Mountain Scholar :: Browsing by Author "Thompson, David W. J., advisor"

Browsing by Author "Thompson, David W. J., advisor"

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Open Access
Connections between climate sensitivity and large-scale extratropical dynamics
(Colorado State University. Libraries, 2019) Davis, Luke L. B., author; Thompson, David W. J., advisor; Birner, Thomas, advisor; Randall, David A., committee member; Pinaud, Olivier, committee member
The response of the extratropical storm tracks to anthropogenic forcing is one of the most important but poorly understood aspects of climate change. The direct, thermodynamic effects of climate change are relatively well understood, but their two-way interactions with large-scale extratropical dynamics are extremely difficult to predict. There is thus continued need for a robust understanding of how this coupling evolves in space and time. The dry dynamical core represents one of the simplest possible numerical models for studying the response of the extratropical storm tracks to climate change. In the model, the extratropical circulation is forced by relaxing to a radiative equilibrium profile using linear damping. The linear damping coefficient plays an essential role in governing the structure of the circulation. But despite decades of research with the dry dynamical core, the role of the damping coefficient in governing the circulation has received relatively little scrutiny. In this thesis, we systematically vary the damping rate and the equilibrium temperature field in a dry dynamical core in order to understand how the amplitude of the damping influences extratropical dynamics. Critically, we prove that the damping rate is a measure of the climate sensitivity of the dry atmosphere. The key finding is that the structure of the extratropical circulation is a function of the climate sensitivity. Larger damping timescales – which are equivalent to higher climate sensitivities – lead to a less dynamically active extratropical circulation, equatorward shifts in the jet, and a background state that is almost neutral to baroclinic instability. They also lead to increases in the serial correlation and relative strength of the annular modes of climate variability. It is argued that the climate sensitivity of the dry atmosphere may be identifiable from its dynamical signatures, and that understanding the response of the circulation to climate change is critically dependent on understanding its climate sensitvity.
Open Access
Links between climate feedbacks and the large-scale circulation across idealized and complex climate models
(Colorado State University. Libraries, 2023) Davis, Luke L. B., author; Thompson, David W. J., advisor; Maloney, Eric, committee member; Randall, David, committee member; Pinaud, Olivier, committee member; Gerber, Edwin, committee member
The circulation response to anthropogenic forcing is typically considered in one of two distinct frameworks: One that uses radiative forcings and feedbacks to investigate the thermodynamics of the response, and another that uses circulation feedbacks and thermodynamic constraints to investigate the dynamics of the response. In this thesis, I aim to help bridge the gap between these two frameworks by exploring direct links between climate feedbacks and the atmospheric circulation across ensembles of experiments from idealized and complex general circulation models (GCMs). I first demonstrate that an existing, widely-used type of idealized GCM — the dynamical core model — has climate feedbacks that are explicitly prescribed and determined by a single parameter: The thermal relaxation timescale. The dynamical core model may thus help to fill gaps in the model hierarchies commonly used to study climate forcings and climate feedbacks. I then perform two experiments: One that explores the influence of prescribed feedbacks on the unperturbed, climatological circulation; and a second that explores their influence on the circulation response to a horizontally uniform, global warming-like forcing perturbation. The results indicate that more stabilizing climate feedbacks are associated with 1) a more vigorous climatological circulation with increased thermal diffusivity, and 2) a weaker poleward displacement of the circulation in response to the global warming-like forcing. Importantly, since the most commonly-used relaxation timescale field resembles the real-world clear-sky feedback field, the uniform forcing perturbations produce realistic warming patterns, with amplified warming in the tropical upper troposphere and polar lower troposphere. The warming pattern and circulation response disappear when the relaxation timescale field is instead spatially uniform, demonstrating the critical role of spatially-varying feedback processes on shaping the response to anthropogenic forcing. I next explore circulation-feedback relationships in more complex GCMs using results from the most recent Coupled Model Intercomparison Projects (CMIP5 and CMIP6). Here, I estimate climate feedbacks by regressing top-of-atmosphere radiation against surface temperature for both 1) an unperturbed pre-industrial control experiment and 2) a perturbed global warming experiment forced by an abrupt quadrupling of CO2 concentrations. I find that across both ensembles, the cloud component of the perturbed climate feedback is closely related to the cloud component of the unperturbed climate feedback. Critically, the relationship is much stronger in CMIP6 than CMIP5, contrasting with many previously proposed constraints on the perturbation response. The relationship also explains the slow part of the CO2 response better than the fast, transient response. In general, the strength of the relationship depends on the degree to which the spatial pattern of the response resembles ENSO-dominated internal variability, with "El Niño-like" East Pacific warming and related tropical cloud changes. This is consistent with fluctuation-dissipation theory: Regions with stronger deep ocean heat exchange and weaker net feedbacks must always dominate both 1) internal fluctuations in the global energy budget, and 2) the slow part of the response to forcing perturbations. The stronger CMIP6 inter-model relationships are due to both an amplification of this mechanism and higher inter-model correlations between tropical cloud changes and extratropical cloud changes. Finally, I present emergent constraints on the slow response using a recent observational estimate of the unperturbed cloud feedback. I conclude by discussing some implications of these results. I consider how the relaxation feedback framework might be further developed and reconciled with traditional climate feedbacks to provide future research opportunities with climate model hierarchies.
