High-frequency observations of wind-forced onshore transport at a coastal site in Baja California

We investigated the effect of the daily sea breeze on the surface flow at Bahia Salsipuedes, Baja California, Mexico. Drifter-tracking experiments were conducted over a 2-week period in July, 1999. Surface drifters were deployed daily and their trajectories tracked from a small boat for 3-7h. Wind speed and direction, as well as the vertical distribution of temperature and current velocities were measured throughout the observation period. Our results indicate that strong winds associated with the daily sea breeze at the study area can produce onshore surface currents with velocities of up to 10cms^-1 (net transport of up to 2km in a diurnal cycle). There was a hyperbolic relationship between onshore wind stress and net onshore transport of the drifters. A decrease in stratification and more consistent pattern of onshore transport were associated with sustained winds >6ms^-1 towards the end of the study. Progressive vector diagrams computed from near-surface currents failed to predict the trajectory of drifters, especially on days where wind forcing was more consistent in terms of direction and magnitude. We discuss the implications of these results regarding the collection of settlement data and their interpretation in terms of larval supply to coastal populations.