Inertial and tidal shear variability above Reykjanes Ridge

Abstract

Yearlong 75 kHz acoustic Doppler current profiler (ADCP) data were obtained well above Reykjanes Ridge (northern extension of the Mid-Atlantic Ridge (MAR)). The area is characterized by relatively large semidiurnal tidal (‘ D 2’) currents that have (at lunar M 2) more than half a decade larger variance than inertial ( f) currents. However, despite the relatively weak near-inertial kinetic energy, its vertical current shear shows larger magnitudes than at M 2 in an otherwise flat f− D 2 band limited between frequencies 0.74 and 1.35 f, which equals the inertio-gravity wave bounds [ σ min, σ max]( N= f). N represents the buoyancy frequency. The shear in this band dominates all shear computed at 20 m effective vertical scale. As the kinetic energy spectrum peaks at M 2, but not (significantly) at S 2 and N 2, a difference in tidal (and inertial) scales and hence sources is observed. M 2-tides contribute mostly to large-scale coherent motions. The dominant incoherent f− D 2 shear is highly variable in time (∼2-day periodicity). Furthermore, inertial and tidal shear are more or less completely separated in space and time, each occurring in different layers in the vertical. The thin shear layers reflect the rapidly varying short vertical scale N s profile, to within the ∼20 m limitation of ADCP data, rather than the large-scale smooth N L. In each of large- N s layers Ri≈1, probably. The yearlong smoothed shear magnitude follows N L, but only as stable Ri≈5. The shear polarization is more circular than rectilinear, albeit varying with time, and highly symmetric around f. During transitions, e.g., between stratified and homogeneous layers and between waves from varying sources, near-circular motions can generate near-rectilinear shear in the direction of wave propagation (in the direction of the minor axis of the current ellipse). This contrasts with the possibility of near-rectilinear barotropic oscillatory motions generating near-circular shear under viscosity in shallow seas.