Open Access
On quasi-periodic Baroclinic variability in the extratropical circulation
(Colorado State University. Libraries, 2016) Crow, Brian, author; Thompson, David W. J., advisor; Barnes, Elizabeth A., committee member; Aster, Richard C., committee member
A number of recent studies have demonstrated that large-scale extratropical wave activity is characterized by quasi-periodic behavior on timescales of 20-30 days, particularly in the Southern Hemisphere. This phenomenon has been termed the Baroclinic Annular Mode (BAM), and is responsible for the modulation of eddy heat fluxes, eddy kinetic energy, and precipitation on large scales. However, the extent to which this periodic modulation is discernable or significant on smaller spatial scales had not yet been established. Using data from the ECMWF Interim Reanalysis for the period 1979-2014, this study extensively examines the spatial structure of the BAM. Spectral analyses reveal the spatial limitations of the periodic behavior, while lag-correlation analyses reveal the patterns of propagation and development of anomalies that give rise to the observed periodicity. Periodic behavior is more robust in the Southern Hemisphere than in the Northern Hemisphere, but filtering out low wavenumbers from NH data helps clarify the BAM signal. Additionally, it is demonstrated that the BAM appears very differently in two relatively similar global climate models, suggesting further study is needed to determine how modern GCMs capture the BAM. Supplementing our analyses of observed and modeled data is a simple two-way linear feedback model, which is utilized to demonstrate the principal mechanism underlying periodic behavior in the BAM. The model makes it apparent that the BAM can be modeled as a simple linear feedback between baroclinicity and eddy heat fluxes. The periodicity seen on larger scales is a product of differential advection rates affecting the development of spatially overlapping, out-of-phase anomalies. The large-scale nature of the periodic behavior, however, makes it difficult to draw conclusions about the potential utility of the BAM for weather analysts and forecasters, and the limitations of this study limit our ability to describe its role in the climate system. It is hoped that the research presented here will pave the way to future studies which may more thoroughly answer such questions.
