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Paul A. O’Gorman

Abstract

Water vapor directly affects the dynamics of atmospheric eddy circulations through the release of latent heat. But it is difficult to include latent heat release in dynamical theories because of the associated nonlinearity (precipitation generally occurs where there is upward motion). A new effective static stability is derived that fundamentally captures the effect of latent heat release on moist eddy circulations. It differs from the usual dry static stability by an additive term that depends on temperature and a parameter measuring the up–down asymmetry of vertical velocity statistics. Latent heat release reduces the effective static stability experienced by eddies but cannot reduce it to zero so long as there are nonprecipitating regions of the eddies. Evaluation based on reanalysis data indicates that the effective static stability in the lower troposphere ranges from ∼80% of the dry static stability at high latitudes to ∼25% in the tropics.

The effective static stability provides a solution to the longstanding problem of how to adapt dry dynamical theories to the moist circulations in the atmosphere. Its utility for climate change problems is illustrated based on simulations with an idealized general circulation model. It is shown to help account for changes in the thermal stratification of the extratropical troposphere, the extent of the Hadley cells, the intensity of extratropical transient eddies, and the extratropical eddy length.

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Paul A. O’Gorman and Tapio Schneider

Abstract

The general circulation and the behavior of multiple jets and baroclinic eddies are described for an atmosphere in which meridional potential temperature gradients and eddies are confined to a weather layer. The weather layer is separated from the frictional lower boundary by a statically stable barotropic layer with significant mass. Closure of the zonal momentum budget in the resulting circulation is achieved through ageostrophic meridional cells that extend to the lower boundary, at which momentum is dissipated. In a series of simulations with a multilevel primitive equation model, dynamic changes in the static stability of the weather layer are found to be critical in determining the scaling of the baroclinic eddies, an effect not captured in quasigeostrophic models. For simulations with a single jet in each hemisphere, the static stability of the weather layer adjusts so that a significant inverse energy cascade to scales larger than the Rossby deformation radius does not occur. The eddy length is found to scale with both the Rossby deformation radius and the Rhines scale. Simulations with larger planetary radii and low pole-to-equator temperature gradients exhibit multiple jets in each hemisphere. Eddy lengths and energies for the jet nearest the equator in each hemisphere have the same scaling as those in the single-jet simulations. Similar scalings are found for jets farther poleward but with different constants of proportionality that are consistent with more supercritical eddies. The local eddy length is found to have only a weak variation with latitude, and the local meridional jet spacing is found to scale with the local eddy length in all cases. Insights from the weather-layer simulations may be relevant to circulations in gas giant planets and the ocean.

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Paul A. O’Gorman and Tapio Schneider

Abstract

Extremes of precipitation are examined in a wide range of climates simulated with an idealized aquaplanet GCM. The high percentiles of daily precipitation increase as the climate warms. Their fractional rate of increase with global-mean surface temperature is generally similar to or greater than that of mean precipitation, but it is less than that of atmospheric (column) water vapor content. A simple scaling is introduced for precipitation extremes that accounts for their behavior by including the effects of changes in the moist-adiabatic lapse rate, the circulation strength, and the temperature when the extreme events occur. The effects of changes in the moist-adiabatic lapse rate and circulation strength on precipitation extremes are important globally, whereas the difference in the mean temperature and the temperature at which precipitation extremes occur is important only at middle to high latitudes.

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John G. Dwyer and Paul A. O’Gorman

