Browsing by Author "Thompson, David, committee member"
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Item Open Access A statistical prediction model for east Pacific and Atlantic tropical cyclone genesis(Colorado State University. Libraries, 2012) Slade, Stephanie A., author; Maloney, Eric D., advisor; Thompson, David, committee member; Chong, Edwin, committee memberA statistical model is developed via multiple logistic regression for the prediction of weekly tropical cyclone activity over the East Pacific and Atlantic Ocean regions using data from 1975 to 2009. The predictors used in the model include a climatology of tropical cyclone genesis for each ocean basin, an El Niño-Southern Oscillation (ENSO) index derived from the first principal component of sea surface temperature over the Equatorial East Pacific region, and two indices representing the propagating Madden-Julian Oscillation (MJO). These predictors are suggested as useful for the prediction of East Pacific and Atlantic cyclogenesis based on previous work in the literature and are further confirmed in this study using basic statistics. Univariate logistic regression models are generated for each predictor in each region to ensure the choice of prediction scheme. Using all predictors, cross-validated hindcasts are developed out to a seven week forecast lead. A formal stepwise predictor selection procedure is implemented to select the predictors used in each region at each forecast lead. Brier skill scores and reliability diagrams are used to assess the skill and dependability of the models. Results show a significant increase in model skill at predicting tropical cyclogenesis by the inclusion of the MJO out to a three week forecast lead for the East Pacific and a two week forecast lead for the Atlantic. The importance of ENSO for Atlantic genesis prediction is highlighted, and the uncertain effects of ENSO on East Pacific tropical cyclogenesis are re-visited using the prediction scheme. Future work to extend the prediction model with other predictors is discussed.Item Open Access Impacts of Arctic warming and sea ice loss on the Northern Hemisphere mid-latitude large-scale circulation(Colorado State University. Libraries, 2020) Ronalds, Bryn, author; Barnes, Elizabeth A., advisor; Thompson, David, committee member; Randall, David A., committee member; Eykholt, Richard, committee memberThe consequences of the rapid warming of the Arctic and associated sea ice loss on the Northern Hemisphere atmospheric circulation is still largely debated. The uncertainty in the circulation response stems from a poor understanding of the underlying physical mechanisms of the remote response, regional and seasonal differences, differences between models and experimental set-ups, the large internal variability of the system, and the short observational record. This research seeks to address some of this uncertainty, specifically the uncertainty related to the physical mechanisms, regionality, and modeling differences. The wintertime Northern Hemisphere eddy-driven jet streams over the North Pacific and North Atlantic basins exhibit differing responses to Arctic warming and sea ice loss in a fully coupled climate model. In the North Atlantic the jet weakens, narrows along the poleward flank, and shifts slightly equatorward. This response is similar to previous studies examining the Northern Hemisphere zonal mean jet response. In contrast, the North Pacific jet strengthens and extends eastward in response to Arctic sea ice loss, with no change in latitude, and narrows slightly along the poleward flank. In both cases, there are high latitude anomalous easterlies in the region of sea ice loss, where the local surface temperature gradients are weakening. This can lead to changes in locations and frequency of wave-breaking, thus leading to changes in the mean zonal winds further south, in the vicinity of the jet. This work relates the differing changes in the North Pacific and North Atlantic to these changes in wave-breaking in a simplified atmospheric model, and posits that the location of the jet relative to the region of Arctic sea ice loss is a dominant factor in determining the mean jet response to the sea ice loss and local warming. Changes in the mean wintertime Northern Hemisphere midlatitude zonal winds are found to be indicative of changes to the sub-seasonal variability of the wintertime zonal winds. The sub-seasonal circulation patterns over the ocean basins are closely linked with continental weather regimes, including changes in temperature and precipitation. While establishing a causal link between Arctic sea ice loss and changes to remote weather regimes in the observational record remains difficult, the Polar Amplification Model Intercomparison Project (PAMIP) provides insight into possible relationships and consequences. The design of the project eliminates differences in experimental set-ups across models and aids in addressing the uncertainty in regional responses. Across four climate models, Arctic sea ice loss leads to a strengthened and extended North Pacific jet in the January-February mean. This mean change is also associated with changes to the sub-seasonal, wintertime North Pacific zonal wind variability. All four models show an increase in strengthened and extended North Pacific eddy-driven jet stream events and a decrease in weakened, retracted and equatorward-shifted North Pacific jet events in January-February. Previous work has also established the relationships between North Pacific jet stream variability and downstream, North American weather regimes, and changes