This study investigates the impact of quasi-stationary atmospheric rivers (QSARs)--slow-moving, narrow moisture bands--on Rossby wave activity in the subtropical Pacific and Atlantic Oceans. While most Rossby waves are known to originate from large-scale topography, land-sea thermal contrasts, and baroclinic instability in extratropics as well as tropical convection, we highlight a less-understood source: subtropical QSARs. We find that the latent heat released by QSARs plays a key role in amplifying downstream Rossby waves. These amplified waves then propagate eastward along the subtropical jet stream, leading to persistent wave patterns. Our findings highlight a previously underappreciated mechanism through which subtropical moisture transport shapes large-scale Rossby wave patterns.