Abstract
Semidiurnal variability of alongshore currents on the inner shelf of the
Southern California Bight is investigated using a seven 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.