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Abstract Semidiurnal variability of alongshore currents on the inner shelf of the Southern California Bight is investigated using a 7‐year velocity and pressure time series. Analysis reveals that the ‐frequency alongshore current varies significantly over spatial scales of O(10 km), inconsistent with the expected progressive surface tide. Instead, the observed variability is attributed to the influence of a northward‐propagating, superinertial baroclinic coastal trapped wave (CTW) that generates a quasi‐barotropic flow, defined as the portion of the depth‐averaged alongshore current that is not directly driven by the surface tide. A superinertial CTW model, forced by realistic bathymetry and stratification conditions, suggests that the dominant mode of variability is likely a mode‐1 CTW with a wavelength of approximately 40 km. The observations and model also reveal that seasonal changes in stratification modulate the wavelength and phase speed of the CTW, leading to a seasonal pattern in the phasing of the quasi‐barotropic alongshore flow. These findings provide a new perspective on the complex dynamics governing semidiurnal variability of alongshore currents on the inner shelf of the Southern California Bight and highlight the importance of considering the effects of superinertial CTWs when examining coastal dynamics.