*Journal of Climate*,
**23**, 1760-1778.

Gilles Bellon

Department of Applied Physics and Applied Mathematics,
Columbia University, New York, NY.

Adam H. Sobel

Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences,
Columbia University, New York, NY.

**Abstract**

A model of intermediate complexity based on quasi-equilibrium theory --- a version of
the Quasi-equilibrium Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture
and temperature --- is used in a zonally-symmetric aquaplanet configuration to study the
sensitivity of the Hadley circulation to the Sea Surface Temperature (SST) forcing. For
equatorially symmetric SST forcing with large SST gradients in the tropics, the model
simulates the classical double Hadley cell with one equatorial Intertropical Convergence
Zone (ITCZ). For small SST gradients in the tropics, the model exhibits multiple equilib-
ria, with one equatorially symmetric equilibrium and two asymmetric equilibria (mirror
images of each other) with an off-equatorial ITCZ.

Further investigation of the feedbacks at play in our model shows that the vertical struc-
ture of the wind is crucial to the existence and stability of the asymmetric equilibria in
our model. The free-tropospheric moisture-convection feedback must also be sufficiently
strong to sustain asymmetric equilibria. The symmetry-breaking mechanism and result-
ing multiple equilibria have their origin in the column multiple equilibria that can be
described by a single column model using the weak temperature gradient approximation.
An additional experiment using an SST forcing with a relative minimum at the equator
shows that the feedbacks controlling these multiple equilibria might be somewhat relevant
to the double ITCZ problem.