The Experts below are selected from a list of 10878 Experts worldwide ranked by ideXlab platform
Ronald J Stouffer - One of the best experts on this subject based on the ideXlab platform.
-
a model intercomparison of changes in the atlantic Thermohaline Circulation in response to increasing atmospheric co2 concentration
Geophysical Research Letters, 2005Co-Authors: Andrew J Weaver, Ronald J Stouffer, Keith W Dixon, Jonathan M Gregory, E Driesschaert, Michael Eby, Thierry Fichefet, Hiroyasu Hasumi, Johann H JungclausAbstract:[ 1] As part of the Coupled Model Intercomparison Project, integrations with a common design have been undertaken with eleven different climate models to compare the response of the Atlantic Thermohaline Circulation ( THC) to time-dependent climate change caused by increasing atmospheric CO2 concentration. Over 140 years, during which the CO2 concentration quadruples, the Circulation strength declines gradually in all models, by between 10 and 50%. No model shows a rapid or complete collapse, despite the fairly rapid increase and high final concentration of CO2. The models having the strongest overturning in the control climate tend to show the largest THC reductions. In all models, the THC weakening is caused more by changes in surface heat flux than by changes in surface water flux. No model shows a cooling anywhere, because the greenhouse warming is dominant.
-
equilibrium response of Thermohaline Circulation to large changes in atmospheric co2 concentration
Climate Dynamics, 2003Co-Authors: Ronald J Stouffer, Syukuro ManabeAbstract:This study evaluates the equilibrium response of a coupled ocean–atmosphere model to the doubling, quadrupling, and halving of CO2 concentration in the atmosphere. Special emphasis in the study is placed upon the response of the Thermohaline Circulation in the Atlantic Ocean to the changes in CO2 concentration of the atmosphere. The simulated intensity of the Thermohaline Circulation (THC) is similar among three quasi-equilibrium states with the standard, double the standard, and quadruple the standard amounts of CO2 concentration in the atmosphere. When the model atmosphere has half the standard concentration of CO2, however, the THC is very weak and shallow in the Atlantic Ocean. Below a depth of 3 km, the model oceans maintain very thick layer of cold bottom water with temperature close to –2 °C, preventing the deeper penetration of the THC in the Atlantic Ocean. In the Circumpolar Ocean of the Southern Hemisphere, sea ice extends beyond the Antarctic Polar front, almost entirely covering the regions of deepwater ventilation. In addition to the active mode of the THC, there exists another stable mode of the THC for the standard, possibly double the standard (not yet confirmed), and quadruple the standard concentration of atmospheric carbon dioxide. This second mode is characterized by the weak, reverse overturning Circulation over the entire Atlantic basin, and has no ventilation of the entire subsurface water in the North Atlantic Ocean. At one half the standard CO2 concentration, however, the intensity of the first mode is so weak that it is not certain whether there are two distinct stable modes or not. The paleoceanographic implications of the results obtained here are discussed as they relate to the signatures of the Cenozoic changes in the oceans.
-
transient response of a coupled model to estimated changes in greenhouse gas and sulfate concentrations
Geophysical Research Letters, 1997Co-Authors: James M. Haywood, R T Wetherald, Ronald J Stouffer, Syukuro Manabe, V. RamaswamyAbstract:This study investigates changes in surface air temperature (SAT), hydrology and the Thermohaline Circulation due to the the radiative forcing of anthropogenic greenhouse gases and the direct radiative forcing (DRF) of sulfate aerosols in the GFDL coupled ocean-atmosphere model. Three 300-year model integrations are performed with increasing greenhouse gas concentrations only, increasing sulfate aerosol concentrations only and increasing greenhouse gas and sulfate aerosol concentrations. A control integration is also performed keeping concentrations of sulfate and carbon dioxide fixed. The global annual mean SAT change when both greenhouse gases and sulfate aerosols are included is in better agreement with observations than when greenhouse gases alone are included. When the global annual mean SAT change from a model integration that includes only increases in greenhouse gases is added to that from a model integration that includes only increases in sulfate, the resulting global SAT change is approximately equal to that from a model integration that includes increases in both greenhouse gases and sulfate aerosol throughout the integration period. Similar results are found for global annual mean precipitation changes and for the geographical distribution of both SAT and precipitation changes indicating that the climate response is linearly additive for the two types of forcing considered here. Changes in the mid-continental summer dryness and Thermohaline Circulation are also briefly discussed.
