin variations in the future projection of the tropical Hydrological sensitivity (HS) regardless of how SST warms. I have further demonstrated the remote impact of the inter-basin discrepancy in HS on land precipitation and surface temperature by understanding the corresponding tropical-extra-tropical teleconnections. Specifically, I have analyzed the atmospheric circulation response induced by tropical precipitation with and without inter-basin discrepancy in HS by conducting diabatic heating adjustment experiments in CAM5.
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ItemObservations of peroxyacyl nitrates in polluted and remote troposphere(Georgia Institute of Technology, 2023-08-01) Lee, Young RoEmissions of volatile organic compounds (VOCs) and their photooxidation with nitrogen oxides (NOx) play a significant role in atmospheric chemistry and have substantial effects on air quality. Understanding these processes in the ambient environments is a challenge, in part, due to uncertainties in emission sources and the complex chemical evolution of emissions. This dissertation leverages a comprehensive suite of ground-based and airborne observations to investigate the impacts of VOCs-NOx photochemistry on atmospheric trace gas compositions, both in heavily polluted and pristine environments. In particular, this work focuses on observations of peroxyacyl nitrates (PANs) to provide a detailed diagnosis of photochemistry in the regions discussed throughout the dissertation. East Asian countries such as South Korea and China have experienced severe air pollution problems. In this work, extensive observations of primary and secondary pollutants were conducted in two locations: a remote ground site in the Yellow River Delta, China, during the Ozone Photochemistry and Export from China Experiment (OPECE) in 2018, and a petrochemical producing region in South Korea during the Korea-United States Air quality (KORUS-AQ) campaign in 2016. Our findings during the field observations indicated that both regions are characterized by heterogeneous VOC composition with substantial emissions of alkenes and aromatics. Photooxidation of these VOCs led to efficient ozone production in a radical-limited environment. In addition, elevated levels of peroxyacetic nitric anhydride (PAN), as well as rarely measured homologs such as peroxybenzoic nitric anhydride (PBzN) and peroxyacrylic nitric anhydride (APAN), illustrated the unique atmospheric chemistry in East Asian environments. This dissertation presents global-scale airborne observations of PAN from the NASA DC-8 research aircraft during the ATmospheric Tomography (ATom) campaign. The focus of this investigation was on PAN observations in remote tropospheric regions such as over the Pacific and Atlantic Oceans. We found that PAN over remote oceans is significantly influenced by relatively simple sources including anthropogenic and biomass burning emissions. Notably, biomass burning has a dominant and persistent impact on the global distribution of PAN. Based on a diagnostic evaluation using observations, this work suggests that accurate model treatment of biomass burning can improve prediction of PAN in the remote troposphere. Lastly, the characterization of a low-activity 210Po ion source with an initial activity of 1.5 mCi was performed for use with iodide-chemical ionization mass spectrometry (I--CIMS). We demonstrated that the low activity source is a viable substitute of higher activity radioactive source, offering advantages in terms of reduced regulatory burden during storage and shipping. The performance of the low activity source is illustrated using airborne measurements of PANs during the ATom campaign.
