Terrence R. Nathan

Barotropic Stability of Realistic Stratospheric Jets

Abstract The stability of realistic jets is examined in a linearized barotropic model on a sphere. Approximately non-dispersive modes associated with a region of negative basic state absolute vorticity gradient on the poleward side of the jet are examined in detail. As in previous studies, broader jets and those which peak at higher latitudes produce poleward modes that are less dispersive. Jet profiles derived from observational data at 10, 5, and 2 mb for three Southern Hemisphere winter months are used in the model, and the results are compared with quasi-nondispersive features (QNDF) which have been observed in satellite data in the Southern Hemisphere winter stratosphere. Characteristics of the barotropically unstable modes compare remarkably well with those of the observed modes. The barotropic model results for a month in which these features are not observed indicate the presence of equatorward modes at wavenumbers 3 and 4 which grow considerably faster than the quasi-nondispersive poleward modes....

Effects of Low-Frequency Tropical Forcing on Intraseasonal Tropical–Extratropical Interactions

Abstract A spherical nondivergent barotropic model, linearized about a 300-mb climatological January flow, is used to examine the extratropical response to low-frequency tropical forcing. A two-dimensional WKB analysis shows that the energy propagation depends on the sum of three vectors: the basic state wind vector, a vector that is parallel to the absolute vorticity contours, and the local wave vector. The latter two vectors are functions of the slowly varying background flow and forcing frequency ω. As ω decreases, the ray paths approach that of the local wave vector, so that the energy propagates in a direction perpendicular to the wave fronts. The extratropical jet streams have a stronger influence on the long period (>30 day) ray paths than on those of intermediate period (∼10–30 day). Global and local energetics calculations show that the energy conversion from the zonally varying basic flow increases as ω decreases. The local energetics show that for the long period disturbances, both the energy c...

Linear Stability of Free Planetary Waves in the Presence of Radiative–Photochemical Feedbacks

Abstract A simple β-plane model that couples radiative transfer, ozone advection, and ozone photochemistry with the quasi-geostrophic dynamical circulation is used to study the diabatic effects of Newtonian cooling and ozone–dynamics interaction on the linear stability of free planetary waves in the atmosphere. Under the assumption that the diabatic processes are sufficiently weak, an analytical expression is derived for the eigenfrequencies of these waves valid for arbitrary vertical distributions of background wind and ozone volume mixing ratio (g 2) vertical ozone advection is locally (de)stabilizing when dg 3) photochemically accelerated cooling, which predominates in the upper stratosphere, augments the Newtonian cooling rate and is stabilizing. The on...

The Global Atmospheric Response to Low-Frequency Tropical Forcing: Zonally Averaged Basic States

Abstract The extratropical response to localized, low-frequency tropical forcing is examined using a linearized, non-divergent barotropic model on a sphere. Zonal-mean basic states characterized by solid-body rotation or critical latitudes are considered. An analytical analysis based on WKB and ray tracing methods shows that, in contrast to stationary Rossby waves, westward moving, low-frequency Rossby waves can propagate through the tropical easterlies into the extratropics. It is shown analytically that the difference between the stationary and low-frequency ray paths is proportional to the forcing frequency and inversely proportional to the zonal wavenumber cubed. An expression for the disturbance amplitude is derived that shows the ability of the forced waves to maintain their strength well into middle latitudes depends on their meridional wave scale and northward group velocity, both of which are functions of the slowly varying background flow. A local energetics analysis shows that the combination o...

Pathways for Communicating the Effects of Stratospheric Ozone to the Polar Vortex: Role of Zonally Asymmetric Ozone

A mechanistic model that couples quasigeostrophic dynamics, radiative transfer, ozone transport, and ozone photochemistry is used to study the effects of zonal asymmetries in ozone (ZAO) on the model’s polar vortex. The ZAO affect the vortex via two pathways. The first pathway (P1) hinges on modulation of thepropagation anddampingof aplanetarywaveby ZAO; the second pathway(P2)hinges onmodulation of the wave‐ozone flux convergences by ZAO. In the steady state, both P1 and P2 play important roles in modulating the zonal-mean circulation. The relative importance of wave propagation versus wave damping in P1 is diagnosed using an ozone-modified refractive index and an ozone-modified vertical energy flux. In the lower stratosphere, ZAO cause wave propagation and wave damping to oppose each other. The result is a small change in planetary wave drag but a large reduction in wave amplitude. Thus in the lower stratosphere, ZAO ‘‘precondition’’ the wave before it propagates into the upper stratosphere, where damping due to photochemically accelerated coolingdominates, causing a large reduction in planetary wave drag and thus a colder polar vortex. The ability of ZAO within the lower stratosphere to affect the upper stratosphere and lower mesosphere is discussed in light of secular and episodic changes in stratospheric ozone.