-
simulation of abrupt climate change induced by freshwater input to the north atlantic ocean
Nature, 1995Co-Authors: Syukuro Manabe, Ronald J StoufferAbstract:TEMPERATURE records from Greenland ice cores1,2 suggest that large and abrupt changes of North Atlantic climate occurred frequently during both glacial and postglacial periods; one example is the Younger Dryas cold event. Broecker3 speculated that these changes result from rapid changes in the Thermohaline Circulation of the Atlantic Ocean, which were caused by the release of large amounts of melt water from continental ice sheets. Here we describe an attempt to explore this intriguing phenomenon using a coupled ocean–atmosphere model. In response to a massive surface flux of fresh water to the northern North Atlantic of the model, the Thermohaline Circulation weakens abruptly, intensifies and weakens again, followed by a gradual recovery, generating episodes that resemble the abrupt changes of the ocean–atmosphere system recorded in ice and deep-sea cores4. The associated change of surface air temperature is particularly large in the northern North Atlantic Ocean and its neighbourhood, but is relatively small in the rest of the world.
-
interdecadal variations of the Thermohaline Circulation in a coupled ocean atmosphere model
Journal of Climate, 1993Co-Authors: Thomas L Delworth, Syukuro Manabe, Ronald J StoufferAbstract:Abstract A fully coupled ocean-atmosphere model is shown to have irregular oscillations of the Thermohaline Circulation in the North Atlantic Ocean with a time scale of approximately 50 years. The irregular oscillation appears to be driven by density anomalies in the sinking region of the Thermohaline Circulation (approximately 52°N to 72°N) combined with much smaller density anomalies of opposite sign in the broad, rising region. The spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the North Atlantic. The anomalies of sea surface temperature induce model surface air temperature anomalies over the northern North Atlantic, Arctic, and northwestern Europe.
Jonathan M Gregory - One of the best experts on this subject based on the ideXlab platform.
-
a model intercomparison of changes in the atlantic Thermohaline Circulation in response to increasing atmospheric co2 concentration
Geophysical Research Letters, 2005Co-Authors: Andrew J Weaver, Ronald J Stouffer, Keith W Dixon, Jonathan M Gregory, E Driesschaert, Michael Eby, Thierry Fichefet, Hiroyasu Hasumi, Johann H JungclausAbstract:[ 1] As part of the Coupled Model Intercomparison Project, integrations with a common design have been undertaken with eleven different climate models to compare the response of the Atlantic Thermohaline Circulation ( THC) to time-dependent climate change caused by increasing atmospheric CO2 concentration. Over 140 years, during which the CO2 concentration quadruples, the Circulation strength declines gradually in all models, by between 10 and 50%. No model shows a rapid or complete collapse, despite the fairly rapid increase and high final concentration of CO2. The models having the strongest overturning in the control climate tend to show the largest THC reductions. In all models, the THC weakening is caused more by changes in surface heat flux than by changes in surface water flux. No model shows a cooling anywhere, because the greenhouse warming is dominant.
-
mechanisms determining the atlantic Thermohaline Circulation response to greenhouse gas forcing in a non flux adjusted coupled climate model
Journal of Climate, 2001Co-Authors: Robert Thorpe, Jonathan M Gregory, T C Johns, Richard Wood, J F B MitchellAbstract:Abstract Models of the North Atlantic Thermohaline Circulation (THC) show a range of responses to the high-latitude warming and freshening characteristic of global warming scenarios. Most simulate a weakening of the THC, with some suggesting possible interruption of the Circulation, but others exhibit little change. The mechanisms of the THC response to climate change using the HadCM3 coupled ocean–atmosphere general Circulation model, which gives a good simulation of the present-day THC and does not require flux adjustment, were studied. In a range of climate change simulations, the strength of the THC in HadCM3 is proportional to the meridional gradient of steric height (equivalent to column-integrated density) between 30°S and 60°N. During an integration in which CO2 increases at 2% per year for 70 yr, the THC weakens by about 20%, and it stabilizes at this level if the CO2 is subsequently held constant. Changes in surface heat and water fluxes are the cause of the reduction in the steric height gradie...
