Climate change impacts hydrological systems differently in different parts of the world. In this study I assess the ability of various global climate models (GCMs) to accurately reproduce observed historical trends in the frequency of flooding at the global scale. I use a peak-over-threshold (POT) method and focus on the period 1985-2014. First, I compare trends in regions belonging to different climate zone types and find that tropical and arid areas adhere to the “wet-get-wetter, dry-get-drier” pattern, with significant increasing trends in tropical areas and significant decreasing trends in arid areas; the results are less definite in temperate, boreal, and polar regions. Second, I assess whether the outputs from 12 different GCMs are consistent with the results from the reference data, considering both annual and seasonalized time scales. I find that, in general, the GCMs are not able to accurately model the observed regional trends. Northern Australia (NAU), however, is an exception to this, and I use a subset of ten GCMs that perform well in this region to determine the projected changes in the frequency of flood events by the end of the 21st century. My results indicate that flooding is projected to decrease in frequency under all four emissions scenarios considered.

Stable isotopes are useful tools to describe how elements move through plants, as transport mechanisms often favor certain isotopes. In the past, stable isotope research on plants has focused primarily on agricultural plants and the transport of elements with high concentrations. Only recently have improvements in mass spectrometry made natural-abundance measurements of heavier metal isotope systems, including magnesium (Mg) and calcium (Ca), possible. Work to characterize these systems in plants is ongoing, and no study measures both Ca and Mg in the same plants. The new potential to study Ca and Mg in plants motivates this thesis, as does the application of stable isotopes to paleodietary study. Ca and Mg isotopes within dinosaur tooth enamel have been used to hypothesize trophic level differences, spatial niche partitioning, feeding height stratification, and preferential feeding on plant groups and plant parts. Without a more complete understanding of isotopic fractionation in plants, however, it is difficult to support these conclusions.

In this thesis, I aim to advance understanding of Mg and Ca isotope systems through a survey of plant materials gathered from two field sites in the California redwoods. I observe Ca and Mg isotope signals (1) at ecosystem scale (from water to soil to plants and across plant species), (2) within plants, across different plant parts, and (3) across leaves up the height of a single 107-meter redwood tree. When considering Ca and Mg isotopes separately, differences in isotopic ratios occur across multiple scales (location, species, plant parts, leaf height), complicating paleodietary interpretations. A model for Mg and Ca transport is proposed as a framework to quantify some of these differences. When considering Ca and Mg isotope measurements together, species form distinct clusters, which may be explained by the proportion of different pools of nutrients, mediated by the presence of potassium (K). Thus, the dual measurement of the two isotope systems in plants and tooth enamel emerges as a promising tool to inform paleodietary reconstructions, for dinosaurs as well as other animals, and to improve understanding of Ca and Mg biogeochemical cycling. Future work should clarify the role of K in fractionation and further classify plant material in Mg-Ca isospace.

The coastal provinces of Ecuador are some of the country’s most climate-vulnerable and socioeconomically vulnerable places in the country. These areas also brace most climate impacts related to El Niño–Southern Oscillation (ENSO) through precipitation and flooding extremes. There is a significant literature gap on local and coast-specific ENSO impacts in all of Latin America, but Ecuador in particular is under-studied. This research characterizes the relation- ship between ENSO indices in the Central Pacific and Eastern Pacific (Niño 3.4 and Niño 1+2) and ENSO meteorological and social impacts on the coast of Latin America (Niño 1+2), to understand further the local meteorological variables that affect the livelihood of communities living in this under-studied and vulnerable area. Using locally collected meteorological and precipitation-linked weather event casualty data to visualize the formation of extreme El Niño events, we assess ENSO climate impacts at multiple scales. At the source-to-hazard level, Niño 1+2 showed stronger and more consistent correlations with coastal precipitation anomalies than Niño 3.4, particularly during the peak rainy season. At the hazard-to-exposure level, precipitation anomalies aligned with increases in people and homes affected, with stronger coupling in southern coastal regions. At the community level, household surveys revealed that limited hazard knowledge, weak institutional trust, and economic dependence on climate-sensitive sectors collectively amplify vulnerability to ENSO events. These findings shed light on potential local policy measures that would help risk mitigation and preparedness for an El Niño event for small coastal communities.

