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By the end of this century, under the current trajectory of greenhouse gas emissions, climate models project a substantial decline in both Arctic and Antarctic sea ice extent. There has [...]

This paper considers the effect of time-dependent lower boundary wave forcing on the internal variability found to appear spontaneously in a stratosphere-only model when the forcing is perfectly steady. While the time-dependent forcing is found to modulate the internal variability, leading in some cases to frequency locking of the upper-stratospheric response to the forcing, the temporal and spatial structure of the variability remains similar to the case when the forcing is time independent. Experiments with a time-periodic modulation of the forcing amplitude indicate that the wave flux through the lower boundary is only partially related to the instantaneous forcing, but is more significantly influenced by the condition of the polar vortex itself. In cases of purely random wave forcing with zero time mean, the stratospheric response is similar to that obtained with steady forcing of magnitude equal to the root-mean-square of the time-varying forcing.

Although possibly the simplest model for the atmospheres of the giant planets, the turbulent forced-dissipative shallow-water system in spherical geometry has not, to date, been investigated: the present study aims to fill this gap. Unlike the freely decaying shallow-water system described by Cho and Polvani, equilibrium states in the forced-dissipative system are highly dependent on details of the forcing and the dissipation. For instance, it is found that for a given equilibrated energy level, the steadiness of zonal jets depends crucially on the balance between forcing and dissipation. With long (up to 100 000 days) high-resolution (T170) calculations, the dependence of the equilibrium states on Rossby number Ro and Rossby deformation radius [L.sub.D] is explored, for the case when the dissipation takes the form of hypodiffusion (acting predominantly at large scales) and the random forcing at small scales is [delta] correlated in time. When [L.sub.D] is large compared to the planetary radius, zonal jets are verified to scale closely with the Rhines scale over a wide range of Ro: furthermore, the jets at the equator are found to be both prograde and retrograde with approximately equal likelihood. As [L.sub.D] is decreased, the equatorial jets become increasingly and consistently retrograde, in agreement with the freely decaying turbulence results. Also, the regime recently discussed by Theiss, where zonal jets are confined to low latitudes, is illustrated to emerge robustly in the limit of small [L.sub.D]. Finally, specific calculations with parameter values typical of the giant planets are presented, confirming many of the earlier results obtained in the freely decaying case.

This paper examines the nature and robustness of internal stratospheric variability, namely the variability resulting from the internal dynamics of the stratosphere itself, as opposed to that forced by external sources such as the natural variability of the free troposphere. Internal stratospheric variability arises from the competing actions of radiative forcing, which under perpetual winter conditions strengthens the polar vortex, and planetary wave breaking, which weakens it. The results from a stratosphere-only model demonstrate that strong internal stratospheric variability, consisting of repeated sudden warming-type events, exists over a wide range of realistic radiative and wave forcing conditions, and is largely independent of other physical and numerical parameters. In particular, the coherent form of the variability persists as the number of degrees of freedom is increased, and is therefore not an artifact of severe model truncation. Various diagnostics, including three-dimensional representations of the potential vorticity, illustrate that the variability is determined by the vertical structure of the vortex and the extent to which upward wave propagation is favored or inhibited. In this paper, the variability arising from purely internal stratosphere dynamics is isolated by specifying thermal and wave forcings that are completely time independent, in a second paper, the authors investigate the relative importance of internal and external variability by considering time-dependent wave forcing as a simple representation of tropospheric variability.

The linear and nonlinear dynamics of layers of anomalously high potential vorticity (PV) are studied in detail. It is well known that PV layers are subject to slow, balanced, mixed barotropic-baroclinic instabilities. In this paper, it is shown that, in addition, PV layers are subject to a Kelvin-Helmholtz instability, operating on much smaller spatial and faster temporal scales. For simplicity, spatially infinite layers of uniform anomalous PV are considered. Such layers are characterized by two key parameters: the ratio [DELTA] q of their anomalous PV to the background PV, and the angle a between the layer and the direction of the ambient stratification gradient (in suitably scaled coordinates). It is found that Kelvin-Helmholtz appears, for certain values of a, whenever [DELTA] q > 8. Of notable interest is the case of an initially vertical PV layer embedded in a weak ambient shear flow: for sufficiently large [DELTA] q, once the PV layer is tilted past a critical angle, Kelvin-Helmholtz instability becomes possible. It is argued that the breakdown of PV layers due to a Kelvin-Helmholtz instability induced by ambient shear might be an important systematic mechanism leading to irreversible mixing during stratosphere troposphere exchange events. This is discussed in the context of an example of Kelvin-Helmholtz instability observed near a tropopause fold.

