Investigating the Hadley Cell and eddies with varying sea surface temperature gradients

We examine the tropical meridional overturning circulation in an aquaplanet GCM with fixed orbital parameters and uniform insolation angle. The atmosphere is forced by an imposed non-interactive sea surface temperature (SST) distribution which is varied between present-day Earth-like to a latitudinally uniform profile. A conventional Hadley Cell (HC) -like flow is observed in all experiments along with the poleward transport of energy 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. Rossby wave activity generated near the extratropical surface propagates upward and turns equatorward on reaching the tropopause. These waves break on the edge of the HC, fluxing heat and momentum poleward and reinforcing a thermally direct cell in the same sense as the HC. When the SST distribution becomes globally uniform, the traditional midlatitude Rossby waves are trapped near the surface as the mean flow inhibits their upward propagation. But, near the 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 a dominant MJO-like mode, which facilitates near-equatorial convergence, this leads to a conventional tropical overturning circulation. As the SST gradient weakens, the HC moves from a regime intermediate to thermally and eddy-driven to one that's strongly influenced by eddies. Moreover, the thermal structure of the troposphere becomes uniform with weak gradients, and for flat SSTs, the tropopause in the midlatitudes is also set by convection. A Transformed Eulerian Mean perspective is consistent with this view and highlights the diabatic nature of the midlatitude circulation in the limit of flat sea surface temperatures.