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Peter H. Stone and Boaz Nemet

Abstract

Isentropic slopes calculated from Northern Hemisphere analyses of the zonal mean state of the atmosphere are compared with isentropic slopes calculated from baroclinic adjustment theory. In midlatitudes, the isentropic slopes are relatively close to the adjusted values in the layer from 2 to 7 km (800 to 400 mb). They also have very little variation in latitude and season, which implies that baroclinic eddy fluxes supply a strong negative feedback to changes in isentropic slopes. The vertical structure of the temperature field in the “adjusted” layers and the location of the layers suggest that vertical eddy heat fluxes play a significant role in this feedback process.

Isentropic slopes are also calculated from simulations of the current climate by the GISS Model II GCM and the NCAR CCM2. Both models have regions of apparent baroclinic adjustment similar to that in the Northern Hemisphere atmosphere. However, compared to the observations, the agreement of the simulations with baroclinic adjustment theory is not as good, and the isentropic slopes show stronger latitudinal and seasonal variations. The discrepancies are associated primarily with errors in the models’ meridional temperature gradients in the lower troposphere in midlatitudes. The seasonal changes in these gradients are much larger than in the observations, particularly in the CCM2, even though the model simulations were constrained by climatological sea surface temperatures.

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Peter H. Stone and Lee Branscome

Abstract

We use a scaling analysis and numerical solutions of a quasigeostrophic two-level model on a β plane to explore what kind of eddy regimes can occur when the eddies are forced solely by differential diabatic heating, and friction is negligible outside a boundary layer near the lower surface. The eddy regimes all have a convergent eddy momentum flux and a Ferrel cell near the center of the channel flow, and a downgradient eddy heat flux that exceeds the upgradient heat flux by the Ferrel cell. Quantitatively the regimes can be characterized by R, the ratio of the heat transport by the Ferrel cell to the heat transport by the eddies. This ratio depends on a nondimensional parameter, δ, which is proportional to the characteristic relaxation time associated with the diabatic heating and to the convergence of the eddy momentum flux. If δ → ∞, then R → 1, the divergence of the Eliassen-Palm flux approaches zero, and the temperature structure approaches equilibrium; that is, the motions have very little effect on the temperature structure. If δ → 0, then R → 0, and the eddy forcing of the average, or climate-mean zonal-mean zonal wind and temperature fields is dominated by the vertical component of the Eliassen-Palm flux, that is, by the eddy heat flux. The midlatitude troposphere is near this second asymptotic limit, with δ ≈ 0.03, R ≈ 0.14. The two-level model solutions show that the eddy regimes are substantially supercritical for baroclinic instability, unless the forcing is weak. Nevertheless, the parametric behavior closely approximates that expected from the baroclinic adjustment hypothesis, that is, the vertical shear near the center of the channel flow is proportional to the static stability and is very insensitive to the forcing, the surface friction, and the diabatic relaxation time.

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Yang Zhang and Peter H. Stone

Abstract

Baroclinic eddy equilibration and the roles of different boundary layer processes in limiting the baroclinic adjustment are studied using an atmosphere–ocean thermally coupled model. Boundary layer processes not only affect the dynamical constraint of the midlatitude baroclinic eddy equilibration but also are important components in the underlying surface energy budget. The authors' study shows that baroclinic eddies, with the strong mixing of the surface air temperature, compete against the fast boundary layer thermal damping and enhance the meridional variation of surface sensible heat flux, acting to reduce the meridional gradient of the surface temperature. Nevertheless, the requirement of the surface energy balance indicates that strong surface baroclinicity is always maintained in response to the meridionally varying solar radiation. With the strong surface baroclinicity and the boundary layer processes, the homogenized potential vorticity (PV) suggested in the baroclinic adjustment are never observed near the surface or in the boundary layer.

Although different boundary layer processes affect baroclinic eddy equilibration differently with more dynamical feedbacks and time scales included in the coupled system, their influence in limiting the PV homogenization is more uniform compared with the previous uncoupled runs. The boundary layer PV structure is more determined by the strength of the boundary layer damping than the surface baroclinicity. Stronger boundary layer processes always prevent the lower-level PV homogenization more efficiently. Above the boundary layer, a relatively robust PV structure with homogenized PV around 600–800 hPa is obtained in all of the simulations. The detailed mechanisms through which different boundary layer processes affect the equilibration of the coupled system are discussed in this study.

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Peter J. Gierasch and Peter H. Stone

Abstract

No abstract available.

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Peter J. Gierasch and Peter H. Stone

Abstract

No abstract available.

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Peter H. Stone and John H. Carlson

Abstract

Lapse rates, moist adiabatic lapse rates and the critical lapse rate for baroclinic adjustment are calculated and compared for the mean annual, January and July states in the Northern Hemisphere. In the troposphere above the planetary boundary layer zonal mean lapse rates are within 20% of the moist adiabatic lapse rate from the equator up to about 30°N in January and 50°N in July, but are appreciably more stable in higher latitudes. The latitudinal distribution of tropospheric mean lapse rates clearly delineates two regimes in the atmosphere—a low-latitude regime where the lapse rates are essentially moist adiabatic, and a high-latitude regime where the lapse rates are essentially the critical lapse rate for baroclinic adjustment. The dividing point between the two regimes shifts from 28°N in January to 47°N in July, and the transition is less sharp in July than in January. The absence of appreciable seasonal changes in lapse rates in midlatitudes can be attributed to counterbalancing seasonal changes in the strength of moist convection and baroclinic eddies. Hemispheric mean lapse rates in the mid and lower troposphere are within 0.4 K km−1 of the moist adiabatic lapse rate in July, but are as much as 1.9 K km−1 less in January. Implications for simple climate models are discussed. A principal conclusion is that the vertical temperature structure could be well approximated by a radiative-convective equilibrium model with two critical lapse rates—the moist adiabatic lapse rate and the critical lapse rate for baroclinic adjustment.

