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- Author or Editor: Jochem Marotzke x
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Abstract
A barotropic ocean circulation model coupled to a mixed-layer temperature equation is used to study the poleward heat transport by the horizontal wind-driven ocean currents. Through scaling analyses, two different regimes of the heat transport are found, characterized by λ→∞ and λ→0, respectively, where λ is the ratio between the ocean advection timescale and the restoring timescale of Newtonian cooling at the air–sea interface. In the regime λ→∞, the heat transport is proportional to λ−1 and to the second power both of the basin east–west width and of the magnitude of wind stress curl. In the regime λ→0, the heat transport is proportional to λ and to the basin width, and insensitive to the magnitude of wind stress curl. The heat transport is a maximum for intermediate values of λ, and the numerical experiments show that a restoring timescale of 5 months maximizes the heat transport for a barotropic square basin 500 m deep and 400 km wide (North Atlantic size). The corresponding maximum heat transport is about 0.32 PW. If the basin width is doubled (North Pacific size), the maximum poleward heat transport by the modeled ocean currents is estimated to be 0.73 PW. The numerical experiments also show that the heat transport can be underestimated if the model resolution is too coarse—for example, for a horizontal resolution of 4° × 4°, the heat transport is underestimated by about 50%.
Abstract
A barotropic ocean circulation model coupled to a mixed-layer temperature equation is used to study the poleward heat transport by the horizontal wind-driven ocean currents. Through scaling analyses, two different regimes of the heat transport are found, characterized by λ→∞ and λ→0, respectively, where λ is the ratio between the ocean advection timescale and the restoring timescale of Newtonian cooling at the air–sea interface. In the regime λ→∞, the heat transport is proportional to λ−1 and to the second power both of the basin east–west width and of the magnitude of wind stress curl. In the regime λ→0, the heat transport is proportional to λ and to the basin width, and insensitive to the magnitude of wind stress curl. The heat transport is a maximum for intermediate values of λ, and the numerical experiments show that a restoring timescale of 5 months maximizes the heat transport for a barotropic square basin 500 m deep and 400 km wide (North Atlantic size). The corresponding maximum heat transport is about 0.32 PW. If the basin width is doubled (North Pacific size), the maximum poleward heat transport by the modeled ocean currents is estimated to be 0.73 PW. The numerical experiments also show that the heat transport can be underestimated if the model resolution is too coarse—for example, for a horizontal resolution of 4° × 4°, the heat transport is underestimated by about 50%.
Abstract
The interhemispheric thermohaline circulation is examined using Rooth’s three-box ocean model, whereby overturning strength is parameterized from density differences between high-latitude boxes. Recent results with general circulation models indicate that this is a better analog of the Atlantic thermohaline circulation than a single-hemisphere box model. The results are compared with those of hemispheric box model studies, where possible, and the role of asymmetrical freshwater forcing is explored.
Using both analytical and numerical methods, the linear and nonlinear stability of the model is examined. Although freshwater forcing in the Southern Hemisphere alone governs overturning strength, increasing freshwater forcing in the Northern Hemisphere leads to a heretofore unrecognized instability in the northern sinking branch due to an increasingly positive ocean salinity feedback. If the northern forcing is instead made weaker than the southern forcing, this feedback becomes negative. In contrast, the ocean salinity feedback is always positive in single-hemisphere models. Nonlinear stability, as measured by the size of the perturbation necessary to induce a permanent transition to the southern sinking equilibrium, is also observed to depend similarly on the north–south forcing ratio.
The model is augmented with explicit atmospheric eddy transport parameterizations, allowing examination of the eddy moisture transport (EMT) and eddy heat transport (EHT) feedbacks. As in the hemispheric model, the EMT feedback is always destabilizing, whereas the EHT may stabilize or destabilize. However, in this model whether the EHT stabilizes or destabilizes depends largely on the sign of the ocean salinity feedback and the size of the perturbation. Since oceanic heat transport in the Southern Hemisphere is weak, the Northern Hemisphere EMT and EHT feedbacks dominate.