The influence of wave- and zonal mean-ozone feedbacks on the quasi-biennial oscillation

The effects of wave and zonal mean ozone heating on the evolution of the quasi-biennial oscillation (QBO) are examined using a two-dimensional mechanistic model of the equatorial stratosphere. The model atmosphere is governed by coupled equations for the zonal mean and (linear) wave fields of ozone, temperature, and wind, and is driven by specifying the amplitudes of a Kelvin wave and a Rossby‐gravity wave at the lower boundary. Wave‐mean flow interactions are accounted for in the model, but not wave‐wave interactions. A reference simulation (RS) of the QBO, in which ozone feedbacks are neglected, is carried out and the results compared with Upper Atmosphere Research Satelliteobservations. The RS is then compared with three model experiments, which examine separately and in combination the effects of wave ozone and zonal mean ozone feedbacks. Wave‐ozone feedbacks alone increase the driving by the Kelvin and Rossby‐gravity waves by up to 10%, producing stronger zonal wind shear zones and a stronger meridional circulation. Zonal mean‐ ozone feedbacks (ozone QBO) alone decrease the magnitude of the temperature QBO by up to 15%, which in turn affects the momentum deposition by the wave fields. Overall, the zonal mean‐ozone feedbacks increase the magnitude of the meridional circulation by up to 30%. The combined effects of wave‐ozone and ozone QBO feedbacks generally produce a larger response then either process alone. Moreover, these combined ozone feedbacks produce a temperature QBO amplitude that is up to 30% larger than simulations without the feedbacks. Correspondingly, significant changes are also observed in the zonal wind and ozone QBOs. When ozone feedbacks are included in the model, the Kelvin and Rossby‐gravity wave amplitudes can be reduced by ;10% and still produce a QBO similar to simulations without ozone.

Ozone heating and the destabilization of traveling waves during summer

The effects of ozone heating on the linear stability of lower stratospheric traveling waves of the summertime, extratropical circulation are examined. Based on coupled equations for the quasigeostrophic potential vorticity and ozone volume mixing ratio, it is shown that the diabatic heating arising from ozone advection can offset the damping due to Newtonian cooling, leading to wave amplification and significant changes in the structure and zonally rectified fluxes of the wave fields in both the lower stratosphere and troposphere. The vertical profile of the zonal mean wind plays a crucial role in determining whether the ozone heating destabilizes eastward and/or westward traveling disturbances.

Effects of Ozone Heating on Forced Equatorial Kelvin Waves

Abstract An equatorial beta-plane model of the stratosphere is used to examine the effects of longwave radiational cooling, ozone photochemistry, and ozone advection on the linear spatial modulation of forced equatorial Kelvin waves. The model atmosphere is described by coupled equations for the zonal and meridional momentum, temperature, mass continuity, and ozone volume mixing ratio. For basic states characterized by a vertically sheared zonal wind that is in radiative–photochemical equilibrium, the linearized form of these equations is solved analytically and numerically. Results show that in the lower stratosphere, where the background vertical ozone gradient is positive, the wave amplitude is enhanced, whereas in the upper stratosphere, where temperature-dependent ozone photochemistry predominates, the wave amplitude is reduced. For vertical wind shears typical of the descending quasi-biennial oscillation (QBO) and semiannual oscillation (SAO) westerlies, it is shown that in the lower (upper) stratos...

On the role of ozone in the stability of Rossby normal modes

Abstract The role of ozone in the linear stability of Rossby normal modes is examined in a continuously stratified, extratropical baroclinic atmosphere. The flow is described by coupled equations for the quasi-geostrophic potential vorticity and ozone volume mixing ratio. A perturbation analysis is carded out under the assumption of weak diabatic heating, which is generated by Newtonian cooling and dynamics–ozone interaction. An expression for the propagation and growth characteristics is obtained analytically in terms of the vertically averaged wave activity, which depends explicitly on the wave spatial structure, photochemistry, and basic state distributions of wind, temperature, and ozone mixing ratio. Calculations show that stationary internal modes, whose amplitudes are largest in the stratosphere, are destabilized by dynamics–ozone interaction and Newtonian cooling, with e-folding times on the order of 20–40 days.

An Analytical Study of Ozone Feedbacks on Kelvin and Rossby–Gravity Waves: Effects on the QBO

An equatorial beta-plane model of the middle atmosphere is used to analytically examine the effects of radiative cooling and ozone heating on the spatial and temporal evolution of the quasi-biennial oscillation (QBO). Under the assumption that the diabatic heating is weak and the background fields of wind, temperature, and ozone are slowly varying, a perturbation analysis yields expressions describing the vertical spatial modulation of Kelvin and Rossby‐gravity waves in the presence of ozone. These expressions show that wave-induced changes in the diabatic heating arising from the advection of basic-state ozone reduce the local radiative damping rate by up to 15% below 35 km. In a one-dimensional model of the QBO, eddy ozone heating increases the amplitude of the zonal wind QBO by 1‐2 m s21 and increases the oscillation period by about two months. The significance of these results to the observed QBO is discussed.