Abstract

The Eliassen–Palm (EP) flux is an important diagnostic for wave propagation and wave–mean flow interaction in the atmosphere. Here, two moist formulations of the EP flux are compared with the traditional dry EP flux, and their links to the surface westerlies are analyzed using reanalysis data and simulations with GCMs. The first moist formulation of the EP flux modifies only the static stability to account for latent heat release by eddies, while the second moist formulation simply replaces all potential temperatures with equivalent potential temperatures. For reanalysis data, the peak upward EP flux in the lower troposphere is farther equatorward and stronger when the moist formulations are used, with greater changes for the second moist formulation. The moist formulations have the advantage of giving a closer relationship over the seasonal cycle between the latitudes of the peak surface westerlies and the peak upward EP flux. In simulations with a comprehensive GCM, the dry and moist upward EP fluxes shift poleward by a similar amount as the climate warms. In simulations over a wider range of climates with an idealized GCM, the surface westerlies can shift both poleward and equatorward with warming, and they are influenced by an anomalous region of dry EP flux divergence near the subtropical jet. Using moist EP fluxes weakens this anomalous divergence in the idealized GCM simulations, and the shifts in the surface westerlies can then be understood through changes in the preference for equatorward or poleward wave propagation.

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Paul A. O’Gorman and Tapio Schneider

Abstract

As the climate changes, changes in static stability, meridional temperature gradients, and availability of moisture for latent heat release may exert competing effects on the energy of midlatitude transient eddies. This paper examines how the eddy kinetic energy in midlatitude baroclinic zones responds to changes in radiative forcing in simulations with an idealized moist general circulation model. In a series of simulations in which the optical thickness of the longwave absorber is varied over a wide range, the eddy kinetic energy has a maximum for a climate with mean temperature similar to that of present-day earth, with significantly smaller values both for warmer and for colder climates. In a series of simulations in which the meridional insolation gradient is varied, the eddy kinetic energy increases monotonically with insolation gradient. In both series of simulations, the eddy kinetic energy scales approximately linearly with the dry mean available potential energy averaged over the baroclinic zones. Changes in eddy kinetic energy can therefore be related to the changes in the atmospheric thermal structure that affect the mean available potential energy.

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Martin S. Singh and Paul A. O’Gorman

Abstract

Many features of the general circulation of the atmosphere shift upward in response to warming in simulations of climate change with both general circulation models (GCMs) and cloud-system-resolving models. The importance of the upward shift is well known, but its physical basis and the extent to which it occurs coherently across variables are not well understood. A transformation is derived here that shows how an upward shift of a solution to the moist primitive equations gives a new approximate solution with higher tropospheric temperatures. According to the transformation, all variables shift upward with warming but with an additional modification to the temperature and a general weakening of the pressure velocity. The applicability of the vertical-shift transformation is explored using a hierarchy of models from adiabatic parcel ascents to comprehensive GCMs. The transformation is found to capture many features of the response to climate change in simulations with an idealized GCM, including the mid- and upper-tropospheric changes in lapse rate, relative humidity, and meridional wind. The transformation is less accurate when applied to simulations with more realistic GCMs, but it nonetheless captures some important features. Deviations from the simulated response are primarily due to the surface boundary conditions, which do not necessarily conform to the transformation, especially in the case of the zonal winds. The results allow for a physical interpretation of the upward shift in terms of the governing equations and suggest that it may be thought of as a coherent response of the general circulation of the mid- and upper troposphere.

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Michael P. Byrne and Paul A. O’Gorman

Abstract

Climate models simulate a strong land–ocean contrast in the response of near-surface relative humidity to global warming; relative humidity tends to increase slightly over oceans but decrease substantially over land. Surface energy balance arguments have been used to understand the response over ocean but are difficult to apply over more complex land surfaces. Here, a conceptual box model is introduced, involving atmospheric moisture transport between the land and ocean and surface evapotranspiration, to investigate the decreases in land relative humidity as the climate warms. The box model is applied to simulations with idealized and full-complexity (CMIP5) general circulation models, and it is found to capture many of the features of the simulated changes in land humidity. The simplest version of the box model gives equal fractional increases in specific humidity over land and ocean. This relationship implies a decrease in land relative humidity given the greater warming over land than ocean and modest changes in ocean relative humidity, consistent with a mechanism proposed previously. When evapotranspiration is included, it is found to be of secondary importance compared to ocean moisture transport for the increase in land specific humidity, but it plays an important role for the decrease in land relative humidity. For the case of a moisture forcing over land, such as from stomatal closure, the response of land relative humidity is strongly amplified by the induced change in land surface–air temperature, and this amplification is quantified using a theory for the link between land and ocean temperatures.