Abstract
The interhemispheric thermohaline circulation is examined using Rooth’s three-box ocean model, whereby overturning strength is parameterized from density differences between high-latitude boxes. Recent results with general circulation models indicate that this is a better analog of the Atlantic thermohaline circulation than a single-hemisphere box model. The results are compared with those of hemispheric box model studies, where possible, and the role of asymmetrical freshwater forcing is explored.
Using both analytical and numerical methods, the linear and nonlinear stability of the model is examined. Although freshwater forcing in the Southern Hemisphere alone governs overturning strength, increasing freshwater forcing in the Northern Hemisphere leads to a heretofore unrecognized instability in the northern sinking branch due to an increasingly positive ocean salinity feedback. If the northern forcing is instead made weaker than the southern forcing, this feedback becomes negative. In contrast, the ocean salinity feedback is always positive in single-hemisphere models. Nonlinear stability, as measured by the size of the perturbation necessary to induce a permanent transition to the southern sinking equilibrium, is also observed to depend similarly on the north–south forcing ratio.
The model is augmented with explicit atmospheric eddy transport parameterizations, allowing examination of the eddy moisture transport (EMT) and eddy heat transport (EHT) feedbacks. As in the hemispheric model, the EMT feedback is always destabilizing, whereas the EHT may stabilize or destabilize. However, in this model whether the EHT stabilizes or destabilizes depends largely on the sign of the ocean salinity feedback and the size of the perturbation. Since oceanic heat transport in the Southern Hemisphere is weak, the Northern Hemisphere EMT and EHT feedbacks dominate.
Abstract
This paper analyzes regional sea level changes in a climate change simulation using the Max Planck Institute for Meteorology (MPI) coupled atmosphere–ocean general circulation model ECHAM5/MPI-OM. The climate change scenario builds on observed atmospheric greenhouse gas (GHG) concentrations from 1860 to 2000, followed by the International Panel on Climate Change (IPCC) A1B climate change scenario until 2100; from 2100 to 2199, GHG concentrations are fixed at the 2100 level. As compared with the unperturbed control climate, global sea level rises 0.26 m by 2100, and 0.56 m by 2199 through steric expansion; eustatic changes are not included in this simulation. The model’s sea level evolves substantially differently among ocean basins. Sea level rise is strongest in the Arctic Ocean, from enhanced freshwater input from precipitation and continental runoff, and weakest in the Southern Ocean, because of compensation of steric changes through dynamic sea surface height (SSH) adjustments. In the North Atlantic Ocean (NA), a complex tripole SSH pattern across the subtropical to subpolar gyre front evolves, which is consistent with a northward shift of the NA current. On interannual to decadal time scales, the SSH difference between Bermuda and the Labrador Sea correlates highly with the combined baroclinic gyre transport in the NA but only weakly with the meridional overturning circulation (MOC) and, thus, does not allow for estimates of the MOC on these time scales. Bottom pressure increases over shelf areas by up to 0.45 m (water column equivalent) and decreases over the Atlantic section in the Southern Ocean by up to 0.20 m. The separate evaluation of thermosteric and halosteric sea level changes shows that thermosteric anomalies are positive over most of the World Ocean. Because of increased atmospheric moisture transport from low to high latitudes, halosteric anomalies are negative in the subtropical NA and partly compensate thermosteric anomalies, but are positive in the Arctic Ocean and add to thermosteric anomalies. The vertical distribution of thermosteric and halosteric anomalies is highly nonuniform among ocean basins, reaching deeper than 3000 m in the Southern Ocean, down to 2200 m in the North Atlantic, and only to depths of 500 m in the Pacific Ocean by the end of the twenty-first century.
