Effects of low‐frequency current variability on near‐inertial submesoscale vortices

Abstract

From September 30 to October 14, 1993, and from May 18 to June 13, 1994, ocean surface currents along the inshore boundary of the Florida Current were measured using a land‐based ocean surface current radar (OSCR) deployed along the Florida Keys. A dual‐station OSCR mapped the current fields at 20‐min intervals with a horizontal resolution of 1.2 km over a 30×44 km domain using the high‐frequency (HF, 25.4 MHz) mode. An upward looking, narrowband acoustic Doppler current profiler (ADCP) was moored in 150 m of water seaward of the shelf break concurrently sampling the three‐dimensional current vector between 15 and 130 m at 30‐min intervals. Regression analyses between surface and subsurface currents at 15 m depth from the ADCP indicated biases of 3–12 cm s−1 and slopes of O(0.8–1). Root‐mean‐square differences were about 18 cm s−1 because of a 15‐m‐depth separation between the two measurements, and bulk current shears were O(10−3 s−1) in the Florida Current where maximum velocities exceeded 150 cm s−1 between 40 and 70 m. Surface tidal currents were 2–10 cm s−1 in the diurnal and semidiurnal bands and revealed fine‐scale variability in the tidal ellipses due to topographical variations. Differences in the tidal current amplitudes between the surface and 15 m ranged from 1 to 3 cm s−1 in these constituents and explained more of the current variance with depth. During the second experiment a submesoscale surface feature was detected along the shelf break and progressed eastward at a rate of about 30 cm s−1. This feature was embedded in the near‐inertial wave band where the local inertial period was ≈29 hours that may have been forced by an abrupt change in the wind direction. This transient feature was trapped and advected along the inshore edge of the Florida Current where the anticyclonic vorticity of the subinertial flow shifted the frequency of the nearinertial motions by a few percent of f. These energetic near‐inertial currents of 15–20 cm s−1 had horizontal and vertical wavelengths of 40 km and 50–100 m, respectively.