Papers by Christine Shields
npj Climate and Atmospheric Science
Atmospheric rivers (ARs) impacting western North America are analyzed under climate intervention ... more Atmospheric rivers (ARs) impacting western North America are analyzed under climate intervention applying stratospheric aerosol injections (SAI) using simulations produced by the Whole Atmosphere Community Climate Model. Sulfur dioxide injections are strategically placed to maintain present-day global, interhemispheric, and equator-to-pole surface temperatures between 2020 and 2100 using a high forcing climate scenario. Three science questions are addressed: (1) How will western North American ARs change by the end of the century with SAI applied, (2) How is this different from 2020 conditions, and (3) How will the results differ with no future climate intervention. Under SAI, ARs are projected to increase by the end of the 21st century for southern California and decrease in the Pacific Northwest and coastal British Columbia, following changes to the low-level wind. Compared to 2020 conditions, the increase in ARs is not significant. The character of AR precipitation changes under ...
The low-resolution fully coupled configuration of the Community Climate System Model version 3 (C... more The low-resolution fully coupled configuration of the Community Climate System Model version 3 (CCSM3) is described and evaluated. In this most economical configuration, an ocean at nominal 3° resolution is coupled to an atmosphere model at T31 resolution. There are climate biases associated with the relatively coarse grids, yet the coupled solution remains comparable to higher-resolution CCSM3 results. There are marked improvements in the new solution compared to the low-resolution configuration of CCSM2. In particular, the CCSM3 simulation maintains a robust meridional overturning circulation in the ocean, and it generates more realistic El Niño variability. The improved ocean solution was achieved with no increase in computational cost by redistributing deep ocean and midlatitude resolution into the upper ocean and the key water formation regions of the North Atlantic, respectively. Given its significantly lower resource demands compared to higher resolutions, this configuration ...
Bulletin of the American Meteorological Society
national laboratories met to discuss progress with the Atmospheric River Tracking Method Intercom... more national laboratories met to discuss progress with the Atmospheric River Tracking Method Intercomparison Project (ARTMIP). ARTMIP aims to quantify the uncertainty in AR climatology, precipitation, and related impacts that arise from a wide range of AR tracking methods developed by the community and how these AR-related metrics may change in the future.
Geophysical Research Letters
Atmospheric rivers are recognized as major contributors to the poleward transport of water vapor.... more Atmospheric rivers are recognized as major contributors to the poleward transport of water vapor. Upon reaching land, these phenomena also play a critical role in extreme precipitation and flooding events. The Pineapple Express (PE) is defined as an atmospheric river extending out of the deep tropics and reaching the west coast of North America. Community Climate System Model (CCSM4) high-resolution ensemble simulations for the twentieth and 21st centuries are diagnosed to identify the PE. Analysis of the twentieth century simulations indicated that the CCSM4 accurately captures the spatial and temporal climatology of the PE. Analysis of the end 21st century simulations indicates a significant increase in storm duration and intensity of precipitation associated with landfall of the PE. Only a modest increase in the number of atmospheric rivers of a few percent is projected for the end of 21st century.
Geophysical Research Letters
The latitude of landfall for atmospheric rivers (ARs) is examined in the fully coupled half-degre... more The latitude of landfall for atmospheric rivers (ARs) is examined in the fully coupled half-degree version of the Community Climate System Model, version 4 (CCSM4) for warm future climate simulations. Two regions are examined: U.S. West Coast/North Pacific ARs and United Kingdom/North Atlantic ARs. Changes in AR landfall-latitude reflect changes in the atmospheric steering flow. West Coast U.S. ARs are projected to push equatorward in response to the subtropical jet climate change. UK AR response is dominated by eddy-driven jets and is seasonally dependent. UK simulated AR response is modest in the winter with the largest relative changes occurring in the seasonal transition months. Precipitation associated with ARs is also projected to increase in intensity under global warming. CCSM4 projects a marked shift to higher rainfall rates for Southern California. Small to modest rainfall rates may increase for all UK latitudes, for the Pacific Northwest, and central and northern California.
