  The main objective of this article is to study the effect of the moisture on
the planetary scale atmospheric circulation over the tropics. The modeling we
adopt is the Boussinesq equations coupled with a diffusive equation of humidity
and the humidity dependent heat source is modeled by a linear approximation of
the humidity. The rigorous mathematical analysis is carried out using the
dynamic transition theory. In particular, we obtain the same types of
transitions and hence the scenario of the El Ni\~no mechanism as described in
\cite{MW2,MW3}. The effect of the moisture only lowers slightly the magnitude
of the critical thermal Rayleigh number.
