Seasonal variation of wind-driven diurnal current cycling on the Texas-Louisiana Continental Shelf

We describe observations of large amplitude wind-driven current oscillations of 24-hr period occurring in the near-surface layer of waters of the Texas-Louisiana continental shelf. The near-surface anti-cyclonic current amplitudes can reach 60 cm s(-1) and represent the largest non-storm induced high-frequency currents on the shelf. These currents can persist for a week or more, as long as driving diurnal winds persist with uninterrupted phase. The latitude of the shelf and the diurnal period of the wind-forcing combine to produce conditions for a near-resonant response of the surface currents to the wind stress, such that, the resulting currents are almost an order of magnitude greater than those found from steady Ekman drift. The oscillations are phase-locked to time of day suggesting a connection to the daily cycle of heating and cooling. The oscillations generally occur during the summer months when there is a shallow mixed layer, strong vertical stratification, maximum insolation, and infrequent frontal passages.