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- Author or Editor: Max J. Suarez x
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Abstract
When sufficiently large zonally asymmetric tropical heating is introduced in a two-level model of global atmospheric flow, its general circulation becomes strongly superrotating. The nature of the superrotating solutions is studied by examining momentum and heat budgets for a range of values of thermal forcing. Changes in the transport of zonal momentum by transient eddies appear to play the key role in the transition to superrotation. The dramatic bifurcation of the solutions of this model may help explain the maintenance and variability of the zonal mean flow in the tropics.
Abstract
When sufficiently large zonally asymmetric tropical heating is introduced in a two-level model of global atmospheric flow, its general circulation becomes strongly superrotating. The nature of the superrotating solutions is studied by examining momentum and heat budgets for a range of values of thermal forcing. Changes in the transport of zonal momentum by transient eddies appear to play the key role in the transition to superrotation. The dramatic bifurcation of the solutions of this model may help explain the maintenance and variability of the zonal mean flow in the tropics.
Abstract
Results are presented from a 35-year integration of a coupled ocean-atmosphere model. Both ocean and atmosphere are two-level, nonlinear primitive equations models. The global atmospheric model is forced by a steady, zonally symmetric Newtonian heating. The ocean model is solved in a rectangular tropical basin. Heat fluxes between ocean and atmosphere are linear in air-sea temperature differences, and the interfacial stress is proportional to lower-level atmospheric winds.
The coupled models produce ENSO-like variability on time scales of 3 to 5 years. Since there is no external time-dependent forcing, these are self-sustained vacillations of the nonlinear system. It is argued that the energetics of the vacillations is that of unstable coupled modes and that the time scale is crucially dependent on the effects of ocean waves propagating in a closed basin.
Abstract
Results are presented from a 35-year integration of a coupled ocean-atmosphere model. Both ocean and atmosphere are two-level, nonlinear primitive equations models. The global atmospheric model is forced by a steady, zonally symmetric Newtonian heating. The ocean model is solved in a rectangular tropical basin. Heat fluxes between ocean and atmosphere are linear in air-sea temperature differences, and the interfacial stress is proportional to lower-level atmospheric winds.
The coupled models produce ENSO-like variability on time scales of 3 to 5 years. Since there is no external time-dependent forcing, these are self-sustained vacillations of the nonlinear system. It is argued that the energetics of the vacillations is that of unstable coupled modes and that the time scale is crucially dependent on the effects of ocean waves propagating in a closed basin.
Abstract
A simple nonlinear model is proposed for the El Niño/Southern Oscillation (ENSO) phenomenon. Its key feature is the inclusion of oceanic wave transit effects through a negative, delayed feedback. A linear stability analysis and numerical results are presented to show that the period of the oscillation is typically several times the delay. It is argued such an effect can account for the long time scale of ENSO.
Abstract
A simple nonlinear model is proposed for the El Niño/Southern Oscillation (ENSO) phenomenon. Its key feature is the inclusion of oceanic wave transit effects through a negative, delayed feedback. A linear stability analysis and numerical results are presented to show that the period of the oscillation is typically several times the delay. It is argued such an effect can account for the long time scale of ENSO.
Abstract
A useful but as yet under-utilized tool for climatic studies is an atmospheric model in which the time evolution of large-scale eddies is resolved explicitly, but in a relatively simple dynamical framework. One such model is described in detail in this study–a two-level primitive equation model on a sphere with variable static stability, finite-differenced in the meridional direction but Fourier analyzed and then very severely truncated in the zonal direction. Two versions of the model-moist and dry–are developed, the maintenance of the model's static stability being markedly different in the two versions.
Statistically steady states are obtained for a variety of spectral truncations For both versions of the model in order to determine the fewest zonal wavenumbers one can retain and still obtain a reasonable zonally averaged circulation. Including only one wave, of wavelength typical of strongly unstable waves in mid-latitudes, results in a circulation with a subpolar jet as well as a subtropical jet in the zonal wind. The addition of a longer wave (i.e., the addition of wavenumber 3 to wavenumber 6) results in the destruction of the subpolar jet.No further dramatic changes in the zonally averaged flow occur as more waves are added to the system.
