Semidiurnal inner shelf flow in the Southern California Bight

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 M2-frequency alongshore current varies significantly over spatial scales of O(10km), inconsistent with the expected progressive surface tide. Instead, the observed variability is attributed to the influence of a northward-propagating, superinertial coastal trapped wave (CTW) that generates a quasi-barotropic (QBT) flow, defined as the portion of the depth-averaged alongshore current that is out of phase with 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 50km. 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.