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

Lynn K. Shay Division of Meteorology and Physical Oceanography, RSMAS/University of Miami, Miami, Florida

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Steven J. Lentz Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Hans C. Graber Division of Applied Marine Physics, RSMAS/University of Miami, Miami, Florida

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Brian K. Haus Division of Applied Marine Physics, RSMAS/University of Miami, Miami, Florida

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Abstract

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 × 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 s−1, slopes of O(1), and rms differences of 7–9 cm s−1. 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 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 column. An abrupt wind change following this event excited near-inertial (≈20.3 h) currents with amplitudes of about 20 cm s−1 rotating clockwise with time and depth. 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. 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 s−1 over the inner to midshelf.

Corresponding author address: Dr. Lynn K. Shay, MPO/RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149.

Email: lshay@rsmas.miami.edu

Abstract

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 × 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 s−1, slopes of O(1), and rms differences of 7–9 cm s−1. 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 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 column. An abrupt wind change following this event excited near-inertial (≈20.3 h) currents with amplitudes of about 20 cm s−1 rotating clockwise with time and depth. 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. 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 s−1 over the inner to midshelf.

Corresponding author address: Dr. Lynn K. Shay, MPO/RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149.

Email: lshay@rsmas.miami.edu

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