Current variability on a narrow shelf with large ambient vorticity

Abstract

Surface and subsurface currents and stratification were observed on and near the narrow shelf off Fort Lauderdale, Florida, in June-August 1999. The Florida Current flowed past, occasionally on the shelf, with speeds at times exceeding 2 m s-1 only 8 km offshore. The typical vorticity associated with the lateral shear of the Florida Current was 4f, where f is the local Coriolis parameter. Two dominant modes of higher frequency current variability were embeded in this low-frequency flow, a 10-hour signal with amplitudes of up to 0.5 m s-1 and an equally as strong, almost rectilinear signal with a 27-hour period, the same as the local inertial period. Both signals were propagating northward along the shore with wavelengths of ∼27 and ∼170 km for the 10- and 27-hour signals, respectively, and with corresponding phase speeds of 0.85 and 1.7 m s-1. The phase trend in east-west direction of the 10-hour signal is consistent with unstable, growing waves. The 10- and 27-hour signals appear dominantly barotropic near the coast out to depths of at least 50 m, but the 10-hour signal displayed a 180° phase change between surface and bottom farther away from the shelf break at 160 m depth. The two dominant signals cannot be attributed to barotropic instability.