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Browsing by Author "Thompson, David, advisor"

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    Estimating the likelihood of significant climate change with the NCAR 40-member ensemble
    (Colorado State University. Libraries, 2014) Foust, William Eliott, author; Thompson, David, advisor; Randall, David, committee member; Barnes, Elizabeth, committee member; Cooley, Daniel, committee member
    Increasing greenhouse gas concentrations are changing the radiative forcing on the climate system, and this forcing will be the key driver of climate change over the 21st century. One of the most pressing questions associated with climate change is whether certain aspects of the climate system will change significantly. Climate ensembles are often used to estimate the probability of significant climate change, but they struggle to produce accurate estimates of significant climate change because they sometimes require more realizations than what is feasible to produce. Additionally, the ensemble mean suggests how the climate will respond to an external forcing, but since it filters out the variability, it cannot determine if the response is significant. In this study, the NCAR CCSM 40-member ensemble and a lag-1 autoregressive model (AR1 model) are used to estimate the likelihood that climate trends will be significant. The AR1 model generates an analytic solution for what the distribution of trends should be if the NCAR model was run an infinite number of times. The analytical solution produced by the AR1 model is used to assess the significance of future climate trends. The results of this study demonstrate that an AR1 model can aid in making a probabilistic forecast. Additionally, the results give insight into the certainty of the trends in the surface temperature field, precipitation field, and atmospheric circulation, the probability of climate trends being significant, and whether the significance of climate trends is dependent on the internal variability or anthropogenic forcing.
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    Mechanisms of observed sea surface temperature variability in the extratropical southern hemisphere
    (Colorado State University. Libraries, 2008) Ciasto, Laura M., author; Thompson, David, advisor
    The physical mechanisms that drive sea surface temperature (SST) variability in the extratropical Southern Hemisphere (SH) are examined using multiple ocean temperature datasets. The first part of the study provides a detailed analysis of the relationships between variability in SH SST anomalies, the Southern Annular Mode (SAM) and the El-NiƱo/Southern Oscillation (ENSO) during the warm (November-April) and cold (May-October) seasons. It is shown that the signatures of the SAM and ENSO in the SST field vary as a function of season, both in terms of their amplitudes and structures. SAM-related SST anomalies are primarily driven by surface turbulent heat fluxes with a smaller contribution from heat advection by Ekman currents. The role of turbulent heat fluxes in generating ENSO-related SST anomalies is less clear. Analyses of the temporal evolution of the relationships between the SAM and the SST field demonstrate that SST anomalies are largest when SSTs lag by ~1 week and persist for up to 8 weeks. In the absence of ENSO, cold season SAM-related SST anomalies persist longer than their warm season counterparts, consistent with seasonal variations in the depth of the mixed layer. The second part of the study uses observations of subsurface temperatures to examine the winter-to-winter "reemergence" of SST anomalies in the extratropical South Pacific. Reemergence is the mechanism whereby SST anomalies formed in the late winter are sequestered beneath the shallow summer mixed layer and then re-entrained into the deepening mixed layer during the following fall/winter. The results exhibit a pronounced reemergence signal in which surface temperature anomalies during the late winter season are strongly correlated with surface temperature anomalies during the subsequent early winter months, but are only significantly correlated with temperature anomalies beneath the mixed layer during the intervening summer months. The results are robust to small changes in the period of analysis and are qualitatively similar to existing evidence of reemergence in the Northern Hemisphere. The signal of reemergence evident in the subsurface data is readily apparent in SST data in the western South Pacific. Reemergence is less evident in SST data in the eastern South Pacific.
