Graduate Thesis Or Dissertation
 

Exploring the Relationships Between Stable Water Isotope Ratios and Large Scale Atmospheric Circulation in Paleoclimate Settings Using an Isotope-Enabled General Circulation Model

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1n79hc72d

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  • Climate model simulations and paleoclimate proxies are two tools that enable an understanding of the climate history of the Earth. When utilized together, they form a powerful paradigm for understanding past changes. Proxies are the only physical link to the past conditions on Earth, and models “fill in the gaps” that are intrinsic to the non-uniform spatial and temporal distribution of proxies. Precipitation changes are a major component of past climates, but proxies do not directly record precipitation amount. Instead, many proxies of precipitation are based on the stable oxygen or hydrogen isotopic composition of precipitation (δ18Op and δDp, or δp). Hydroclimate reconstructions from isotope-based proxies are complicated because the relationship between δp and climate is not well understood in time and space and is highly variable in both domains. Following in the footsteps of recent work, the studies presented in this dissertation focus on constraining some of the uncertainty surrounding the δp-climate relationship using results from general circulation model (GCM) simulations of two paleoclimate states, the mid-Holocene (MH, 6 ka), and the last glacial maximum (LGM, 21 ka). The MH and LGM contain a deluge of δp-based proxies and are historical benchmarks for GCM paleoclimate simulations, making them ideal case studies for this work. In all chapters, the isotope-enabled Community Earth System Model (iCESM) is employed in an “atmosphere-only” setup. In chapter 1, an overview of the MH tropical hydrology is presented, and simulated δp results are compared to the suite of available cave stalagmite (speleothem) proxies. Model results indicate the presence of a relationship between large-scale mean atmospheric circulation and simulated δp through changes in the Hadley and Walker circulation. At the scale of individual speleothems, however, shifts in local- and regional-scale hydrology are more important for simulated δp than large-scale mean atmospheric changes. These results are discussed in the context of model validation, i.e. the extent to which simulated δp resembles the proxy-suggested spatial distribution of δp, and the isotopic role of various climate variables in regions where model minus proxy errors are high. In chapter 2, the influence of the African Humid Period (AHP) on East Asian Summer Monsoon δp is investigated by increasing the vegetation coverage in northern Africa during the MH. Location-based water tracers, or “tags”, are added to the MH simulation, and a technique is developed for deconstructing Δδp into two components: 1) changes in vapor source contribution, and 2) changes in vapor source isotope ratio. Shifts in EASM δp are found to be driven by an increased contribution from vapor of Pacific origin by shifts in atmospheric circulation that favor the convergence of Pacific vapor onto East Asia. In this experimental setup, circulation changes are induced only because of vegetation changes in North Africa, suggesting a teleconnection between the AHP and EASM δp. These results highlight the relative importance of changes in vapor source contribution over changes in vapor source isotope ratio, and demonstrate that small vapor sources (i.e. those that account for < 5% of total rainfall) can have significant impacts on δp. In chapter 3, iCESM is used to simulate the millennial-scale variability in global temperature that occurred during the most recent glacial period, by inducing changes in the southern extent of North Atlantic sea ice. A warm and cold glacial climate state is simulated by shifting sea ice anomalously northward for the warm and southward for the cold, reminiscent of sea ice extent during Greenland interstadials (GI, warm) and Greenland stadials (GS, cold). GI – GS changes in simulated δp are compared to the GI – GS changes in Greenland Summit ice core δp. GI-GS variability in simulated δp is induced simply by capping the Greenland, Icelandic, and Norwegian seas (GIN) with sea ice, preventing evaporation. Using the same decomposition technique from chapter 2, it is shown that by removing the presence of GIN sea vapor in Greenland precipitation, δp of Greenland is reduced without any other factors changing, including the atmospheric circulation and the isotope ratio of vapor sources from other regions. Results from these chapters highlight the specific role of changes in changes to the spatial variability of moisture sourcing. As long as precipitation is created from vapor source regions with distinct isotopic compositions, shifts towards certain regions over others is the main driving force behind total isotope ratio change. Furthermore, the potential of low-contribution vapor sources as the main drivers of total isotope ratio change is shown to be a significant factor for tropical rainfall, suggesting that non-local and seemingly insignificant vapor sources can dramatically impact total isotope ratios.
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