  The umbral oscillations of velocity are commonly observed in the chromosphere
of a sunspot. Their sources are considered to be either the external p-mode
driving or the internal excitation by magnetoconvection. Even though the
possibility of the p-mode driving has been often considered, the internal
excitation has been rarely investigated. We report the identification of the
oscillation patterns that may be closely related to the events of internal
excitation from the observations of velocity oscillations in the temperature
minimum region of two sunspots. The velocities were determined from the
spectral data of the Fe I 5435 A line, a magnetically insensitive line, taken
with the Fast Imaging Solar Spectrograph of the 1.6 m Goode Solar Telescope at
the Big Bear Solar Observatory. As a result, we identified 4 oscillation
patterns of 2.0 * 10^3 km coherent size that were clearly identified for about
7.9 minutes with oscillation amplitude of 9.3 * 10^-2 km/s. The power of the
oscillations in these centers was concentrated in the 3 minute band. All the
oscillation centers were located above the umbral dots undergoing noticeable
morphological and dynamical changes that may be regarded as an observable
signature of small-scale magnetoconvection inside the umbrae. Our results
support the notion that magnetoconvection associated with umbral dots inside
sunspots can drive the 3 minute umbral oscillations.