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ItemSurface Gravity Waves in Global Climate Models: Development, Evaluation and Optimization(Georgia Institute of Technology, 2023-07-25) Ikuyajolu, Olawale JamesSurface gravity waves play a critical role in several processes at the air-sea interface, including mixing, coastal inundation, and surface fluxes. Yet wind–wave processes are usually excluded from Earth system models partly due to a lack of physical understanding and the high computational costs of spectral wave models. Most wave modeling studies utilize uncoupled short-term simulations and focus on the upper ocean. The impacts of wind-wave processes on coupled climate variability have yet to be thoroughly evaluated. This all underscores the need to advance surface gravity wave modeling frameworks within general circulation models (GCMs). Herein, the first half of this thesis partly addresses the high computational cost of running spectral wave models on a global grid. I identify the wave action source terms as the most computationally intensive part of the spectral wave model WAVEWATCH III (WW3), and then accelerate them on Graphics Processing Units (GPUs) using OpenACC. An average speedup of 1.4x was achieved, resulting in a reduction of 35-40% in runtime and resource usage. In the second half of this thesis, I incorporated a wave-state dependent bulk formula by fully coupling WW3 to the Energy Exascale Earth System Model (E3SM). Current state of the science GCM bulk parameterizations estimate the sea-state roughness as a function of surface wind speed, ignoring wave effects. The newly implemented parameterization includes two primary wave effects: first, a wave-state dependent surface roughness computed by WW3; second, the alteration of momentum flux from the atmosphere to the ocean due to wave growth and dissipation. I conducted numerical experiments with this new parameterization to investigate the sensitivity of the mean climate and Madden-Julian Oscillation (MJO) to different bulk flux parameterizations and the role of waves in air-sea coupling. My results highlight that discrepancies between bulk algorithms have nonnegligible impacts on mean climate such as ocean heat content, sea-ice concentration and a 2℃ difference in sea surface temperature in the North Atlantic. Also, the proper treatment of air-sea coupling via the inclusion of wave-induced effects improves the simulation of MJO. Most importantly, the analysis emphasizes the importance of considering the role of waves in redistributing momentum flux between the atmosphere and the ocean, especially in coastal and high-latitude regions. This work is key to enhancing the capability of future GCMs to simulate coastal changes and extreme events.
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ItemDecreased Dissolved Oxygen Content of the Pacific Deep Water During the Last Glacial Maximum(Georgia Institute of Technology, 2023-05-05) Kim, GraceThe mechanisms responsible for lowering atmospheric CO2 levels during glaciation have yet to be constrained, but the deep ocean is the most likely reservoir of CO2 drawdown. Deep ocean carbon export and storage are suggested to have increased cyclically during glacial periods due to greater biological pump efficiency and ocean stratification, and poor ventilation. Increased respired carbon in the Eastern Equatorial Pacific Ocean (EEP) would be evident with depleted dissolved oxygen content, but there is insufficient paleo-oxygen data in this region. This study uses a benthic foraminifera Δδ13C proxy to provide quantitative assessments of changes in oxygen concentration between the Holocene and LGM. The proxy relies on the empirical relationship between bottom water oxygen and the carbon isotope gradient between the sediment-water interface and oxic-anoxic interface preserved in benthic foraminifera. Cibicidoides wuellerstorfi and Globobulimina spp. are benthic foraminifera that preferentially reside at these interfaces and record δ13C at equilibrium with bottom water and pore water dissolved inorganic carbon, respectively. The findings of this paper provided oxygen concentrations in the Holocene and LGM that indicate a more depleted bottom water oxygen content and higher mid-depth oxygen concentrations during the last glacial period. This suggests increased carbon storage, poorer ventilation, and greater water mass stratification and supports the respired carbon deepening hypothesis and corresponds to oxygen trends of qualitative paleo-oxygen proxies in the EEP.
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ItemAssessment of High Resolution Numerical Weather Prediction (NWP) Parameters and their Contribution to Weather Integration Prototype (WIP) Performance to Aid High Energy Laser (HEL) Testing(Georgia Institute of Technology, 2023-05-01) Murdock, Jordan N.Since it was first discussed in 1960, testing and development of high energy lasers (HELs) has only continued to increase in interest for the U.S. military. To yield the most effective engagements with these HEL systems, there are many parameters that must be evaluated including atmospheric variables. The Weather Integration Prototype (WIP) is an instrument and software suite that measures and ingests various atmospheric data to produce a HEL performance assessment. In this work a study was conducted to determine whether mesoscale, high resolution numerical weather Prediction (NWP) model data can more accurately predict observational atmospheric data reported to the WIP from field atmospheric sensors, relative to lower resolution NWP models. Data from the WIP testing conducted during March 2022 are used for this study. The WIP-reported observed atmospheric conditions include temperature, pressure, humidity (expressed in