-
changing spatial structure of the Thermohaline Circulation in response to atmospheric co2 forcing in a climate model
Nature, 1999Co-Authors: Richard A Wood, A B Keen, J F B Mitchell, Jonathan M GregoryAbstract:The heat transported northwards by the North Atlantic Thermohaline Circulation warms the climate of western Europe1,2,3. Previous model studies4,5,6 have suggested that the Circulation is sensitive to increases in atmospheric greenhouse-gas concentrations, but such models have been criticised for the use of unphysical ‘flux adjustments’7,8,9 (artificial corrections that keep the model from drifting to unrealistic states), and for their inability to simulate deep-water formation both north and south of the Greenland–Iceland–Scotland ridge, as seen in observations10,11. Here we present simulations of today's Thermohaline Circulation using a coupled ocean–atmosphere general Circulation model without flux adjustments. These simulations compare well with the observed Thermohaline Circulation, including the formation of deep water on each side of the Greenland–Iceland–Scotland ridge. The model responds to forcing with increasing atmospheric greenhouse-gas concentrations by a collapse of the Circulation and convection in the Labrador Sea, while the deep-water formation north of the ridge remains stable. These changes are similar intwo simulations with different rates of increase of CO2 concentrations. The effects of increasing atmospheric greenhouse-gas concentrations that we simulate are potentially observable, suggesting that it is possible to set up an oceanic monitoring system for the detection of anthropogenic influence on ocean Circulation.
-
changing spatial structure of the Thermohaline Circulation in response to atmospheric co2 forcing in a climate model
Nature, 1999Co-Authors: Richard A Wood, J F B Mitchell, Ann Keen, Jonathan M GregoryAbstract:The heat transported northwards by the North Atlantic Thermohaline Circulation warms the climate of western Europe1,2,3. Previous model studies4,5,6 have suggested that the Circulation is sensitive to increases in atmospheric greenhouse-gas concentrations, but such models have been criticised for the use of unphysical ‘flux adjustments’7,8,9 (artificial corrections that keep the model from drifting to unrealistic states), and for their inability to simulate deep-water formation both north and south of the Greenland–Iceland–Scotland ridge, as seen in observations10,11. Here we present simulations of today's Thermohaline Circulation using a coupled ocean–atmosphere general Circulation model without flux adjustments. These simulations compare well with the observed Thermohaline Circulation, including the formation of deep water on each side of the Greenland–Iceland–Scotland ridge. The model responds to forcing with increasing atmospheric greenhouse-gas concentrations by a collapse of the Circulation and convection in the Labrador Sea, while the deep-water formation north of the ridge remains stable. These changes are similar intwo simulations with different rates of increase of CO2 concentrations. The effects of increasing atmospheric greenhouse-gas concentrations that we simulate are potentially observable, suggesting that it is possible to set up an oceanic monitoring system for the detection of anthropogenic influence on ocean Circulation.
Syukuro Manabe - One of the best experts on this subject based on the ideXlab platform.
-
equilibrium response of Thermohaline Circulation to large changes in atmospheric co2 concentration
Climate Dynamics, 2003Co-Authors: Ronald J Stouffer, Syukuro ManabeAbstract:This study evaluates the equilibrium response of a coupled ocean–atmosphere model to the doubling, quadrupling, and halving of CO2 concentration in the atmosphere. Special emphasis in the study is placed upon the response of the Thermohaline Circulation in the Atlantic Ocean to the changes in CO2 concentration of the atmosphere. The simulated intensity of the Thermohaline Circulation (THC) is similar among three quasi-equilibrium states with the standard, double the standard, and quadruple the standard amounts of CO2 concentration in the atmosphere. When the model atmosphere has half the standard concentration of CO2, however, the THC is very weak and shallow in the Atlantic Ocean. Below a depth of 3 km, the model oceans maintain very thick layer of cold bottom water with temperature close to –2 °C, preventing the deeper penetration of the THC in the Atlantic Ocean. In the Circumpolar Ocean of the Southern Hemisphere, sea ice extends beyond the Antarctic Polar front, almost entirely covering the regions of deepwater ventilation. In addition to the active mode of the THC, there exists another stable mode of the THC for the standard, possibly double the standard (not yet confirmed), and quadruple the standard concentration of atmospheric carbon dioxide. This second mode is characterized by the weak, reverse overturning Circulation over the entire Atlantic basin, and has no ventilation of the entire subsurface water in the North Atlantic Ocean. At one half the standard CO2 concentration, however, the intensity of the first mode is so weak that it is not certain whether there are two distinct stable modes or not. The paleoceanographic implications of the results obtained here are discussed as they relate to the signatures of the Cenozoic changes in the oceans.
