• Assessing the 20th Century Performance of Global Climate Models and Application to Climate Change Adaptation Planning

      Zeng, Xubin; Geil, Kerrie L.; Zeng, Xubin; Crimmins, Michael; Ferguson, Daniel; Marsh, Stuart (The University of Arizona., 2017)
      Rapid environmental changes linked to human-induced increases in atmospheric greenhouse gas concentrations have been observed on a global scale over recent decades. Given the relative certainty of continued change across many earth systems, the information output from climate models is an essential resource for adaptation planning. But in the face of many known modeling deficiencies, how confident can we be in model projections of future climate? It stands to reason that a realistic simulation of the present climate is at least a necessary (but likely not sufficient) requirement for a model’s ability to realistically simulate the climate of the future. Here, I present the results of three studies that evaluate the 20th century performance of global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The first study examines precipitation, geopotential height, and wind fields from 21 CMIP5 models to determine how well the North American monsoon system (NAMS) is simulated. Models that best capture large-scale circulation patterns at low levels usually have realistic representations of the NAMS, but even the best models poorly represent monsoon retreat. Difficulty in reproducing monsoon retreat results from an inaccurate representation of gradients in low-level geopotential height across the larger region, which causes an unrealistic flux of low-level moisture from the tropics into the NAMS region that extends well into the post-monsoon season. The second study examines the presence and severity of spurious Gibbs-type numerical oscillations across the CMIP5 suite of climate models. The oscillations can appear as unrealistic spatial waves near discontinuities or sharp gradients in global model fields (e.g., orography) and have been a known problem for decades. Multiple methods of oscillation reduction exist; consequently, the oscillations are presumed small in modern climate models and hence are rarely addressed in recent literature. Here we quantify the oscillations in 13 variables from 48 global climate models along a Pacific ocean transect near the Andes. Results show that 48% of nonspectral models and 95% of spectral models have at least one variable with oscillation amplitude as large as, or greater than, atmospheric interannual variability. The third study is an in-depth assessment model simulations of 20th century monthly minimum and maximum surface air temperature over eight US regions, using mean state, trend, and variability bias metrics. Transparent model performance information is provided in the form of model rankings for each bias type. A wide range in model skill is at the regional scale, but no strong relationships are seen between any of the three bias types or between 20th century bias and 21st century projected change. Using our model rankings, two smaller ensembles of models with better performance over the southwestern U.S. are selected, but they result in negligible differences from the all-model ensemble in the average 21st century projected temperature change and model spread. In other words, models of varied quality (and complexity) are projecting very similar changes in temperature, implying that the models are simulating warming for different physical reasons. Despite this result, we suggest that models with smaller 20th century biases have a greater likelihood of being more physically realistic and therefore, more confidence can be placed in their 21st century projections as compared to projections from models that have demonstrably poor skill over the observational period. This type of analysis is essential for responsibly informing climate resilience efforts.
    • Flash Flood Causing Mechanisms of the North American Monsoon System in the Sonoran Desert

      Comrie, Andrew C.; Bieda, Stephen W.; Crimmins, Michael A.; Guertin, D. Philip; Marsh, Stuart E.; Byerle, Lee A.; Comrie, Andrew C. (The University of Arizona., 2012)
      The North American Monsoon System (NAMS) is a significant weather and climate phenomenon that brings critical rainfall to the southwestern United States and northwestern Mexico. As a result of the North American Monsoon Experiment, and research efforts surrounding the field campaign, the understanding of the NAMS has increased considerably over the last 15 years. In addition questions concerning potential flash flood causing mechanisms of the NAMS have not been thoroughly investigated. This dissertation is comprised of two papers that collectively address the aspects of the literary understanding of the NAMS as we know it today and conduct an investigation into the complex interactions between various weather systems that may influence the NAMS. In the first paper, a review of the major research of the NAMS literature since the last comprehensive review 15 years ago is conducted. The results of his review are assessed for where our understanding has been improved and where future research needs to be guided for purposes of the second paper. Based upon the results from the literature review, the second paper focuses on identification of inverted troughs and gulf surges based upon lower- and mid-level atmospheric parameters for purposes of assessing the impacts on National Weather Service Storm Report flash flood dates. This research contributes to the synthesis of the current knowledge of the NAMS in general and to the specific regional impacts that do occur during periods of heavy precipitation over the NAMS region for purposes of improving meteorological predictability of flash flooding. The results can (1) gauge our understanding of the NAMS literature to date and (2) improve meteorological forecasts through the recognition of synoptic and sub-synoptic patterns related to the NAMS that are most likely to cause flash floods.
    • Glacial-Interglacial Variability of Eastern Pacific Hydroclimate

