The complex transport phenomena and their effects on the weld pool dynamics and surface rippling in moving gas tungsten arc welding (GTAW) under pulsed currents are studied by using a 3D transient numerical model. The distributions of the melt-flow velocity and temperature, and weld bead formation are simulated. The effects of welding conditions, including welding current waveform, pulse frequency and welding speed, on the weld penetration, formation and final appearance of ripples are discussed. It is found that surface ripples are formed under pulsed current due to the up-and-down weld pool motion, caused mainly by the periodically varied current and solidification rate of weld pool. The results show that for the cases with the same average current, the pulsed current leads to the deeper weld penetration than continuous current, and the higher peak current corresponds to the higher ripples and deeper penetration. The larger pulse frequency results in the more uniform thermal energy distributions on the workpiece and tends to decrease the solidification rate, leading to the more uniform penetration depths, the smaller pitch and height of the ripples. A slow travel speed is helpful to reduce pitch of the ripples but also at the risk of the reduced effective penetration. Finally, a GTAW experiment for the case of continuous current is conducted to validate the modeling predictions in terms of weld width, penetration depth and the formation of ripples. (C) 2015 Elsevier Ltd. All rights reserved.