  Oscillations are ubiquitous in sunspots and the associated higher
atmospheres. However, it is still unclear whether these oscillations are driven
by the external acoustic waves (p-modes) or generated by the internal
magnetoconvection. To obtain clues about the driving source of umbral waves in
sunspots, we analyzed the spiral wave patterns (SWPs) in two sunspots
registered by IRIS MgII 2796 {\AA} slit-jaw images. By tracking the motion of
the SWPs, we find for the first time that two one-armed SWPs coexist in the
umbra, and they can rotate either in the same or opposite directions.
Furthermore, by analyzing the spatial distribution of the oscillation centers
of the one-armed SWPs within the umbra (the oscillation center is defined as
the location where the SWP first appears), we find that the chromospheric
umbral waves repeatedly originate from the regions with high oscillation power
and most of the umbral waves occur in the dark nuclei and strong magnetic field
regions of the umbra. Our study results indicate that the chromospheric umbral
waves are likely excited by the p-mode oscillations.
