Doctor of Philosophy

dissertationAmong the most societally important trends in observations and models are poleward shifts in global circulation features. In order to investigate the magnitude and mechanisms of such circulation shifts, we explore the general circulation response to imposed forcings, using the Geophysical Fluid Dynamics Laboratories Atmospheric Model version 2.1. Present-day simulations exhibit robust poleward shifts in zonal mean circulation features, compared to a preindustrial control, as do future simulations compared to presentday. These responses vary by season, and the response to combined forcings is wellapproximated by the sum of the individual responses. Results suggest that warming sea surface temperatures are the main driver of circulation change over both hemispheres. This work also projects that the southern hemisphere jet will continue to shift poleward, albeit more slowly during the summer due to expected ozone recovery in the stratosphere. The relationship between shifts in the position of the eddy-driven jet and of the Hadley cell edge are examined. From year to year, the eddy-driven jet shifts more than the Hadley cell edge, with a ratio of approximately 1.5:1 between the two depending on season, hemisphere, and simulation. Furthermore, the mean position of the Hadley cell edge explains a substantial portion of the variability of this ratio. The author attributes this to the varying susceptibility of the Hadley cell to the influence of midlatitude eddies. Finally, the transient response to suddenly imposed forcings is examined. For both direct radiative forcings and sea surface warming, broad tropospheric and stratospheric temperature changes can be seen almost immediately. Once zonal winds near the tropopause accelerate a few days later, zonal winds and eddies respond throughout the rest of the troposphere, shifting the surface circulation. The transient changes in the Ferrel cell center and the Hadley cell edge appear simultaneous, illustrating the need for consideration of both tropical and extratropical processes in the theory for the latitude of either the Hadley cell edge or the eddy-driven jet

Breaking down the tropospheric circulation response by forcing

pre-printThis study describes simulated changes in the general circulation during the twentieth and twenty -first centuries due to a number of individual direct radiat ive forcings and warming sea surface temperatures, by examining very long time-slice simulations created with an enhanced version of the Geophysical Fluid Dynamics Laboratories Atmospheric Model AM 2.1. We examine the effects of changing stratospheric ozone, greenhouse gas concentrations, and sea surface temperatures individually and in combination over both hemispheres . Data reveal robust poleward shifts in zonal mean circulation features in present -day simulations compared to a pre-industrial control, and in future simulations compared to present -day. We document the seasonality and significance of these shifts, and find that the combined response is well approximated by the sum of the individual responses. In contrast with other recent studies, we find that circulation shifts due to changing sea surface temperatures - not ozone depletion or recovery - dominate the combined southern hemisphere response during all seasons, and accordingly project that the southern hemisphere jet will continue to shift poleward during the twenty -first century

Atlantic Multidecadal Variability modulates the climate impacts of El Niño-Southern Oscillation in Australia

Atlantic Multidecadal Variability (AMV) modulates El Niño-Southern Oscillation (ENSO) dynamics. Here, we explore the effect of warm (AMV+) and cold (AMV-) AMV conditions on the austral summer teleconnection of ENSO to Australia using idealized simulations performed with the NCAR-CESM1 model. AMV+ strengthens the mean and extreme precipitation and temperature responses to El Niño in south-western Australia and weakens the mean precipitation and temperature impacts in north-eastern Australia. The modulation of La Niña impacts by AMV is asymmetric to El Niño, with a weakening of the mean and extreme precipitation and temperature responses in eastern Australia. Decomposing the total difference in ENSO response between AMV phases, we find that the signals are mainly explained by the direct AMV modulation of ENSO and its teleconnections rather than by changes in background climate induced by AMV. The exception is ENSO-driven fire impacts, where there is a significant increase in burned area in south-eastern Australia only when El Niño and AMV+ co-occur. However, modulation of ENSO between AMV+ and AMV- does offset ~37% of the decrease in burned area extent during La Niña summers. The altered surface climate response to ENSO in Australia by AMV is attributed to variations in large-scale atmospheric circulation. Under AMV+, there is increased subsidence over western Australia during El Niño associated with a westward shift of the local Walker circulation. A weakening of the upwelling branch of the local Hadley circulation over north-eastern Australia is responsible for the weakening of La Niña impacts in AMV+, accompanied by a strengthening of subsidence in south central Australia due to a weakening of the local Hadley circulation, amplifying La Niña impacts over this region. The results suggest the potential for AMV to drive multidecadal variability in ENSO impacts over Australia.P.T.-C. was supported by a PhD scholarship from the Natural Environment Research Council PANORAMA Doctoral Training Partnership (NE/S007458/1). Y.R.-R. received the support of a fellowship from ”la Caixa” Foundation (ID 100010434) and from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 847648. The fellowship code is LCF/BQ/PR21/11840016. A.C.M. was supported by the European Union’s Horizon 2020 research and innovation program under grant agreement no. 820829 (CONSTRAIN project) and The Leverhulme Trust (PLP-2018-278). M.T. acknowledges funding by the Spanish Ministry of Science, Innovation and Universities through the Ramón y Cajal Grant Reference RYC2019-027115-I and through the project ONFIRE, grant PID2021-123193OB-I00, funded by MCIN/AEI/10.13039/501100011033. Computing facilities were provided by the Barcelona Supercomputing Center and the University of LeedsPeer ReviewedPostprint (author's final draft