terms of relative humidity or dewpoint), wind speed and direction, aerosol particle counts, and optical turbulence measurements and NWP model data provide similar outputs. Both measured and NWP atmospheric data are then used as inputs to both the Laser Environmental Effects Definition and Reference (LEEDR) and High Energy Laser End-to-End Operational Simulation (HELEEOS) models to determine the accuracy of NWP model data and ultimately the effectiveness in modeling HEL performance. The WIP takes in field sensor data and ingests it to LEEDR and HELEEOS and then the values output from the WIP are compared to the Weather Research Forecast (WRF) high resolution model data to determine mesoscale NWP model data accuracy and both are compared to the HEL diagnostic measurements. The WRF model is the selected high-resolution NWP used in this study. In addition to WRF model data, climatology data that are utilized in both HELEEOS and LEEDR are also examined to determine which atmospheric modeling method would best forecast the observed atmospheric conditions, with an emphasis on optical turbulence. The main goals of this study are to 1) determine whether using mesoscale NWP model data is a more or equally reliable method of forecasting observed atmospheric conditions needed for HEL operations as compared to the WIP; 2) prove that both the WIP and NWP model can accurately predict optical turbulence values; and 3) evaluate if mesoscale NWP forecasts and local measurements can provide the optimal HEL performance assessment as compared to laser diagnostic measurements. This study demonstrates that the WIP can evolve with the addition of higher resolution NWP model data, and become a reliable method to determine HEL performance measurements. The results of this study support the capability of the WIP to provide, with reasonable accuracy, forecasted HEL performance assessments well prior to HEL execution.
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ItemThe Habitable Ocean World Box (HOWBox) Model: Coupling Europa's Thermal and Chemical Evolution(Georgia Institute of Technology, 2023-04-30) Spiers, Elizabeth MarieEuropa, a moon of Jupiter, has evidence for a liquid water ocean in contact with a rocky mantle beneath its icy surface. Due to the presence of this ocean, Europa has been a primary target for investigations of habitability within our solar system. Europa’s ocean has remained liquid, in part, due to tidal heating in the interior. However, this interior tidal heat production has likely varied and potentially even been oscillatory over Europa’s history due to evolving orbital eccentricity. Such thermal oscillations will affect Europa’s interior geochemistry, defining changes to chemical and energy inventories for putative biology. Therefore, Europa’s long-term habitability depends on both geochemical pathways and thermodynamics—modulated by tidal evolution—that define chemical and energy inventories. Here, we demonstrate how thermally-modulated volumetric changes to Europa’s ice shell, ocean, and fluid-accessible rocky mantle affect oceanic composition in Europa’s interior due to variation in salinity, reduced volatile flux from the seafloor, and oxidized material delivery from the ice shell. We find that Europa may have undergone major oxidation events in its past during periods of increased orbital eccentricity, with oxidant flux increasing by up to 10 orders of magnitude, affecting the composition of the ocean. We also find that stable hydrogen flux into the ocean from serpentinization is tied to the water activity of the ocean and pore fluids, which also vary with heat production. Periods of fluctuating heat production, driven by orbital dynamics, can result in substantial oscillations to hydrogen flux ranging from 100 -1013 moles of H2 per year, with the degree of variability controlled by the dissolved ion composition and concentration. These results suggest new ways that water activity and salinity of the ocean are important to stability of habitable conditions in ocean worlds. By demonstrating that Europa’s ocean composition will be sensitive to its orbital dynamics and the degree to which it undergoes oscillatory tidal heat production, we reinforce that Europa’s long-term habitability is directly tied to its thermal-orbital history.
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ItemAntarctic diatom frustules: XRF and XAS analysis of structural zinc incorporation and the implications for marine zinc cycling(Georgia Institute of Technology, 2023-04-30) Griffith, Erin KathleenOur understanding of the composition and uptake of zinc into Southern Ocean diatom frustules has been enhanced by X-ray fluorescence (XRF) microscopy and X-ray absorption spectroscopy (XAS). These findings potentially resolve decades of debate about the close correlation of zinc and silicate in the global ocean. In most of our sampled diatoms, there is a high correlation of zinc to silica within the diatom’s frustule, regardless of oceanic conditions. Consistent with this hypothesis is the enrichment of zinc in the silica frustule, with frustule Zn/Si ratios approximately 15 times higher than dissolved Zn/Si ratios in the surrounding water. Furthermore, frustule zinc occurs in a form similar to a zinc silicate or a zinc sulfide in all of the samples. The XRF and XAS results suggest a structural incorporation of zinc into the siliceous frustule, even if the precise reason is unknown. The low solubility of zinc silicates and zinc sulfides results in much of the incorporated zinc surviving transport through the global ocean with eventual burial in the sediments. The burial of zinc associated with silica frustules is a new, major, ocean zinc sink which adjusts the residence time of oceanic dissolved zinc to 5,200 years.
