Previous: Introduction Next: The meridional circulation in a zonally symmetric model Up: Ext. Abst.

2. The meridional circulation in a zonally asymmetric model

The model used in this section is the same as that described in Scott & Haynes (2000b) based on the primitive equations on the sphere. There are 31 vertical levels equally spaced in , where p is the pressure, and a spectral representation in the horizontal comprising 31 meridional modes, and the first (wavenumber-one) zonal mode. The model is forced radiatively by including Newtonian relaxation to a zonally symmetric radiative basic state that may be either constant in time or time-varying, the latter case providing an approximation to the annual cycle. Waves are forced in the model by applying a wave-one perturbation to the geopotential height at the lower boundary, in the nothern hemisphere only. Horizontal hyperdiffusion and vertical diffusion are included for model stability, with values of 4/day on the smallest wavenumber and 0.1m/s/s respectively.

We present two sets of experiments, one in which the radiative basic state is constant in time, W1, corresponding to a perpetual winter (Figure 1a), the other in which the radiative basic state is time varying, W2, corresponding to an annual cycle (Figure 1b). Each set is forced with geopotential wave amplitudes h₀=210m, h₀=240m and h₀=270m. From Figure 1a we see that, althrough the increments in forcing amplitude are equal, there is a much greater increase in the tropical upwelling (and extratropical downwelling) when between h₀=240m and h₀=270m than between h₀=210m and h₀=240m. The reason is that between h₀=240m and h₀=270m the model response changes suddenly from a quiescient, southern hemisphere type evolution to a more disturbed northern hemisphere type evolution, similar to the model transitions found in Scott & Haynes (2000a). The increase in upwelling at h₀=270m is consistent with a similar increase in the vertical component of the Eliassen-Palm flux through the lower boundary (not shown).

In Figure 1b we see a similar small increase between h₀=210m and h₀=240m for the case of a time varying radiative basic state. For h₀=270m, however, a new model regime is found in which there is a biennial oscillation of the type investigated in Scott & Haynes (1998) (see also Scott & Haynes, 2000b), the two dotted lines in Figure 1b representing the meridional circulation produced by consecutive years with alternating quiescient and disturbed winters. Again, the disturbed year is characterized by larger values of the vertical component of the Eliassen-Palm flux through the lower boundary than those of the quiescient year.

In all cases note that there is significant upwelling at low latitudes, and that the magnitude is consistent with those values measured in the real atmosphere. Since the wave induced zonal momentum forcing decreases to very nearly zero south of around N, straightforward application of (1) fails to explain the upwelling response.

Figure 1: Residual vertical velocity, , at 20km altitude derived from heating rates according to , where Q is the diabatic heating, and is a reference potential temperature profile: (a) steady state response to wave-one forcing with amplitude h₀=210m, 240m and 270m; (b) annually averaged, seasonally varying response to wave-one forcing with amplitude h₀=210m, 240m and 270m. The dotted lines in (b) represent two consecutive annual cycles of the h₀=270m case.