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Peter R. Gent
,
Frank O. Bryan
,
Gokhan Danabasoglu
,
Keith Lindsay
,
Daisuke Tsumune
,
Matthew W. Hecht
, and
Scott C. Doney

Abstract

An ensemble of nine simulations for the climate of the twentieth century has been run using the Community Climate System Model version 3 (CCSM3). Three of these runs also simulate the uptake of chlorofluorocarbon-11 (CFC-11) into the ocean using the protocol from the Ocean Carbon Model Intercomparison Project (OCMIP). Comparison with ocean observations taken between 1980 and 2000 shows that the global CFC-11 uptake is simulated very well. However, there are regional biases, and these are used to identify where too much deep-water formation is occurring in the CCSM3. The differences between the three runs simulating CFC-11 uptake are also briefly documented.

The variability in ocean heat content in the 1870 control runs is shown to be only a little smaller than estimates using ocean observations. The ocean heat uptake between 1957 and 1996 in the ensemble is compared to the recent observational estimates of the secular trend. The trend in ocean heat uptake is considerably larger than the natural variability in the 1870 control runs. The heat uptake down to 300 m between 1957 and 1996 varies by a factor of 2 across the ensemble. Some possible reasons for this large spread are discussed. There is much less spread in the heat uptake down to 3 km. On average, the CCSM3 twentieth-century ensemble runs take up 25% more heat than the recent estimate from ocean observations. Possible explanations for this are that the model heat uptake is calculated over the whole ocean, and not just in the regions where there are many observations and that there is no parameterization of the indirect effects of aerosols in CCSM3.

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Gretchen Keppel-Aleks
,
James T. Randerson
,
Keith Lindsay
,
Britton B. Stephens
,
J. Keith Moore
,
Scott C. Doney
,
Peter E. Thornton
,
Natalie M. Mahowald
,
Forrest M. Hoffman
,
Colm Sweeney
,
Pieter P. Tans
,
Paul O. Wennberg
, and
Steven C. Wofsy

Abstract

Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ecosystem responses to climate change and have implications for the design of carbon monitoring programs. This paper describes changes in the three-dimensional structure of atmospheric CO2 for several representative concentration pathways (RCPs 4.5 and 8.5) using the Community Earth System Model–Biogeochemistry (CESM1-BGC). CO2 simulated for the historical period was first compared to surface, aircraft, and column observations. In a second step, the evolution of spatial and temporal gradients during the twenty-first century was examined. The mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net flux in the Community Land Model (the land component of CESM) was too weak. Consistent with weak summer drawdown in Northern Hemisphere high latitudes, simulated CO2 showed correspondingly weak north–south and vertical gradients during the summer. In the simulations of the twenty-first century, CESM predicted increases in the mean annual cycle of atmospheric CO2 and larger horizontal gradients. Not only did the mean north–south gradient increase due to fossil fuel emissions, but east–west contrasts in CO2 also strengthened because of changing patterns in fossil fuel emissions and terrestrial carbon exchange. In the RCP8.5 simulation, where CO2 increased to 1150 ppm by 2100, the CESM predicted increases in interannual variability in the Northern Hemisphere midlatitudes of up to 60% relative to present variability for time series filtered with a 2–10-yr bandpass. Such an increase in variability may impact detection of changing surface fluxes from atmospheric observations.

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Keith Lindsay
,
Gordon B. Bonan
,
Scott C. Doney
,
Forrest M. Hoffman
,
David M. Lawrence
,
Matthew C. Long
,
Natalie M. Mahowald
,
J. Keith Moore
,
James T. Randerson
, and
Peter E. Thornton

Abstract

Version 1 of the Community Earth System Model, in the configuration where its full carbon cycle is enabled, is introduced and documented. In this configuration, the terrestrial biogeochemical model, which includes carbon–nitrogen dynamics and is present in earlier model versions, is coupled to an ocean biogeochemical model and atmospheric CO2 tracers. The authors provide a description of the model, detail how preindustrial-control and twentieth-century experiments were initialized and forced, and examine the behavior of the carbon cycle in those experiments. They examine how sea- and land-to-air CO2 fluxes contribute to the increase of atmospheric CO2 in the twentieth century, analyze how atmospheric CO2 and its surface fluxes vary on interannual time scales, including how they respond to ENSO, and describe the seasonal cycle of atmospheric CO2 and its surface fluxes. While the model broadly reproduces observed aspects of the carbon cycle, there are several notable biases, including having too large of an increase in atmospheric CO2 over the twentieth century and too small of a seasonal cycle of atmospheric CO2 in the Northern Hemisphere. The biases are related to a weak response of the carbon cycle to climatic variations on interannual and seasonal time scales and to twentieth-century anthropogenic forcings, including rising CO2, land-use change, and atmospheric deposition of nitrogen.

