Stirring and Mixing in the Wake of Velasco Reef, Palau

Abstract

AbstractOver 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.