Vorticity Recirculation and Asymmetric Generation at a Small Headland With Broadband Currents

Abstract

Fixed acoustic Doppler current profiler (ADCP) velocity measurements are used to investigate headland vorticity generation and recirculation in ∼20 m depth around the small (∼1 km) central California headland Pt. Sal. To reduce vorticity estimation noise, velocities are reconstructed from the first two Empirical Orthogonal Function modes representing ≈73% of the variance. Using fixed ADCPs, depth-averaged vorticity is estimated west and south of Pt. Sal. Only one west-location vorticity component is estimated, leading to negative vorticity bias for northward flow. The south location vorticity is consistent with estimates from parallel vessel transects on one day. The observed depth-averaged flow V was primarily along-bathymetric contours and varied ±0.2 ms−1 across subtidal and tidal frequency bands. The depth-averaged normalized vorticity (Formula presented.) varied ±8 across all frequency bands. Vorticity distributions are skewed with opposite sign at west and south locations, and (Formula presented.) is more likely at the west location. At both locations, depth-averaged vorticity and velocity are inversely related, with relationship asymmetric with sign of V, indicating headland and farther upstream vorticity generation. Binned-mean (Formula presented.) depends on both V and its time-derivative, and indicates vorticity recirculation across the headland. The ∼2 h vorticity adjustment timescale and the associated short excursion distances indicate vorticity generation between south and west locations. Potential vorticity changes across the headland are different for positive and negative V indicating headland asymmetric vorticity generation. Pt. Sal occupies a nondimensional parameter space that is unique relative to other well studied headlands.