Open Access
On the observed and simulated responses of the extratropical atmosphere to surface thermal forcing
(Colorado State University. Libraries, 2019) Wills, Samantha M., author; Thompson, David W. J., advisor; Alexander, Michael, committee member; Barnes, Elizabeth, committee member; Maloney, Eric, committee member; Venayagamoorthy, Subhas Karan, committee member
The ocean is an integral part of the climate system, and its closely-coupled interactions with the atmosphere system have wide-ranging impacts on the large-scale and local patterns of climate and weather variability from one region of the globe to another. Improvements in the resolution of satellite observations and numerical models over the past decade have led to a series of advances in understanding the role of the ocean in extratropical air-sea interaction. While the influence of the extratropical ocean can be relatively subtle and difficult to detect, recent studies have provided a growing body of evidence suggesting that the extratropical ocean has a potentially important influence on the atmospheric circulation on a wide variety of timescales. The aim of this thesis is to improve the current understanding on the role of the extratropical ocean in climate by 1) presenting new observational analyses on the relationships between midlatitude SST anomalies and the atmospheric circulation on subseasonal timescales and 2) providing a new, simplistic framework for interpreting the atmospheric response to surface thermal forcing across the globe in an idealized global climate model. In the first theme of this thesis, observational analyses of daily-mean data are exploited to re-examine the evidence for midlatitude air-sea interaction over the Kuroshio-Oyashio Extension region, and important comparisons are drawn to a previous companion study over the Gulf Stream Extension region. The results indicate that during the boreal winter season, SST anomalies in both the Gulf Stream and Kuroshio-Oyashio Extension regions are associated with distinct spatial and temporal patterns of atmospheric variability that precede and follow peak amplitude in the SST field on daily-mean timescales. In particular, a very similar pattern of low pressure anomalies that develops over the warm SST anomalies is viewed as the most robust common aspect of the atmospheric "response" over both ocean basins. The least common aspect of the "response" is characterized by robust high pressure anomalies that develop over the North Atlantic and have a seemingly unique relationship to positive lower-tropospheric temperature anomalies generated over the Gulf Stream Extension region. These results suggest that extratropical SST anomalies on subseasonal timescales are capable of forcing significant changes in the large-scale atmospheric circulation through the transfer of heat from the ocean to the atmosphere. Partially motivated by the results from the observational analyses, the second theme of this thesis presents a simplified model framework to critically assess the one-way influence of the ocean on the atmosphere at different locations across the globe. A series of steady-state and transient numerical experiments are designed to explore the atmospheric response to surface thermal forcing in an idealized "aquaplanet" configuration of the NCAR Community Atmosphere Model, Version 5.3. The results indicate that in each of the extratropical SST perturbation experiments, there is a consistent and robust steady-state atmospheric response (of similar sign and amplitude) to surface thermal forcing. The response is characterized by a hemispheric-scale, equivalent-barotropic pattern of atmospheric circulation anomalies reminiscent of the model's leading mode of internal variability and is seemingly independent of the latitudinal placement of the heat source. This result is explored further, and a possible explanation of the consistent steady-state atmospheric circulation response is discussed.
Open Access
On the observed relationships between variability in sea surface temperatures and the atmospheric circulation in the Northern Hemisphere
(Colorado State University. Libraries, 2015) Wills, Samantha M., author; Thompson, David W. J., advisor; Barnes, Elizabeth, committee member; Venayagamoorthy, Subhas Karan, committee member
The advent of increasingly high-resolution satellite observations and numerical models has led to a series of advances in our understanding of the role of midlatitude sea surface temperature (SST) in climate variability, especially near western boundary currents (WBC). For example, recent observational analyses suggest that ocean dynamics play a central role in driving interannual SST variability over the Kuroshio-Oyashio and Gulf Stream Extension regions, and recent numerical experiments suggest that variations in the SST field in the Kuroshio-Oyashio Extension region may have a much more pronounced influence on the atmospheric circulation than previously thought. We assess the observational support for (or against) a robust atmospheric response to midlatitude ocean variability in the Kuroshio-Oyashio and Gulf Stream Extension regions. We apply lead/lag analysis based on daily data to assess relationships between SST anomalies and the atmospheric circulation on transient timescales, building off of previous studies that have applied a similar methodology to weekly data. In addition, we employ a novel approach to separate the regressions into an "atmospheric forcing" pattern and an "atmospheric response" pattern through spatial linear decomposition. The analysis reveals two distinct patterns associated with midlatitude atmosphere/ocean interaction in the vicinity of the major Northern Hemisphere WBCs: 1) a pattern that peaks 2-3 weeks before the SST anomalies (the "atmospheric forcing") and 2) a pattern that peaks after the SST anomalies (the "atmospheric response"). The latter pattern is independent of the former, and is interpreted as the signature of SST variability in the atmospheric circulation. Further analysis is required to understand if the "atmospheric response" pattern truly reflects the response to the SST anomalies within the WBC regions.
