  In this paper we consider the following modified quasi-geostrophic equation
  \partial_{t}\theta+u\cdot\nabla\theta+\nu |D|^{\alpha}\theta=0,
  \quad u=|D|^{\alpha-1}\mathcal{R}^{\bot}\theta,\quad x\in\mathbb{R}^2 with
$\nu>0$ and $\alpha\in ]0,1[\,\cup \,]1,2[$. When $\alpha\in]0,1[$, the
equation was firstly introduced by Constantin, Iyer and Wu in \cite{ref
ConstanIW}. Here, by using the modulus of continuity method, we prove the
global well-posedness of the system with the smooth initial data. As a
byproduct, we also show that for every $\alpha\in ]0,2[$, the Lipschitz norm of
the solution has a uniform exponential bound.
