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

Abstract

Ocean surface current measurements from high-frequency (HF) radar are assessed by comparing these data s1 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  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 , slopes of O(1), and rms differences of 7–9 cm s1 salinity units (psu) were associated with a coastally trapped buoyant jet superposed on tidal currents. This tidal forcing consisted of diurnal (K1 m) speeds were 3 and 8 cm s1 surface winds reached 14 m s1 s1 ) and semidiurnal (M2 , and 2and7cms1 . Episodic freshwater intrusions of about 30 practical ) tidal constituents where the surface and subsurface (4 , respectively. During the passage of a nor’easter, near- column. An abrupt wind change following this event excited near-inertial (20.3 h) currents with amplitudes of about 20 cm s1 O(102 , which induced vertical mixing that caused weak stratification in the water rotating clockwise with time and depth. Bulk current shears over 4- and 6-m layers were 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 s1 midshelf. ) at the 25-m mooring where the correlation coefficients exceeded 0.8. Similar results were found at over the inner to