-
transient response of a coupled model to estimated changes in greenhouse gas and sulfate concentrations
Geophysical Research Letters, 1997Co-Authors: James M. Haywood, R T Wetherald, Ronald J Stouffer, Syukuro Manabe, V. RamaswamyAbstract:This study investigates changes in surface air temperature (SAT), hydrology and the Thermohaline Circulation due to the the radiative forcing of anthropogenic greenhouse gases and the direct radiative forcing (DRF) of sulfate aerosols in the GFDL coupled ocean-atmosphere model. Three 300-year model integrations are performed with increasing greenhouse gas concentrations only, increasing sulfate aerosol concentrations only and increasing greenhouse gas and sulfate aerosol concentrations. A control integration is also performed keeping concentrations of sulfate and carbon dioxide fixed. The global annual mean SAT change when both greenhouse gases and sulfate aerosols are included is in better agreement with observations than when greenhouse gases alone are included. When the global annual mean SAT change from a model integration that includes only increases in greenhouse gases is added to that from a model integration that includes only increases in sulfate, the resulting global SAT change is approximately equal to that from a model integration that includes increases in both greenhouse gases and sulfate aerosol throughout the integration period. Similar results are found for global annual mean precipitation changes and for the geographical distribution of both SAT and precipitation changes indicating that the climate response is linearly additive for the two types of forcing considered here. Changes in the mid-continental summer dryness and Thermohaline Circulation are also briefly discussed.
-
simulation of abrupt climate change induced by freshwater input to the north atlantic ocean
Nature, 1995Co-Authors: Syukuro Manabe, Ronald J StoufferAbstract:TEMPERATURE records from Greenland ice cores1,2 suggest that large and abrupt changes of North Atlantic climate occurred frequently during both glacial and postglacial periods; one example is the Younger Dryas cold event. Broecker3 speculated that these changes result from rapid changes in the Thermohaline Circulation of the Atlantic Ocean, which were caused by the release of large amounts of melt water from continental ice sheets. Here we describe an attempt to explore this intriguing phenomenon using a coupled ocean–atmosphere model. In response to a massive surface flux of fresh water to the northern North Atlantic of the model, the Thermohaline Circulation weakens abruptly, intensifies and weakens again, followed by a gradual recovery, generating episodes that resemble the abrupt changes of the ocean–atmosphere system recorded in ice and deep-sea cores4. The associated change of surface air temperature is particularly large in the northern North Atlantic Ocean and its neighbourhood, but is relatively small in the rest of the world.
-
interdecadal variations of the Thermohaline Circulation in a coupled ocean atmosphere model
Journal of Climate, 1993Co-Authors: Thomas L Delworth, Syukuro Manabe, Ronald J StoufferAbstract:Abstract A fully coupled ocean-atmosphere model is shown to have irregular oscillations of the Thermohaline Circulation in the North Atlantic Ocean with a time scale of approximately 50 years. The irregular oscillation appears to be driven by density anomalies in the sinking region of the Thermohaline Circulation (approximately 52°N to 72°N) combined with much smaller density anomalies of opposite sign in the broad, rising region. The spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the North Atlantic. The anomalies of sea surface temperature induce model surface air temperature anomalies over the northern North Atlantic, Arctic, and northwestern Europe.