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William D. Collins
,
Cecilia M. Bitz
,
Maurice L. Blackmon
,
Gordon B. Bonan
,
Christopher S. Bretherton
,
James A. Carton
,
Ping Chang
,
Scott C. Doney
,
James J. Hack
,
Thomas B. Henderson
,
Jeffrey T. Kiehl
,
William G. Large
,
Daniel S. McKenna
,
Benjamin D. Santer
, and
Richard D. Smith

Abstract

The Community Climate System Model version 3 (CCSM3) has recently been developed and released to the climate community. CCSM3 is a coupled climate model with components representing the atmosphere, ocean, sea ice, and land surface connected by a flux coupler. CCSM3 is designed to produce realistic simulations over a wide range of spatial resolutions, enabling inexpensive simulations lasting several millennia or detailed studies of continental-scale dynamics, variability, and climate change. This paper will show results from the configuration used for climate-change simulations with a T85 grid for the atmosphere and land and a grid with approximately 1° resolution for the ocean and sea ice. The new system incorporates several significant improvements in the physical parameterizations. The enhancements in the model physics are designed to reduce or eliminate several systematic biases in the mean climate produced by previous editions of CCSM. These include new treatments of cloud processes, aerosol radiative forcing, land–atmosphere fluxes, ocean mixed layer processes, and sea ice dynamics. There are significant improvements in the sea ice thickness, polar radiation budgets, tropical sea surface temperatures, and cloud radiative effects. CCSM3 can produce stable climate simulations of millennial duration without ad hoc adjustments to the fluxes exchanged among the component models. Nonetheless, there are still systematic biases in the ocean–atmosphere fluxes in coastal regions west of continents, the spectrum of ENSO variability, the spatial distribution of precipitation in the tropical oceans, and continental precipitation and surface air temperatures. Work is under way to extend CCSM to a more accurate and comprehensive model of the earth's climate system.

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Maurice Blackmon
,
Byron Boville
,
Frank Bryan
,
Robert Dickinson
,
Peter Gent
,
Jeffrey Kiehl
,
Richard Moritz
,
David Randall
,
Jagadish Shukla
,
Susan Solomon
,
Gordon Bonan
,
Scott Doney
,
Inez Fung
,
James Hack
,
Elizabeth Hunke
,
James Hurrell
,
John Kutzbach
,
Jerry Meehl
,
Bette Otto-Bliesner
,
R. Saravanan
,
Edwin K. Schneider
,
Lisa Sloan
,
Michael Spall
,
Karl Taylor
,
Joseph Tribbia
, and
Warren Washington

The Community Climate System Model (CCSM) has been created to represent the principal components of the climate system and their interactions. Development and applications of the model are carried out by the U.S. climate research community, thus taking advantage of both wide intellectual participation and computing capabilities beyond those available to most individual U.S. institutions. This article outlines the history of the CCSM, its current capabilities, and plans for its future development and applications, with the goal of providing a summary useful to present and future users.

The initial version of the CCSM included atmosphere and ocean general circulation models, a land surface model that was grafted onto the atmosphere model, a sea-ice model, and a “flux coupler” that facilitates information exchanges among the component models with their differing grids. This version of the model produced a successful 300-yr simulation of the current climate without artificial flux adjustments. The model was then used to perform a coupled simulation in which the atmospheric CO2 concentration increased by 1 % per year.

In this version of the coupled model, the ocean salinity and deep-ocean temperature slowly drifted away from observed values. A subsequent correction to the roughness length used for sea ice significantly reduced these errors. An updated version of the CCSM was used to perform three simulations of the twentieth century's climate, and several projections of the climate of the twenty-first century.

The CCSM's simulation of the tropical ocean circulation has been significantly improved by reducing the background vertical diffusivity and incorporating an anisotropic horizontal viscosity tensor. The meridional resolution of the ocean model was also refined near the equator. These changes have resulted in a greatly improved simulation of both the Pacific equatorial undercurrent and the surface countercurrents. The interannual variability of the sea surface temperature in the central and eastern tropical Pacific is also more realistic in simulations with the updated model.

Scientific challenges to be addressed with future versions of the CCSM include realistic simulation of the whole atmosphere, including the middle and upper atmosphere, as well as the troposphere; simulation of changes in the chemical composition of the atmosphere through the incorporation of an integrated chemistry model; inclusion of global, prognostic biogeochemical components for land, ocean, and atmosphere; simulations of past climates, including times of extensive continental glaciation as well as times with little or no ice; studies of natural climate variability on seasonal-to-centennial timescales; and investigations of anthropogenic climate change. In order to make such studies possible, work is under way to improve all components of the model. Plans call for a new version of the CCSM to be released in 2002. Planned studies with the CCSM will require much more computer power than is currently available.

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