Past literature has identified several factors that limit deep convection, including sea surface temperature (SST) and the humidity within and around an atmospheric column. These limiting factors often have a nonlinear relationship to deep convection, imposing thresholds below which convection is rare. Empirical values for these thresholds are well known, but there is not a well-known theoretical basis for the values themselves. Why is deep convection rare below SSTs of 26°C rather than 24°C? Why is it rare for water vapor content below 35 kg/m2? This paper uses reanalysis data to compute moist static energy (MSE), a metric which accounts for temperature and humidity simultaneously. We test the hypothesis that deep convection begins when local MSE surpasses the rainy tropical maximum MSE (i.e. “lid” MSE) and find that in many regions, a local MSE within 5 kJ/kg of the lid MSE is enough for deep convection onset to occur. However, it is hard to find universal support for the hypothesis, as behavior varies widely between regions. Factors including latitude, topography, land-sea ratio, distance to lid origin location, and dry air entrainment may play a role in this variability.

Oxygen minimum zones (OMZs) are regions of the world’s ocean that are permanently depleted in dissolved oxygen. Nitrite oxidation, traditionally considered an obligately aerobic process, has been observed in the anoxic core of these OMZs challenging longstanding assumptions about nitrogen cycling in low-oxygen environments. This paper investigates nitrite oxidation within the Eastern Tropical South Pacific (ETSP) oxygen minimum zone through ¹⁵NO₂⁻ tracer incubations on samples collected at three stations spanning from the oxygen minimum zone’s anoxic core to oxygenated waters outside of the OMZ. Experiments tested nitrite oxidation rates under anoxic, low oxygen (1 μM O2) and oxygen-spike treatments designed to mimic transient lateral oxygen intrusions. Results revealed unexpectedly high nitrite oxidation rates under anoxic treatment in samples from the anoxic core of the OMZ (92.52 ± 19.32 nM/day at station PS3 depth 230 m), with little to no activity under oxic or spike conditions. These findings contradict the hypothesis that episodic oxygen availability sustains nitrite oxidation in OMZs, suggesting instead that some nitrite-oxidizing bacteria (NOB) possess a previously unrecognized anaerobic metabolism. This discovery supports growing evidence of niche-adapted NOB within OMZs, distinct from their aerobic counterparts. Potential mechanisms include nitrite dismutation or alternate oxidants, though further work is needed to test these theories. As OMZs expand under climate change, understanding these alternative pathways is critical to predicting nitrogen cycle dynamics and fixed nitrogen loss in the ocean.

Potassium (K), magnesium (Mg), and calcium (Ca) are vital macronutrients in plant biochemistry, regulating plant growth, nutrient and metabolite transport, responses to environmental stresses, etc. K and N fertilizers are frequently applied to crops while Mg and Ca are usually applied when deficiency is expected. This thesis investigates how varying K concentrations affect nutrient uptake in plants, particularly in Arabidopsis thaliana, using a controlled hydroponic experiment setup. Plants regulate potassium uptake through specialized transport systems. When potassium is limited (<0.1 mM), plants tend to depend on high-affinity transport systems (HATS) to uptake potassium, whereas low-affinity transport systems (LATS) dominate when potassium levels are plentiful (>0.1 mM). Previous experiments revealed that at 0.10 mM external potassium, plants exhibited δ⁴¹K isotopic signatures in line with HATS, employed under stressed conditions without showing significant reductions in plant K acquisition relative to plants grown in more replete potassium environments. To address the gaps from the previous experiments, we examined intermediate potassium concentrations to evaluate whether a gradual transition or a step change would occur between HATS and LATS. Our results demonstrate that δ⁴¹K values become more negative with rising potassium availability. Plants grown at 0.33 mM potassium displayed intermediate δ⁴¹K, suggesting the concurrent operation of both transport systems. In contrast, δ⁴⁴/⁴⁰Ca and δ²⁶Mg remained stable across varying potassium conditions, indicating that their uptake is not dependent on external potassium availability. Additionally, based on the observed relationship between δ⁴¹K and δ15N, potassium availability may have an influence on nitrogen source utilization in plants (NO3 or NH+4). These findings demonstrate how nutrient availability affects uptake mechanisms and isotopic fractionation, providing new insights to achieve more sustainable agriculture practices.

