Internal Waves, Shear, and Mixing at the Equator

Data from a month long expedition at the equator in October-November 2012 ("EquatorMix," near 140W) are used to examine relations between high-frequency internal waves, strong shears, and mixing in the upper 200 m. During this time, the Equatorial Undercurrent was unusually strong, peaking over 1.5 m/s relative to the surface at about 120 m depth. This corresponds to a mean shear of order 0.0125 s^-1 (1/80s)! A turbulent eddy rotating at the same rate would have a turn-over time of pi*80s, a little over 4 minutes. This is quite close to the buoyancy frequency over the same depth range, so there exists the possibility of direct interactions between such eddies and internal waves. The mean Richardson number over this depth range is often under 0.5; with such a low mean Richardson number, almost any perturbation can excite turbulence and mixing. One analysis issue in the presence of such strong shear is to separate the effects of vertical advection of the mean shear from the horizontal velocity perturbations due to high-frequency internal waves. With a Phased-Array Doppler Sonar (PADS) looking at a vertical slice on a vertical plane parallel to the ship's track, we get robust estimates of the vertical velocity that are not subject to the troubles of beam-separation becoming comparable to the internal waves' length; integrating this to obtain vertical displacements over a narrow frequency band proves fairly effective at making the above distinction; this also helps us to refine our estimate of the temporal offset between isopycnal depths and the HDSS-based horizontal velocities (due mainly to being measured at different locations along the ship). We find that "packets" of high-frequency internal waves often occur near times of "shear events" associated with mixing, though we can't say whether there is a causal relation (or, if so, in which direction).