Browsing by Author "Rugenstein, Jeremy, committee member"
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Item Open Access Effects of warming and stratospheric aerosol injection on tropical cyclone distribution and frequency in a high-resolution global circulation model(Colorado State University. Libraries, 2024) Feder, Andrew, author; Randall, David, advisor; Hurrell, James, committee member; Rugenstein, Jeremy, committee memberTropical cyclones (TCs) occur stochastically in any given TC season, with varying numbers and intensities within basins over time. Nevertheless, they arise out of fundamental laws of thermodynamics and fluid physics, and in recent years, as global circulation models (GCMs) have increased in spatial resolution, increasingly realistic TCs and TC distributions have emerged from them. Where prior research on TC climatologies has relied on proxies like Potential Intensity (PI) and synthetic storm models, the cyclones emerging from the dynamics of newer GCMs can now be analyzed directly, using native model variables. Such direct analysis may be particularly useful in studying possible global storm distributions under radically altered future climates, including high-emissions warming scenarios, and even those shaped by climate interventions. These interventions include various directed changes in global albedo, such as Stratospheric Aerosol Injection (SAI), with only limited precedent in the historical period. GCMs simulating realistic climate intervention scenarios, have not as of yet paired storm-resolving resolution with realistic intervention scenario construction. This has left gaps in our understanding as to how interventions might affect global storm/TC distributions, and whether ameliorating warming in this way could also substantially lessen related natural disaster risk profiles. In this paper, we utilize a new high-resolution model configuration to conduct experiments examining the effects of SAI, on tropical cyclones and global storm physics more broadly. These experiments are constructed based on prior work on SAI using the GLENS GCM ensemble (Tilmes et al. 2020; Danabasoglu 2019a,b). Our analysis centers on 3 10-year experiments conducted using 30-km grid spacing. These include a recent-past calibration run; the Intergovernmental Panel on Climate Change climate pathway SSP 8.5 (IPCC 2021), for the years 2090-2099, with no SAI; and SSP 8.5, with SAI having begun in 2020 to maintain a global temperature rise of no more than 1.5° C, also simulated for the years 2090-2099. With the resulting data sets, we deploy a novel TC-tracking algorithm to analyze resulting changes in storm tracks and properties. Based on our results for these different scenarios, we find that SAI, while in some ways restoring global storm patterns to a pre-warming state, may also create unique basin-scale TC distribution features and pose novel related hazards.Item Open Access Impacts of historic anthropogenic aerosol forcing on large climate ensembles through the lens of poleward energy transport(Colorado State University. Libraries, 2024) Needham, Michael Robert, author; Randall, David A., advisor; Rugenstein, Maria, committee member; van Leeuwen, Peter Jan, committee member; Rugenstein, Jeremy, committee memberIn discussions of the human impact on Earth's climate, aerosols receive much less attention than greenhouse gases. And yet, the change in the global mean effective radiative forcing from anthropogenic aerosols was roughly of the same magnitude (but of opposite sign) as the change in greenhouse gases throughout much of the twentieth century. Aerosols also represent the largest uncertainty in the effective radiative forcing, due to their complex interactions with clouds and solar radiation. Complicating this even further, aerosols are relatively short-lived within the atmosphere, and thus exhibit a large degree of variability in space and time. This dissertation presents a set of studies which investigate the ways in which historic anthropogenic aerosols may have impacted the Earth's weather and climate, through the analysis of a large number historic climate model simulations which comprise so-called large ensembles. Analysis of these ensembles allows for the isolation of some forced signal (e.g., the influence of aerosols) from the noise (i.e., the background variability of the model). This leads to conclusions through the analysis of summary statistics across members of the ensemble population which would be impossible to make based on only one or a few simulations. In particular, these studies show that the emission of aerosol precursors from Europe and North America increased the northward transport of heat from the southern into the northern hemisphere in an ensemble of simulations performed with version 2 of the Community Earth System Model (CESM2). The additional heat transport was in excess of 0.25 PW. This is an increase of at least 4-5% compared to the baseline maximum transport of between 5-6 PW which occurs in the mid-latitudes. At latitudes away from these maxima, the increase was a much larger percentage of the total. This anomalous northward energy transport was accomplished by changes in both atmospheric and oceanic processes. These include a southward shift of the Intertropical Convergence Zone (ITCZ) associated with changes in the Hadley cells; an increase in the frequency of extratropical cyclones in the north Atlantic; a strengthening of the Atlantic Meridional Overturning Circulation (AMOC); as well as changes to multiple ocean processes across the Indo-Pacific. Comparison of these results to the literature indicates that this modeled response to aerosols in CESM2 is likely too large. Furthermore, analysis of two additional large ensembles reveals that this over-sensitivity of CESM2 cannot be due to some deficiency in the model. Instead, it is demonstrated that the difference is the result of changes to the historical emission estimates between phase 5 and phase 6 of the Coupled Model Intercomparison Project (i.e., CMIP5 and CMIP6). This finding leads to the hypothesis that the higher interannual variability associated with a change from decadal-scale CMIP5 emissions to annual-scale CMIP6 emissions is the ultimate cause of the overzealous response of the model. Testing this hypothesis likely will provide the most fertile