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David J. Karoly

Abstract

The vacillatory behavior of the general circulation model described by Hunt (1978a,b) is analyzed using transformed Eulerian-mean diagnostics. This model was shown by Hunt to have large time variations in the troposphere and stratosphere with a period of ∼20 days. These diagnostics are used to show the coupling between the troposphere and the stratosphere and the forcing of mean state changes during the vacillation cycle.

The time variations of the wave-induced form on the mean flow and the Coriolis torque are in approximate balance throughout the vacillation cycle. Thus mean flow changes are small and the effect of the mean state on wave propagation is approximately constant. The vacillation cycle in the model is apparently due to variations in baroclinic wave activity in the troposphere and not to wave, mean flow interaction in the upper troposphere and stratosphere.

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David J. Karoly

Abstract

Eliassen-Palm cross sections and residual meridional circulations are presented for the Northern and Southern Hemispheres for summer and winter based on the data of Newell et al. (1972, 1974). The cross sections are similar to those presented by Edmon et al. (1980) for the Northern Hemisphere except that, in the Southern Hemisphere, there is much less variation between summer and winter than in the Northern Hemisphere.

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Brian J. Hoskins and David J. Karoly

Abstract

Motivated by some results from barotropic models, a linearized steady-state five-layer baroclinic model is used to study the response of a spherical atmosphere to thermal and orographic forcing. At low levels the significant perturbations are confined to the neighborhood of the source and for midlatitude thermal forcing these perturbations are crucially dependent on the vertical distribution of the source. In the upper troposphere the sources generate wavetrains which are very similar to those given by barotropic models. For a low-latitude source, long wavelengths propagate strongly polewards as well as eastwards. Shorter wavelengths are trapped equatorward of the poleward flank of the jet, resulting in a split of the wave-trains at this latitude. Using reasonable dissipation magnitudes, the easiest way to produce an appreciable response in middle and high latitudes is by subtropical forcing. These results suggest an explanation for the shapes of patterns described in observational studies.

The theory for waves propagating in a slowly varying medium is applied to Rossby waves propagating in a barotropic atmosphere. The slow variation of the medium is associated with the sphericity of the domain and the latitudinal structure of the zonal wind. Rays along which wave activity propagates, the speeds of propagation, and the amplitudes and phases along these rays are determined for a constant angular velocity basic flow as well as a more realistic jet flow. They agree well with the observational and numerical model results and give a simple interpretation of them.

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David J. Karoly, R. Alan Plumb, and Mingfang Ting

Abstract

Examples of the diagnostic of the horizontal propagation of stationary wave activity proposed by Plumb are presented for a simple model of the atmospheric response to thermal forcing in the tropics, for the observed Southern Hemisphere winter mean stationary waves and for several cases of anomalous quasi-stationary waves in both the Northern and Southern hemispheres. For the simple model, the propagation of wave activity out of the tropics is clear. From the observational data, the apparent sources of anomalous stationary wave activity are located in the regions of the major middle latitude jets and storm tracks in both hemispheres in most cases. The results suggest that midlatitude processes, such as instabilities of the jet stream or interaction with transient eddies, are the major mechanisms for forcing anomalous stationary waves. There are indications that Rossby-like wave propagation from low latitudes plays a role in forcing anomalous stationary waves associated with Southern Oscillation events and with some cases of anomalous stationary waves in the Southern Hemisphere.

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Greg C. Tyrrell, David J. Karoly, and John L. McBride

Abstract

Data from the Intensive Observation Period of the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (November 1992–February 1993) have been used to investigate the links between intraseasonal variations in tropical convection and those in forcing of upper-tropospheric Rossby waves in the extratropics. The primary databases are Geostationary Meteorological Satellite imagery and tropical wind analyses from the Bureau of Meteorology, Australia. A number of 5-day periods showing convection in different locations were chosen. For each period, mean fields of divergence, cloud-top temperature, and upper-tropospheric Rossby wave source are presented. Vorticity budgets are used to demonstrate the processes responsible for the Rossby wave source patterns. The approach follows earlier studies of links between interannual variations of tropical convection associated with the Southern Oscillation and variations of the extratropical circulation.

It is shown that the regions of tropical convection correspond to longitudinally localized Hadley cells. In the subtropics, at the higher-latitude end of each cell, there is a Rossby wave source dipole with anticyclonic and cyclonic forcing. The anticyclonic forcing of Rossby waves is associated with advection of vorticity by the divergent outflow, while the cyclonic forcing is due to the region of convergence immediately above the down-ward branch of the local Hadley cell. Hence, the authors provide a dynamical basis for tropical-midlatitude interactions associated with intraseasonal variations of tropical convection.

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Fiona M. Guest, Michael J. Reeder, Crispin J. Marks, and David J. Karoly

Abstract

This study examines the properties of inertia–gravity waves observed in the lower stratosphere over Macquarie Island, how these properties vary with season, and the likely source of the waves. The waves are observed in high-resolution upper-air ozonesonde soundings of wind and temperature released from Macquarie Island during the 1994 ASHOE–MAESA program.

The properties of the inertia–gravity waves observed in the soundings are quantified using hodograph and rotary spectral analyses. The analyzed waves have horizontal wavelengths between 100 and 1000 km, vertical wavelengths between about 1 and 7 km, intrinsic frequencies between f and 2f, and horizontal trace speeds between −50 and 30 m s−1. There appears to be a seasonal cycle in the inertia–gravity wave activity in the lower stratosphere, the minimum being in the austral winter when the background zonal flow is strong and westerly and its vertical shear is positive. In contrast, the variance of the horizontal perturbation winds does not show a similar seasonal cycle.

Inertia–gravity waves are detected over Macquarie Island on days with a common synoptic pattern. Two features define this synoptic pattern: 1) an upper-level jet and associated surface front lying upstream of Macquarie Island, and 2) a 300-hPa height field with Macquarie Island located between the inflection axis and the downstream ridge. This common synoptic pattern is observed on 16 of the 21 days on which inertia–gravity waves were detected. Moreover, the pattern is not observed on 15 of the 21 days in which inertia–gravity waves are not identified. This common synoptic pattern shows a seasonal cycle similar to that found for the inertia–gravity wave activity. Analyses of the ozonesonde soundings suggest also that the source of the inertia–gravity waves is in the troposphere. Using GROGRAT, the ray-tracing model developed by Marks and Eckermann, a cone of rays is released 21 km above Macquarie Island and traced backward in time. These rays suggest that the inertia–gravity waves are generated in the jet–front system southwest of Macquarie Island.

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