Features of the model's dynamics which might limit its utility are emphasized, notably the dependence of the strength of the Hadley cell on the details of the convective adjustment scheme. We find, however, that the total energy transported by the Hadley cell is insensitive to such details.
Climatic sensitivity experiments with thee models will be described in forthcoming papers.
Abstract
A useful but as yet under-utilized tool for climatic studies is an atmospheric model in which the time evolution of large-scale eddies is resolved explicitly, but in a relatively simple dynamical framework. One such model is described in detail in this study–a two-level primitive equation model on a sphere with variable static stability, finite-differenced in the meridional direction but Fourier analyzed and then very severely truncated in the zonal direction. Two versions of the model-moist and dry–are developed, the maintenance of the model's static stability being markedly different in the two versions.
Statistically steady states are obtained for a variety of spectral truncations For both versions of the model in order to determine the fewest zonal wavenumbers one can retain and still obtain a reasonable zonally averaged circulation. Including only one wave, of wavelength typical of strongly unstable waves in mid-latitudes, results in a circulation with a subpolar jet as well as a subtropical jet in the zonal wind. The addition of a longer wave (i.e., the addition of wavenumber 3 to wavenumber 6) results in the destruction of the subpolar jet.No further dramatic changes in the zonally averaged flow occur as more waves are added to the system.
Features of the model's dynamics which might limit its utility are emphasized, notably the dependence of the strength of the Hadley cell on the details of the convective adjustment scheme. We find, however, that the total energy transported by the Hadley cell is insensitive to such details.
Climatic sensitivity experiments with thee models will be described in forthcoming papers.
Abstract
Result obtained with a mixed layer model are used to study the dynamics of stratomulus formation and dissipation in subtropical marine stratocumulus cloud regimes. The model used allows entrainment to be driven by shear as well as buoyancy, and includes a very crude parameterization of the partial blackness of thin cloud layers. Model results show that for some values of the large-scale divergence there are three equilibrium mixed layer structures, two of which are stable. One of the stable equilibria is cloudy, deep, and buoyancy-driven, while the other is clear, shallow, and shear-driven. It is found that as a result of hysteresis effects a transient increase in the large-scale divergence can produce a long-lasting break in the clouds.
Abstract
Result obtained with a mixed layer model are used to study the dynamics of stratomulus formation and dissipation in subtropical marine stratocumulus cloud regimes. The model used allows entrainment to be driven by shear as well as buoyancy, and includes a very crude parameterization of the partial blackness of thin cloud layers. Model results show that for some values of the large-scale divergence there are three equilibrium mixed layer structures, two of which are stable. One of the stable equilibria is cloudy, deep, and buoyancy-driven, while the other is clear, shallow, and shear-driven. It is found that as a result of hysteresis effects a transient increase in the large-scale divergence can produce a long-lasting break in the clouds.
Abstract
A detailed examination of the atmospheric momentum budget over the equatorial Pacific and its relation to oceanic wind stresses is undertaken using the results of a 20-yr (1979–99), forced-SST, AGCM experiment. The results show that free-tropospheric pressure gradients play a significant role in forcing boundary layer flow in the model. In particular, the time-mean and interannual variability of wind stress at the surface is found to be dominated by forcing from the free troposphere. The NCEP reanalyses from 1979–99 are also examined and a similar result is found, although the relative importance of this free-tropospheric forcing is somewhat higher in the model.
The seasonal cycle of free-tropospheric forcing in the model is found to be substantially stronger in the model than in the reanalysis, and that has a clearly negative impact on the simulated seasonal cycle of surface wind stresses. In the model, these free-tropospheric pressure gradients are not balanced by turbulent stresses or other dissipative forces. Rather, the momentum budget analysis shows that they are balanced by advective momentum tendencies, with vertical advection of momentum in the descending branch of the Walker circulation playing an important role.