Open Access
On the structure of climate variability near the tropopause and its relationship to equatorial planetary waves
(Colorado State University. Libraries, 2011) Grise, Kevin M., author; Thompson, David W. J., advisor; Birner, Thomas, committee member; Eykholt, Richard E., committee member; Randall, David A., committee member; Randel, William J., committee member
The tropopause is an important interface in the climate system, separating the unique dynamical, chemical, and radiative regimes of the troposphere and stratosphere. Previous studies have demonstrated that the long-term mean structure and variability of the tropopause results from a complex interaction of stratospheric and tropospheric processes. This project provides new insight into the processes involved in the global tropopause region through two perspectives: 1) a high vertical resolution climatology of static stability and 2) an observational analysis of equatorial planetary waves. High vertical resolution global positioning system radio occultation profiles are used to document fine-scale features of the global static stability field near the tropopause. Consistent with previous studies, a region of enhanced static stability, known as the tropopause inversion layer (TIL), exists in a narrow layer above the extratropical tropopause and is strongest over polar regions during summer. However, in the tropics, the TIL possesses a unique horizontally and vertically varying structure with maxima located at ~17 and ~19 km. The upper feature peaks during boreal winter and has its largest magnitude between 10° and 15° latitude in both hemispheres; the lower feature exhibits a weaker seasonal cycle and is centered at the Equator. The spatial structure of both features resembles the equatorial planetary wave response to the climatological distribution of deep convection. Equatorial planetary waves not only dominate the climatological-mean general circulation near the tropical tropopause but also play an important role in its intraseasonal and interannual variability. The structure of the equatorial planetary waves emerges as the leading pattern of variability of the zonally asymmetric tropical atmospheric circulation. Regressions on an index of the equatorial planetary waves reveal that they are associated with a distinct pattern of equatorially symmetric climate variability characterized by variations in: 1) the distribution of convection in the deep tropics; 2) the eddy momentum flux convergence and the zonal-mean zonal wind in the tropical upper troposphere; 3) the mean meridional circulation of the tropical and subtropical troposphere; 4) temperatures in the tropical upper troposphere, the tropical lower stratosphere, and the subtropical troposphere of both hemispheres; and 5) the amplitude of the upper tropospheric anticyclones that straddle the Equator over the western tropical Pacific Ocean. The pulsation of the equatorial planetary waves in time provides a framework for interpreting a broad range of climate phenomena. Variability in the equatorial planetary waves is associated with variability in the tropical TIL and is linked to both the El Niño-Southern Oscillation and the Madden-Julian Oscillation (MJO). Evidence is presented that suggests that the MJO can be viewed as the linear superposition of: 1) the pulsation of the equatorial planetary waves at a fixed location and 2) a propagating component. Variability in the equatorial planetary waves may also contribute to variability in troposphere/stratosphere exchange and the width of the tropical belt.
Open Access
Quantifying internal climate variability and its changes using large-ensembles of climate change simulations
(Colorado State University. Libraries, 2020) Li, Jingyuan, author; Thompson, David W. J., advisor; Barnes, Elizabeth A., committee member; Ravishankara, A. R., committee member; Cooley, Daniel, committee member
Increasing temperatures over the last 50 years have led to a multitude of studies on observed and future impacts on surface climate. However, any changes on the mean need to be placed in the context of its variability to be understood and quantified. This allows us to: 1) understand the relative impact of the mean change on the subsequent environment, and 2) detect and attribute the external change from the underlying "noise" of internal variability. One way to quantify internal variability is through the use of large ensemble models. Each ensemble member is run on the same model and with the same external forcings, but with slight differences in the initial conditions. Differences between ensemble members are due solely to internal variability. This research exploits one such large ensemble of climate change simulations (CESM-LE) to better understand and evaluate surface temperature variability and its effects under external forcing. One large contribution to monthly and annual surface temperature variability is the atmospheric circulation, especially in the extratropics. Dynamical adjustment seeks to determine and remove the effects of circulation on temperature variability in order to narrow the range of uncertainty in the temperature response. The first part of this work compares several commonly used dynamical adjustment methods in both a pre-industrial control run and the CESM-LE. Because there are no external forcings in the control run, it is used to provide a quantitative metric by which the methods are evaluated. We compare and assess these dynamical adjustment methods on the basis of 2 attributes: 1) the method should remove a maximum amount of internal variability while 2) preserving the true forced signal. While the control run is excellent for assessing the methods in an "ideal" environment, results from the CESM-LE show biases in the dynamically-adjusted trends due to a forced response in the circulation fields themselves. This work provides a template from which to assess the various dynamical adjustment methods available to the community. A less studied question is how internal variability itself will respond to climate change. Past studies have found regional changes in surface temperature variance and skewness. This research also investigates the impacts of climate change on day-to-day persistence of surface temperature. Results from the CESM-LE suggest that external warming generally increases surface temperature persistence, with the largest changes over the Arctic and ocean regions. The results are robust and distinct from internal variability. We suggest that persistence changes are mostly due to an increase in the optical thickness of the atmosphere due to increases in both carbon dioxide and water vapor. This increased optical thickness reduces the thermal damping of surface temperatures, increasing their persistence. Model results from idealized aquaplanet simulations with different radiation schemes support this hypothesis. The results thus reflect a robust thermodynamic and radiative constraint on surface temperature variability.
