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Sarah M. Kang, Dargan M. W. Frierson, and Isaac M. Held

Abstract

The response of tropical precipitation to extratropical thermal forcing is reexamined using an idealized moist atmospheric GCM that has no water vapor or cloud feedbacks, simplifying the analysis while retaining the aquaplanet configuration coupled to a slab ocean from the authors’ previous study. As in earlier studies, tropical precipitation in response to high-latitude forcing is skewed toward the warmed hemisphere. Comparisons with a comprehensive GCM in an identical aquaplanet, mixed-layer framework reveal that the tropical responses tend to be much larger in the comprehensive GCM as a result of positive cloud and water vapor feedbacks that amplify the imposed extratropical thermal forcing.

The magnitude of the tropical precipitation response in the idealized model is sensitive to convection scheme parameters. This sensitivity as well as the tropical precipitation response can be understood from a simple theory with two ingredients: the changes in poleward energy fluxes are predicted using a one-dimensional energy balance model and a measure of the “total gross moist stability” [Δm, which is defined as the total (mean plus eddy) atmospheric energy transport per unit mass transport] of the model tropics converts the energy flux change into a mass flux and a moisture flux change. The idealized model produces a low level of compensation of about 25% between the imposed oceanic flux and the resulting response in the atmospheric energy transport in the tropics regardless of the convection scheme parameter. Because Geophysical Fluid Dynamics Laboratory Atmospheric Model 2 (AM2) with prescribed clouds and water vapor exhibits a similarly low level of compensation, it is argued that roughly 25% of the compensation is dynamically controlled through eddy energy fluxes. The sensitivity of the tropical response to the convection scheme in the idealized model results from different values of Δm: smaller Δm leads to larger tropical precipitation changes for the same response in the energy transport.

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Dargan M. W. Frierson, Isaac M. Held, and Pablo Zurita-Gotor

Abstract

A simplified moist general circulation model is used to study changes in the meridional transport of moist static energy by the atmosphere as the water vapor content is increased. The key assumptions of the model are gray radiation, with water vapor and other constituents having no effect on radiative transfer, and mixed layer aquaplanet boundary conditions, implying that the atmospheric meridional energy transport balances the net radiation at the top of the atmosphere. These simplifications allow the authors to isolate the effect of moisture on energy transports by baroclinic eddies in a relatively simple setting.

The authors investigate the partition of moist static energy transport in the model into dry static energy and latent energy transports as water vapor concentrations are increased, by varying a constant in the Clausius–Clapeyron relation. The increase in the poleward moisture flux is rather precisely compensated by a reduction in the dry static energy flux. These results are interpreted with diffusive energy balance models (EBMs). The simplest of these is an analytic model that has the property of exact invariance of total energy flux as the moisture content is changed, but the assumptions underlying this model are not accurately satisfied by the GCM. A more complex EBM that includes expressions for the diffusivity, length scale, velocity scale, and latitude of maximum baroclinic eddy activity provides a better fit to the GCM’s behavior.

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Rong Zhang, Sarah M. Kang, and Isaac M. Held

Abstract

A variety of observational and modeling studies show that changes in the Atlantic meridional overturning circulation (AMOC) can induce rapid global-scale climate change. In particular, a substantially weakened AMOC leads to a southward shift of the intertropical convergence zone (ITCZ) in both the Atlantic and the Pacific Oceans. However, the simulated amplitudes of the AMOC-induced tropical climate change differ substantially among different models. In this paper, the sensitivity to cloud feedback of the climate response to a change in the AMOC is studied using a coupled ocean–atmosphere model [the GFDL Coupled Model, version 2.1 (CM2.1)]. Without cloud feedback, the simulated AMOC-induced climate change in this model is weakened substantially. Low-cloud feedback has a strong amplifying impact on the tropical ITCZ shift in this model, whereas the effects of high-cloud feedback are weaker. It is concluded that cloud feedback is an important contributor to the uncertainty in the global response to AMOC changes.