Abstract
This paper analyzes regional sea level changes in a climate change simulation using the Max Planck Institute for Meteorology (MPI) coupled atmosphere–ocean general circulation model ECHAM5/MPI-OM. The climate change scenario builds on observed atmospheric greenhouse gas (GHG) concentrations from 1860 to 2000, followed by the International Panel on Climate Change (IPCC) A1B climate change scenario until 2100; from 2100 to 2199, GHG concentrations are fixed at the 2100 level. As compared with the unperturbed control climate, global sea level rises 0.26 m by 2100, and 0.56 m by 2199 through steric expansion; eustatic changes are not included in this simulation. The model’s sea level evolves substantially differently among ocean basins. Sea level rise is strongest in the Arctic Ocean, from enhanced freshwater input from precipitation and continental runoff, and weakest in the Southern Ocean, because of compensation of steric changes through dynamic sea surface height (SSH) adjustments. In the North Atlantic Ocean (NA), a complex tripole SSH pattern across the subtropical to subpolar gyre front evolves, which is consistent with a northward shift of the NA current. On interannual to decadal time scales, the SSH difference between Bermuda and the Labrador Sea correlates highly with the combined baroclinic gyre transport in the NA but only weakly with the meridional overturning circulation (MOC) and, thus, does not allow for estimates of the MOC on these time scales. Bottom pressure increases over shelf areas by up to 0.45 m (water column equivalent) and decreases over the Atlantic section in the Southern Ocean by up to 0.20 m. The separate evaluation of thermosteric and halosteric sea level changes shows that thermosteric anomalies are positive over most of the World Ocean. Because of increased atmospheric moisture transport from low to high latitudes, halosteric anomalies are negative in the subtropical NA and partly compensate thermosteric anomalies, but are positive in the Arctic Ocean and add to thermosteric anomalies. The vertical distribution of thermosteric and halosteric anomalies is highly nonuniform among ocean basins, reaching deeper than 3000 m in the Southern Ocean, down to 2200 m in the North Atlantic, and only to depths of 500 m in the Pacific Ocean by the end of the twenty-first century.
Abstract
In this paper, the overturning responses to wind stress changes of an eddying ocean and a non-eddying ocean are compared. Differences are found in the deep overturning cell in the low-latitude North Atlantic Ocean with substantial implications for the deep western boundary current (DWBC). In an ocean-only twin experiment with one eddying and one non-eddying configuration of the MPI ocean model, two different forcings are being applied: the standard NCEP forcing and the NCEP forcing with 2× surface wind stress. The response to the wind stress doubling in the Atlantic meridional overturning circulation is similar in the eddying and the non-eddying configuration, showing an increase by about 4 Sv (~25%; 1 Sv ≡ 106 m3 s−1). In contrast, the DWBC responds with a speedup in the non-eddying configuration and a slowdown in the eddying configuration. This paper demonstrates that the DWBC slowdown in the eddying configuration is largely balanced by eddy vorticity fluxes. Because those fluxes are not resolved and also not captured by an eddy parameterization in the non-eddying configuration, such a DWBC slowdown is likely not to occur in non-eddying ocean models, which therefore might not capture the whole range of overturning responses. Furthermore, evidence is provided that the balancing effect of the eddies is not a passive reaction to a remotely triggered DWBC slowdown. Instead, deep eddies that are sourced from the upper ocean provide an excess input of relative vorticity that then actively forces the DWBC mean flow to slow down.
Abstract
In this paper, the overturning responses to wind stress changes of an eddying ocean and a non-eddying ocean are compared. Differences are found in the deep overturning cell in the low-latitude North Atlantic Ocean with substantial implications for the deep western boundary current (DWBC). In an ocean-only twin experiment with one eddying and one non-eddying configuration of the MPI ocean model, two different forcings are being applied: the standard NCEP forcing and the NCEP forcing with 2× surface wind stress. The response to the wind stress doubling in the Atlantic meridional overturning circulation is similar in the eddying and the non-eddying configuration, showing an increase by about 4 Sv (~25%; 1 Sv ≡ 106 m3 s−1). In contrast, the DWBC responds with a speedup in the non-eddying configuration and a slowdown in the eddying configuration. This paper demonstrates that the DWBC slowdown in the eddying configuration is largely balanced by eddy vorticity fluxes. Because those fluxes are not resolved and also not captured by an eddy parameterization in the non-eddying configuration, such a DWBC slowdown is likely not to occur in non-eddying ocean models, which therefore might not capture the whole range of overturning responses. Furthermore, evidence is provided that the balancing effect of the eddies is not a passive reaction to a remotely triggered DWBC slowdown. Instead, deep eddies that are sourced from the upper ocean provide an excess input of relative vorticity that then actively forces the DWBC mean flow to slow down.
