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- Author or Editor: Peter R. Gent x
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Abstract
A nonlocal K-profile parameterization (KPP) of the upper-ocean boundary layer is tested for the equatorial regions. First, the short-term performance of a one-dimensional model with KPP is found to compare favorably to large eddy simulations (LES), including nonlocal countergradient heat flux. The comparison is clean because both the surface forcing and the large-scale flow are identical in the two models. The comparison is direct because the parameterized turbulent flux profiles are explicitly computed in LES. A similar comparison is less favorable when KPP is replaced by purely downgradient diffusion with Richardson-number-dependent viscosity and diffusivity because of the absence of intense convection after sunset. Sensitivity experiments are used to establish parameter values in the interior mixing of KPP.
Second, the impact of the parameterization on annual means and the seasonal cycle in a general circulation model of the upper, equatorial Pacific Ocean is described. The results of GCM runs with and without KPP are compared to annual mean profiles of zonal velocity and temperature from the TOGA-TAO array. The two GCM solutions are closer to each other than to the observations, with biases in zonal velocity in the western Pacific and in subsurface temperature in the eastern Pacific. Such comparisons are never clean because neither the wind stress and the surface heat flux nor the forcing by the large-scale flow are known to sufficient accuracy.
Finally, comparisons are made of the equatorial Pacific Ocean GCM results when different heat flux formulations are used. These include bulk forcing where prescribed air temperature and humidity are used, SST forcing where the use of such ocean-controlled parameters is avoided, and a fully coupled atmospheric general circulation model where there is no prescribed control over any surface fluxes. It is concluded, especially in the eastern Pacific, that the use of specified air temperature and humidity does not overly constrain the model sea surface temperature.
Abstract
A nonlocal K-profile parameterization (KPP) of the upper-ocean boundary layer is tested for the equatorial regions. First, the short-term performance of a one-dimensional model with KPP is found to compare favorably to large eddy simulations (LES), including nonlocal countergradient heat flux. The comparison is clean because both the surface forcing and the large-scale flow are identical in the two models. The comparison is direct because the parameterized turbulent flux profiles are explicitly computed in LES. A similar comparison is less favorable when KPP is replaced by purely downgradient diffusion with Richardson-number-dependent viscosity and diffusivity because of the absence of intense convection after sunset. Sensitivity experiments are used to establish parameter values in the interior mixing of KPP.
Second, the impact of the parameterization on annual means and the seasonal cycle in a general circulation model of the upper, equatorial Pacific Ocean is described. The results of GCM runs with and without KPP are compared to annual mean profiles of zonal velocity and temperature from the TOGA-TAO array. The two GCM solutions are closer to each other than to the observations, with biases in zonal velocity in the western Pacific and in subsurface temperature in the eastern Pacific. Such comparisons are never clean because neither the wind stress and the surface heat flux nor the forcing by the large-scale flow are known to sufficient accuracy.
Finally, comparisons are made of the equatorial Pacific Ocean GCM results when different heat flux formulations are used. These include bulk forcing where prescribed air temperature and humidity are used, SST forcing where the use of such ocean-controlled parameters is avoided, and a fully coupled atmospheric general circulation model where there is no prescribed control over any surface fluxes. It is concluded, especially in the eastern Pacific, that the use of specified air temperature and humidity does not overly constrain the model sea surface temperature.
Abstract
The concepts of residual-mean circulation, transformed Eulerian-mean equations, and Eliassen–Palm fluxes are generalized when the averaging is a low-pass operator in time and space rather than a zonal average. Thus, the eddy motions being considered are ocean eddies on short time and small space scales rather than either purely transient eddies or steady, zonally averaged, standing eddies as commonly considered for the atmosphere.
The generalized Eliassen–Palm fluxes are then parameterized as downgradient momentum diffusion plus the appropriate Coriolis term. This gives a momentum equation for use in non-eddy-resolving ocean circulation models. The resulting potential vorticity equation is then analyzed and the quasigeostrophic limit taken. When the adiabatic tracer parameterization of Gent and McWilliams is also used, this equation is close to showing that quasigeostrophic potential vorticity is advected by the geostrophic velocity and diffused by a Laplacian operator.