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ItemA Deep Look into Continental Tectonic Processes Using High-resolution Earthquake Catalogs(Georgia Institute of Technology, 2023-04-28) Gois Ferreira Gaspar Neves, Miguel JoaoAccording to the theory of plate tectonics, the Earth’s crust is formed by rigid blocks (tectonic plates) that move relative to each other along linear faults that bound the blocks. The model further assumes that deformation occurs mainly at plate boundaries. In the past decades the definition of plate boundaries has evolved, and it is now recognized that plate boundaries can also be broad areas of deformation, and slowly deforming regions in plate interiors can also generate significant and destructive earthquakes. But even at linear plate boundaries seismologists still struggle to understand the seismic cycle, and improvements in seismic monitoring also revealed different stress sources capable of interacting with faults and earthquakes, such as tidal stresses and stress variations generated by anthropogenic activities. The work presented here focuses on how we can systemically and efficiently compile high-resolution earthquake catalogs and use them to better understand earthquake sequences, the kinematics of faults, and stress interactions in different continental regions. Earthquake catalogs are one of the most important tools in seismology, but these catalogs are often incomplete at lower magnitudes, limiting the information available for understanding the sequence and faulting processes. To improve earthquake catalogs, the past decades have seen the development of different techniques to automatically analyze seismic data, such as matched filter and deep learning earthquake detectors. I first present a study where I use matched filter detection and cross-correlation derived differential travel times to study the 2004 M6 Parkfield, California, earthquake sequence along the San Andreas Fault. I improve the earthquake catalog by about 3 times the number of events listed in the Northern California Seismic Network catalog and use the new catalog to study the period prior to the 2004 mainshock and interactions with tidal stresses. No clear precursory signals to the 2004 mainshock are identified, but an increase in the seismic activity is observed in the creeping section of the San Andreas Fault (about 30 km northwest of the mainshock epicenter) in the weeks prior to the mainshock. This activity increase is also accompanied by a decrease in the b-value parameter in the Gutenberg-Richter relationship in the creeping section. These results suggest stress is increasingly released seismically in the creeping section, accompanied by a decreasing aseismic creeping rate before the mainshock occurrence. However, seismicity rates remain stable in the Parkfield section where the 2004 mainshock ruptured. The analysis of tidal stress variations in the Parkfield segment during the 2004 sequence also reveals that microearthquakes at Parkfield are modulated by tidal stresses, but with different impacts before and after the mainshock. I then describe a project using a deep learning earthquake detector in Iberia, a mostly slowly deforming region in Southwest Europe. I analyze 7 years of seismic data from 552 stations in Iberia to improve the quality of the regional earthquake catalog and gain new insights into the seismicity behavior in the region. I fine-tune a deep-learning earthquake detector and phase picker to analyze the Iberian datasets, using a small dataset of 28,622 waveforms from the region. Using the new phase picker, I compile an earthquake catalog with 56,354 events. Additional analysis to identify anthropogenic signals in the catalog reveals that most clusters in slowly deforming Iberia are connected to areas of anthropogenic signals, which suggests that induced activity is widespread in the region. Using the new catalog, I was also able to identify new lineaments in Iberia, likely illuminating new fault structures in that region. Lastly, combining matched filter detection and a deep learning detector, I study the August 9, 2020, Mw5.1 Sparta, North Carolina, earthquake sequence. This earthquake ruptured the uppermost crust near the town of Sparta and is likely the first reported surface-rupturing event in the Eastern United States. The new catalog reveals that the mainshock nucleated near an intersection point of two fault strands, a blind strike-slip fault where the rupture was possibly initiated, and a reverse fault associated with the identified surface rupture, possibly part of a flower structure like diffuse fault zone. The high-resolution catalogs developed in these studies still present some limitations, but they highlight how high-resolution earthquake catalogs can reveal fault structures and kinematics, stress interactions and seismicity patterns. These findings give insights into the seismic cycle and can have important implications on seismic hazard estimation, particularly in slowly deforming settings. Additionally, the earthquake catalogs presented here hold further opportunities for future studies. For example, they can be used to extend focal mechanism catalogs, and further constrain the kinematics of the structures in the studied regions. The magnitude calibration procedures used in some of the catalog compilation procedures can hold important information to resolve the inconsistencies between different magnitude metrics and potentially improve hazard assessments. The improved catalogs can also be compared with current physics-based models and possibly better constrain the limitations of these models or even improve them.