Abstract
A detailed examination of the atmospheric momentum budget over the equatorial Pacific and its relation to oceanic wind stresses is undertaken using the results of a 20-yr (1979–99), forced-SST, AGCM experiment. The results show that free-tropospheric pressure gradients play a significant role in forcing boundary layer flow in the model. In particular, the time-mean and interannual variability of wind stress at the surface is found to be dominated by forcing from the free troposphere. The NCEP reanalyses from 1979–99 are also examined and a similar result is found, although the relative importance of this free-tropospheric forcing is somewhat higher in the model.
The seasonal cycle of free-tropospheric forcing in the model is found to be substantially stronger in the model than in the reanalysis, and that has a clearly negative impact on the simulated seasonal cycle of surface wind stresses. In the model, these free-tropospheric pressure gradients are not balanced by turbulent stresses or other dissipative forces. Rather, the momentum budget analysis shows that they are balanced by advective momentum tendencies, with vertical advection of momentum in the descending branch of the Walker circulation playing an important role.
A benchmark calculation is proposed for evaluating the dynamical cores of atmospheric general circulation models independently of the physical parameterizations. The test focuses on the long-term statistical properties of a fully developed general circulation; thus, it is particularly appropriate for intercomparing the dynamics used in climate models. To illustrate the use of this benchmark, two very different atmospheric dynamical cores—one spectral, one finite difference—are compared. It is found that the long-term statistics produced by the two models are very similar. Selected results from these calculations are presented to initiate the intercomparison.
A benchmark calculation is proposed for evaluating the dynamical cores of atmospheric general circulation models independently of the physical parameterizations. The test focuses on the long-term statistical properties of a fully developed general circulation; thus, it is particularly appropriate for intercomparing the dynamics used in climate models. To illustrate the use of this benchmark, two very different atmospheric dynamical cores—one spectral, one finite difference—are compared. It is found that the long-term statistics produced by the two models are very similar. Selected results from these calculations are presented to initiate the intercomparison.
Abstract
An equation that describes the partitioning of annual mean precipitation into annual mean evaporation and runoff, developed decades ago by Budyko, is used to derive a second equation that relates the interannual variability of evaporation to gross characteristics of the atmospheric forcing. Both Budyko’s original equation and the new variability equation perform well when tested against results from a 20-yr GCM simulation. In these tests, using knowledge of the climatological mean precipitation and net radiation alone, the authors predict the ratio of annual evaporation to annual precipitation with a standard error of 0.10 in nondesert regions, and they predict the ratio of the standard deviation of annual evaporation to that of annual precipitation there with a standard error of 0.14. In analogy with Budyko’s conclusion for the mean hydrological cycle, water and energy availability appear to be the critical factors controlling the interannual variability of surface moisture fluxes. The derived equations suggest, and the GCM results confirm, that the ratio of an evaporation anomaly to the corresponding precipitation anomaly tends to be significantly less than the ratio of mean evaporation to mean precipitation.
Abstract
An equation that describes the partitioning of annual mean precipitation into annual mean evaporation and runoff, developed decades ago by Budyko, is used to derive a second equation that relates the interannual variability of evaporation to gross characteristics of the atmospheric forcing. Both Budyko’s original equation and the new variability equation perform well when tested against results from a 20-yr GCM simulation. In these tests, using knowledge of the climatological mean precipitation and net radiation alone, the authors predict the ratio of annual evaporation to annual precipitation with a standard error of 0.10 in nondesert regions, and they predict the ratio of the standard deviation of annual evaporation to that of annual precipitation there with a standard error of 0.14. In analogy with Budyko’s conclusion for the mean hydrological cycle, water and energy availability appear to be the critical factors controlling the interannual variability of surface moisture fluxes. The derived equations suggest, and the GCM results confirm, that the ratio of an evaporation anomaly to the corresponding precipitation anomaly tends to be significantly less than the ratio of mean evaporation to mean precipitation.