Abstract
This study aims at improving the forecast skill of climate predictions through the use of ocean synthesis data for initial conditions of a coupled climate model. For this purpose, the coupled model of the Max Planck Institute (MPI) for Meteorology, which consists of the atmosphere model ECHAM5 and the MPI Ocean Model (MPI-OM), is initialized with oceanic synthesis fields available from the German contribution to Estimating the Circulation and Climate of the Ocean (GECCO) project. The use of an anomaly coupling scheme during the initialization avoids the main problems with drift in the climate predictions. Thus, the coupled model is continuously forced to follow the density anomalies of the GECCO synthesis over the period 1952–2001. Hindcast experiments are initialized from this experiment at constant intervals. The results show predictive skill through the initialization up to the decadal time scale, particularly over the North Atlantic. Viewed over the time scales analyzed here (annual, 5-yr, and 10-yr mean), greater skill for the North Atlantic sea surface temperature (SST) is obtained in the hindcast experiments than in either a damped persistence or trend forecast. The Atlantic meridional overturning circulation hindcast closely follows that of the GECCO oceanic synthesis. Hindcasts of global-mean temperature do not obtain greater skill than either damped persistence or a trend forecast, owing to the SST errors in the GECCO synthesis, outside the North Atlantic. An ensemble of forecast experiments is subsequently performed over the period 2002–11. North Atlantic SST from the forecast experiment agrees well with observations until the year 2007, and it is higher than if simulated without the oceanic initialization (averaged over the forecast period). The results confirm that both the initial and the boundary conditions must be accounted for in decadal climate predictions.
Abstract
This study aims at improving the forecast skill of climate predictions through the use of ocean synthesis data for initial conditions of a coupled climate model. For this purpose, the coupled model of the Max Planck Institute (MPI) for Meteorology, which consists of the atmosphere model ECHAM5 and the MPI Ocean Model (MPI-OM), is initialized with oceanic synthesis fields available from the German contribution to Estimating the Circulation and Climate of the Ocean (GECCO) project. The use of an anomaly coupling scheme during the initialization avoids the main problems with drift in the climate predictions. Thus, the coupled model is continuously forced to follow the density anomalies of the GECCO synthesis over the period 1952–2001. Hindcast experiments are initialized from this experiment at constant intervals. The results show predictive skill through the initialization up to the decadal time scale, particularly over the North Atlantic. Viewed over the time scales analyzed here (annual, 5-yr, and 10-yr mean), greater skill for the North Atlantic sea surface temperature (SST) is obtained in the hindcast experiments than in either a damped persistence or trend forecast. The Atlantic meridional overturning circulation hindcast closely follows that of the GECCO oceanic synthesis. Hindcasts of global-mean temperature do not obtain greater skill than either damped persistence or a trend forecast, owing to the SST errors in the GECCO synthesis, outside the North Atlantic. An ensemble of forecast experiments is subsequently performed over the period 2002–11. North Atlantic SST from the forecast experiment agrees well with observations until the year 2007, and it is higher than if simulated without the oceanic initialization (averaged over the forecast period). The results confirm that both the initial and the boundary conditions must be accounted for in decadal climate predictions.
