Tomographic particle image velocimetry and proper orthogonal decomposition are used to investigate the three-dimensional instantaneous flow organization of an incident shock wave/turbulent boundary layer interaction at Mach 2.1. The results show that the incoming boundary layer contains largescale coherent motions, in the form of streamwise-elongated regions of low- and high-speed fluid, similar to what has been observed in incompressible turbulent boundary layers. The instantaneous reflected shock foot pattern appears to respond to these regions as they enter the interaction, conforming to the lowand high-speed regions. Its behaviour can be qualitatively decomposed into two types of patterns: an overall streamwise translation and a spanwise rippling. These observations are substantiated by the global eigenmodes of the proper orthogonal decomposition, which contain subspace representations of this phenomenology.
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