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We present the first results of sunspot oscillations from observations by theInterface Region Imaging Spectrograph. The strongly nonlinear oscillation isidentified in both the slit-jaw images and the spectra of several emissionlines formed in the transition region and chromosphere. We first apply a singleGaussian fit to the profiles of the Mgii 2796.35 {\AA}, Cii 1335.71 {\AA}, andSi iv 1393.76 {\AA} lines in the sunspot. The intensity change is about 30%.The Doppler shift oscillation reveals a sawtooth pattern with an amplitude ofabout 10 km/s in Si iv. In the umbra the Si iv oscillation lags those of Ciiand Mgii by about 3 and 12 s, respectively. The line width suddenly increasesas the Doppler shift changes from redshift to blueshift. However, wedemonstrate that this increase is caused by the superposition of two emissioncomponents. We then perform detailed analysis of the line profiles at a fewselected locations on the slit. The temporal evolution of the line core isdominated by the following behavior: a rapid excursion to the blue side,accompanied by an intensity increase, followed by a linear decrease of thevelocity to the red side. The maximum intensity slightly lags the maximumblueshift in Si iv, whereas the intensity enhancement slightly precedes themaximum blueshift in Mgii. We find a positive correlation between the maximumvelocity and deceleration, a result that is consistent with numericalsimulations of upward propagating magnetoacoustic shock waves.

High-resolution Observations of the Shock Wave Behavior for Sunspot Oscillations with the Interface Region Imaging Spectrograph