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    The spatial scale of convective aggregation in cloud-resolving simulations of radiative-convective equilibrium
    (Colorado State University. Libraries, 2017) Patrizio, Casey, author; Randall, David, advisor; Thompson, David, advisor; Kirkpatrick, Allan, committee member
    A three-dimensional cloud-resolving model (CRM) was used to investigate the preferred separation distance between humid, rainy regions formed by convective aggregation in radiative--convective equilibrium without rotation. We performed the simulations with doubly-periodic square domains of widths 768 km, 1536 km and 3072 km over a time period of about 200 days. The simulations in the larger domains were initialized using multiple copies of the results in the small domain at day 90, plus a small perturbation. With all three domain sizes, the simulations evolved to a single statistically steady convective cluster surrounded by a broader region of dry, subsiding air by about day 150. In the largest domain case, however, we found that an additional convective cluster formed when we the simulation was run for an extended period of time. Specifically, a smaller convective cluster formed at around day 185 at a maximum radial distance from the larger cluster and then re-merged with the larger cluster after about 10 days. We explored how the aggregated state was different in each domain case, before the smaller cluster formed in the large domain. In particular, we investigated changes in the radial structure of the aggregated state by calculating profiles for the water, dynamics and radiation as a function of distance from the center of the convective region. Changes in the vertical structure were also investigated by compositing on the convective region and dry, subsiding region at each height. We found that, with increasing domain size, the convective region boundary layer became more buoyant, the convective cores reached deeper into the troposphere, the mesoscale convective updraft became weaker, and the mesoscale convective region spread out. Additionally, as the domain size was increased, conditions in the remote environment became favorable for convection. We describe a physical mechanism for the weakening of the mesoscale convective updraft and associated broadening of the convective region with increasing domain size, which involves mid-level stable layer enhancement as a result of the deeper convection. Finally, a simple analytical model of the aggregated state was used to explore the dependency of the convective fractional area on the domain size. The simple model solutions that had net radiative cooling and surface evaporation in the convective region were consistent with the simulation results. In particular, the solutions captured the broadening of the convective region, the weakening of the convective region updraft, as well as the positive and declining gross moist stability (GMS) that occurred with increasing domain size in the simulations. Furthermore, the simple model transitioned from positive to negative GMS at a domain length of about 7000 km because the convective region boundary layer became progressively more humid with increasing domain size. This suggests that the spatial scale of the aggregated RCE state in the simulations would be limited to a length scale of about 7000 km, as convectively-active areas are commonly observed to have positive GMS. This work additionally suggests that the processes that influence the water vapor content in the convective region boundary layer, such as convectively-driven turbulent water vapor fluxes, are important for determining the spatial scale of the aggregated RCE state.
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    Towards understanding the processes that govern variability in the Southern Hemisphere
    (Colorado State University. Libraries, 2013) Woodworth, Jonathan D., author; Thompson, David, advisor; Birner, Thomas, committee member; Yalin, Azer, committee member
    The climate at extratropical latitudes is strongly a result of the behavior of the zonal mean zonal wind and its inherent variability. This variability is dominated largely by the north-south fluctuation of the midlatitude jet and is identified in the Southern Hemisphere as the Southern Annular Mode (SAM). Recent observations have shown a tendency of the jet to move poleward due to, in part, the forcing associated with stratospheric cooling due to ozone loss and the tropical tropospheric warming from increasing greenhouse gases. Two dominant processes drive variability in the midlatitude jet: anomalies in the eddy momentum flux (EMF) and the eddy heat flux (EHF). In an attempt to link these processes, this study aims to diagnose a relationship in the observational data via two aspects: 1) To assess the extent to which feedbacks between the EMF and EHF give rise to the annular modes; and 2) To understand, in the context of the atmospheric energy cycle, the dominant patterns of variability of the EMF and EHF fields. Preliminary results reveal that the variability observed in the extratropical flow may exhibit a slight feedback between these processes. Additionally, it has been found that this variability may be viewed in the context of two distinct structures: (i) those that owe their existence to conversions between zonal-mean and eddy kinetic energy and (ii) those that owe their existence to conversions between zonal-mean and eddy potential energy. Past studies have largely focused on the former's impact on the extratropical circulation. However, not much emphasis has been placed on the latter, despite arguably playing an equally important role in driving the variability.
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    Towards understanding the processes that influence global mean temperature
    (Colorado State University. Libraries, 2011) Mullin, Kathryn A., author; Thompson, David, advisor; Denning, Scott, committee member; Klein, Julia, committee member
    Global mean surface temperature variability is largely determined by the global mean surface energy budget, which is driven by many natural and anthropogenic forcings. In theory, if all natural sources of global mean temperature variability could be removed from the global mean temperature time series the anthropogenic signal would be clearer. Previous studies have exploited this reasoning to remove the signature of volcanoes, the El-NiƱo Southern Oscillation (ENSO), and dynamic variability from the global mean temperature time series. This thesis extends previous work by 1) examining the linkages between global mean temperature and natural variability as a function of timescale; and 2) examining the two-way coupling between area-averaged surface temperatures and sea ice concentration. The results reveal a series of unique spatial structures in surface temperatures that drive intraannual, interannual, and decadal variability in global mean temperature. The results confirm the apparent role of hemispheric mean temperatures in driving sea ice variability, and also point to a possible feedback between wintertime sea ice concentration and springtime surface temperatures over the Northern Hemisphere. Linkages between sea ice concentration and surface temperature in the Southern Hemisphere are much weaker, and it can be argued that the hemispheric difference in these linkages may aid in explaining the different trends in sea ice between the two hemispheres.
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    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 member
    The 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.