Dynamic effects are commonly added offline to pre-recorded videos. In this work, we propose to synthesize online dynamic effects with controlled vector fields by using mobile augmented reality. By modelling typical primitives of a flow field to oriented markers, we use image detection and tracking techniques to build and control a virtual vector field in the real world. Virtual objects are then added and animated in real time. We prototype our system and the results show the possibility that dynamic effects can be created in mobile augmented reality to enhance the visual communication with the real world.