  High-resolution observations of the solar chromosphere and transition region
often reveal surge-like oscillatory activities above sunspot light bridges.
These oscillations are often interpreted as intermittent plasma jets produced
by quasi-periodic magnetic reconnection. We have analyzed the oscillations
above a light bridge in a sunspot using data taken by the Interface Region
Imaging Spectrograph (IRIS). The chromospheric 2796\AA{}~images show surge-like
activities above the entire light bridge at any time, forming an oscillating
wall. Within the wall we often see that the Mg~{\sc{ii}}~k 2796.35\AA{}~line
core first experiences a large blueshift, and then gradually decreases to zero
shift before increasing to a red shift of comparable magnitude. Such a behavior
suggests that the oscillations are highly nonlinear and likely related to
shocks. In the 1400\AA{}~passband which samples emission mainly from the
Si~{\sc{iv}}~ion, the most prominent feature is a bright oscillatory front
ahead of the surges. We find a positive correlation between the acceleration
and maximum velocity of the moving front, which is consistent with numerical
simulations of upward propagating slow-mode shock waves. The Si~{\sc{iv}}
1402.77\AA{}~line profile is generally enhanced and broadened in the bright
front, which might be caused by turbulence generated through compression or by
the shocks. These results, together with the fact that the oscillation period
stays almost unchanged over a long duration, lead us to propose that the
surge-like oscillations above light bridges are caused by shocked p-mode waves
leaked from the underlying photosphere.
