Decay of weak solutions to the 2D dissipative quasi-geostrophic equation

We address the decay of the norm of weak solutions to the 2D dissipative quasi-geostrophic equation. When the initial data is in $L^2$ only, we prove that the $L^2$ norm tends to zero but with no uniform rate, that is, there are solutions with arbitrarily slow decay. For the initial data in $L^p\cap L^2$, with $1\le p<2$, we are able to obtain a uniform decay rate in $L^2$. We also prove that when the $L^{\frac{2}{2\alpha -1}}$ norm of the initial data is small enough, the $L^q$ norms, for $q>\frac{2}{2\alpha -1}$ have uniform decay rates. This result allows us to prove decay for the $L^q$ norms, for $q \geq \frac{2}{2 \alpha -1}$,whentheinitialdataisin$L^2 \cap L^{\frac{2}{2 \alpha -1}}$.