  The purpose of this short communication is to show that the theory of
Breit-Wigner resonances can be used as an efficient numerical tool to compute
black hole quasinormal modes. For illustration we focus on the Schwarzschild
anti-de Sitter (SAdS) spacetime. The resonance method is better suited to small
SAdS black holes than the traditional series expansion method, allowing us to
confirm that the damping timescale of small SAdS black holes for scalar and
gravitational fields is proportional to r_+^(-2l-2), where r_+ is the horizon
radius. The proportionality coefficients are in good agreement with analytic
calculations. We also examine the eikonal limit of SAdS quasinormal modes,
confirming quantitatively Festuccia and Liu's prediction of the existence of
very long-lived modes in asymptotically AdS spacetimes. Our results are
particularly relevant for the AdS/CFT correspondence, since long-lived modes
presumably dominate the decay timescale of the perturbations.
