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John E. Geisler

Abstract

It has recently been suggested that exchange of sensible heat with the underlying surface is necessary for the baroclinic instability of waves on the scale of observed blocking ridge activity over the North Pacific. Theoretical support for this idea comes from studies of baroclinic instability in a two-layer model with diabatic heating proportional to the wave temperature. In this paper the problem is treated in a continuously stratified model. It is shown that such diabatic heating is not needed for instability at long wavelengths and that when it is present the growth rate of unstable modes is decreased.

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John E. Geisler and Robert E. Dickinson

Abstract

Free oscillations of small amplitude and low frequency in an isothermal atmosphere at rest have the vertical structure of an edge wave, energy density decaying exponentially away from the ground. In this paper we treat these external Rossby modes on a middle latitude β-plane to see how their vertical structure and phase speed are changed by the presence of a winter zonal wind with realistic vertical structure. We find that for planetary-scale modes the basic state interior potential vorticity gradient and the temperature gradient at the lower boundary produce roughly equal but opposite changes in phase speed. Energy density decays less rapidly with altitude when the basic state temperature gradient is included in the lower boundary condition. As wavenumber is increased, the phase speed of the external Rossby mode is found to approach the speed of the zonal wind at the surface, and there is a limiting wavenumber beyond which the mode does not exist. At this limiting value of wavenumber, the mode becomes the low-wavenumber bound for the rapidly growing unstable modes first investigated by Charney, which have similar vertical structure. In addition to the, neutral external free mode, we find also neutral internal free modes that exist because of wave trapping in regions of relatively weak zonal winds. The relationship between the neutral free modes and unstable modes is explored numerically for the realistic zonal wind profile and for a zonal wind increasing linearly with altitude. In contrast to past studies of the stability of the linear profile, the present study indicates the possibility of stability for a band of wavelengths on the low-wavenumber side of a critical stability curve.

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Keith D. Sashegyi and John E. Geisler

Abstract

A linear model of the steady response of a stratified fluid to isolated heat sources on a sphere is developed. The model is used to examine the response to diabatic heating associated with summer monsoon precipitation in India and to low-level diabatic heating along the northeast coast of Africa. In a laterally unbounded, spherical domain, the summer monsoon heat source forces a cross-equatorial meridional cell that is about half as strong as the main response feature, which is a cell oriented zonally and situated on the west side of the source. The imposition of a meridional wall concentrates the cross-equatorial flow in the meridional cell into a western boundary current. For representative summer monsoon heating the northward transport in this simple East African Jet is comparable to what is observed. The cross-equatorial flew pattern forced by low-level, diabatic heating along the African coast consists of a western boundary current near the equator that turns into a geostrophically balanced sea breeze in low latitudes away from the equator. The northward mass flux in this locally forced jet is about an order of magnitude smaller than that which is forced by the summer monsoon heat source.

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Rolando R. Garcia and John E. Geisler

Abstract

A quasi-geostrophic β-plane channel model is used to study the response of the stratosphere to planetary waves forced at the ground. The forcing consists of a standing field of eddy vertical velocity whose amplitude fluctuates randomly about a time-average value. Cross-spectrum analysis of model results reveals the presence of westward traveling waves and, among these, of normal modes in the model solution. The superposition of the traveling waves and a stationary wave maintained by the time-average forcing gives rise to oscillating eddy heat fluxes. The relationship of these fluxes to changes in the zonal-mean temperature gradient is investigated by means of the squared coherence statistic. We examine how the occurrence of high levels of coherence depends on the existence of the external normal mode and on the presence of noise introduced into the model output.

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Adilson W. Gandu and John E. Geisler

Abstract

A nonlinear, primitive equations model with five levels in a σ coordinate is applied to study the effects of idealized topography on the summer circulation pattern over tropical South America. The model circulation is forced by prescribed latent heating centered over the Amazon Basin. Both the steady and the transient components of the response are considered and compared with the corresponding components in the absence of topography. It is found that topography blocks low-level inflow from the equatorial Pacific and leads to the development of a steady, northerly jet that is fed from the tropical Atlantic. Other important effects of topography on the response to steady heating are found in the low-level field of vertical motion. The effects of topography on the response to transient heating are described in terms of a component reflected to the east and a component transmitted to the west by the topography.