to the former are expected to impact the latter. Again, there is model agreement in an increase of a warm west/cold east temperature dipole over North America, associated with the strengthened and extended jet events. There is also a decrease in cold air temperature anomalies over North America, associated with weakened and equatorward-shifted jet events.Item Open Access Seasonal to multi-decadal variability of the width of the tropical belt(Colorado State University. Libraries, 2013) Davis, Nicholas Alexander, author; Birner, Thomas, advisor; Thompson, David, committee member; Venayagamoorthy, Karan, committee memberAn expansion of the tropical belt has been extensively reported in observations, reanalyses, and climate model simulations, but there is a great deal of uncertainty in estimates of the rate of widening as different diagnostics give a wide range of results. This study critically examines robust diagnostics for the width of the tropical belt to explore their seasonality, interannual variability, and multi-decadal trends. These diagnostics are motivated by an exploration of two simple models of the Hadley circulation and subtropical jets. The width based on the latitudes of the maximum tropospheric dry bulk static stability, measuring the difference in potential temperature between the tropopause and the surface, is found to be closely coupled to the width based on the subtropical jet cores on all timescales. In contrast, the tropical belt width and Northern Hemisphere edge latitudes based on the latitudes at which the vertically-averaged streamfunction vanishes, a measure of the Hadley circulation's poleward edges, lags those of the other diagnostics by approximately one month. The tropical belt width varies by up to ten degrees latitude among the diagnostics, with trends in the tropical belt width ranging from -0.5 to 2.0 degrees per decade over the 1979-2012 period. Nevertheless, in agreement with previous studies nearly all diagnostics exhibit a widening trend, although the streamfunction diagnostic exhibits a significantly stronger widening than either the jet or dry bulk stability diagnostics. Finally, GPS radio occultation observations are used to assess the ability of the reanalyses to reproduce the tropical belt width, finding that they better situate the latitudes of maximum bulk stability versus those of the subtropical jets.Item Open Access Studies of oceanic, atmospheric, cryospheric, and fluvial processes through spectral analysis of seismic noise(Colorado State University. Libraries, 2016) Anthony, Robert Ernest, author; Aster, Richard, advisor; Schutt, Derek, committee member; Thompson, David, committee member; Reusch, David, committee memberDuring the past decade, there has been rapidly growing interest in using the naturally occurring seismic noise field to study oceanic, atmospheric, and surface processes. As many seismic noise sources, are non-impulsive and vary over a broad range of time scales (e.g., minutes to decades), they are commonly analyzed using spectral analysis or other hybrid time-frequency domain methods. The PQLX community data analysis program, and the recently released Noise Tool Kit that I co-developed with Incorporated Research Institutions for Seismology’s Data Management Center are used here to characterize seismic noise for a variety of environmental targets across a broad range of frequencies. The first two chapters of the dissertation place a strong emphasis on analysis of environmental microseism signals, which occur between 1-25 s period and are dominated by seismic surface waves excited by multiple ocean-solid Earth energy transfer processes. I move away from microseisms in Chapter 3 to investigate the generally higher frequency seismic signals (> 0.33 Hz) generated by fluvial systems. In Chapter 1, I analyze recently collected, broadband data from temporary and permanent Antarctic stations to quantitatively assess background seismic noise levels across the continent between 2007-2012, including substantial previously unsampled sections of the Antarctic continental interior. I characterize three-component noise levels between 0.15-150 s using moving window probability density function-derived metrics and analyze seismic noise levels in multiple frequency bands to examine different noise sources. These metrics reveal and quantify patterns of significant seasonal and geographic noise variations across the continent, including the strong effects of seasonal sea ice variation on the microseism, at a new level of resolution. Thorough analysis of the seismic noise environment and its relation to instrumentation and siting techniques in the Polar Regions facilitates new science opportunities and the optimization of deployment strategies for future seismological research in the Polar Regions, and in mountain glacier systems. Chapter 2 details the analysis of 23 years of microseism observations on the Antarctic Peninsula to investigate wave-sea ice interactions and assess the influence of the Southern Annular Mode (SAM) on storm activity and wave state in the Drake Passage. The lack of landmasses, climatological low pressure, and strong circumpolar westerly winds between latitudes of 50°S to 65°S produce exceptional Southern Ocean storm-driven wave conditions. This combination makes the Antarctic Peninsula one of Earth's most notable regions of high amplitude wave activity and one of the planet’s strongest sources of ocean-swell driven microseism noise in both the primary (direct wave-coastal region interactions) and secondary (direct ocean floor forcing due to interacting wave