In this paper we study cluster algebras $\myAA$ of type $A_2^{ (1)}$. We solve the recurrence relations among the cluster variables (which form a T--system of type $A_2^{ (1)}$). We solve the recurrence relations among the coefficients of $\myAA$ (which form a Y--system of type $A_2^{ (1)}$). In $\myAA$ there is a natural notion of positivity. We find linear bases $\BB$ of $\myAA$ such that positive linear combinations of elements of $\BB$ coincide with the cone of positive elements. We call these bases \emph{ atomic bases} of $\myAA$. These are the analogue of the "canonical bases" found by Sherman and Zelevinsky in type $A_{ 1}^{ (1)}$. Every atomic basis consists of cluster monomials together with extra elements. We provide explicit expressions for the elements of such bases in every cluster. We prove that the elements of $\BB$ are parameterized by $\ZZ^3$ via their $\mathbf{ g}$--vectors in every cluster. We prove that the denominator vector map in every acyclic seed of $\myAA$ restricts to a bijection between $\BB$ and $\ZZ^3$. In particular this gives an explicit algorithm to determine the "virtual" canonical decomposition of every element of the root lattice of type $A_2^{ (1)}$. We find explicit recurrence relations to express every element of $\myAA$ as linear combinations of elements of $\BB$.
Categories: math.RT math.RA