Abstract
Recent numerical experiments indicate that the rate of meridional overturning associated with North Atlantic Deep Water is partially controlled by wind stress in the Southern Ocean, where the zonal periodicity of the domain alters the nature of the flow. Here, the authors solve the cubic scale relationship of Gnanadesikan to find a simple expression for meridional overturning that is used to clarify the relative strength of the wind-forced component. The predicted overturning is compared with coarse-resolution numerical experiments with an idealized Atlantic Ocean–Southern Ocean geometry. The scaling accurately predicts the sensitivity to forcing for experiments with a level model employing isopycnal diffusion of temperature, salinity, and “layer thickness.” A layer model produces similar results, increasing confidence in the numerics of both models. Level model experiments with horizontal diffusivity have similar qualitative behavior but somewhat different sensitivity to forcing. The paper highlights the difference in meridional overturning induced by changes in wind stress or vertical diffusivity. Strengthening the Southern Ocean wind stress induces a circulation anomaly in which most of the water is subducted in the Ekman layer of the wind perturbation region, follows isopycnals down into the thermocline, and changes density again when the isopycnals near the surface in the Northern Hemisphere. Approximating the circulation anomaly by this subduction route allows for a surprisingly accurate prediction of the resulting heat transport anomaly, based on the surface temperature distribution. Some of the induced flow follows a second, near-surface northward route through low-latitude water that is lighter than the subducted flow. Overturning anomalies far from the wind stress perturbations are not completely determined by wind stress in the zonally periodic Southern Ocean: wind stress outside the periodic region strongly influences the transport of heat across the equator primarily by changing the temperature of the flow across the equator.
Abstract
Recent numerical experiments indicate that the rate of meridional overturning associated with North Atlantic Deep Water is partially controlled by wind stress in the Southern Ocean, where the zonal periodicity of the domain alters the nature of the flow. Here, the authors solve the cubic scale relationship of Gnanadesikan to find a simple expression for meridional overturning that is used to clarify the relative strength of the wind-forced component. The predicted overturning is compared with coarse-resolution numerical experiments with an idealized Atlantic Ocean–Southern Ocean geometry. The scaling accurately predicts the sensitivity to forcing for experiments with a level model employing isopycnal diffusion of temperature, salinity, and “layer thickness.” A layer model produces similar results, increasing confidence in the numerics of both models. Level model experiments with horizontal diffusivity have similar qualitative behavior but somewhat different sensitivity to forcing. The paper highlights the difference in meridional overturning induced by changes in wind stress or vertical diffusivity. Strengthening the Southern Ocean wind stress induces a circulation anomaly in which most of the water is subducted in the Ekman layer of the wind perturbation region, follows isopycnals down into the thermocline, and changes density again when the isopycnals near the surface in the Northern Hemisphere. Approximating the circulation anomaly by this subduction route allows for a surprisingly accurate prediction of the resulting heat transport anomaly, based on the surface temperature distribution. Some of the induced flow follows a second, near-surface northward route through low-latitude water that is lighter than the subducted flow. Overturning anomalies far from the wind stress perturbations are not completely determined by wind stress in the zonally periodic Southern Ocean: wind stress outside the periodic region strongly influences the transport of heat across the equator primarily by changing the temperature of the flow across the equator.
Abstract
Zonal wind stress over the Southern Ocean may be responsible for a significant fraction of the meridional overturning associated with North Atlantic Deep Water. Numerical experiments by Tsujino and Suginohara imply that the zonal periodicity of the Southern Ocean is not necessary for midlatitude westerly winds to drive strong remote meridional overturning. Here, idealized numerical experiments examine the importance of zonal periodicity and other factors in setting the sensitivity of this overturning to the wind stress. These experiments support the conclusion that the wind can drive remote overturning in the absence of zonal periodicity. However, making the subpolar ocean zonally periodic roughly doubles the strength of the overturning induced by the wind there. Tsujino and Suginohara's experiments are especially sensitive to wind stress because their basin has a relatively small meridional range, which increases the Ekman transport associated with the wind stress. Depending on the stratification in the wind-forcing region, the heating associated with the westerly winds can occur almost exclusively near the surface or deeper in the thermocline as well. Subsurface cooling in the wind-forcing region reduces the remote effects and can occur through both vertical or horizontal diffusion. A scale analysis of the heat budget suggests that sufficiently strong subpolar westerlies produce remote overturning because there is no way for local cooling to balance wind-induced surface heating. Tsujino and Suginohara suggested that wind increases the overturning by enhancing the mixing-driven thermohaline circulation. However, an increase in thermohaline circulation is associated with increased conversion of turbulent kinetic energy to potential energy. This increase in the energy conversion is absent in the wind-driven case, indicating an important qualitative difference between mixing-driven thermohaline overturning and remote wind-driven overturning.