trains) period bands. Microseism observations are examined from 1993-2015 from long running seismographs located at Palmer Station (PMSA), on the west coast of the Antarctic Peninsula, and from the sub-Antarctic East Falkland Island (EFI). These records provide a spatially integrative measure of Southern Ocean amplitudes and of the degree of coupling between ocean waves and the solid earth with and without the presence of sea ice (which can reduce wave coupling with the continental shelf). A spatiotemporal correlation-based approach illuminates how the distribution of sea ice influences seasonal primary and secondary microseism power. I characterize primary and secondary microseism power due to variations in sea ice, and find that primary microseism energy is both more sensitive to sea ice and more capable of propagating across ocean basins than secondary microseism energy. During positive phases of the SAM, sea ice is reduced in the Bellingshausen Sea and overall storm activity in the Drake Passage increases, resulting in strongly increased microseism power levels. The field of fluvial seismology has emerged during the past decade, with seismic recordings near fluvial systems showing potential for a continuous, inexpensive, and non-invasive method of measuring flow and, in some cases, bed-load transport, in streams and rivers. In Chapter 3, I extend this research to the South Fork of the Cache la Poudre River in Northern Colorado where I deployed a small seismoaccoustic array while simultaneous measurements of discharge, suspended sediment concentrations, and precipitation were obtained. By placing seismometers within unprecedented proximity to the channel (~ 1 m, and during some time periods submerged), I found a broad range of frequencies excited by discharge, including novel, low-frequency (< 1 Hz) signals. After calibrating horizontal seismic power with flow rates over the course of a rainstorm event for individual sensors, I show that horizontal seismogram power in the 0.33-2 Hz band can be used to accurately invert for fluvial discharge with simple regressions, once a site is properly calibrated to a traditional hydrograph. These signals likely arise from local sensor tilt as the seismometer is directly forced by channel flow and show promise for augmenting seismic monitoring of fluvial systems by introducing a technique to estimate discharge rates from outside the channel with easily deployed noninvasive instrumentation.Item Open Access Tropical cyclone inner core structure and intensity change(Colorado State University. Libraries, 2011) Musgrave, Kate D., author; Schubert, Wayne, advisor; Davis, Christopher, advisor; Johnson, Richard, committee member; Thompson, David, committee member; Kirby, Michael, committee memberThis dissertation focuses on two projects that examine aspects of the relationship between tropical cyclone (TC) storm-scale dynamics and intensity. TC intensity change is a forecast challenge combining influences from the large-scale environment, the underlying ocean state, and the storm-scale dynamics within the TC. In particular structures and processes involving the TC eye are observed to have an impact on current and future intensity. The first project examines observations of TC eyes from aircraft reconnaissance flown into Atlantic basin TCs over the period 1989-2008. Relationships between TC eye diameter and type and intensity and intensity change are investigated. Consistent with previous studies, eye diameter does not display a direct relationship with intensity. Smaller eye diameters are observed at all intensities, though both the most and least intense TCs with eyes have smaller average eye diameters. Smaller eyes also have the largest variability in intensity change. Larger eyes show smaller ranges for intensity change, and the largest eyes tend to maintain or weaken in intensity. TCs with eyes reported had higher intensification rates and higher probabilities of undergoing rapid intensification. The second project takes a theoretical approach to examining the TC response to the location of the convection within the vortex structure using the balanced vortex model. An annular ring of heating is placed along an idealized axisymmetric vortex. The largest increase in intensity is produced when the heating is placed within the radius of maximum winds. Intensification still occurs at a lessened rate when the heating is contained within the vorticity skirt, and when the heating is outside the vorticity skirt the vortex does not intensify. The strength of the vortex increases in all cases, though less so than the intensity when the heating is within the radius of maximum winds.Item Open Access Wave-mean flow positive feedbacks associated with sudden stratospheric warmings(Colorado State University. Libraries, 2014) Sjoberg, Jeremiah P., author; Birner, Thomas, advisor; Eykholt, Richard, committee member; Garcia, Rolando, committee member; Schubert, Wayne, committee member; Thompson, David, committee memberSudden stratospheric warmings - most often characterized by zonal mean zonal wind easterlies at 60°N, 10 hPa - represent the largest dynamical perturbations to the wintertime polar stratosphere. Despite this, the predictability of sudden warmings remains low, in part because the forcing of these warming events involves a nonlinear positive feedback between planetary scale waves and the zonal wind of the stratosphere. In the wave-mean flow positive feedback, wave forcing decelerates the mean flow, allowing enhanced upward wave propagation, which then further decelerates the mean flow, etc., until the mean flow no