An annular synthetic jet was investigated experimentally, both with and without an opposing impingement wall. The experiments involved smoke visualization and mass transfer measurement on the wall by means of naphthalene sublimation technique. Two qualitatively different flow field patterns were identified, depending upon the driving amplitude level. With small amplitudes, vortical puffs maintain their identity for a relatively long time. If the amplitudes are large, breakdown and coalescence of the vortical train is much faster. Also the resultant mass transfer to the impingement wall is then much higher. Furthermore, a fundamental change of the whole flow field was observed at the high end of the investigated frequency range, associated with radical reduction of the size of the recirculation bubble. (C) 2003 Elsevier Science Ltd. All rights reserved.