The Time-Mean Flow and Variability in a Nonlinear Model of the Atmosphere with Tropical Diabatic Forcing

The response to steady tropical diabatic forcing in a nonlinear model of the atmosphere is analyzed. For sufficiently small diabatic heating, upper tropospheric anticyclones develop along the equator to the west of the heating, as predicted by linear theory. For sufficiently large diabatic heating, the anticyclones shift eastward to the same longitude as the heating (nonlinear response). The structure of the nonlinear response agrees more favorably with the observations than does the linear response. Inclusion of strong tropospheric dissipation causes the weaker diabatic forcing to produce the structural characteristics of the nonlinear response. Analysis of the time-mean vorticity budget reveals that the relative magnitude of the steady nonlinear flux divergence is substantially larger for the strong forcing as compared to the weak forcing. This appears to be the mechanism responsible for the marked difference between the two responses. Significant differences in the extratropical response exist for the two cases. The tropical variability in the presence of the time-mean asymmetries in the basic state is examined. Strong maxima and minima in the transient kinetic energy are observed in the regions of equatorial westerlies and easterlies, respectively. The penetration into the regions of reduced easterlies by equatorward propagating extratropical waves is shown to be the major cause of the asymmetry in the variance. Most incident midlatitude waves are seen to be absorbed at their low-latitude critical line. Because the spectrum of atmospheric waves produced in midlatitudes includes some westward moving waves, the existence of equatorial westerlies is not required for the asymmetry to occur.