A discussion of the Antarctic Circumpolar Current and the meridional-plane circulation, the Deacon cell, in the Southern Hemisphere ocean follows. In an eddy-resolving model with nearly adiabatic interior dynamics, the Deacon cell essentially does not appear when the zonal averaging of the meridional velocity is taken along a constant density surface. This result has a counterpart in non-eddy-resolving ocean model simulations in that the Deacon cell is partially cancelled by the parameterized eddy-induced mass transport.
Abstract
The concepts of residual-mean circulation, transformed Eulerian-mean equations, and Eliassen–Palm fluxes are generalized when the averaging is a low-pass operator in time and space rather than a zonal average. Thus, the eddy motions being considered are ocean eddies on short time and small space scales rather than either purely transient eddies or steady, zonally averaged, standing eddies as commonly considered for the atmosphere.
The generalized Eliassen–Palm fluxes are then parameterized as downgradient momentum diffusion plus the appropriate Coriolis term. This gives a momentum equation for use in non-eddy-resolving ocean circulation models. The resulting potential vorticity equation is then analyzed and the quasigeostrophic limit taken. When the adiabatic tracer parameterization of Gent and McWilliams is also used, this equation is close to showing that quasigeostrophic potential vorticity is advected by the geostrophic velocity and diffused by a Laplacian operator.
A discussion of the Antarctic Circumpolar Current and the meridional-plane circulation, the Deacon cell, in the Southern Hemisphere ocean follows. In an eddy-resolving model with nearly adiabatic interior dynamics, the Deacon cell essentially does not appear when the zonal averaging of the meridional velocity is taken along a constant density surface. This result has a counterpart in non-eddy-resolving ocean model simulations in that the Deacon cell is partially cancelled by the parameterized eddy-induced mass transport.
Abstract
An anisotropic generalization of the Gent–McWilliams (GM) parameterization is presented for eddy-induced tracer transport and diffusion in ocean models, and it is implemented in an ocean general circulation model using a functional formalism to derive the spatial discretization. This complements the anisotropic viscosity parameterization recently developed by Smith and McWilliams. The anisotropic GM operator is potentially useful in both coarse- and high-resolution ocean models, and in this study the focus is on its application in high-resolution eddying solutions, for which it provides an adiabatic alternative to the more commonly used biharmonic horizontal diffusion operators. It is shown that realistically high levels of eddy energy can be simulated using harmonic anisotropic diffusion and friction operators. Isotropic forms can also be used, but these tend either to overly damp the solution when a large diffusion coefficient is used or to introduce unacceptable levels of numerical noise when a small coefficient is used. A series of numerical simulations of the North Atlantic Ocean are conducted at 0.2° resolution using anisotropic viscosity, anisotropic GM, and biharmonic mixing operators to investigate the effects of the anisotropic forms and to isolate changes in the solutions specifically associated with anisotropic GM. A high-resolution 0.1° simulation is then conducted using both anisotropic forms, and the results are compared with a similar run using biharmonic mixing. Modest improvements are seen in the mean wind-driven circulation with the anisotropic forms, but the largest effects are due to the anisotropic GM parameterization, which eliminates the spurious diapycnal diffusion inherent in horizontal tracer diffusion. This leads to significant improvements in the model thermohaline circulation, including the meridional heat transport, meridional overturning circulation, and deep-water formation and convection in the Labrador Sea.
Abstract
An anisotropic generalization of the Gent–McWilliams (GM) parameterization is presented for eddy-induced tracer transport and diffusion in ocean models, and it is implemented in an ocean general circulation model using a functional formalism to derive the spatial discretization. This complements the anisotropic viscosity parameterization recently developed by Smith and McWilliams. The anisotropic GM operator is potentially useful in both coarse- and high-resolution ocean models, and in this study the focus is on its application in high-resolution eddying solutions, for which it provides an adiabatic alternative to the more commonly used biharmonic horizontal diffusion operators. It is shown that realistically high levels of eddy energy can be simulated using harmonic anisotropic diffusion and friction operators. Isotropic forms can also be used, but these tend either to overly damp the solution when a large diffusion coefficient is used or to introduce unacceptable levels of numerical noise when a small coefficient is used. A series of numerical simulations of the North Atlantic Ocean are conducted at 0.2° resolution using anisotropic viscosity, anisotropic GM, and biharmonic mixing operators to investigate the effects of the anisotropic forms and to isolate changes in the solutions specifically associated with anisotropic GM. A high-resolution 0.1° simulation is then conducted using both anisotropic forms, and the results are compared with a similar run using biharmonic mixing. Modest improvements are seen in the mean wind-driven circulation with the anisotropic forms, but the largest effects are due to the anisotropic GM parameterization, which eliminates the spurious diapycnal diffusion inherent in horizontal tracer diffusion. This leads to significant improvements in the model thermohaline circulation, including the meridional heat transport, meridional overturning circulation, and deep-water formation and convection in the Labrador Sea.
