Abstract

In Part I of this investigation, we described the stochastic, near-field behavior of disturbances excited by randomly evolving tropical heating. In the present paper, we examine how these disturbances are modified as they propagate through the far field in the presence of spatially-varying background states. Although the behavior can no longer be broken down into individual Hough modes, it can still be understood in terms of projection and barotropic components of the response.

Responses to fast heating, as may be produced by daily fluctuations in convection, and to slow heating, evolving over seasonal time scales, are studied separately. For fast heating the projection response consists mainly of a spectrum of Kelvin waves which, in the lower stratosphere, is centered at frequencies corresponding to twice the effective depth of the heating. The spectrum shifts to higher frequency with increasing altitude due to differential damping. As a result, the slow, fast and ultrafast Kelvin waves identified in observations all appear in our calculations as manifestations of the same response modified by dissipation. The barotropic response to fast forcing is dominated by the (1,1) Rossby normal mode throughout the tropics and in the stratosphere. In the extratropical troposphere, a transient barotropic wavetrain composed of low frequency Rossby waves of zonal wavenumber 1–3 is also present.

For slowly evolving heating projection and barotropic components from various modes overlap in the spectrum, coalescing into a continuum near zero frequency. Nevertheless, it is still possible to distinguish projection from barotropic responses because the former are dominant in the tropics while the latter are responsible for the extratropical behavior. The projection response to slow heating does not propagate effectively in the vertical and is largely confined to the troposphere, where its behavior is dictated by the particular part of the solution and assumes the form of a slowly evolving Walker circulation. The barotropic response is dominated by the same transient wavetrain found in the fast forcing case, but its amplitude is larger as a result of the greater amount of power available at low frequencies. Radiation of the barotropic response to higher latitudes is strongly dependent on the presence of westerly shear near the source region. Thus, maximum radiation takes place in the winter hemisphere, where the subtropical jet is closest to the source. The evolution of the wavetrain is also sensitive to the wind within the source region. Given the variability of winds in the tropical troposphere, the extratropical wavetrain can be expected to be a highly variable feature of the response to tropical heating. By contrast, the tropical Walker cell, which is essentially a forced response, is the most robust feature found in our slow heating calculations.

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