Finescale Shear at 1660 and 2850 dbar over the Mid-Atlantic Ridge in the Eastern Brazil Basin

Abstract

Two drifting Shearmeter instruments have provided records of the magnitude of shear (the magnitude of the vertical derivative of horizontal velocity), measured over a 10-m aperture, in the vicinity of the Brazil Basin Tracer Release Experiment. The floats drifted at roughly 1660 and 2850 dbar, where the buoyancy frequencies were N = 1.1 and 0.5 cph, respectively. Two additional floats provided deep trajectories but no shear data. The records of hourly mean values of shear magnitude (with mean values denoted Sh) exhibit strong temporal variability, with time scales comparable to those of fortnightly tidal modulation. The deep record exhibits stronger mean Sh near the seafloor than above, with the exception of the bin closest to the seafloor; Sh from neither float correlates strictly with tidal forcing. Shallow Sh varies in apparent response to atmospheric forcing. The mean squared shear of the shallow record is 4 times the N-dependent value associated with the Garrett?Munk spectral model, and that of the deep record is 6.4 times the comparable model value. The enhancement of Sh within 1000 m of the bottom suggests a bottom source for internal waves. A moderate correlation between tides and Sh for this small dataset provides an estimate of the relative importance of tidal versus subtidal forcing of such bottom-generated waves, although the data are limited.