Stephen Yeager | National Center for Atmospheric Research (NCAR) (original) (raw)
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Papers by Stephen Yeager
Ocean Modelling, 2009
Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behavi... more Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behaviour of global ocean-ice models under forcing from a common atmospheric dataset. We highlight issues arising when designing coupled global ocean and sea ice experiments, such as difficulties formulating a consistent forcing methodology and experimental protocol. Particular focus is given to the hydrological forcing, the details of which are key to realizing simulations with stable meridional overturning circulations.
Nature Geoscience, 2010
Global sea level has risen during the past decades as a result of thermal expansion of the warmin... more Global sea level has risen during the past decades as a result of thermal expansion of the warming ocean and freshwater addition from melting continental ice 1 . However, sea-level rise is not globally uniform 1-5 . Regional sea levels can be affected by changes in atmospheric or oceanic circulation. As long-term observational records are scarce, regional changes in sea level in the Indian Ocean are poorly constrained. Yet estimates of future sea-level changes are essential for effective risk assessment 2 . Here we combine in situ and satellite observations of Indian Ocean sea level with climate-model simulations, to identify a distinct spatial pattern of sea-level rise since the 1960s. We find that sea level has decreased substantially in the south tropical Indian Ocean whereas it has increased elsewhere. This pattern is driven by changing surface winds associated with a combined invigoration of the Indian Ocean Hadley and Walker cells, patterns of atmospheric overturning circulation in the north-south and east-west direction, respectively, which is partly attributable to rising levels of atmospheric greenhouse gases. We conclude that-if ongoing anthropogenic warming dominates natural variability-the pattern we detected is likely to persist and to increase the environmental stress on some coasts and islands in the Indian Ocean.
Journal of Climate, 2014
Surface forcing perturbation experiments are examined to identify the key forcing elements associ... more Surface forcing perturbation experiments are examined to identify the key forcing elements associated with late-twentieth-century interannual-to-decadal Atlantic circulation variability as simulated in an ocean-sea ice hindcast configuration of the Community Earth System Model, version 1 (CESM1). Buoyancy forcing accounts for most of the decadal variability in both the Atlantic meridional overturning circulation (AMOC) and the subpolar gyre circulation, and the key drivers of these basin-scale circulation changes are found to be the turbulent buoyancy fluxes: evaporation as well as the latent and sensible heat fluxes. These three fluxes account for almost all of the decadal AMOC variability in the North Atlantic, even when applied only over the Labrador Sea region. Year-to-year changes in surface momentum forcing explain most of the interannual AMOC variability at all latitudes as well as most of the decadal variability south of the equator. The observed strengthening of Southern Ocean westerly winds accounts for much of the simulated AMOC variability between 308S and the equator but very little of the recent AMOC change in the North Atlantic. Ultimately, the strengthening of the North Atlantic overturning circulation between the 1970s and 1990s, which contributed to a pronounced SST increase at subpolar latitudes, is explained almost entirely by trends in the atmospheric surface state over the Labrador Sea.
Journal of Climate, 2012
Résumé/Abstract Air-sea fluxes from the Community Climate System Model version 4 (CCSM4) are comp... more Résumé/Abstract Air-sea fluxes from the Community Climate System Model version 4 (CCSM4) are compared with the Coordinated Ocean-Ice Reference Experiment (CORE) dataset to assess present-day mean biases, variability errors, and late twentieth-century trend differences. CCSM4 is improved over the previous version, CCSM3, in both air-sea heat and freshwater fluxes in some regions; however, a large increase in net shortwave radiation into the ocean may contribute to an enhanced hydrological cycle. The authors ...
Ocean Modelling, 2009
Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behavi... more Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behaviour of global ocean-ice models under forcing from a common atmospheric dataset. We highlight issues arising when designing coupled global ocean and sea ice experiments, such as difficulties formulating a consistent forcing methodology and experimental protocol. Particular focus is given to the hydrological forcing, the details of which are key to realizing simulations with stable meridional overturning circulations.
Nature Geoscience, 2010
Global sea level has risen during the past decades as a result of thermal expansion of the warmin... more Global sea level has risen during the past decades as a result of thermal expansion of the warming ocean and freshwater addition from melting continental ice 1 . However, sea-level rise is not globally uniform 1-5 . Regional sea levels can be affected by changes in atmospheric or oceanic circulation. As long-term observational records are scarce, regional changes in sea level in the Indian Ocean are poorly constrained. Yet estimates of future sea-level changes are essential for effective risk assessment 2 . Here we combine in situ and satellite observations of Indian Ocean sea level with climate-model simulations, to identify a distinct spatial pattern of sea-level rise since the 1960s. We find that sea level has decreased substantially in the south tropical Indian Ocean whereas it has increased elsewhere. This pattern is driven by changing surface winds associated with a combined invigoration of the Indian Ocean Hadley and Walker cells, patterns of atmospheric overturning circulation in the north-south and east-west direction, respectively, which is partly attributable to rising levels of atmospheric greenhouse gases. We conclude that-if ongoing anthropogenic warming dominates natural variability-the pattern we detected is likely to persist and to increase the environmental stress on some coasts and islands in the Indian Ocean.
Journal of Climate, 2014
Surface forcing perturbation experiments are examined to identify the key forcing elements associ... more Surface forcing perturbation experiments are examined to identify the key forcing elements associated with late-twentieth-century interannual-to-decadal Atlantic circulation variability as simulated in an ocean-sea ice hindcast configuration of the Community Earth System Model, version 1 (CESM1). Buoyancy forcing accounts for most of the decadal variability in both the Atlantic meridional overturning circulation (AMOC) and the subpolar gyre circulation, and the key drivers of these basin-scale circulation changes are found to be the turbulent buoyancy fluxes: evaporation as well as the latent and sensible heat fluxes. These three fluxes account for almost all of the decadal AMOC variability in the North Atlantic, even when applied only over the Labrador Sea region. Year-to-year changes in surface momentum forcing explain most of the interannual AMOC variability at all latitudes as well as most of the decadal variability south of the equator. The observed strengthening of Southern Ocean westerly winds accounts for much of the simulated AMOC variability between 308S and the equator but very little of the recent AMOC change in the North Atlantic. Ultimately, the strengthening of the North Atlantic overturning circulation between the 1970s and 1990s, which contributed to a pronounced SST increase at subpolar latitudes, is explained almost entirely by trends in the atmospheric surface state over the Labrador Sea.
Journal of Climate, 2012
Résumé/Abstract Air-sea fluxes from the Community Climate System Model version 4 (CCSM4) are comp... more Résumé/Abstract Air-sea fluxes from the Community Climate System Model version 4 (CCSM4) are compared with the Coordinated Ocean-Ice Reference Experiment (CORE) dataset to assess present-day mean biases, variability errors, and late twentieth-century trend differences. CCSM4 is improved over the previous version, CCSM3, in both air-sea heat and freshwater fluxes in some regions; however, a large increase in net shortwave radiation into the ocean may contribute to an enhanced hydrological cycle. The authors ...