This study examines the spatial correlation between low-level rotation fields from the Multi-Radar Multi-Sensor (MRMS) system and tornado reports from the Storm Prediction Center (SPC) between 2021 and 2023. Tornadoes were analyzed by overlaying reported paths onto MRMS RotationTrack60min fields, which capture maximum low-level azimuthal shear over a rolling one-hour window. Two spatial search methods were used: a radial search centered on each tornado’s reported start point and an interpolated path-based search along the full track. Results show that MRMS-detected rotation signatures were found within 5 kilometers of approximately 95% of tornado reports using the radial method, with marginal improvement from path interpolation. Regional differences were evident; tornadoes in Tornado Alley exhibited stronger rotation and higher detection rates than those in Dixie Alley. Tornadoes associated with stronger radar-indicated rotation were also more likely to align with reports, revealing a detection bias toward more intense systems. These findings underscore the strengths and limitations of MRMS azimuthal shear for tornado detection. While MRMS enhances identification of well-organized tornadoes, weaker or short-lived events remain harder to detect. This research contributes to improving tornado climatology and real-time warning systems by cross-validating radar data with observational records. Future work expanding the MRMS archive and incorporating environmental classifications will further refine understanding of tornado detection efficiency.

This thesis seeks to identify a relationship between the phase of the Madden Julian Oscillation (MJO) and the probability of rapid intensification (RI) of tropical cyclones (TCs) in the Gulf of Mexico and the Caribbean Sea. The following methods were used to investigate the effect of the MJO on the likelihood of RI in the Gulf and Caribbean: the probabilities of TCs undergoing RI in the Gulf of Mexico and Caribbean (and in the entire North Atlantic) were calculated for each phase of the MJO (using the eight-phase index developed by Wheeler and Hendon 2004). Phases 2 and 8 exhibited enhanced probabilities of RI in the Gulf of Mexico and Caribbean (and the entire North Atlantic). The probabilities of TC genesis were also calculated for each MJO phase in the Gulf of Mexico and Caribbean (and the North Atlantic) in order to determine whether the increased probability of RI in certain phases is mostly attributable to an increased probability of TC genesis or whether other environmental conditions largely independent of genesis are responsible for enhanced RI in certain phases. Additionally, vertical wind shear anomaly maps, outgoing longwave radiation (OLR) anomaly maps, sea surface temperature (SST) anomaly maps, and maps of the temperature anomaly at 250 hPa were made for each phase of the MJO in order to determine which large-scale conditions affected RI the most and whether there was a constructive relationship between wind shear and OLR for certain phases. None of these variables explains the MJO’s modulation of RI, although there is a constructive relationship between negative OLR anomalies, negative wind shear anomalies, and positive SST anomalies in phase 2 that (at least partially) explain its high probability of RI. Finally, relationship between the Madden Julian Oscillation and the El Niño Southern Oscillation (ENSO) was briefly discussed. Diverging theories about how ENSO is expected to change with warming were introduced, and future projections of the MJO were made (using bandpass-filtered variances of OLR and monthly SST climatology maps) and analyzed in the context of the relationship between the MJO and ENSO.