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Isaac M. Held and Brian J. Soden

Abstract

Using the climate change experiments generated for the Fourth Assessment of the Intergovernmental Panel on Climate Change, this study examines some aspects of the changes in the hydrological cycle that are robust across the models. These responses include the decrease in convective mass fluxes, the increase in horizontal moisture transport, the associated enhancement of the pattern of evaporation minus precipitation and its temporal variance, and the decrease in the horizontal sensible heat transport in the extratropics. A surprising finding is that a robust decrease in extratropical sensible heat transport is found only in the equilibrium climate response, as estimated in slab ocean responses to the doubling of CO2, and not in transient climate change scenarios. All of these robust responses are consequences of the increase in lower-tropospheric water vapor.

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De-Zheng Sun and Isaac M. Held

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The correlations between interannual variations of tropical mean water vapor and temperature in the simulations by a low resolution (R15) GCM are stronger than those in the rawinsonde observations. The rate of fractional increase of tropical mean water vapor with temperature in the model simulations is also larger than that from the observations. The largest discrepancies are found in the region immediately above the tropical convective boundary layer (850–600 mb). The rate of fractional increase of tropical mean water vapor with temperature in the model simulations is close to that for a constant relative humidity. The correlations between variations of water vapor in the upper troposphere and those in the lower troposphere are also stronger in the model simulations than in the observations. In the horizontal, the characteristic spatial patterns of the normalized water vapor variations in the model simulations and observations are similar. The water vapor–temperature relationship in simulations by a GCM with a somewhat higher spatial resolution (R30) is almost identical to that in the simulations by the low resolution (R15) GCM. The implications of these findings for the radiative feedback of water vapor are discussed.

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Paul J. Kushner and Isaac M. Held

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The use of eddy flux of thickness between density surfaces has become a familiar starting point in oceanographic studies of adiabatic eddy effects on the mean density distribution. In this study, a dynamical analogy with the density thickness flux approach is explored to reexamine the theory of nonzonal wave–mean flow interaction in two-dimensional horizontal flows. By analogy with the density thickness flux, the flux of thickness between potential vorticity (PV) surfaces is used as a starting point for a residual circulation formulation for nonzonal mean flows. Mean equations for barotropic PV dynamics are derived in which a modified mean velocity with an eddy-induced component advects a modified mean PV that also has an eddy-induced component. For small-amplitude eddies, the results are analogous to recent results of McDougall and McIntosh derived for stratified flow.

The dynamical implications of this approach are then examined. The modified mean PV equation provides a decomposition of the eddy forcing of the mean flow into contributions from wave transience, wave dissipation, and wave-induced mass redistribution between PV contours. If the mean flow is along the mean PV contours, the contribution from wave-induced mass redistribution is “workless” in Plumb’s sense that it is equivalent to an eddy-induced stress that is perpendicular to the mean flow. This contribution is also associated with the convergence along the mean streamlines of a modified PV flux that is equal to the difference between the PV flux and the rotational PV flux term identified by Illari and Marshall. The cross-stream component of the modified PV flux is related to wave transience and dissipation.

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Olivier Pauluis, V. Balaji, and Isaac M. Held

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The frictional dissipation in the shear zone surrounding falling hydrometeors is estimated to be 2–4 W m−2 in the Tropics. A numerical model of radiative–convective equilibrium with resolved three-dimensional moist convection confirms this estimate and shows that the precipitation-related dissipation is much larger than the dissipation associated with the turbulent energy cascade from the convective scale. Equivalently, the work performed by moist convection is used primarily to lift water rather than generate kinetic energy of the convective airflow. This fact complicates attempts to use the entropy budget to derive convective velocity scales.

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Dale B. Haidvogel and Isaac M. Held

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Statistically steady states consistent with a horizontally uniform time-averaged temperature gradient in a two-layer quasi-geostrophic model on a beta-plane are found by numerically integrating the equations for deviations from this mean state in a doubly periodic domain. Based on the result that the flow statistics are not strongly dependent on the size of the domain, it is suggested that this homogeneous flow is physically realizable. The dependence of the eddy heat and potential vorticity fluxes and eddy energy level on various model parameters (the beta effect, surface drag, small-scale horizontal mixing) is described. Implications for eddy flux parameterization theories am discussed.

