  We report three-minute oscillations in the solar chromosphere driven by a
strong downflow event in a sunspot. We used the Fast Imaging Solar Spectrograph
of the 1.6 m New Solar Telescope and the Interface Region Imaging Spectrograph
(IRIS). The strong downflow event is identified in the chromospheric and
transition region lines above the sunspot umbra. After the event, oscillations
occur at the same region. The amplitude of the Doppler velocity oscillations is
2 km/s, and gradually decreases with time. In addition, the period of the
oscillations gradually increases from 2.7 minutes to 3.3 minutes. In the IRIS
1330 slit-jaw images, we identify a transient brightening near the footpoint of
the downflow detected in the Ha+0.5A image. The characteristics of the
downflowing material are consistent with those of sunspot plumes. Based on our
findings, we suggest that the gravitationally stratified atmosphere came to
oscillate with three minute period in response to the impulsive downflow event
as was theoretically investigated by Chae & Goode(2015).