Abstract
The trapped equatorial standing modes described theoretically by Gent (1979) are reproduced in a single vertical-mode numerical ocean model. integrations are carried out in domains whose longitudinal extents are characteristic of the widths of the Atlantic and Pacific Oceans, as well as in a narrow ocean in which the simplest possible standing mode can exist. The modes are shown to be very insensitive to small changes in basin width and to the inclusion of friction, and somewhat sensitive to the inclusion of the nonlinear terms and to rotation of the rectangular basin relative to the equator. Moreover, they arise spontaneously from simple atmospheric forcing or from random initial conditions. Typically, 20–40% of the energy input to the equatorial ocean remains trapped in a number of distinct standing modes after about nine years of integration time. The wider the ocean domain, the more energy remains trapped near the equator. These unexpected results have important implications for equatorial ocean dynamics and tropical air-sea interaction.
Abstract
The trapped equatorial standing modes described theoretically by Gent (1979) are reproduced in a single vertical-mode numerical ocean model. integrations are carried out in domains whose longitudinal extents are characteristic of the widths of the Atlantic and Pacific Oceans, as well as in a narrow ocean in which the simplest possible standing mode can exist. The modes are shown to be very insensitive to small changes in basin width and to the inclusion of friction, and somewhat sensitive to the inclusion of the nonlinear terms and to rotation of the rectangular basin relative to the equator. Moreover, they arise spontaneously from simple atmospheric forcing or from random initial conditions. Typically, 20–40% of the energy input to the equatorial ocean remains trapped in a number of distinct standing modes after about nine years of integration time. The wider the ocean domain, the more energy remains trapped near the equator. These unexpected results have important implications for equatorial ocean dynamics and tropical air-sea interaction.
Abstract
The emergence of a spatial pattern in the externally forced response (FR) of dynamic sea level (DSL) during the altimeter era has recently been demonstrated using climate models but our understanding of its initial emergence, drivers, and implications for the future is poor. Here the anthropogenic forcings of the DSL pattern are explored using the Community Earth System Model Large Ensemble (CESM-LE) and Single-Forcing Large Ensemble, a newly available set of simulations where values of individual forcing agents remain fixed at 1920 levels, allowing for an estimation of their effects. Statistically significant contributions to the DSL FR are identified for greenhouse gases (GHGs) and industrial aerosols (AERs), with particularly strong contributions resulting from AERs in the mid-twentieth century and GHGs in the late twentieth and twenty-first century. Secondary, but important, contributions are identified for biomass burning aerosols in the equatorial Atlantic Ocean in the mid-twentieth century, and for stratospheric ozone in the Southern Ocean during the late twentieth century. Key to understanding regional DSL patterns are ocean heat content and salinity anomalies, which are driven by surface heat and freshwater fluxes, ocean dynamics, and the spatial structure of seawater thermal expansivity. Potential implications for the interpretation of DSL during the satellite era and the longer records from tide gauges are suggested as a topic for future research.
Abstract
The emergence of a spatial pattern in the externally forced response (FR) of dynamic sea level (DSL) during the altimeter era has recently been demonstrated using climate models but our understanding of its initial emergence, drivers, and implications for the future is poor. Here the anthropogenic forcings of the DSL pattern are explored using the Community Earth System Model Large Ensemble (CESM-LE) and Single-Forcing Large Ensemble, a newly available set of simulations where values of individual forcing agents remain fixed at 1920 levels, allowing for an estimation of their effects. Statistically significant contributions to the DSL FR are identified for greenhouse gases (GHGs) and industrial aerosols (AERs), with particularly strong contributions resulting from AERs in the mid-twentieth century and GHGs in the late twentieth and twenty-first century. Secondary, but important, contributions are identified for biomass burning aerosols in the equatorial Atlantic Ocean in the mid-twentieth century, and for stratospheric ozone in the Southern Ocean during the late twentieth century. Key to understanding regional DSL patterns are ocean heat content and salinity anomalies, which are driven by surface heat and freshwater fluxes, ocean dynamics, and the spatial structure of seawater thermal expansivity. Potential implications for the interpretation of DSL during the satellite era and the longer records from tide gauges are suggested as a topic for future research.