Abstract
Two contrasting representations of land surface variability used in general circulation models (GCMS) are compared through an analysis of their corresponding surface energy balance equations. In one representation (the “mixture” approach), different vegetation types are assumed to be homogeneously mixed over a grid square, so that the GCM atmosphere sees near-surface conditions pertaining to the mixture only. In the second representation (the “mosaic” approach), different vegetation types are viewed as separate “tiles” of a grid-square “mosaic,” and each tile interacts with the atmosphere independently. The mosaic approach is computationally simpler and in many ways more flexible than the mixture approach.
Analytical solutions to the linearized energy balance equations and numerical solutions to the nonlinear equations both demonstrate that the mixture strategy, when applied to two coexisting vegetation types that differ only in canopy transpiration resistance, promotes both total turbulent flux and latent beat flux relative to the mosaic strategy. The effective differences between the strategies, however, are small over a wide range of conditions. In particular, the strategies are effectively equivalent when the transpiration resistances of the different vegetation types are of the saint order of magnitude.
Abstract
Two contrasting representations of land surface variability used in general circulation models (GCMS) are compared through an analysis of their corresponding surface energy balance equations. In one representation (the “mixture” approach), different vegetation types are assumed to be homogeneously mixed over a grid square, so that the GCM atmosphere sees near-surface conditions pertaining to the mixture only. In the second representation (the “mosaic” approach), different vegetation types are viewed as separate “tiles” of a grid-square “mosaic,” and each tile interacts with the atmosphere independently. The mosaic approach is computationally simpler and in many ways more flexible than the mixture approach.
Analytical solutions to the linearized energy balance equations and numerical solutions to the nonlinear equations both demonstrate that the mixture strategy, when applied to two coexisting vegetation types that differ only in canopy transpiration resistance, promotes both total turbulent flux and latent beat flux relative to the mosaic strategy. The effective differences between the strategies, however, are small over a wide range of conditions. In particular, the strategies are effectively equivalent when the transpiration resistances of the different vegetation types are of the saint order of magnitude.
Abstract
The retention of precipitation water in land surface reservoirs damps higher frequencies of evaporation variability and can thereby influence the feedback of evaporation on precipitation. The extent of this influence is examined in a series of general circulation model simulations in which the timescale of surface moisture retention is very carefully controlled. Shorter timescales lead to increased daily precipitation variance and one-day-lagged precipitation autocorrelations but to decreased autocorrelations at longer lags.
An explanation for the simulated precipitation statistics is offered in the form of a heuristic model of evaporation feedback that describes precipitation variance and autocorrelation in terms of three parameters: (i) the timescale of precipitation persistence in the absence of feedback; (ii) the surface retention timescale; and (iii) a parameter describing the atmosphere's responsiveness to variations in evaporation. The heuristic model reproduces the statistical trends seen in the GCM diagnostics, and it can be used to explain geographical variations in precipitation statistics generated by a CYCM coupled to a full biosphere model.
Abstract
The retention of precipitation water in land surface reservoirs damps higher frequencies of evaporation variability and can thereby influence the feedback of evaporation on precipitation. The extent of this influence is examined in a series of general circulation model simulations in which the timescale of surface moisture retention is very carefully controlled. Shorter timescales lead to increased daily precipitation variance and one-day-lagged precipitation autocorrelations but to decreased autocorrelations at longer lags.
An explanation for the simulated precipitation statistics is offered in the form of a heuristic model of evaporation feedback that describes precipitation variance and autocorrelation in terms of three parameters: (i) the timescale of precipitation persistence in the absence of feedback; (ii) the surface retention timescale; and (iii) a parameter describing the atmosphere's responsiveness to variations in evaporation. The heuristic model reproduces the statistical trends seen in the GCM diagnostics, and it can be used to explain geographical variations in precipitation statistics generated by a CYCM coupled to a full biosphere model.