This work investigates the extent to which potential vorticity gradients affect the vertical propagation of planetary-scale Rossby waves on the edge of a stratospheric polar vortex and their eventual nonlinear saturation and breaking. Using two different numerical modeling approaches, it is shown that wave propagation and wave breaking are significantly reduced when the potential vorticity gradients at the vortex edge are anomalously weak. The efficiency of the first model, based on high-resolution contour dynamics, permits a full exploration of the parameter space of wave forcing amplitude and edge steepness. A more realistic primitive equation model in spherical geometry both confirms the contour dynamics results and highlights some direct implications for stratospheric modeling in more comprehensive models. The results suggest that stratospheric models using horizontal resolutions of spectral T42 or less may significantly underestimate the vertical propagation and breaking of planetary waves, and may consequently misrepresent such important stratospheric processes as the mean meridional circulation, sudden warmings, and the mixing of chemically distinct polar and midlatitude air.

The weak temperature gradient (WTG) approximation is applied to simple shallow-water models of the Hadley circulation. While it is difficult to formally justify the use of the WTG approximation for this problem, the derived WTG solutions are shown to agree well with numerical solutions of the full equations and to converge to the traditional angular momentum conserving (AMC) solutions in the inviscid limit. Heuristic arguments are given to explain this. The WTG method also provides semianalytical solutions in the case of nonvanishing viscosity, in contrast to the AMC solutions, which are strictly inviscid.

Juckes' observations regarding recent research on the link between nonlinear wave breaking and thermal forcing were unmerited since transport into, rather than within, the stratospheric surf zone was the focus of the work. Although his assertion that the shallow water model is inadequate for modeling the stratosphere in tropical areas is correct, he erred in stating that the model cannot capture the tropical atmospheric response to midlatitude Rossby waves.

The nonlinear response of a barotropic, nondivergent, spherical flow representative of the upper troposphere (but without a tropical Hadley cell) to localized, extratropical topographic forcing is examined using high-resolution contour surgery calculations. The response is shown to vary greatly with forcing amplitude and can be significantly different from the linear response. At large amplitude, Rossby wave breaking occurs in the tropics irrespective of the direction of the equatorial winds, and leads to small-scale stirring and the formation of a 'tropical surf zone,' which inhibits the meridional propagation of the disturbance.

The behavior of an isolated vortex perturbed by topographically forced Rossby waves is studied using the method of Contour Dynamics. For a single-contour vortex a distinct forcing threshold exists above which the wave breaks in a dynamically significant way, leading to a disruption of the vortex. This breaking is distinguished from the process of weak filamentary a breaking described by Dritschel and classified here as microbreaking; the latter occurs in nondivergent flow even at very small forcing amplitudes but does not affect the vortex in a substantial manner. In cases with finite Rossby deformation radius (comparable with the vortex radius) neither breaking nor microbreaking occurs below the forcing threshold. In common with previous studies using high-resolution spectral models, the vortex is not diluted by intrusion of outside air, except during remerger with a secondary vortex shed previously from the main vortex during a breaking event. The kinematics of the breaking process and of the vortex interior and the morphology of material ejected from the vortex are described. When the Rossby radius is finite there is substantial mixing in the deep interior of the vortex, even when the vortex is only mildly disturbed. Implications for the stratospheric polar vortex are discussed., The method of Contour dynamics was used in a study of behavior dynamics of an isolated vortex perturbed by quasi-stationary driven Rosssby waves. The model is restricted to a single-contour vortex and investigation of its dynamics revealed Rossby wave breaking characteristics and ejection of material from the vortex identical to other modeling studies of the stratospheric polar vortex. Neither breaking nor microbreaking occured below the forcing threshold in Rossby finite deformation cases.

The large amplitude oscillations of the shape of Neptune's Great Dark Spot are well reproduced by simple dynamical models of an isolated vortex embedded in a background shear flow. From [...]

Provides information on a study which analyzed the three-dimensional nature of breaking Rossby waves in the polar wintertime stratosphere. Description of the polar wintertime stratosphere; Model used in the study; Discussions on the baroclinic vortex.

Investigates dynamics of initially balanced decaying turbulence in a shallow rotating fluid layer. Case of incompressible two-dimensional decaying turbulence; Analysis of the emergence of coherent structures; Quantification of the evolution of the flow; Relevance of the study to the vortices observed in Jupiter's atmosphere.

Examines the nonlinear response of a barotropic, nondivergent, spherical flow characteristic of the upper troposphere to localized, extratropical topographic forcing. Use of high resolution contour surgery resolutions; Effect of a tropical surf zone to the propagation of Rossby waves.