Open Access
The atmospheric circulation response to climate change-like thermal forcings in a simple general circulation model
(Colorado State University. Libraries, 2009) Butler, Amy Hawes, author; Thompson, David W. J., advisor
Temperature changes due to increased greenhouse gases and depleted stratospheric ozone are associated with robust changes in the large-scale atmospheric circulation. In this thesis we explore how these anthropogenically-driven temperature changes affect the atmospheric circulation. Our approach is to force a simple dry dynamical general circulation model (GCM) with idealized thermal forcings that resemble three key effects of greenhouse gas increases and stratospheric ozone depletion: warming at the polar surface, warming of the tropical upper troposphere, and cooling of the polar stratosphere.
Open Access
The contribution of clouds to global surface temperature variability on monthly to decadal timescales
(Colorado State University. Libraries, 2022) Boehm, Chloe, author; Thompson, David W. J., advisor; Randall, David, committee member; McGrath, Daniel, committee member
Cloud radiative effects (CREs) have well documented impacts on the mean climate, and have recently been found to play a key role in climate variability in the tropics. This thesis expands on previous work to probe the role of CREs on extratropical surface temperature variability. The impact of CREs on climate variability is isolated using the 'cloud-locking' method run on the Max Planck Earth System Model. This method involves comparing the output from two climate simulations: one in which clouds are coupled to the atmospheric circulation, and another in which clouds are prescribed and thus decoupled from the flow. Results show that coupling between CREs and the atmospheric circulation leads to widespread increases in extratropical surface temperature variability, particularly over the North Atlantic and North Pacific. This work then explores on what timescales surface temperature variability is increased. In general, CREs play an increasingly large role in surface temperature variability at increasingly long timescales. Importantly, cloud-circulation coupling leads to enhanced decadal temperature variability of ~25–45% over most of the Northern Hemisphere oceans and ~10–15% over most of the land areas. Finally, using a simple expression for temperature variance in terms of the surface energy balance, the mechanisms driving these variability changes are identified. This variability enhancement derives from 'reddening' of surface temperature variability by cloud shortwave radiative effects. These results demonstrate the dominant effect that cloud-circulation coupling has on interannual and decadal temperature variability across much of the globe. This work has implications for the interpretation of observed decadal variability, and for the importance of cloud-circulation coupling in climate model simulations.