Atheisms and the purification of faith

Philosophers of religion have distinguished between ‘negative’ and ‘positive’ atheism. This article considers further conceptions of atheism, especially the idea that atheism can facilitate a faith in God purified of idolatrous assumptions. After introducing Bultmann’s contention that a ‘conscious atheist’ can find something transcendent in the world, this contention is interpreted through reflection on Ricoeur’s claim that the atheisms of Nietzsche and Freud serve to mediate a transition to a purified faith – a faith involving heightened receptivity to agapeic love. The troubling question of what differentiates atheism from belief in God is then discussed in the light of Simone Weil’s meditations on God’s secret presence

Regional and Temporal Variability of Atmospheric River Seasonality: Influences of Detection Algorithms and Moisture Transport Dynamics

Understanding the regional and temporal variability of atmospheric river (AR) seasonality is crucial for preparedness and mitigation of extreme events. While ARs were thought to peak in winter, recent research shows they exhibit region-specific seasonality and are heavily influenced by the chosen detection algorithm. This study examines the link between the year-to-year consistency of peak-AR activity to the presence of a dominant seasonal pattern, considering both location and algorithm choice. Regions are categorized by their temporal characteristics: consistent patterns (e.g., East Asia), patterns with occasional outliers (e.g., British Columbia coast), and regions lacking a clear dominant peak season (e.g., South Atlantic, parts of Australia). Hence, not all regions display a consistent seasonal cycle of AR activity. This study quantifies the extent to which a region experiences a dominant peak season of AR activity (or lacks one) and offers insights to enhance decision-making in water management, natural hazard preparedness, and forecasting. Furthermore, given our finding that detection algorithms influence the peak season of AR activity, we also examine two diagnostic variables representative of moisture transport to corroborate our results. Integrated vapor transport, which captures meridional and zonal moisture transport, and Moist Wave Activity, representing moisture intrusions from lower to higher latitudes, are examined. Our analysis indicates that inconsistencies in the seasonal cycle of AR activity are not solely due to discrepancies in detection algorithms but also arise from changes in moisture transport

The TropD software package (v1): standardized methods for calculating tropical-width diagnostics

Observational and modeling studies suggest that Earth's tropical belt has widened over the late 20th century and will continue to widen throughout the 21st century. Yet, estimates of tropical-width variations differ significantly across studies. This uncertainty, to an unknown degree, is partly due to the large variety of methods used in studies of the tropical width. Here, methods for eight commonly used metrics of the tropical width are implemented in the Tropical-width Diagnostics (TropD) code package in the MATLAB programming language. To consolidate the various methods, the operations used in each of the implemented methods are reduced to two basic calculations: finding the latitude of a zero crossing and finding the latitude of a maximum. A detailed description of the methods implemented in the code and of the code syntax is provided, followed by a method sensitivity analysis for each of the metrics. The analysis provides information on how to reduce the methodological component of the uncertainty associated with fundamental aspects of the calculations, such as monthly vs. seasonal averaging biases, grid dependence, sensitivity to noise, and sensitivity to threshold criteria

Overview of the instrumentation for the Dark Energy Spectroscopic Instrument

The Dark Energy Spectroscopic Instrument (DESI) embarked on an ambitious 5 yr survey in 2021 May to explore the nature of dark energy with spectroscopic measurements of 40 million galaxies and quasars. DESI will determine precise redshifts and employ the baryon acoustic oscillation method to measure distances from the nearby universe to beyond redshift z > 3.5, and employ redshift space distortions to measure the growth of structure and probe potential modifications to general relativity. We describe the significant instrumentation we developed to conduct the DESI survey. This includes: a wide-field, 3.°2 diameter prime-focus corrector; a focal plane system with 5020 fiber positioners on the 0.812 m diameter, aspheric focal surface; 10 continuous, high-efficiency fiber cable bundles that connect the focal plane to the spectrographs; and 10 identical spectrographs. Each spectrograph employs a pair of dichroics to split the light into three channels that together record the light from 360–980 nm with a spectral resolution that ranges from 2000–5000. We describe the science requirements, their connection to the technical requirements, the management of the project, and interfaces between subsystems. DESI was installed at the 4 m Mayall Telescope at Kitt Peak National Observatory and has achieved all of its performance goals. Some performance highlights include an rms positioner accuracy of better than 0.″1 and a median signal-to-noise ratio of 7 of the [O ii] doublet at 8 × 10−17 erg s−1 cm−2 in 1000 s for galaxies at z = 1.4–1.6. We conclude with additional highlights from the on-sky validation and commissioning, key successes, and lessons learned

Editorial “The Impacts of Climate Change on Atmospheric Circulations”

Understanding the atmospheric general circulation is, in a way, analogous to cleaning a large home [...

MS

thesisResearch suggests that changes in tropopause structure can both indicate and impact changes in the global climate system. The Global Positioning System radio occultation (RO) technique shows tremendous potential for monitoring the global tropopause due to its precision, temporal consistency, and global measurement density. This study examines the capability of RO to monitor the global tropopause by addressing three specific objectives: (1) quantify sources of uncertainty in individual RO tropopause measurements, (2) examine mean biases and long-term stability of RO tropopause parameters with respect to those obtained from radiosondes, and (3) distinguish between errors due to processing and RO instrument differences by comparing tropopause parameters from different RO products. Measured tropopause uncertainty is shown to be due largely to the nonlinear nature of the tropopause. Global mean temperature and height biases between RO instruments and radiosondes are small. One long-term RO dataset examined in this study showed spurious temperature trends, but these have since been corrected. Tropopause measurements from different RO instruments are generally very small as long as datasets are processed similarly. These results confirm the precision of RO data, but also demonstrate the importance of careful, consistent processing for long-term tropopause temperature studies. Unlike tropopause temperatures, tropopause heights do not appear to be significantly affected by the differences in processing examined in this study