Intraseasonal, Seasonal, and Interannual Characteristics of Regional Monsoon Simulations in CESM2
Journal of Advances in Modeling Earth Systems
Characteristics of Future Warmer Base States in CESM2
Earth and Space Science
Simulated Eocene Hothouse Climate - a Deepmip Study
Effects of model resolution, physics, and coupling on Southern Hemisphere storm tracks in CESM1.3
Geophysical Research Letters
Geoscientific Model Development
The Atmospheric River Tracking Method Intercomparison Project (ARTMIP) is an international collab... more The Atmospheric River Tracking Method Intercomparison Project (ARTMIP) is an international collaborative effort to understand and quantify the uncertainties in atmospheric river (AR) science based on detection algorithm alone. Currently, there are many AR identification and tracking algorithms in the literature with a wide range of techniques and conclusions. ARTMIP strives to provide the community with information on different methodologies and provide guidance on the most appropriate algorithm for a given science question or region of interest. All ARTMIP participants will implement their detection algorithms on a specified common dataset for a defined period of time. The project is divided into two phases: Tier 1 will utilize the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) reanalysis from January 1980 to June 2017 and will be used as a baseline for all subsequent comparisons. Participation in Tier 1 is required. Tier 2 will be optional and include sensitivity studies designed around specific science questions, such as reanalysis uncertainty and climate change. High-resolution reanalysis and/or model output will be used wherever possible. Proposed metrics include AR frequency, duration, intensity, and precipitation attributable to ARs. Here, we present the ARTMIP experimental design, timeline, project requirements, and a brief description of the variety of methodologies in the current literature. We also present results from our 1-month "proof-of-concept" trial run designed to illustrate the utility and feasibility of the ART-MIP project.
Geoscientific Model Development
Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenari... more Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high ( > 800 ppmv) atmospheric CO<sub>2</sub> concentrations. Although a post hoc intercomparison of Eocene ( ∼ 50 Ma) climate model simulations and geological data has been carried out previously, models of past high-CO<sub>2</sub> periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the early Eocene and the latest Paleocene (the EECO, PETM, and pre-PETM). Together with the CMIP6 pre-industrial control and abrupt 4 × CO<sub>2</sub> simulations, and additional sensitivity studies, these form the first phase of DeepMIP – the Deep-time Model Intercomparison Project, itself a group within the wider Paleoclimate Modelling Intercomparison Project (PMIP). The experimental design specifies and provides guidance on boundary condit...
Climate of the Past Discussions
Simulations of the late Permian (251Ma) are analyzed with respect to the northern hemispheric Pan... more Simulations of the late Permian (251Ma) are analyzed with respect to the northern hemispheric Pangean megamonsoon. We find that the presence and spatial distribution of the warm pool, and not land-sea temperature differences, are the primary forcing agents for the megamonsoon. The land-sea temperature gradient, as a monsoonal mechanism, is tested by eliminating the Cathyasian peninsula and is found to have little impact on the spatial character of the monsoon. Furthermore, the response of the monsoon to the warm pool was tested by removing all Paleo-Tethys equatorial islands, which allows the warm pool to expand and migrate westward thus shifting the pattern of monsoonal precipitation. Additionally, different CO<sub>2</sub> regimes are presented, in which a 10-fold change in forcing produces diverging climates and therefore different warm pool and monsoon locales. Atmospheric mass flux for the monsoonal regime is characterized and shown to change with warm pool movement....
Siberian Traps Sulfur and Carbon Degassing and End-Permian Climate Feedbacks
RAbstract: Earth's future climate is expected to warm considerably due to increased atmospheric c... more RAbstract: Earth's future climate is expected to warm considerably due to increased atmospheric carbon dioxide. Paleoclimate records indicate that pre-Quaternary time periods provide the best possible view of Earth under warm greenhouse conditions. Thus, past warm greenhouse climates provide an important tool to evaluate fully coupled climate models that are currently used to study future climate change. In this study, we use the Community Climate System Model (CCSM3) to investigate the climate of the latest Cretaceous (Maastrichtian). CCSM3 is a fully coupled threedimensional global model that includes atmospheric, oceanic, sea-ice and terrestrial processes. The CCSM3 simulations employ the paleogeographic and global vegetation reconstructions used in earlier simulations of the late Maastrichtian with the GENESIS Earth System Model (Upchurch, Otto-Bliesner, and Scotese, 1999). CCSM3 simulations include two levels of atmospheric carbon dioxide (2XPAL and 6XPAL), elevated levels of atmospheric methane, changes to low level liquid cloud properties based on the hypothesis of Kump and Pollard (2008), and different paleoelevations for the interior of Siberia. A coupled simulation of multi-century length is carried out to study steady state conditions for the oceans. For terrestrial regions, model mean annual temperatures and seasonality are compared with data from angiosperm leaf physiognomy, plant life form distribution, and other climatic indicators to determine how well the model represents high latitude warmth on a zonal and regional basis. Model precipitation is compared with a database of climatically restricted sediments and angiosperm leaf physiognomy for specific sites. For oceanic regions, the CCSM3 simulations are compared to marine proxies of surface and benthic temperatures to study how well the model captures global Cretaceous ocean circulation patterns. Our results underscore the need for accurate boundary conditions in model simulations and provide a series of baseline simulations for the study of climatic change at the Cretaceous-Paleogene boundary.