Open Access
The role of Earth system interactions in large-scale atmospheric circulation and climate
(Colorado State University. Libraries, 2023) Yook, Simchan, author; Thompson, David W. J., advisor; Ravishankara, A. R., committee member; Hurrell, James, committee member; Ebert-Uphoff, Imme, committee member
The complex interactions among different components of the Earth system play a key role in governing the climate variability through various physical processes. For example, an interaction between the fluctuations in one component of the Earth system and associated variations in another component of the Earth system can either amplify or dampen the climate variability depending on the nature of their two-way feedback mechanisms. Thus, understanding the role of various physical interactions among components of the Earth system is critical to understand the changes in climate as well as to reduce the uncertainty in future climate projections. This dissertation focuses on discovering the key processes and interactions among different components of the Earth system on the climate variability using observations and model hierarchies. In Part 1, the interactions between the atmospheric circulation and western North Pacific SST anomalies are explored in two sets of simulations: 1) a simulation run on a coupled atmosphere-ocean general circulation model (GCM), and 2) a simulation forced with prescribed, time-evolving SST anomalies over the western North Pacific. The results support the interpretation of the observed lead/lag relationships between western North Pacific Sea Surface Temperature (SST) anomalies and the atmospheric circulation, and provide numerical evidence that SST variability over the western North Pacific has a demonstrable effect on the large-scale atmospheric circulation throughout the North Pacific sector. In Part 2, the role of moist lapse rate in altering the temperature variability under climate change is explored. To reduce the complexity of the problem, the changes in the temperature variance under global warming are first analyzed in the simplest version of model hierarchy: a single column Rapid Radiative Transfer Model with a simplified convective adjustment. Similar analyses were repeated with varying model hierarchies with additional complexities: a global general circulation model in global Radiative Convective Equilibrium (RCE) setting with fixed SST, and fully coupled Earth system models. The results highlight the role of moist lapse rate as a potential constraint for climate variability in the tropical atmosphere simulated by different model hierarchies. In Part 3, the effects of coupled chemistry-climate interactions on the amplitude and structure of stratospheric temperature variability are quantified in two numerical simulations: A "free running" simulation that includes fully coupled chemistry-climate interactions; and a "specified chemistry" version of the model forced with prescribed chemical composition. The results indicate that the inclusion of coupled chemistry-climate interactions increases the internal variability of temperature by a factor of ~two in the lower tropical stratosphere through dynamically driven ozone-temperature feedbacks. The results highlight the fundamental role of two-way feedbacks between the atmospheric circulation and chemistry in driving climate variability in the lower stratosphere. In Part 4, the effects of coupled chemistry-climate interactions on the large-scale atmospheric circulation are further explored based on two observational case studies of the Antarctic ozone holes of 2020 and 2021. The 2020 and 2021 were marked by two of the largest Antarctic ozone holes on record. It has been demonstrated that the ozone holes of 2020 and 2021 were associated with large changes in the atmospheric circulation consistent with the climate impacts of Antarctic ozone depletion. The ozone holes were also unusual for their associations with aerosol burdens due to two extraordinary events: the Australian wildfires of early 2020 and the eruption of La Soufriere in 2021. The results provide suggestive evidence that injections of both wildfire smoke and volcanic emissions into the stratosphere can lead to hemispheric-scale changes in surface climate. This dissertation provides a detailed look at the complex aspects of the coupled interactions among different components of the Earth system and their roles on climate variability and large-scale dynamics. To clarify the role of the different physical processes contributing to the climate responses, this study performed a comprehensive analysis based on observations as well as a series of numerical experiments run on different configurations of climate model hierarchies. The findings herein improve our understanding of different Earth system interactions and their influences on global climate and large-scale atmospheric dynamics.
Open Access
Towards understanding the role of natural variability in climate change
(Colorado State University. Libraries, 2017) Li, Jingyuan, author; Thompson, David W. J., advisor; Barnes, Elizabeth A., committee member; Cooley, Daniel, committee member
Natural variability plays a large role in determining surface climate on local and regional scales. Understanding the role of natural variability is crucial for accurately assessing and attributing climate trends, both past and future. One successful way to examine the role of natural variability in climate change has been through large ensembles of climate models. This thesis uses one such large ensemble (the NCAR CESM-LE) to test various methods used to quantify natural variability in the context of climate change. We first introduce a simple analytic expression for calculating the lead time required for a linear trend to emerge in a Gaussian first order autoregressive process. The expression is derived from the standard error of the regression and is tested using the CESM-LE. It is shown to provide a robust estimate of the point in time when the forced signal of climate change has emerged from the natural variability of the climate system with a predetermined level of statistical confidence. The expression provides a novel analytic tool for estimating the time of emergence of anthropogenic climate change and its associated regional climate impacts from either observed or modeled estimates of natural variability and trends. We next compare and analyze various methods for calculating the effects of internal circulation dynamics on surface temperature. Dynamical adjustment seeks to separate out dynamical contribution to temperature trends, thus reducing the amplitude of natural variability that obscures the signal of anthropogenic forcing. Three specific methods used in the climate literature are examined: principal component analysis (PCR), maximum covariance analysis (MCA), and constructed circulation analogs. An assessment of these methods are given with their respective results from the CESM control run and large ensemble.

Browsing by Author "Thompson, David W. J., advisor"

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