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Dargan M. W. Frierson, Isaac M. Held, and Pablo Zurita-Gotor

Abstract

In this paper, a simplified moist general circulation model is developed and used to study changes in the atmospheric general circulation as the water vapor content of the atmosphere is altered. The key elements of the model physics are gray radiative transfer, in which water vapor and other constituents have no effect on radiative fluxes, a simple diffusive boundary layer with prognostic depth, and a mixed layer aquaplanet surface boundary condition. This GCM can be integrated stably without a convection parameterization, with large-scale condensation only, and this study focuses on this simplest version of the model. These simplifications provide a useful framework in which to focus on the interplay between latent heat release and large-scale dynamics. In this paper, the authors study the role of moisture in determining the tropospheric static stability and midlatitude eddy scale. In a companion paper, the effects of moisture on energy transports by baroclinic eddies are discussed.

The authors vary a parameter in the Clausius–Clapeyron relation to control the amount of water in the atmosphere, and consider circulations ranging from the dry limit to 10 times a control value. The typical length scale of midlatitude eddies is found to be remarkably insensitive to the amount of moisture in the atmosphere in this model. The Rhines scale evaluated at the latitude of the maximum eddy kinetic energy fits the model results for the eddy scale well. Moist convection is important in determining the extratropical lapse rate, and the dry stability is significantly increased with increased moisture content.

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Isaac M. Held, Raymond T. Pierrehumbert, and R. Lee Panetta

Abstract

External Rossby waves in vertical shear can be destabilized by thermal damping. They can also be destabilized by damping of potential vorticity if this damping is larger in the lower than in the upper troposphere. Results are described in detail for Charney's model. Implications for the effects of diabatic heating and mixing due to smaller scale transients on equivalent barotropic stationary or quasi-stationary long waves are discussed. It is painted out that energy or potential enstrophy budgets may indicate that transients are damping the long waves while, in fact, their presence is destabilizing these waves.

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Sumant Nigam, Isaac M. Held, and Steven W. Lyons

Abstract

The quantitative validity of linear stationary wave theory is examined by comparing the results from a linear primitive equation model on the sphere with the stationary eddies produced by a general circulation model (GCM). The GCM simulated has a flat lower boundary, so that the stationary eddies can be thought of as forced by heating (sensible. latent and radiative) and time-averaged transient eddy flux convergences. Orographic forcing is examined in the second part of this study. The distribution of the diabatic heating and transient eddy flux convergences and the zonally symmetric basic state are taken directly from the GCM's climatology for Northern winter (DJF). Strong Rayleigh friction is included in the linear model wherever the zonal mean wind is small, as well as near the surface.

The linear model is found to simulate the stationary eddy pattern of the GCM with considerable skin in both midlatitudes and the tropics. Some deficiencies include the inaccurate simulation of the upper tropospheric geopotential over North America and distortion of the wind field near the low-level zero-wind line in the subtropics. Decomposition of the linear solution shows that 1) the extratropical upper tropospheric eddy pattern generated by tropical forcing is significant but smaller than due to extratropical forcing, 2) the upper-level extratropical pattern deteriorates somewhat when forcing by transients is removed, while the low-level pattern deteriorates dramatically and 3) there is considerable compensation between the effects of low-level thermal transients and extratropical sensible heating, to the point that we argue that this decomposition is not physically meaningful. The sensitivity of the results to the Rayleigh friction.formulation is discussed, as is the effect of replacing the transients with thermal damping.

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Stephen T. Garner, Noboru Nakamura, and Isaac M. Held

Abstract

The equilibration of two-dimensional baroclinic waves differs fundamentally from equilibration in three dimensions because two-dimensional eddies cannot develop meridional temperature or velocity structure. It was shown in an earlier paper that frontogenesis together with diffusive mixing in a two-dimensional Eady wave brings positive potential vorticity (PV) anomalies deep into the atmosphere from both boundaries and allows the disturbance to settle into a steady state without meridional gradients. Here we depart from the earlier explanation of this equilibration and associate the PV intrusions with essentially the same kind of vortex “roll-up” that characterizes the evolution of barotropic shear layers.

