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

Abstract

In order to clarify the extent to which the two-layer model can successfully simulate the remote tropospheric response to localized stationary forcing, the structure of stationary Rossby waves in the two-layer model is compared with that in continuous models. One finds a close correspondence when the two-layer flow is supercritical in the sense of the Phillips' criterion, except for the possibility of upstream propagation in the two-layer model when the lower-layer wind is small. When the two-layer flow is subcritical, the stationary waves can be very seriously distorted. The manner in which neutral modes are spatially or temporally destabilized by damping in the two-layer model is contrasted with similar results for Charney's model.

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Wenyu Zhou, Isaac M. Held, and Stephen T. Garner

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Tropical cyclones are studied under the idealized framework of rotating radiative–convective equilibrium, achieved in a large doubly periodic f plane by coupling the column physics of a global atmospheric model to rotating hydrostatic dynamics. Unlike previous studies that prescribe uniform sea surface temperature (SST) over the domain, SSTs are now predicted by coupling the atmosphere to a simple slab ocean model. With coupling, SSTs under the eyewall region of tropical cyclones (TCs) become cooler than the environment. However, the domain still fills up with multiple long-lived TCs in all cases examined, including at the limit of the very small depth of the slab. The cooling of SSTs under the eyewall increases as the depth of the slab ocean layer decreases but levels off at roughly 6.5 K as the depth approaches zero. At the eyewall, the storm interior is decoupled from the cooler surface and moist entropy is no longer well mixed along the angular momentum surface in the boundary layer. TC intensity is reduced from the potential intensity computed without the cooling, but the intensity reduction is smaller than that estimated by a potential intensity taking into account the cooling and assuming that moist entropy is well mixed along angular momentum surfaces within the atmospheric boundary layer.

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Gang Chen, Isaac M. Held, and Walter A. Robinson

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The sensitivity to surface friction of the latitude of the surface westerlies and the associated eddy-driven midlatitude jet is studied in an idealized dry GCM. The westerlies move poleward as the friction is reduced in strength. An increase in the eastward phase speed of midlatitude eddies is implicated as playing a central role in this shift.

This shift in latitude is mainly determined by changes in the friction on the zonal mean flow rather than the friction on the eddies. If the friction on the zonal mean is reduced instantaneously, the response reveals two distinctive adjustment time scales. In the fast adjustment over the first 10–20 days, there is an increase in the barotropic component of zonal winds and a substantial decrease in the eddy kinetic energy; the shift in the surface westerlies and jet latitude occurs in a slower adjustment. The space–time eddy momentum flux spectra suggest that the key to the shift is a poleward movement in the subtropical critical latitude associated with the faster eastward phase speeds in the dominant midlatitude eddies. The view is supported by simulating the upper-tropospheric dynamics in a stochastically stirred nonlinear shallow water model.

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Ngar-Cheung Lau, Isaac M. Held, and J. David Neelin

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The structure of the intraseasonal oscillations in the tropics of an idealized general circulation model with a zonally symmetric climate is described. Space-time spectra show a peak in zonal winds and velocity potential at the equator in zonal wavenumbers 1 and 2, corresponding to eastward-propagating power at phase speeds of ≈18 m s−1. This speed is significantly greater than that of the observed oscillation but comparable to that obtained in similar models by Hayashi and Sumi and Swinbank et al. The corresponding eastward-propagating power in the precipitation spectrum is concentrated in wavenumbers 2–5. A composite procedure is used to describe the three-dimensional structure of the model's oscillation. The oscillation is characterized by circulation cells oriented along the equatorial zonal plane, with enhanced precipitation in the region of rising motion. Zonal wind changes tend to be positively correlated with geopotential height changes at the same level. Positive perturbations in the water vapor mixing ratio, evaporation, and lower tropospheric horizontal moisture convergence all exhibit distinct eastward displacements from the center of convection.

Two different linear models are used to interpret the GCM results. The response to the GCM's composited diabatic heating field is first computed using a linear primitive equation model on the sphere. This linear model requires strong damping above the heated region, as well as near the surface, to produce a pattern in rough agreement with the GCM. A simple Kelvin wave-CISK model, in which the vertical structure of the heating is taken from the composite, is then shown to be capable of reproducing the phase speed simulated in the GCM.

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Ming Zhao, Isaac M. Held, and Shian-Jiann Lin

Abstract

High-resolution global climate models (GCMs) have been increasingly utilized for simulations of the global number and distribution of tropical cyclones (TCs), and how they might change with changing climate. In contrast, there is a lack of published studies on the sensitivity of TC genesis to parameterized processes in these GCMs. The uncertainties in these formulations might be an important source of uncertainty in the future projections of TC statistics.