Abstract
Zonal wind stress over the Southern Ocean may be responsible for a significant fraction of the meridional overturning associated with North Atlantic Deep Water. Numerical experiments by Tsujino and Suginohara imply that the zonal periodicity of the Southern Ocean is not necessary for midlatitude westerly winds to drive strong remote meridional overturning. Here, idealized numerical experiments examine the importance of zonal periodicity and other factors in setting the sensitivity of this overturning to the wind stress. These experiments support the conclusion that the wind can drive remote overturning in the absence of zonal periodicity. However, making the subpolar ocean zonally periodic roughly doubles the strength of the overturning induced by the wind there. Tsujino and Suginohara's experiments are especially sensitive to wind stress because their basin has a relatively small meridional range, which increases the Ekman transport associated with the wind stress. Depending on the stratification in the wind-forcing region, the heating associated with the westerly winds can occur almost exclusively near the surface or deeper in the thermocline as well. Subsurface cooling in the wind-forcing region reduces the remote effects and can occur through both vertical or horizontal diffusion. A scale analysis of the heat budget suggests that sufficiently strong subpolar westerlies produce remote overturning because there is no way for local cooling to balance wind-induced surface heating. Tsujino and Suginohara suggested that wind increases the overturning by enhancing the mixing-driven thermohaline circulation. However, an increase in thermohaline circulation is associated with increased conversion of turbulent kinetic energy to potential energy. This increase in the energy conversion is absent in the wind-driven case, indicating an important qualitative difference between mixing-driven thermohaline overturning and remote wind-driven overturning.
Abstract
Using a 0.1° ocean model, this paper establishes a consistent picture of the interaction of mesoscale eddy density fluxes with the geostrophic deep western boundary current (DWBC) in the Atlantic between 26°N and 20°S. Above the DWBC core (the level of maximum southward flow, ~2000-m depth), the eddies flatten isopycnals and hence decrease the potential energy of the mean flow, which agrees with their interpretation and parameterization in the Gent–McWilliams framework. Below the core, even though the eddy fluxes have a weaker magnitude, they systematically steepen isopycnals and thus feed potential energy to the mean flow, which contradicts common expectations. These two vertically separated eddy regimes are found through an analysis of the eddy density flux divergence in stream-following coordinates. In addition, pathways of potential energy in terms of the Lorenz energy cycle reveal this regime shift. The twofold eddy effect on density is balanced by an overturning in the plane normal to the DWBC. Its direction is clockwise (with upwelling close to the shore and downwelling further offshore) north of the equator. In agreement with the sign change in the Coriolis parameter, the overturning changes direction to anticlockwise south of the equator. Within the domain covered in this study, except in a narrow band around the equator, this scenario is robust along the DWBC.
Abstract
Using a 0.1° ocean model, this paper establishes a consistent picture of the interaction of mesoscale eddy density fluxes with the geostrophic deep western boundary current (DWBC) in the Atlantic between 26°N and 20°S. Above the DWBC core (the level of maximum southward flow, ~2000-m depth), the eddies flatten isopycnals and hence decrease the potential energy of the mean flow, which agrees with their interpretation and parameterization in the Gent–McWilliams framework. Below the core, even though the eddy fluxes have a weaker magnitude, they systematically steepen isopycnals and thus feed potential energy to the mean flow, which contradicts common expectations. These two vertically separated eddy regimes are found through an analysis of the eddy density flux divergence in stream-following coordinates. In addition, pathways of potential energy in terms of the Lorenz energy cycle reveal this regime shift. The twofold eddy effect on density is balanced by an overturning in the plane normal to the DWBC. Its direction is clockwise (with upwelling close to the shore and downwelling further offshore) north of the equator. In agreement with the sign change in the Coriolis parameter, the overturning changes direction to anticlockwise south of the equator. Within the domain covered in this study, except in a narrow band around the equator, this scenario is robust along the DWBC.
