Stirring and Mixing in the Wake of Velasco Reef, Palau.

Over 2 years, underwater gliders measured currents, temperature, and salinity along lines 40 km up‐ and downstream of the abrupt topography of Palau to examine topographic effects on the incident flow. The mean flow is westward and is associated with the North Equatorial Current. Salinity on isopycnals is a passive tracer. During westward flow, this tracer variance is elevated downstream in the strongest flow north of the topography at scales of 36 km (evaluated from 12 to 60 km) consistent with a wake eddy at the scale of the topography. During eastward flow, variance is elevated downstream at all measured scales. This result suggests eddies are trapped at the topography because westward eddy propagation is opposed by the incident eastward flow. This time‐mean tracer and current structure up‐ and downstream are used to estimate terms in the advection‐diffusion equation. The mean isopycnal diffusivity is about 17 times greater than background values at 36‐km scales (and 40 times greater within 20 km of the topography), while the mean diapycnal diffusivity is 100 times greater than background values for this latitude. The contrast between up‐and downstream salinity on isopycnals can reach 0.2 psu over 5–10 km, which is seen in the glider measurements and one ship‐based survey. This contrast arises due to eddies detaching from the North Equatorial Countercurrent or its recirculations. These episodes are associated with westward‐propagating, intraseasonal Rossby waves. Plain Language Summary: Two years of underwater glider measurements were repeated along lines up‐ and downstream of Palau to examine topographic effects on the incident flow. For over half of the time, in the upper 200 m, the flow was westward in the North Equatorial Current. Previous work showed an eddy of about 40 km extent formed in the wake of the topography during westward flow. A passive tracer displays elevated variance at these scales as the westward flow passes the north point of Palau. The tracer is salinity on a density surface; density differences lead to currents and are active, but salinity differences at a given density have no effect on currents and instead are stirred by the currents. Diffusion is estimated from the difference in the mean tracer structure between up‐ and downstream. Diffusion along isopycnals is about 17 times greater than background at the scale of the wake eddies. Diffusion across isopycnals is 100 times greater than background values and is often associated with current shear or internal waves. Episodes of tracer contrast between up‐ and downstream come from either recirculation of North Equatorial Countercurrent or eddies detaching from it. These episodes appear linked to westward‐propagating, intraseasonal waves on the thermocline. Key Points: Thermohaline variability on isopycnals is elevated at scales of 12–60 km in westward mean flow past topography consistent with wake eddiesUp‐/downstream thermohaline differences on isopycnals coincide with westward‐moving eddies, which seem related to intraseasonal Rossby wavesMean isopycnal (diapycnal) diffusivities are 17 (100) times greater than background values within 40 km of the topography [ABSTRACT FROM AUTHOR]