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 array s (20 and 25 m) instrumented with vector-measuring current meters (VMC Ms) and Seacat sensors during the Duck94 experiment. A dual-station oc ean surface current radar (OSCR) mapped the current fields at 20-min i ntervals at a horizontal resolution of 1.2 km over a 25 km X 44 km dom ain using the HF (25.4 MHz) mode and directly overlooked these mooring s. In response to wind, tidal, and buoyancy forcing over 29 days, surf ace current observations were acquired 95% of the time in the core of the OSCR domain, decreasing to levels of about 50% in the offshore dir ection. Regression analyses between surface and subsurface measurement s at 4 and 6 m indicated biases of 2-6 cm s(-1) slopes of O(1), and rm s differences of 7-9 cm s(-1). Episodic freshwater intrusions of about 30 practical salinity units (psu) were associated with a coastally tr apped buoyant jet superposed on tidal currents, This tidal forcing con sisted of diurnal (K-1,) and semidiurnal (M-2,) tidal constituents whe re the surface and subsurface (4 m) speeds were 3 and 8 cm s(-1) and 2 and 7 cm s(-1) respectively. During the passage of a nor'easter, near ; surface winds reached 14 m s(-1), which induced vertical mixing that caused weak stratification in the water An abrupt wind change followi ng this event excited near-inertial (approximate to 20.3 h) currents w ith amplitudes of about 20 cm s(-1) rotating clockwise with time and d epth. Bulk current shears over 4- and 6-m layers were O(10(-2) s(-1)) at the 25-m mooring where the correlation coefficients exceeded 0.8, S imilar results were found at the 20-m mooring until the nor'easter whe n correlation coefficients decreased to 0.5 due to the superposition o f storm-induced Rows and the buoyant jet, causing the surface current to exceed 90 cm s(-1) over the inner to midshelf.