Atmospheric General Circulation and its Low Frequency Variance: Radiative Influences

Abstract

 Analyses of satellite radiation budget measurements, atmospheric response times calculations, and general circulation model results are invoked to elucidate the radiatively driven components of the atmospheric general circulation. The clear-sky and cloud radiative processes are shown to exert significant vertical, latitudinal and longitudinal gradients in the diabatic heating within the troposphere and the stratosphere. The meridional heating gradient, which drives the general circulation, is altered significantly by clouds. In the tropics, clouds introduce a dipole heating pattern of convergence of IR radiation in the upper troposphere and a reduction of solar radiation absorbed by the ocean-surface. The magnitudes of the convergence and the reductionn of radiation fluxes can become as large as 100 W·m^-2.
 For the column as a whole, more than 75% of the zonal mean absorbed solar energy in low latitudes is balanced by the IR emission. Likewise, the net diabatic heating in the low-latitude troposphere results from a delicate balance between IR cooling and convective heating. These estimates reveal the strong control of radiative-convective processes in governing the observed thermal structure. The time scales for radiative-convective adjustments are in the range of 10-20 days within the troposphere and are in the range of 20 to 100 days in the lower stratosphere. Hence, adjustments to radiative perturbations occur on time scales of importance to the law frequency variance problem. Furthermore, the atmospheric response to zonal mean radiative heating perturbations normally involves a radiative-convective adjustment. Examples are given to illustrate the mechanisms by which meridional gradients in radiative heating due to cirrus and other processes alter the zonal winds and the propagation of planetary waves.
 The major conclusion of this study is that the observed negative anomalies in the outgoing IR radiation following intense warm episodes of tropical sea-surface temperatures, the so-called El Nino, are indeed anomalies in the cloud-radiative forcing. During the 1982/1983 El Nino phenomenon, over the tropical eastern Pacific, the mid- to upper- tropospheric radiative heating must have increased by 50 to 100 W·m^-2 or more and the solar energy reaching the ocean surface must have decreased by at least 25 to 50 W·m^-2. These changes, which have been ignored in the earlier studies, can account for a substantial fraction of the observed atmospheric changes. The inferred decrease in the solar energy to the ocean surface should play a substantial role in dissipating the warm anomalies in the eastern Pacific ocean.