Coastal oceanic environments act as dispersive waveguides for acoustic propagation, which is conveniently described using normal mode theory. Here we propose a framework to describe and estimate the polarization of normal modes, as measured using a single vector sensor in the water column. We introduce the Stokes parameters, four real-valued parameters widely used to describe polarization properties in wave physics, notably optics, but largely ignored in ocean acoustics. Analytic expressions for modal Stokes parameters are derived, and a signal processing framework to estimate them is introduced. The concept of polarization spectrograms, which enables visualization of the Stokes parameters in the time-frequency domain, is notably introduced. The whole framework is illustrated for dispersed impulsive signals using both simulations and experimental marine data collected during the Seabed Characterization Experiment. [Work supported by the Direction Générale de l’Armement and by the Office of Naval Research.]