Current Structure Variations Detected by High-Frequency Radar and Vector-Measuring Current Meters

Ocean surface current measurements from high-frequency (HF) radar are assessed by comparing these data to near-surface current observations from 1 to 30 October 1994 at two moored subsurface current meter arrays (20 and 25 m) instrumented with vector-measuring current meters (VMCMs) and Seacat sensors during the Duck94 experiment. A dual-station ocean surface current radar (OSCR) mapped the current fields at 20-min intervals at a horizontal resolution of 1.2 km over a 25 km 3 44 km domain using the HF (25.4 MHz) mode and directly overlooked these moorings. In response to wind, tidal, and buoyancy forcing over 29 days, surface current observations were acquired 95% of the time in the core of the OSCR domain, decreasing to levels of about 50% in the offshore direction. Regression analyses between surface and subsurface measurements at 4 and 6 m indicated biases of 2–6 cm s21, slopes of O(1), and rms differences of 7–9 cm s21. Episodic freshwater intrusions of about 30 practical salinity units (psu) were associated with a coastally trapped buoyant jet superposed on tidal currents. This tidal forcing consisted of diurnal (K1) and semidiurnal (M2) tidal constituents where the surface and subsurface (4 m) speeds were 3 and 8 cm s21, and 2 and 7 cm s21, respectively. During the passage of a nor’easter, nearsurface winds reached 14 m s21, which induced vertical mixing that caused weak stratification in the water column. An abrupt wind change following this event excited near-inertial (ø20.3 h) currents with amplitudes of about 20 cm s21 rotating clockwise with time and depth. Bulk current shears over 4and 6-m layers were O(1022 s21) at the 25-m mooring where the correlation coefficients exceeded 0.8. Similar results were found at the 20-m mooring until the nor’easter when correlation coefficients decreased to 0.5 due to the superposition of storm-induced flows and the buoyant jet, causing the surface current to exceed 90 cm s21 over the inner to midshelf.