ground for future work.Item Open Access Surface heat fluxes and MJO propagation through the Maritime Continent(Colorado State University. Libraries, 2022) Hudson, Justin, author; Maloney, Eric, advisor; Rasmussen, Kristen, committee member; Rugenstein, Jeremy, committee memberThe 'barrier effect' of the Maritime Continent (MC) is a known hurdle in understanding the propagation of the Madden-Julian Oscillation (MJO). To understand the differing dynamics of MJO events that propagate versus stall over the MC, a new MJO tracking algorithm utilizing 30-96 day filtered NOAA Interpolated OLR anomalies is presented. Using this algorithm, MJO events can be identified, tracked, and described in terms of their propagation characteristics. Latent heat flux from CYGNSS and OAFLUX as well as CYGNSS surface winds are used to compare large-scale patterns for MJO events that do and do not propagate through the MC. Local area-averaged surface fluxes and OLR anomalies are 7-14% and 18-22% of the value of precipitation anomalies, respectively. While differences in these contributions do not change substantially for propagating versus terminating events, precipitation events that successfully propagate through the MC demonstrate surface flux anomalies that are stronger and more spatially-coherent. The spatial scale of precipitation events that propagate through the MC region is also larger than terminating events. It is also shown that large-scale enhancement of latent heat fluxes near and to the east of the Dateline accompanies MJO events that successfully propagate through the MC. This large-scale enhancement of latent heat fluxes to the east of the Dateline is equally driven by dynamic and thermodynamic effects. These findings are placed in the context of recent theoretical models of the MJO in which latent heat fluxes are important for propagation and destabilization. The tracking algorithm is also used to show for historical and greenhouse gas warming scenarios in CESM2 that MJO propagation speed increases and precipitation anomalies propagate further east with warming. However, the CESM2 inadequately represents the 'barrier effect' of the MC region on propagating MJO events.Item Open Access Understanding the role of ocean dynamics in climate variability(Colorado State University. Libraries, 2021) Patrizio, Casey R., author; Thompson, David, advisor; Randall, David, advisor; Rugenstein, Maria, committee member; Rugenstein, Jeremy, committee member; Small, Richard, committee memberThe ocean plays a key role in regulating Earth's mean climate, both because of its massive heat capacity, but also its heat transport by slow-moving circulations and other dynamics. In principle, fluctuations in such ocean heat transport can influence the variability in the climate, by impacting the sea-surface temperature (SST) variability and in turn the atmospheric variability through surface heat exchange, but this is incompletely understood, particularly in the extratropics. The goal of this dissertation is to clarify the role of ocean dynamics in climate variability, first focusing on the role of ocean dynamics in SST variability across the global oceans (Chapters 1 and 2), and then on the impact of midlatitude ocean-driven SST anomalies on the atmospheric circulation (Chapter 3). In Chapter 1, the contributions of ocean dynamics to ocean-mixed layer temperature variance are quantified on monthly to multiannual timescales across the globe. To do so, two methods are used: 1) a method in which monthly ocean heat transport anomalies are estimated directly from a state-of-the-art ocean state estimate spanning 1992-2015; and 2) a method in which they are estimated indirectly using the energy budget of the mixed layer with monthly observations of SSTs and air-sea heat fluxes between 1980-2017. Consistent with previous studies, both methods indicate that ocean dynamics contribute notably to mixed layer temperature variance in western boundary current regions and tropical regions on monthly to interannual timescales. However, in contrast to previous studies, the results also suggest that ocean dynamics reduce the variance of Northern Hemisphere mixed layer temperatures on timescales longer than a few years. In Chapter 2, the role of ocean dynamics in midlatitude SST variability is further understood using Hasselmann's model of climate variability, wherein midlatitude SST anomalies are driven entirely by atmospheric processes. Motivated by the results of Chapter 1, here Hasselmann's climate model is extended to include the forcing and damping of SST variability by ocean processes, which are estimated indirectly from monthly observations. It is found that the classical Hasselmann model driven only by observed surface heat fluxes generally produces midlatitude SST power spectra that are too red compared to observations. Including ocean processes in the model reduces this discrepancy by decreasing the low-frequency SST variance and increasing the high-frequency SST variance, leading to a whitening of the midlatitude SST spectra. This happens because ocean forcing increases the midlatitude SST variance across many timescales but is outweighed by ocean damping at timescales > 2 years, particularly away from the western boundary currents. It is also shown that the whitening of midlatitude SST variability by ocean dynamical processes operates in NCAR's Community Earth System Model (CESM). In the final chapter, the atmospheric circulation response to midlatitude ocean-forced SST anomalies is explored. In particular, the extended Hasselmann model is used to isolate the oceanic and atmospheric-forced components of the observed SST variability in the Kuroshio-Oyashio Extension (KOE) region. The associated atmospheric circulation anomalies are diagnosed by lagged-regression of monthly sea-level pressure (SLP) anomalies onto the KOE-averaged SST anomalies, and their oceanic and atmospheric-forced components. Consistent with previous studies, a large-scale SLP pattern is found to lag the KOE SST anomalies by one month. Here it is shown that this pattern is linked to the oceanic-forced component of the SST variability, but not the atmospheric-forced component. The results hence suggest that the midlatitude ocean dynamical processes in the North Pacific influence the variability of the large-scale atmospheric circulation.