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John E. Geisler and Eric J. Pitcher

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John E. Geisler and Lawrence A. Mysak

Abstract

This paper describes the zonally propagating wave modes of a homogeneous fluid on an equatorial beta-plane with linear bottom topography. The fluid is bounded above by a free surface and bounded below by a depth profile that increases linearly with distance away from a beach that is parallel to the equator and located some distance either north or south of it. The governing equation for the wave amplitude is solved numerically to obtain dispersion relations and mode structures of waves trapped against the beach. These are interpreted in the light of existing knowledge of trapped modes on a sloping beach on an f-plane and of trapped modes on an equatorial beta plane with a flat bottom. Both gravity modes and low-frequency topographic modes are included in the analysis.

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John E. Geisler and Rolando R. Garcia

Abstract

The problem of the baroclinic instability of an atmospheric zonal flow which is a continuous function of altitude above a horizontal boundary on a, β-plane exhibits two classes of unstable normal mode solutions. One of these consists of the rapidly growing modes discovered by Charney (1947). A second class consisting of the more slowly growing modes at longer wavelengths first found by Green (1960) has received comparatively little attention. This paper presents results of a numerical study of this class of modes that show how their growth rate and vertical structure depend on basic state model parameters. In the absence of dissipation the e-folding times of these modes at planetary wave scales is about one week. The vertical structure at these scales is that of a trapped internal normal mode with associated wind and temperature fields typically an order of magnitude larger in the middle and upper stratosphere than at the ground.

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John D. Horel, Andrea N. Hahmann, and John E. Geisler

Abstract

Outgoing longwave radiation (OLR) is used to describe the annual cycle of convection that resides over the Amazon Basin during austral summer and over Central America and the adjacent waters of the Pacific during austral winter. The preferred locations of the convective activity during the wet season in the respective hemispheres are determined, and the beginning and ending of these seasons is specified. The onset of the wet season over Amazonia usually occurs within a single month, while the onset of the wet season over Central America typically requires from one to three months. The annual cycle of convective activity in this regime is shown to exhibit a seasonal regularity and degree of symmetry with respect to the equator which exceeds those characterizing the other two annually varying regimes in the tropical belt. Analyses produced by the European Centre for Medium-range Weather Forecasts (ECMWF) are superimposed upon OLR fields to illustrate features of the atmospheric circulation in the vicinity of the tropical Americas that are associated with the annual cycle of convection. The onset and demise of the wet season in the Amazon Basin are further described by means of composites of these data. It is found that the Bolivian high inferred from the ECMWF data develops rapidly during the onset transition in a manner that is temporally and spatially consistent with the distribution of OLR.

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Maurice L. Blackmon, John E. Geisler, and Eric J. Pitcher

Abstract

A general circulation model has been run in the perpetual January mode to produce several long-term simulations, each distinguished by a different imposed equatorial Pacific sea surface temperature. From each of them simulations we have extracted an eight-member ensemble of 90-day averaged fields. Ensemble-mean difference maps are presented in this paper, together with an estimate of the statistical significance of features which appear in thee maps. These results are compared with observational studies in the literature that present difference maps of Northern Hemisphere winter fields composited according to some index related to the two extremes of equatorial Pacific sea-surface temperature variation.

The results show many anomaly patterns of high statistical significance that are also in good agreement with those observed. In the tropics, them include 990 mb wind, sea level pressure and rainfall anomalies constituting the Southern Oscillation, as well as a 200 mb height anomaly at all longitudes. In extratropical latitudes there is a 500 mb height anomaly which agrees closely with the observed Pacific/North American (PNA) pattern. Other manifestations of the simulated PNA pattern which are in good agreement with observations are anomalies of height, zonal wind and temperature all at 700 mb. The model also reproduces large anomalies that are observed in the 10 mb height and zonal wind fields in the polar stratosphere.

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