Vertical structure of currents on the northern Grand Bank—a view from a bottom mounted Acoustic Doppler Current Profiler

Abstract

Data obtained from a bottom mounted Acoustic Doppler Current Profiler (ADCP) deployed on the northern Grand Bank for 70 days from mid-March to early July of 1990 were used to study the vertical structure of the currents. Good data were obtained from 70% of the water column sampled by the instrument. Only data near the sea surface were contaminated by the sidelobes of the acoustic signals, and data close to the sea bottom had large errors, possibly caused by the extended transducer ringing due to a protective cover. To determine the vertical current structure in different frequency bands, a rotary spectral analysis and empirical orthogonal function analysis were performed. The results show that the tidal and low-frequency (2–12 days) currents are predominantly barotropic with no significant change in the vertical structure over mooring period. The low-frequency currents had a stronger clockwise component than counter-clockwise component. The asymmetry of the rotary motion is attributed to more frequent storms passage to the north of the ADCP site than to the south of the site. The inertial motion has no barotropic mode. The current structure is a two-layer flow with currents of similar magnitudes but opposite directions in the two layers. Such a structure suggests direct forcing by moving storms. The time change of the structure over the mooring period is consistent with this hypothesis. Implications of the observed current structure for numerical modeling of currents on the Grand Bank are discussed.