To avoid subgrid turbulence parameterizations and computational diffusion, the analogy is developed using Eady's generalized baroclinic instability problem. Eady's generalized model has two semi-infinite regions of large PV surrounding a layer of relatively small PV. Without boundaries, frontal collapse, or strong diffusion the model still produces equilibrated states, with structure similar to the vortex streets that emerge from unstable barotropic shear layers. The similarity is greatest when the baroclinic development is viewed in isentropic coordinates. The contrast between the present equilibrated solutions, which exhibit no vertical tilt, and Blumen's diffusive frontogenesis model, which allows the wave to retain its phase tilt, is briefly discussed.

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Stephen T. Garner, Isaac M. Held, Thomas Knutson, and Joseph Sirutis

Abstract

Atlantic tropical cyclone activity has trended upward in recent decades. The increase coincides with favorable changes in local sea surface temperature and other environmental indices, principally associated with vertical shear and the thermodynamic profile. The relative importance of these environmental factors has not been firmly established. A recent study using a high-resolution dynamical downscaling model has captured both the trend and interannual variations in Atlantic storm frequency with considerable fidelity. In the present work, this downscaling framework is used to assess the importance of the large-scale thermodynamic environment relative to other factors influencing Atlantic tropical storms.

Separate assessments are done for the recent multidecadal trend (1980–2006) and a model-projected global warming environment for the late 21st century. For the multidecadal trend, changes in the seasonal-mean thermodynamic environment (sea surface temperature and atmospheric temperature profile at fixed relative humidity) account for more than half of the observed increase in tropical cyclone frequency, with other seasonal-mean changes (including vertical shear) having a somewhat smaller combined effect. In contrast, the model’s projected reduction in Atlantic tropical cyclone activity in the warm climate scenario appears to be driven mostly by increased seasonal-mean vertical shear in the western Atlantic and Caribbean rather than by changes in the SST and thermodynamic profile.

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Isaac M. Held, Richard S. Hemler, and V. Ramaswamy

Abstract

Radiative-convective statistical equilibria are obtained using a two-dimensional model in which radiative transfer is interactive with the predicted moisture and cloud fields. The domain is periodic in x, with a width of 640 km, and extends from the ground to 26 km. The lower boundary is a fixed-temperature water-saturated surface. The model produces a temperature profile resembling the mean profile observed in the tropics. A number of integrations of several months' duration are described in this preliminary examination of the model's qualitative behavior.

The model generates a QBO-like oscillation in the x-averaged winds with an apparent period of ∼60 days. This oscillation extends into the troposphere and influences the convective organization. In order to avoid the associated large vertical wind shears, calculations are also performed in which the x-averaged winds are constrained to vanish. The convection then evolves into a pattern in which rain falls only within a small part of the domain. The moisture field appears to provide the memory that localizes the convection.

If the vertical shears are fixed at a modest nonzero value, this localization is avoided. Comparing calculations with surface temperatures of 25° and 30°C, the planetary albedo is found to decrease with increasing temperature, primarily due to a reduction in low-level cloudiness.

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Adam H. Sobel, Isaac M. Held, and Christopher S. Bretherton

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Interannual anomalies in tropical tropospheric temperature have been shown to be related to interannual anomalies in tropical mean sea surface temperature (SST) by a simple moist adiabatic relationship. On physical grounds, it is less obvious than it might at first seem that this should be the case. It is expected that the free-tropospheric temperature should be sensitive primarily to SST anomalies in regions in which the mean SST is high and deep convection is frequent, rather than to the tropical mean SST. The tropical mean also includes nonconvecting regions in which the SST has no direct way of influencing the free troposphere. However, interannual anomalies of SST averaged over regions of high monthly mean precipitation are very similar to interannual anomalies of tropical mean SST. Empirical orthogonal function analysis of the monthly SST histograms for the period of 1982–98 reveals a leading mode, well separated from the others, whose structure is very similar to a simple shift of the annual and climatological mean histogram, without change of shape. As a consequence, many different ways of sampling the histogram will yield similar anomaly time series, and the adequacy of the mean SST for predicting the tropospheric temperature appears coincidental from the point of view of the uncoupled atmospheric problem with given SST. There is a suggestion in the results that changes in the histogram shape may be significant for the tropospheric temperature anomalies associated with some large El Niño events and that in those events it is indeed the SST anomalies in the convective regions that are most important in controlling the tropospheric temperature.

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