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James R. Holton

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James R. Holton

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Observational evidence suggests that there may be long-period, large-scale variations in the intensity of the latent heat release in the equatorial zone. In this study a diagnostic numerical model is used to show that a standing wave oscillation of narrow longitudinal extent in the diabatic heat source in the troposphere can generate the global-scale propagating waves observed in the equatorial stratosphere. In particular, a heating oscillation which is symmetric about the equator can account for the eastward-moving Kelvin-wave mode, and a source which is antisymmetric can account for the westward-moving, mixed Rossby-gravity wave mode.

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James R. Holton

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A model is developed to diagnose the response of the atmosphere to a known distribution of diabatic heating. The linearized primitive equations in spherical coordinates are reduced to a single partial differential equation relating the perturbation geopotential to the diabatic heating pattern. The model diagnoses the atmospheric perturbation due to a heating pattern of specified zonal wavenumber, frequency, and distribution in the meridional plane.

The model is used to test the hypothesis that the observed westward propagating wave disturbances in the equatorial Pacific are Rossby waves driven by the latent heat release in the cloud clusters embedded within the waves. For a diabatic heating pattern designed to model the heating in cloud clusters the model duplicates many features of the observed waves. The computed perturbation meridional wind field has maxima in the upper and lower troposphere, separated by a relatively undisturbed region in the mid-troposphere. The structure of the disturbance is quite sensitive to vertical shear of the mean zonal wind. In particular, with westerly shear in the lower troposphere the precipitation occurs to the east of the surface trough, but with easterly shear the precipitation zone is west of the trough. These features are all in qualitative agreement with observations in the western and central Pacific.

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James R. Holton

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James R. Holton

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James R. Holton

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A semi-spectral numerical model is used to study the influence of a longitudinally varying gravity wave source on the general circulation of the winter mesosphere. The gravity wave source consists of stationary (topographic) waves with a longitudinally varying amplitude distribution that is approximated by the first two terms in a zonal harmonic expansion (i.e., the zonal mean plus planetary wavenumber 1). The computed zonal mean circulation in the mesosphere is nearly the same as that computed for a zonally symmetric gravity wave source of equal amplitude. However, the asymmetric source excites a strong stationary wavenumber 1 disturbance near the level of gravity wave breaking (≈71 km). This disturbance has a zonal wind maximum about ¼ cycle upstream from the gravity wave drag maximum. It is concluded that vertically propagating gravity waves produced in the troposphere are a possible source for mesospheric planetary waves.

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James R. Holton

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Vertically stratified stratospheric tracers such as methane and nitrous oxide tend to have constant mixing ratio surfaces that slope downward toward the poles in the meridional plane. The equilibrium tracer slope results from the competition between the slope steepening effects of advection by the diabatic circulation and the slope flattening effects of quasi-isentropic eddy transport and photochemical loss. The diabatic circulation itself is, however, driven primarily by eddy transports, which maintain the departure of stratosphere temperatures from radiative equilibrium. If the eddy transports are weak, the diabatic circulation is also weak and the slope is small. Using a simple beta-plane channel model and an eddy diffusion parameterization for the eddy potential vorticity and tracer transports, we show that the slope is a maximum for a value of eddy diffusion such that the dynamical time scale is between the radiative and chemical time scales.

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James R. Holton

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The equatorial stratospheric quasi-biennial oscillation (QBO) in zonal wind and temperature is observed to be symmetric about the equator. The QBO in column ozone, although it is believed to be caused primarily by vertical displacements due to the meridional circulation associated with the equatorial temperature QBO, is asymmetric with respect to the equator, and is strongly linked to the phase of the annual cycle. In this note a simple one-layer model is used to demonstrate that the gross features of the observed QBO in total ozone can be attributed to meridional advection of the ozone perturbation by the annually reversing mean meridional Hadley circulation. This advection causes a displacement of the equatorial ozone anomaly towards the winter hemisphere, and thus products an asymmetry with respect to the equator. It also modulates the amplitude of the ozone QBO, since the phase of the equatorial wind QBO with respect to the annual cycle may produce either constructive or destructive interference between the effects of the annually reversing meridional transport and the vertical advection by the equatorial wind QBO.

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James R. Holton

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James R. Holton

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It is shown that in the absence of dissipation or of critical levels where the mean zonal flow vanishes the forcing of the mean zonal flow by linearized, quasi-static, stationary long waves is proportional to the Jacobian of the mean zonal flow and the eddy meridional heat flux. However, if the Richardson number for the mean zonal flow is large, this eddy forcing is exactly balanced everywhere by the zonal momentum advection of the mean meridional circulation. Therefore, the local acceleration of the mean zonal flow vanishes. This theorem provides a generalization of the results obtained by Charney and Drazin for quasi-geostrophic perturbations.

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