      Tierney, Jessica E.; Meegan Kumar, Dervla; Anchukaitis, Kevin J.; Lofverstrom, Marcus; Castro, Christopher L. (The University of Arizona., 2022)
      Unprecedented drought conditions in southwest North America (SWNA, defined as the region from southern California to southern Mexico, west of the continental divide) has raised alarms regarding water resource scarcity in a region with dense urban populations and vital agricultural industries. Modern hydroclimate in SWNA is complex as rainfall seasonality and moisture source ranges from the Mediterranean climate of California, bimodal desert southwest, monsoonal northwest Mexico, and Central American tropics, and there is a high degree of interannual rainfall variability due to the importance of short-lived, high-intensity events to regional moisture budgets. A deeper understanding of the spatio-temporal patterns of mean annual and seasonal rainfall variability and their response to global climate perturbations is thus critical for future planning and water resource management in the region. The eastern Pacific is a key modulator of rainfall variability in SWNA. Pacific ocean-atmosphere interactions affect North American monsoon intensity, the seasonal migration of the intertropical convergence zone (ITCZ), the strength, location, and waviness of the mid-latitude storm tracks, and the occurrence of extreme precipitation events associated with atmospheric rivers, tropical cyclones, and the El Niño/Southern Oscillation. Paleoclimate records from the Pacific can provide crucial constraints on how these processes responded to past global climate perturbations. A unified view of Pacific-North American interactions is currently limited by sparse proxy data from the Eastern Pacific Warm Pool (EPWP) and the Gulf of California, specifically of reconstructions that extend beyond the Last Glacial Maximum (LGM; 19--23 ka). The work presented in this dissertation sheds new light onto the late Pleistocene climatic history of SWNA based on novel organic geochemical proxy reconstructions of eastern Pacific sea surface temperatures and North American monsoon rainfall variability. The proxy data indicate that the northern EPWP (along the northwest Mexican Margin near the mouth of the Gulf of California) did not experience glacial cooling. Rather, elevated sea surface temperatures regularly occur during intervals of low greenhouse gases and increased global ice volume across the last glacial cycle. Meanwhile, monsoonal rainfall was elevated during warm interglacials and suppressed during glacial periods. I additionally compare the proxy data to general circulation model simulations of the LGM and Last Interglacial (LIG; $\sim$117--130 ka) periods to investigate the effects of orbital forcing (insolation), greenhouse gases, and ice sheets on moisture source, precipitation seasonality and geographic patterns of rainfall variability across SWNA. This research fills a critical data gap and ultimately provides a mechanistic understanding of the patterns and drivers of hydroclimate variability in SWNA that indicates a complexity in the response of SWNA temperature and precipitation to global climate forcings.
    • Modeling Stream-Aquifer Interactions During Floods and Baseflow: Upper San Pedro River, Southeastern Arizona

      Meixner, Thomas; Simpson, Scott; Meixner, Thomas (The University of Arizona., 2007)
      Streams and groundwaters interact in distinctly different ways during flood versus base flow periods. Recent research in the Upper San Pedro River using isotopic and chemical data shows that (1) near-stream, or 'riparian,' groundwater recharged during high streamflow periods is a major contributor to streamflow for the rest of the year, and (2) the amount of riparian groundwater derived from this flood recharge can vary widely (10-90%) along the river. Riparian groundwater in gaining reaches is almost entirely basin groundwater, whereas losing reaches are dominated by prior streamflow.This description of streamflow gives rise to the questions of (1) how much flood recharge occurs at the river-scale, and (2) subsequently, what is the relative importance of flood recharge and basin groundwater in maintaining the hydrologic state of the riparian system. To address these questions, a coupled hydrologic-solute model was constructed for 45 km of the Upper San Pedro riparian system.
    • Past Climate, Modern Caves, and Future Resource Management in Speleothem Paleoclimatology