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ItemTemperature, Salinity and Velocity Data Derived from the Overturning in the Subpolar North Atlantic Program (OSNAP) Array between August 2014 and May 2018(Georgia Institute of Technology, 2023-03-30) Li, Feili ; Lozier, M. Susan ; Bacon, S. ; Bower, Amy S. ; Cunningham, Stuart A. ; de Jong, M. Femke ; deYoung, Brad ; Fraser, Neil ; Fried, Nora ; Han, Guoqi ; Holliday, N. Penny ; Holte, James ; Houpert, L. ; Inall, Mark E. ; Johns, William E. ; Jones, Sam ; Johnson, C. ; Karstensen, Johannes ; LeBras, I.A. ; Lherminier, P. ; Lin, X. ; Mercier, H. ; Oltmanns, M. ; Pacini, Astrid ; Petit, T. ; Pickart, Robert S. ; Rayner, Darren ; Straneo, Fiammetta ; Thierry, V. ; Visbeck, M. ; Yashayaev, Igor ; Zhou, C.OSNAP (Overturning in the Subpolar North Atlantic Program) is an international program designed to provide a continuous record of the full-water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic.
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ItemProvenance Analysis of the Bouse Formation, Lower Colorado River from Detrital Zircon (U-Th)/Pb Geochronology(Georgia Institute of Technology, 2023-01-18) Motz, Samantha L.The timing and mechanism of Colorado River integration from the Grand Canyon to the Gulf of California have long been debated. Early research proposed river integration developed “bottom-up” due to Pliocene marine incursion and regional uplift. However, mapping, stratigraphy, and geochemical analyses of early Colorado River deposits instead support a “top-down” integration by progressive filling of lake basins connected by the Colorado River. Key to this debate are interpretations of the depositional environment of the Pliocene Bouse Formation. Here we present a new dataset of detrital zircon (U-Th)/Pb geochronology (n = 1774 single-grain ages) to explore the sedimentary provenance of sand horizons in the Bouse Formation. Our results span 13 Bouse samples from four sub-basins in the lower Colorado River corridor: Mohave, Chemehuevi, Parker, and Cibola. Additional samples of underlying Pyramid gravel and modern sediment from the Colorado River, Bill Williams River, and Silver Creek are presented for comparison. Except for three samples from the Mohave sub-basin, statistical comparison of grain-age populations illustrates that the Bouse Formation has a non-local provenance consistent with a large drainage area comparable to the modern Colorado River. The excepted samples reflect derivation from local source rocks. Within the Bouse Formation’s stratigraphy, grain-age populations do not vary. Still, inter-sub-basins vary geographically, which we attribute to the progressive admixture of zircons from local source rocks and tributaries. Overall, our provenance analysis is consistent with the deposition of Bouse sand horizons as delta-front turbidities originating from a river with a well-mixed and lithologically diverse sediment load. Exceptional samples from the Mohave sub-basin may be explained by interbedding of transverse fan-deltas from local tributaries. Our analysis does not support the deposition of the Bouse Formation in separated and locally sourced lake systems. Instead, it promotes deposition by a single, high-discharge river rapidly progressing southward, integrating previously separated sub-basins.