longer supports wave propagation. This positive feedback process is crucial for the initiation of such events. Because the associated low predictability stems from poorly resolving initiation, this dissertation focuses on increasing mechanistic understanding of the wave-mean flow positive feedback associated with sudden stratospheric warmings. A simple model of wave-mean flow interaction is the first tool utilized here. In the original form of the model, constant bottom boundary wave forcing, set by geopotential height perturbations, results in a zonal wind state that oscillates between positive values (westerlies) and negative values (easterlies). We present a reformulation of the bottom boundary condition which allows for specification of the upward wave activity flux. Unlike with the original bottom boundary condition, we may precisely set the wave amplitudes propagating into the model domain. With this reformulated model, steady incoming wave fluxes lead to a steady zonal wind response. The oscillating state from the original model is found to rely on a representation of the positive feedback that is too strong. Transient forcing experiments in the reformulated simple model support previous results that there is a preferential wave forcing time scale on the order of 10 days for sudden stratospheric warmings. Forcing the model near this preferential time scale most efficiently drives the positive feedback. Lower stratospheric wave fields in reanalysis data show supporting evidence for these preferential wave forcing time scales prior to sudden stratospheric warmings. Pulses of wave activity flux are also analyzed in reanalysis data, and a set of pulses which are a novel proxy for strong wave-mean flow positive feedback are found. The zonal wind near these pulses display the expected characteristics of the positive feedback: strong precedent zonal winds and strong subsequent wind decelerations. This proxy is thus a useful diagnostic for the wave-mean flow positive feedback. A general circulation model forced by idealized planetary scale topography is employed to perform high order experiments. By stepwise increasing the height of the topography, we find that the frequency of sudden stratospheric warmings within the model increases nonlinearly to a maximum at moderate topographic heights and then strongly jumps down to a lower, steady value for still higher topography. Analyzing the proxy for positive feedback here reveals that the positive feedback is strongest in the range of topographic heights associated with the largest occurrence of sudden warmings, and also that preferential wave forcing time scales on the order of 10 days are upheld.Item Open Access What controls the variability of oxygen in the subpolar North Pacific(Colorado State University. Libraries, 2011) Takano, Yohei, author; Ito, Takamitsu, advisor; Thompson, David, committee member; Deutsch, Curtis, committee member; Harton, John, committee memberDissolved oxygen is a widely observed chemical quantity in the oceans along with temperature and salinity. Changes in the dissolved oxygen have been observed over the world oceans. Observed oxygen in the Ocean Station Papa (OSP, 50°N, 145°W) in the Gulf of Alaska exhibits strong variability over interannual and decadal timescales, however, the mechanisms driving the observed variability are not yet fully understood. Furthermore, irregular sampling frequency and relatively short record length make it difficult to detect a low-frequency variability. Motivated by these observations, we investigate the mechanisms driving the low-frequency variability of oxygen in the subpolar North Pacific. The specific purposes of this study are 1) to evaluate the robustness of the observed low-frequency variability of dissolved oxygen and 2) to determine the mechanisms driving the observed variability using statistical data analysis and numerical simulations. To evaluate the robustness of the low-frequency variability, we conducted spectral analyses on the observed oxygen at OSP. To address the irregular sampling frequency we randomly sub-sampled the raw data to form 500 ensemble members with a regular time interval, and then performed spectral analyses. The resulting power spectrum of oxygen exhibits a robust low-frequency variability and a statistically significant spectral peak is identified at a timescale of 15-20 years. The wintertime oceanic barotropic streamfunction is significantly correlated with the observed oxygen anomaly at OSP with a north-south dipole structure over the North Pacific. We hypothesize that the observed low-frequency variability is primarily driven by the variability of large-scale ocean circulation in the North Pacific. To test this hypothesis, we simulate the three-dimensional distribution of oxygen anomaly between 1952 to 2001 using data-constrained circulation fields. The simulated oxygen anomaly shows an outstanding variability in the Gulf of Alaska, showing that this region is a hotspot of oxygen fluctuation. Anomalous advection acting on the climatological mean oxygen gradient is the source of oxygen variability in this simulation. Empirical Orthogonal Function (EOF) analyses of the simulated oxygen show that the two dominant modes of the oxygen anomaly explains more than 50% of oxygen variance over the North Pacific, that are closely related to the dominant modes of climate variability in the North Pacific (Pacific Decadal Oscillation and North Pacific Oscillation). Our results imply the important link between large-scale climate fluctuations, ocean circulation and biogeochemical tracers in the North Pacific.