Abstract
Strongly curved fronts occur when the ratios of cross-front to alongfront horizontal velocities and scales are comparably small for an order one distance in the alongfront direction. This means that all three velocity terms in the continuity equation contribute at leading order. The balance equations are shown to be formally accurate through two orders in powers of the small parameter in this situation; however, the semigeostrophic equations are accurate only at leading order. Both approximate models are accurate through two orders of magnitude if the front is more weakly curved.
Abstract
Strongly curved fronts occur when the ratios of cross-front to alongfront horizontal velocities and scales are comparably small for an order one distance in the alongfront direction. This means that all three velocity terms in the continuity equation contribute at leading order. The balance equations are shown to be formally accurate through two orders in powers of the small parameter in this situation; however, the semigeostrophic equations are accurate only at leading order. Both approximate models are accurate through two orders of magnitude if the front is more weakly curved.
Abstract
Robust and nonrobust aspects of Atlantic meridional overturning circulation (AMOC) variability and mechanisms are analyzed in several 600-yr simulations with the Community Earth System Model. The simulations consist of a set of cases where a few loosely constrained ocean model parameter values are changed, a pair of cases where round-off level perturbations are applied to the initial atmospheric temperature field, and a millennium-scale integration. The time scales of variability differ among the cases with the dominant periods ranging from decadal to centennial. These dominant periods are not stationary in time, indicating that a robust characterization of AMOC temporal variability requires long, multimillennium-scale simulations. A robust aspect is that positive anomalies of the Labrador Sea (LS) upper-ocean density and boundary layer depth and the positive phase of the North Atlantic Oscillation lead AMOC strengthening by 2–3 years. Respective contributions of temperature and salinity to these density anomalies vary across the simulations, but in a majority of the cases temperature contributions dominate. Following an AMOC intensification, all cases show that advection of warm and salty waters into the LS region results in near-neutral density anomalies. Analysis of the LS heat budget indicates that temperature acts to increase density in all cases prior to an AMOC intensification, primarily due to losses by sensible and latent heat fluxes. The accompanying salt budget analysis reveals that the salt contribution to density anomalies varies across the cases, taking both positive and negative values.
Abstract
Robust and nonrobust aspects of Atlantic meridional overturning circulation (AMOC) variability and mechanisms are analyzed in several 600-yr simulations with the Community Earth System Model. The simulations consist of a set of cases where a few loosely constrained ocean model parameter values are changed, a pair of cases where round-off level perturbations are applied to the initial atmospheric temperature field, and a millennium-scale integration. The time scales of variability differ among the cases with the dominant periods ranging from decadal to centennial. These dominant periods are not stationary in time, indicating that a robust characterization of AMOC temporal variability requires long, multimillennium-scale simulations. A robust aspect is that positive anomalies of the Labrador Sea (LS) upper-ocean density and boundary layer depth and the positive phase of the North Atlantic Oscillation lead AMOC strengthening by 2–3 years. Respective contributions of temperature and salinity to these density anomalies vary across the simulations, but in a majority of the cases temperature contributions dominate. Following an AMOC intensification, all cases show that advection of warm and salty waters into the LS region results in near-neutral density anomalies. Analysis of the LS heat budget indicates that temperature acts to increase density in all cases prior to an AMOC intensification, primarily due to losses by sensible and latent heat fluxes. The accompanying salt budget analysis reveals that the salt contribution to density anomalies varies across the cases, taking both positive and negative values.
Abstract
Present-day control and 1% yr−1 increasing carbon dioxide runs have been made using two versions of the Community Climate System Model, version 3.5. One uses the standard versions of the ocean and sea ice components where the horizontal resolution is 1° and the effects of mesoscale eddies are parameterized, and the second uses a resolution of 1/10° where the eddies are resolved. This is the first time the parameterization has been tested in a climate change run compared to an eddy-resolving run. The comparison is made not straightforward by the fact that the two control run climates are not the same, especially in their sea ice distributions. The focus is on the Antarctic Circumpolar Current region, where the effects of eddies are of leading order. The conclusions are that many of the differences in the two carbon dioxide transient forcing runs can be explained by the different control run sea ice distributions around Antarctica, but there are some quantitative differences in the meridional overturning circulation, poleward heat transport, and zonally averaged heat uptake when the eddies are parameterized rather than resolved.