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Peter H. Stone and Dennis A. Miller

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The correlation of the zonal mean meridional flux of heat in the atmosphere-ocean system and of its various components with the zonal mean 1000 mb meridional temperature gradient are calculated from analyses of the mean seasonal changes in these quantities. The correlations for the total atmospheric flux and its dominant components are typically 90% or more. The correlation of the eddy flux with the temperature gradient is particularly high (typically 97%) but only if the transient and stationary components are added together. There appears to be no correlation between the ocean flux and the temperature gradient in low latitudes.

In cases where the correlation is high, the seasonal changes are used to derive empirical relations between the fluxes and the gradient. Seasonal changes in the total flux in the atmosphere-ocean system are twice as strong as would be implied by a linear diffusion law. The empirical relations for the seasonal changes in the eddy flux of sensible heat indicate that it is approximately proportional to the square of the gradient in midlatitudes, but to the third or fourth power of the gradient near 30°N. This behavior is consistent with proposed parameterizations if β effects are small in midlatitudes but important near 30°N.

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Peter H. Stone and Mao-Sung Yao

Abstract

The effect of eddy momentum fluxes on the general circulation is investigated with the aid of perpetual January simulations with a two-dimensional, zonally averaged model. Sensitivity experiments with this model show that the vertical eddy flux has a negligible effect on the general circulation, while the meridional eddy flux has a substantial effect. The experiments on the effect of the mefidional eddy flux essentially confirm the resultsfound by Schneider in a similar (but not identical) set of sensitivity experiments, and, in addition, show that the vertical structure of the mefidional eddy flux has a relatively small effect on the general circulation.

In order to parameterize the vertically integrated mefidional eddy momentum flux, we take Green's parameterization of this quantity and generalize it to allow for the effects of condensation. In order to do this, it is necessary to use Leovy's approximation for the eddy fluctuations in specific humidity. With this approximation the equivalent potential vorticity defined by Saltzman is conserved even when condensation occurs. Leovy's approximation also allows one to generalize the relation between quasi-geostrophic potential vorticity and theEliassen-Palm flux by replacing the potential vorticity and potential temperature by the corresponding equivalent quantities. Thus, the eddy momentum flux can be related to the eddy fluxes of two conserved quantities even when condensation is present. The eddy fluxes of the two conserved quantities are parametefized by mixing-length expressions, with the mixing coefficient taken to be the sum of Branscome's mixing coefficient, plus a correction which allows for nonlinear effects onthe eddy structure and ensures global momentum conservation.

The parametefization of the mefidional eddy transport is tested in another perpetual January simulation with the two-dimensional averaged model. The results are compared with a parallel three-dimensional simulation which calculates the eddy transport explicitly. The parameterization reproduces the latitudinal and seasonal (interhemisphefic) variations and the magnitude of the eddy transport calculated in the three-dimensional simulation reasonably well.

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Antonio D. Moura and Peter H. Stone

Abstract

A baroclinic stability analysis is performed for a simple family of zonal shear profiles over a sphere, using a two-layer, quasi-geostrophic model. The stability properties and the structure of the most unstable waves are qualitatively similar to those on a β-plane. However, the spherical geometry plays a major role in locating some of the important features of the most unstable waves. In particular, the locations of the maximum wave amplitude, maximum eddy heat fluxes, and maximum convergence of the eddy angular momentum flux are all well correlated with the location of the maximum excess of the vertical shear over the minimum value necessary for local instability on a sphere. Consequently the eddy momentum flux tends to generate a mid-latitude jet even if there is no preexisting mid-latitude jet in the basic state zonal flow. These findings suggest some of the elements needed for parameterizing the meridional variations of baroclinic eddy fluxes accurately.

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James S. Risbey and Peter H. Stone

Abstract

Data on zonally averaged atmospheric angular momentum and high cloud cover percent are analyzed for the periods April–October 1979 and November 1982–October 1983. The dominant periodicity in both momentum and cloud datasets was the so called “30–60 day atmospheric oscillation” in tropical and subtropical belts. In lag correlations between high cloud belts, both the periodicity and a latitudinally varying correlation structure were evident. In the 1979 period (Northern Hemisphere summer) the cloud-cloud correlations had nodes near 17°S, 5°N, 24°N and 36°N, i.e., anomalously high/low zonal mean convection between 5 and 24°N coincided with anomalously low/high zonal mean convection between 17°S and 5°N, and between 24 and 36°N. In April–October 1983, a similar periodicity and phase structure were present, but not as well defined. The principal node in the northern Hemisphere summer, near 5°N, appears to lie between the belt of maximum cloud cover for the period (which is between 5 and 9°N) and the equator. In an analysis of the period November 1982–April 1983 (Southern Hemisphere summer), the principal node was located in the Southern Hemisphere. Lag correlations between high cloud belts and momentum belts showed strong correlations with the 30–60 day oscillation present. Anomalously high/low zonal mean high cloudiness in the tropics is accompanied by anomalously high/low zonal mean momentum in the tropics, with the latter anomalies subsequently propagating into midlatitudes.

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