This study investigates the sensitivity of the global number of TCs in present-day simulations using the Geophysical Fluid Dynamics Laboratory High Resolution Atmospheric Model (GFDL HIRAM) to alterations in physical parameterizations. Two parameters are identified to be important in TC genesis frequency in this model: the horizontal cumulus mixing rate, which controls the entrainment into convective cores within the convection parameterization, and the strength of the damping of the divergent component of the horizontal flow. The simulated global number of TCs exhibits nonintuitive response to incremental changes of both parameters. As the cumulus mixing rate increases, the model produces nonmonotonic response in global TC frequency with an initial sharp increase and then a decrease. However, storm mean intensity rises monotonically with the mixing rate. As the strength of the divergence damping increases, the model produces a continuous increase of global number of TCs and hurricanes with little change in storm mean intensity. Mechanisms for explaining these nonintuitive responses are discussed.

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Wenyu Zhou, Isaac M. Held, and Stephen T. Garner

Abstract

Rotating radiative–convective equilibrium is studied by extracting the column physics of a mesoscale-resolution global atmospheric model that simulates realistic hurricane frequency statistics and then coupling it to rotating hydrostatic dynamics in doubly periodic domains. The parameter study helps in understanding the tropical cyclones simulated in the global model and also provides a reference point for analogous studies with cloud-resolving models.

The authors first examine the sensitivity of the equilibrium achieved in a large square domain (2 × 104 km on a side) to sea surface temperature, ambient rotation rate, and surface drag coefficient. In such a large domain, multiple tropical cyclones exist simultaneously. The size and intensity of these tropical cyclones are investigated.

The variation of rotating radiative–convective equilibrium with domain size is also studied. As domain size increases, the equilibrium evolves through four regimes: a single tropical depression, an intermittent tropical cyclone with widely varying intensity, a single sustained storm, and finally multiple storms. As SST increases or ambient rotation rate f decreases, the sustained storm regime shifts toward larger domain size. The storm’s natural extent in large domains can be understood from this regime behavior.

The radius of maximum surface wind, although only marginally resolved, increases with SST and increases with f for small f when the domain is large enough. These parameter dependencies can be modified or even reversed if the domain is smaller than the storm’s natural extent.

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Paul J. Kushner, Isaac M. Held, and Thomas L. Delworth

Abstract

The response of the Southern Hemisphere (SH), extratropical, atmospheric general circulation to transient, anthropogenic, greenhouse warming is investigated in a coupled climate model. The extratropical circulation response consists of a SH summer half-year poleward shift of the westerly jet and a year-round positive wind anomaly in the stratosphere and the tropical upper troposphere. Along with the poleward shift of the jet, there is a poleward shift of several related fields, including the belt of eddy momentum-flux convergence and the mean meridional overturning in the atmosphere and in the ocean. The tropospheric wind response projects strongly onto the model’s “Southern Annular Mode” (also known as the “Antarctic oscillation”), which is the leading pattern of variability of the extratropical zonal winds.

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

Abstract

The structure of stationary Rossby waves in the presence of a mean westerly zonal flow with vertical shear is examined. There is typically only one stationary vertical mode, the external mode, trapped within the troposphere. For more than one tropospheric mode to exist, we find that vertical shears must be smaller than those usually observed in extratropical latitudes. The vertical structure, horizontal wavenumber and group velocity of the external mode, and the projection onto this mode of topographic and thermal forcing are studied with continuous models (a linear shear profile as well as more realistic basic states), and a finite-differenced model with resolution and upper boundary condition similar to that used in GCMs. We point out that the rigid-lid upper boundary condition need not create artificial stationary resonances, as the artificial stationary vertical modes that are created are often horizontally evanescent.

The results are presented in a form which allows one to design the equivalent barotropic model that captures the external mode's contribution to the stationary wave field. It is found, in particular, that the wind blowing over the topography in such a barotropic model should generally be larger than the surface wind but smaller than the wind at the equivalent barotropic level. Also, the group velocity of the stationary external mode in realistic vertical shear is found to be considerably greater than that of the stationary Rossby wave in the equivalent barotropic model.

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Spencer A. Hill, Yi Ming, and Isaac M. Held

Abstract

Anthropogenically forced changes to the mean and spatial pattern of sea surface temperatures (SSTs) alter tropical atmospheric meridional energy transport throughout the seasonal cycle—in total, its partitioning between the Hadley cells and eddies and, for the Hadley cells, the relative roles of the mass flux and the gross moist stability (GMS). The authors investigate this behavior using an atmospheric general circulation model forced with SST anomalies caused by either historical greenhouse gas or aerosol forcing, dividing the SST anomalies into two components: the tropical mean SST anomaly applied uniformly and the full SST anomalies minus the tropical mean.

For greenhouse gases, the polar-amplified SST spatial pattern partially negates enhanced eddy poleward energy transport driven by mean warming. Both SST components weaken winter Hadley cell circulation and alter GMS. The Northern Hemisphere–focused aerosol cooling induces northward energy flux anomalies in the deep tropics, which manifest partially via strengthened northern and weakened southern Hadley cell overturning. Aerosol-induced GMS changes also contribute to the northward energy fluxes. A simple thermodynamic scaling qualitatively captures these changes, although it performs less well for the greenhouse gas simulations. The scaling provides an explanation for the tight correlation demonstrated in previous studies between shifts in the intertropical convergence zone and cross-equatorial energy fluxes.

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Spencer A. Hill, Yi Ming, and Isaac M. Held
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