      Cole, Julia E.; Overpeck, Jonathan T.; Truebe, Sarah; Quade, Jay; Comrie, Andrew; Cole, Julia E.; Overpeck, Jonathan T. (The University of Arizona., 2016)
      My research focuses on reconstructing past climate in southern Arizona using cave deposits called speleothems. However, this necessitates a broader perspective than simply a geochemical time series, and therefore, I also investigate modern cave systems using a combination of modeling and observational datasets. Finally, cave deposits are fundamentally non-renewable resources, and sampling for past climate reconstruction can be destructive, unlike other cave uses. My last investigation is focused on developing possible best practice recommendations for paleoclimate scientists and other cave stakeholders moving forward. We developed two new stalagmite records of past climate variability in southern Arizona over the past 7000 years. Past climate reconstruction from two caves (Cave of the Bells and Fort Huachuca Cave) highlights insolation control of southern Arizona hydroclimate from 7000-2000 years before present. Additionally, comparison between two stalagmites with different seasonal sensitivities uncovers a few eras of multi-decade long droughts in southern Arizona, which align with other regional reconstructions of past climates and elucidate forcings on Southwest paleoclimate as emergent from both external (insolation) and internal climate variability in the Pacific and Atlantic Ocean basins. Although the oxygen isotopic signal of cave calcite in speleothems is complex, agreement with these other records indicates that the speleothem records from these caves primarily record a climate signal.Modeling and monitoring of modern caves both helps us interpret paleoclimate records and enhances our understanding of cave systems in their own right. Modeling of Cave of the Bells dripwaters demonstrates the effect of storage and mixing on the dripwater oxygen isotope signal; non-climate processes can imprint on dripwater variability on multidecadal timescales. Monitoring shows that on very small spatial scales, every cave is different, and even sites within the same cave respond uniquely to surface climate. Most notably, calcite oxygen isotopic composition, used to reconstruct past climate, shows seasonal variability unrelated to dripwater and surface rainfall oxygen isotope variability. Substantial oxygen isotope disequilibrium is identified at numerous caves sites in southern Arizona, and this understanding aligns with a growing number of cave studies that demonstrate the long-held assumption of isotopic equilibrium in cave systems may not always be valid or that the way in which we define isotopic equilibrium insufficiently captures the variety of processes controlling the oxygen isotopic composition of speleothems. Overall, however, monitoring can identify stalagmites that are more sensitive to surface climate and less sensitive to these in-cave processes by identifying sites with dripwater variability responses to surface rainfall variability and sites that precipitate close to oxygen isotopic equilibrium. Finally, a major missing component in speleothem research is the fact that speleothems take thousands and sometimes hundreds of thousands of years to form. They are non-renewable resources on human timescales, and habitat for myriad microbes that have yet to be identified. Removal of speleothems for paleoclimate research is one of the only destructive uses of these deposits. With that in mind, I also analyze current methods of collecting speleothems and develop a framework based on two surveys of scientists and stakeholders to assist scientists and managers when evaluating potential methods of incorporating cave conservation into the speleothem sampling process. Thus, I approach caves from a variety of angles and timescales, from the past through the present to the future, illuminating caves as complex scientific and social systems.
    • Southwest Climate Research and Education: Investigating the North American Monsoon in Arizona and Teaching Climate Science on the Tohono O'odham Nation

      Comrie, Andrew C.; Kahn-Thornbrugh, Casey Curtiss; Marsh, Stuart E.; Yool, Steve R.; Hiller, Joseph G.; Parezo, Nancy J.; Comrie, Andrew C. (The University of Arizona., 2013)
      Western science and Indigenous knowledge understand Southwest climate and the North American monsoon from different cultural perspectives. However, scant literature exists relating to climate and Indigenous communities in the Southwest. On the contrary, substantial climate research has occurred with Arctic Indigenous communities; however, a general aspiration among communities is Indigenous-led climate research and education. This requires more Native scientists and culturally responsive climate science curricula. Southwest Indigenous communities are primed to do this. This dissertation examines 1) the current scientific understanding of the North American monsoon, 2) the state of climate research in Indigenous communities, and 3) the development of culturally responsive climate science curricula. The first paper synthesizes the current scientific understanding of the monsoon and its interannual variability. Pacific Ocean-based teleconnections, such as ENSO-PDO combined indices do add skill in early-season monsoon forecasting. However, general circulation models continue to deal with computational-spatial resolution limitations challenging their application in future climate change projections of the monsoon. The second paper focuses on climate-related research in Indigenous communities in the Arctic and the Southwest to highlight lessons-learned. Climate researchers working with Native communities must exercise cultural considerations for Indigenous relationships with the climate and Indigenous protocols for acquiring and disseminating knowledge. Furthermore, increasing the number of Native students in science and Native scientists are ways to improve climate-related research in Indigenous communities. The third paper is a participatory action research approach to develop a culturally responsive climate science curriculum for Tohono O'odham high school and college students. This project worked with a community advisory board as well as Tohono O'odham Community College instructors and student interns. Pre-assessment surveys were given to community members learn of the most relevant weather and climate topics. The curriculum was developed incorporating local, culturally relevant topics. Climate workshops were offered in the communities using activities developed for the curriculum. Workshop evaluations were positive; however, they also addressed the need for more culturally relevant examples. The overlapping theme for these dissertation papers is cultural understanding for climate research and education in Indigenous communities toward a means for Indigenous-led climate research/education within their own communities.