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Tapio Schneider and Paul A. O’Gorman

Abstract

Simulations with an aquaplanet general circulation model show that sensible and latent heat transport by large-scale eddies influences the extratropical thermal stratification over a wide range of climates, even in relatively warm climates with small meridional surface temperature gradients. Variations of the lapse rate toward which the parameterized moist convection in the model relaxes atmospheric temperature profiles demonstrate that the convective lapse rate only marginally affects the extratropical thermal stratification in Earth-like and colder climates. In warmer climates, the convective lapse rate does affect the extratropical thermal stratification, but the effect is still smaller than would be expected if moist convection alone controlled the thermal stratification. A theory for how large-scale eddies modify the thermal stratification of dry atmospheres is consistent with the simulation results for colder climates. For warmer and moister climates, however, theories and heuristics that have been proposed to account for the extratropical thermal stratification are not consistent with the simulation results. Theories for the extratropical thermal stratification will generally have to take transport of sensible and latent heat by large-scale eddies into account, but moist convection may only need to be taken into account regionally and in sufficiently warm climates.

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Paul A. O’Gorman and Tapio Schneider

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A wide range of hydrological cycles and general circulations was simulated with an idealized general circulation model (GCM) by varying the optical thickness of the longwave absorber. While the idealized GCM does not capture the full complexity of the hydrological cycle, the wide range of climates simulated allows the systematic development and testing of theories of how precipitation and moisture transport change as the climate changes. The simulations show that the character of the response of the hydrological cycle to variations in longwave optical thickness differs in different climate regimes.

The global-mean precipitation increases linearly with surface temperature for colder climates, but it asymptotically approaches a maximum at higher surface temperatures. The basic features of the precipitation–temperature relation, including the rate of increase in the linear regime, are reproduced in radiative–convective equilibrium simulations. Energy constraints partially account for the precipitation–temperature relation but are not quantitatively accurate.

Large-scale condensation is most important in the midlatitude storm tracks, and its behavior is accounted for using a stochastic model of moisture advection and condensation. The precipitation associated with large-scale condensation does not scale with mean specific humidity, partly because the condensation region moves upward and meridionally as the climate warms, and partly because the mean condensation rate depends on isentropic specific humidity gradients, which do not scale with the specific humidity itself.

The local water vapor budget relates local precipitation to evaporation and meridional moisture fluxes, whose scaling in the subtropics and extratropics is examined. A delicate balance between opposing changes in evaporation and moisture flux divergence holds in the subtropical dry zones. The extratropical precipitation maximum follows the storm track in warm climates but lies equatorward of the storm track in cold climates.

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Michael P. Byrne and Paul A. O’Gorman

Abstract

Surface temperatures increase at a greater rate over land than ocean in simulations and observations of global warming. It has previously been proposed that this land–ocean warming contrast is related to different changes in lapse rates over land and ocean because of limited moisture availability over land. A simple theory of the land–ocean warming contrast is developed here in which lapse rates are determined by an assumption of convective quasi-equilibrium. The theory predicts that the difference between land and ocean temperatures increases monotonically as the climate warms or as the land becomes more arid. However, the ratio of differential warming over land and ocean varies nonmonotonically with temperature for constant relative humidities and reaches a maximum at roughly 290 K.

The theory is applied to simulations with an idealized general circulation model in which the continental configuration and climate are varied systematically. The simulated warming contrast is confined to latitudes below 50° when climate is varied by changes in longwave optical thickness. The warming contrast depends on land aridity and is larger for zonal land bands than for continents with finite zonal extent. A land–ocean temperature contrast may be induced at higher latitudes by enforcing an arid land surface, but its magnitude is relatively small. The warming contrast is generally well described by the theory, although inclusion of a land–ocean albedo contrast causes the theory to overestimate the land temperatures. Extensions of the theory are discussed to include the effect of large-scale eddies on the extratropical thermal stratification and to account for warming contrasts in both surface air and surface skin temperatures.

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