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Brian J. Soden and Isaac M. Held
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Matthew T. Gliatto and Isaac M. Held

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Rossby waves, propagating from the midlatitudes toward the tropics, are typically absorbed by critical latitudes (CLs) in the upper troposphere. However, these waves typically encounter CLs in the lower troposphere first. We study a two-layer linear scattering problem to examine the effects of lower CLs on these waves. We begin with a review of the simpler barotropic case to orient the reader. We then progress to the baroclinic case using a two-layer quasigeostrophic model in which there is vertical shear in the mean flow on which the waves propagate, and in which the incident wave is assumed to be an external-mode Rossby wave. We use linearized equations and add small damping to remove the critical-latitude singularities. We consider cases in which either there is only one CL, in the lower layer, or there are CLs in both layers, with the lower-layer CL encountered first. If there is only a CL in the lower layer, the wave’s response depends on the sign of the mean potential vorticity gradient at this lower-layer CL: if the PV gradient is positive, then the CL partially absorbs the wave, as in the barotropic case, while for a negative PV gradient, the CL is a wave emitter, and can potentially produce overreflection and/or overtransmission. Our numerical results indicate that overtransmission is by far the dominant response in these cases. When an upper-layer absorbing CL is encountered, following the lower-layer encounter, one can still see the signature of overtransmission at the lower-layer CL.

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Kerry H. Cook and Isaac M. Held

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A linearized, steady state, primitive equation model is used to simulate the climatological zonal asymmetries (stationary eddies) in the wind and temperature fields of the 18 000 YBP climate during winter. We compare these results with the eddies simulated in the ice age experiments of Broccoli and Manabe, who used CLIMAP boundary conditions and reduced atmospheric CO2 in an atmospheric general circulation model (GCM) coupled with a static mixed layer ocean model. The agreement between the models is good, indicating that the linear model can be used to evaluate the relative influences of orography, diabatic heating, and transient eddy heat and momentum transports in generating stationary waves. We find that orographic forcing dominates in the ice age climate. The mechanical influence of the continental ice sheets on the atmosphere is responsible for most of the changes between the present day and ice age stationary eddies. This concept of the ice age climate is complicated by the sensitivity of the stationary eddies to the large increase in the magnitude of the zonal mean meridional temperature gradient simulated in the ice age GCM.

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Isaac M. Held and Peter J. Phillipps

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GCM experiments with zonally symmetric climates are used to demonstrate that the increase in the meridional eddy momentum fluxes and zonal surface winds that occurs when resolution is increased is primarily due to the increase in meridional rather than zonal resolution. It is argued that the sensitivity to meridional resolution reflects the need to resolve the small scales generated in the Rossby wave field as waves radiate from the midlatitude baroclinic eddy source region into regions with small mean winds. Some additional experiments highlight the sensitivity of surface winds and eddy momentum fluxes to the subgrid-scale horizontal mixing formulation in low-revolution models.

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Isaac M. Held and Enda O'Brien

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A three-layer, horizontally homogeneous, quasigeostrophic model is selected as one of the simplest environments in which to study the sensitivity of baroclinic eddy fluxes in the atmosphere to the vertical structure of the basic-state temperature gradients or vertical wind shears. Eddy statistics obtained from the model are interpreted in terms of linear theory and a modified “baroclinic adjustment” hypothesis. Both linear theory and the baroclinic adjustment construction are found to provide useful predictions for the vertical structure of the eddy potential vorticity flux.For equal values of the mean vertical shear, eddy fluxes and energies are greater when the shear is concentrated at lower levels (d 2 U/dz 2 < 0) than when the shear is concentrated at higher levels (d 2 U/dz 2 > 0). Eddy fluxes are more sensitive to lower-than to upper-level mean temperature gradients. This relative sensitivity is a function of γ = f 2Λ/(βN 2 H), where Λ is the mean vertical shear and His the depth of the fluid. It is enhanced as γ is reduced, as the unstable modes become shallower, until the eddies become almost completely insensitive to the strength of the upper-layer wind for γ < 0.5.

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