  The tropical overturning circulation is examined in a moist aquaplanet
general circulation model forced using a non-interactive sea surface
temperature (SST) distribution that varies between a present-day Earth-like
profile and one that is globally uniform. A traditional Hadley Cell (HC)-like
flow is observed in all experiments along with the poleward transport of heat
and angular momentum. In simulations with non-zero SST gradients, latent heat
released from organized convection near the equator sets up a deep tropical
cell; midlatitude baroclinic Rossby waves flux heat and angular momentum
poleward, reinforcing the thermally direct circulation. As the imposed SST
gradient is weakened, the HC transitions from a thermally and eddy-driven
regime to one that's completely eddy-driven. When the SST is globally uniform,
equatorial waves concentrate precipitation in the tropics and facilitate the
lower-level convergence necessary for the ascending branch of the HC.
Conventional midlatitude Rossby waves become very weak, but upper-level
baroclinicity generates waves that cause equatorward transport of heat and
poleward transport of momentum. Moreover, these upper-level waves induce a
circulation that opposes the time-mean HC, thus highlighting the role of
tropical waves in driving a traditional overturning flow for uniform SSTs. In
all cases, anomalies associated with the tropical waves closely resemble those
that sum to give the upper-level zonal mean divergent outflow. Through their
ability to modulate tropical rainfall and the related latent heating,
equatorial waves cause considerable hemispheric asymmetry in the HC and impart
synoptic and intraseasonal variability to the tropical overturning circulation.