Abstract
Present-day control and 1% yr−1 increasing carbon dioxide runs have been made using two versions of the Community Climate System Model, version 3.5. One uses the standard versions of the ocean and sea ice components where the horizontal resolution is 1° and the effects of mesoscale eddies are parameterized, and the second uses a resolution of 1/10° where the eddies are resolved. This is the first time the parameterization has been tested in a climate change run compared to an eddy-resolving run. The comparison is made not straightforward by the fact that the two control run climates are not the same, especially in their sea ice distributions. The focus is on the Antarctic Circumpolar Current region, where the effects of eddies are of leading order. The conclusions are that many of the differences in the two carbon dioxide transient forcing runs can be explained by the different control run sea ice distributions around Antarctica, but there are some quantitative differences in the meridional overturning circulation, poleward heat transport, and zonally averaged heat uptake when the eddies are parameterized rather than resolved.
Abstract
Luyten and Roemmich have shown a strong semiannual signal in zonal velocity in the upper, western part of the equatorial Indian Ocean. Their observations are modeled by assuming that they are directly forced by the observed semiannual component of zonal wind stress, which is relatively large in the equatorial Indian Ocean. The model is linear, periodic, has linear damping, uses the long-wave approximation, and can be solved analytically. A good comparison with the observations is obtained for the phase of the oscillation across the array. The predicted magnitude is somewhat smaller than in the observations. The model sensitivity to friction and the spatial distribution of the wind stress is explored. Some additional model simplifications are discussed, but it is concluded that they all detract substantially from the comparison. The main conclusion is that the observations can be accounted for as a directly forced response to the semiannual component of the near-equatorial zonal winds.
Abstract
Luyten and Roemmich have shown a strong semiannual signal in zonal velocity in the upper, western part of the equatorial Indian Ocean. Their observations are modeled by assuming that they are directly forced by the observed semiannual component of zonal wind stress, which is relatively large in the equatorial Indian Ocean. The model is linear, periodic, has linear damping, uses the long-wave approximation, and can be solved analytically. A good comparison with the observations is obtained for the phase of the oscillation across the array. The predicted magnitude is somewhat smaller than in the observations. The model sensitivity to friction and the spatial distribution of the wind stress is explored. Some additional model simplifications are discussed, but it is concluded that they all detract substantially from the comparison. The main conclusion is that the observations can be accounted for as a directly forced response to the semiannual component of the near-equatorial zonal winds.
Abstract
Numerical solutions are examined for nearly axisymmetric geopotential monopole vortices whose vertical structure is essentially confined to the lowest few vertical modes. The vortex environment is a rotating, stratified fluid with spatially variable Coriolis frequency (the β-plane). Solutions are examined with Rossby numbers in an order one range about zero, and therefore the balance equations are and appropriate model. Solutions From the quasi-geostrophic and primitive equations are also examined, and we find that the balance equations are much more accurate than the former and more efficient, both conceptually and computationally, than the latter. The central parameter regime is one of stable vortex propagation, accompanied by week Rossby wave radiation and slow changes in vortex shape, with the latter due more to the radiation than the weak dissipation. Various types of instability—baroclinic, barotropic, and inertial—act to delimit the stable regime for vortices.
Abstract
Numerical solutions are examined for nearly axisymmetric geopotential monopole vortices whose vertical structure is essentially confined to the lowest few vertical modes. The vortex environment is a rotating, stratified fluid with spatially variable Coriolis frequency (the β-plane). Solutions are examined with Rossby numbers in an order one range about zero, and therefore the balance equations are and appropriate model. Solutions From the quasi-geostrophic and primitive equations are also examined, and we find that the balance equations are much more accurate than the former and more efficient, both conceptually and computationally, than the latter. The central parameter regime is one of stable vortex propagation, accompanied by week Rossby wave radiation and slow changes in vortex shape, with the latter due more to the radiation than the weak dissipation. Various types of instability—baroclinic, barotropic, and inertial—act to delimit the stable regime for vortices.