-
century scale effects of increased atmospheric co2 on the ocean atmosphere system
Nature, 1993Co-Authors: Syukuro Manabe, Ronald J StoufferAbstract:SEVERAL studies have addressed the likely effects of CO2-induced climate change over the coming decades1–10, but the longer-term effects have received less attention. Yet these effects could be very significant, as persistent increases in global mean temperatures may ultimately influence the large-scale processes in the coupled ocean–atmosphere system that are thought to play a central part in determining global climate. The Thermohaline Circulation is one such process—Broecker has argued11 that it may have undergone abrupt changes in response to rising temperatures and ice-sheet melting at the end of the last glacial period. Here we use a coupled ocean-atmosphere climate model to study the evolution of the world's climate over the next few centuries, driven by doubling and quadrupling of the concentration of atmospheric CO2. We find that the global mean surface air temperature increases by about 3.5 and 7 °C, respectively, over 500 years, and that sea-level rise owing to thermal expansion alone is about 1 and 2 m respectively (ice-sheet melting could make these values much larger). The thermal and dynamical structure of the oceans changes markedly in the quadrupled-CO2 climate—in particular, the ocean settles into a new stable state in which the Thermohaline Circulation has ceased entirely and the thermocline deepens substantially. These changes prevent the ventilation of the deep ocean and could have a profound impact on the carbon cycle and biogeochemistry of the coupled system.
Johann H Jungclaus - One of the best experts on this subject based on the ideXlab platform.
-
slowdown of the Thermohaline Circulation causes enhanced maritime climate influence and snow cover over europe
Geophysical Research Letters, 2005Co-Authors: Daniela Jacob, Holger Goettel, Johann H Jungclaus, Michael Muskulus, Ralf Podzun, Jochem MarotzkeAbstract:[1] The ocean Thermohaline Circulation (THC) in the Atlantic is generally accepted to contribute to the comparatively mild climate of western and northern Europe. Global climate models (Manabe and Stouffer, 1995; Vellinga and Wood, 2002) and palaeo-observations (McManus et al., 2004) associate periods of weak or absent Atlantic THC with considerably lower temperatures in and around the northern North Atlantic. However, it is uncertain whether such change would spread longitudinally around the globe or would be limited to a narrow strip near the coast (Vellinga and Wood, 2002; Mikolajewicz et al., 1997). Moreover, the relatively low spatial resolution of global climate models has prohibited more detailed statements about possible or probable THC-induced climate change in Europe. Here, we first use a global climate model to perform a THC-slowdown sensitivity experiment, with a weakening of the MOC by about 50%, and the associated control experiment. The two global simulations are then used to force a regional climate model for Europe; the regional control and sensitivity simulations are analyzed here. We find stronger maritime influence over Europe than in the case with no MOC weakening, which surprisingly results in cooling (because sea surface temperatures drop), contrary to today's conditions, where maritime influence leads to milder conditions in Europe. Lower temperatures cause reduced precipitation, increased snow cover and higher albedo leading to positive feedback.
-
a model intercomparison of changes in the atlantic Thermohaline Circulation in response to increasing atmospheric co2 concentration
Geophysical Research Letters, 2005Co-Authors: Andrew J Weaver, Ronald J Stouffer, Keith W Dixon, Jonathan M Gregory, E Driesschaert, Michael Eby, Thierry Fichefet, Hiroyasu Hasumi, Johann H JungclausAbstract:[ 1] As part of the Coupled Model Intercomparison Project, integrations with a common design have been undertaken with eleven different climate models to compare the response of the Atlantic Thermohaline Circulation ( THC) to time-dependent climate change caused by increasing atmospheric CO2 concentration. Over 140 years, during which the CO2 concentration quadruples, the Circulation strength declines gradually in all models, by between 10 and 50%. No model shows a rapid or complete collapse, despite the fairly rapid increase and high final concentration of CO2. The models having the strongest overturning in the control climate tend to show the largest THC reductions. In all models, the THC weakening is caused more by changes in surface heat flux than by changes in surface water flux. No model shows a cooling anywhere, because the greenhouse warming is dominant.
