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

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

Southern Ocean drives multidecadal atmospheric CO2 rise during Heinrich Stadials

The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here, we present CO2 measurements from the West Antarctic Ice Sheet Divide ice core across Heinrich Stadials 2 to 5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO2 concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 y) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO2 are concurrent with jumps in atmospheric CH4 and abrupt changes in the water isotopologs in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologs can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal- to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO2 uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future.Peer reviewe

Data from: Experimental species removals impact the architecture of pollination networks

Mutualistic networks are key for the creation and maintenance of biodiversity, yet are threatened by global environmental change. Most simulation models assume that network structure remains static after species losses, despite theoretical and empirical reasons to expect dynamic responses. We assessed the effects of experimental single bumblebee species removals on the structure of entire flower visitation networks. We hypothesized that network structure would change following processes linking interspecific competition with dietary niche breadth. We found that single pollinator species losses impact pollination network structure: resource complementarity decreased, while resource overlap increased. Despite marginally increased connectance, fewer plant species were visited after species removals. These changes may have negative functional impacts, as complementarity is important for maintaining biodiversity–ecological functioning relationships and visitation of rare plant species is critical for maintaining diverse plant communities

raw network data, Brosi et al. Biology Letters

pollination network data, with site, experimental state (control or manipulation), and plant and pollinator species. The README file has the identities of the manipulated (removed) bumble bee species for each site

Correlation Matrix for network metrics from Experimental species removals impact the architecture of pollination networks

Given a Bonferroni correction for 15 multiple comparisons for alpha = 0.05, and 28 degrees of freedom, 0.5185 is the critical correlation value for statistical significanc

Southern Ocean drives multi-decadal atmospheric CO2 rise during Heinrich Stadials

The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here we present CO2 measurements from the West Antarctic Ice Sheet (WAIS) Divide (WD) ice core across Heinrich Stadials 2-5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO2 concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 years) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO2 are concurrent with jumps in atmospheric CH4 and abrupt changes in the water isotopologues in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologues can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO2 uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future

Southern Ocean drives multidecadal atmospheric CO₂ rise during Heinrich Stadials

The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here, we present CO2 measurements from the West Antarctic Ice Sheet Divide ice core across Heinrich Stadials 2 to 5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO2 concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 y) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO2 are concurrent with jumps in atmospheric CH4 and abrupt changes in the water isotopologs in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologs can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal- to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO2 uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future