Geoscientific Model Development Discussions, 2016
Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenari... more Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high (> 800 ppmv) atmospheric CO<sub>2</sub> concentrations. Although a post-hoc intercomparison of Eocene (~50 million years ago, Ma) climate model simulations and geological data has been carried out previously, models of past high-CO<sub>2</sub> periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the latest Paleocene and the early Eocene. Together these form the first phase of DeepMIP – the deeptime model intercomparison project, itself a group within the wider Paleoclimate Modelling Intercomparison Project (PMIP). The experimental design consists of three core paleo simulations and a set of optional sensitivity studies. The experimental design specifies and provides guidance on boundary conditions associate...
Monsoons across Epochs: Diagnosing Monsoonal Circulations across Warm, Greenhouse Climates as Simulated by the Community Climate System Model (CCSM3)
ABSTRACT 2009.09 The nature of monsoons has been studied extensively in the scientific community ... more ABSTRACT 2009.09 The nature of monsoons has been studied extensively in the scientific community because of its significance in dominating regional climate and weather. Analyzing the underlying mechanisms behind the monsoon in past climates, in particular, warm greenhouse climates, give us a deeper understanding of what drives the monsoon for today and potentially for the future. In this study, we use the Community Climate System Model (CCSM3) to analyze global monsoonal circulations for three warm, greenhouse climates including the Late Permian (250Ma), the Latest Cretaceous (65Ma), and the Paleocene-Eocene Thermal Maximum (55Ma). A suite of simulations will be presented where surface and boundary forcings appropriate for each time period are used as well as sensitivity studies designed to test the role of CO2 and paleogeography. Paleogeography, CO2 levels, and, consequently the location of the tropical oceanic warm pools will be studied as potential elements for obtaining key wind patterns and manifestation of monsoonal precipitation.
Simulating the Warm Climate of the Early Eocene (Invited)
Past attempts to simulate warm climates of the early Eocene have met with limited success. In par... more Past attempts to simulate warm climates of the early Eocene have met with limited success. In particular, simulating the very warm Arctic region and high southern regions has been very difficult to achieve in climate models. This has been of great concern given that we are entering a time period of extreme warming due to increasing levels of atmospheric carbon dioxide. Thus, the question has arisen as to whether current climate models are missing a fundamental feedback that prevents them from capturing high latitude enhanced warming. We present an equilibrium coupled climate simulation using the Community Climate Model (CCSM3) that agrees in many ways with the paleoclimate proxy data for the time period of the Paleocene-Eocene Thermal Maximum (55 Ma). We compare the model simulation to a range of data including sea surface temperatures, benthic temperatures and surface temperatures from terrestrial locations in both the northern and southern hemisphere. The model agrees to within observational uncertainty with this array of data. The model simulation employs increased levels of atmospheric carbon dioxide and methane. We also include a change to warm cloud properties that is based on present day observations for non-industrially affected locations, i.e. naturally occurring aerosol conditions. The concentrations of atmospheric carbon dioxide and methane are within geochemically modeled PETM estimates for these greenhouse gases. We discuss the implications of a warm stratified ocean for modeling atmospheric carbon dioxide levels from simple biogeochemical models and argue that it is important to account for the physical state of the ocean when estimating warm climate greenhouse gas concentrations.
Modeling El Ni�o and its tropical teleconnections during the last glacial-interglacial cycle
Geophys Res Lett, 2003
Climate of the Past, 2016
A range of proxy observations have recently provided constraints on how Earth's hydrological ... more A range of proxy observations have recently provided constraints on how Earth's hydrological cycle responded to early Eocene climatic changes. However, comparisons of proxy data to general circulation model (GCM) simulated hydrology are limited and inter-model variability remains poorly characterised. In this work, we undertake an intercomparison of GCM-derived precipitation and <i>P</i> − <i>E</i> distributions within the extended EoMIP ensemble (Eocene Modelling Intercomparison Project; Lunt et al., 2012), which includes previously published early Eocene simulations performed using five GCMs differing in boundary conditions, model structure, and precipitation-relevant parameterisation schemes. <br><br> We show that an intensified hydrological cycle, manifested in enhanced global precipitation and evaporation rates, is simulated for all Eocene simulations relative to the preindustrial conditions. This is primarily due to elevated atmospheric ...
Uploads
Papers by Christine Shields