-
reconstructing monitoring and predicting multidecadal scale changes in the north atlantic Thermohaline Circulation with sea surface temperature
Journal of Climate, 2004Co-Authors: Mojib Latif, Johann H Jungclaus, Erich Roeckner, Michael Botzet, Monika Esch, Helmut Haak, Stefan Hagemann, Stephanie Legutke, Simon J Marsland, Uwe MikolajewiczAbstract:Sea surface temperature (SST) observations in the North Atlantic indicate the existence of strong multidecadal variability with a unique spatial structure. It is shown by means of a new global climate model, which does not employ flux adjustments, that the multidecadal SST variability is closely related to variations in the North Atlantic Thermohaline Circulation (THC). The close correspondence between the North Atlantic SST and THC variabilities allows, in conjunction with the dynamical inertia of the THC, for the prediction of the slowly varying component of the North Atlantic climate system. It is shown additionally that past variations of the North Atlantic THC can be reconstructed from a simple North Atlantic SST index and that future, anthropogenically forced changes in the THC can be easily monitored by observing SSTs. The latter is confirmed by another state-ofthe-art global climate model. Finally, the strong multidecadal variability may mask an anthropogenic signal in the North Atlantic for some decades.
Thomas L Delworth - One of the best experts on this subject based on the ideXlab platform.
-
implications of the recent trend in the arctic north atlantic oscillation for the north atlantic Thermohaline Circulation
Journal of Climate, 2000Co-Authors: Thomas L Delworth, Keith W DixonAbstract:Most projections of greenhouse gas‐induced climate change indicate a weakening of the Thermohaline Circulation (THC) in the North Atlantic in response to increased freshening and warming in the subpolar region. These changes reduce high-latitude upper-ocean density and therefore weaken the THC. Using ensembles of numerical experiments with a coupled ocean‐atmosphere model, it is found that this weakening could be delayed by several decades in response to a sustained upward trend in the Arctic/North Atlantic oscillation during winter, such as has been observed over the last 30 years. The stronger winds over the North Atlantic associated with this trend extract more heat from the ocean, thereby cooling and increasing the density of the upper ocean and thus opposing the previously described weakening of the THC. This result is of particular importance if the positive trend in the Arctic/North Atlantic oscillation is a response to increasing greenhouse gases, as has been recently suggested.
-
multidecadal Thermohaline Circulation variability driven by atmospheric surface flux forcing
Journal of Climate, 2000Co-Authors: Thomas L Delworth, Richard J GreatbatchAbstract:Previous analyses of an extended integration of the Geophysical Fluid Dynamics Laboratory coupled climate model have revealed pronounced multidecadal variations of the Thermohaline Circulation (THC) in the North Atlantic. The purpose of the current work is to assess whether those fluctuations can be viewed as a coupled air‐sea mode (in the sense of ENSO), or as an oceanic response to forcing from the atmosphere model, in which large-scale feedbacks from the ocean to the atmospheric Circulation are not critical. A series of integrations using the ocean component of the coupled model are performed to address the above question. The ocean model is forced by suitably chosen time series of surface fluxes from either the coupled model or a companion integration of an atmosphere-only model run with a prescribed seasonal cycle of SSTs and sea-ice thickness. These experiments reveal that 1) the previously identified multidecadal THC variations can be largely viewed as an oceanic response to surface flux forcing from the atmosphere model, although air‐ sea coupling through the thermodynamics appears to modify the amplitude of the variability, and 2) variations in heat flux are the dominant term (relative to the freshwater and momentum fluxes) in driving the THC variability. Experiments driving the ocean model using either high- or low-pass-filtered heat fluxes, with a cutoff period of 20 yr, show that the multidecadal THC variability is driven by the low-frequency portion of the spectrum of atmospheric flux forcing. Analyses have also revealed that the multidecadal THC fluctuations are driven by a spatial pattern of surface heat flux variations that bears a strong resemblance to the North Atlantic oscillation. No conclusive evidence is found that the THC variability is part of a dynamically coupled mode of the atmosphere and ocean models.
-
interdecadal variations of the Thermohaline Circulation in a coupled ocean atmosphere model
Journal of Climate, 1993Co-Authors: Thomas L Delworth, Syukuro Manabe, Ronald J StoufferAbstract:Abstract A fully coupled ocean-atmosphere model is shown to have irregular oscillations of the Thermohaline Circulation in the North Atlantic Ocean with a time scale of approximately 50 years. The irregular oscillation appears to be driven by density anomalies in the sinking region of the Thermohaline Circulation (approximately 52°N to 72°N) combined with much smaller density anomalies of opposite sign in the broad, rising region. The spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the North Atlantic. The anomalies of sea surface temperature induce model surface air temperature anomalies over the northern North Atlantic, Arctic, and northwestern Europe.
