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Zusammenfassung:
We examine the tropical meridional overturning circulation in an aquaplanet forced by a globally uniform solar angle and an imposed non-interactive sea surface temperature (SST) distribution that is varied between present-day Earth-like to a globally uniform profile. A conventional Hadley Cell (HC) is observed in all experiments, along with the poleward transport of heat and momentum. In simulations forced by a non-zero SST gradient, latent heat released from organized convection near the equator sets up a deep tropical cell. Midlatitude Rossby waves generated from the near-surface baroclinicity propagate upward and break on the HC edge, fluxing heat and momentum poleward and reinforcing a thermally direct cell. When the SST distribution becomes globally uniform, the near tropopause baroclinicity generates waves that ride on a sharp upper tropospheric potential vorticity gradient. These waves propagate downwards towards the lower equatorial troposphere and transport angular momentum out of the tropics. Together with eastward propagating convective activity, which facilitates near-equatorial convergence, this leads to a conventional tropical overturning circulation. In terms of a local Rossby number, as the SST gradient weakens, the HC goes from a thermally and eddy-driven cell to one that is strongly forced by eddies. Moreover, the thermal structure of the troposphere becomes uniform with weakening gradients, and for flat SSTs, the tropopause in the mid-latitudes is also set by convection. A Transformed Eulerian Mean perspective is consistent with this view and highlights the diabatic nature of the mid-latitude circulation in the limit of flat sea surface temperatures.
