Near-inertial surface currents at Palau: Island blocking, flow enhancement, and wake formation

As part of the ONR funded project Flow Encountering Abrupt Topography (FLEAT), we explore the modification of wind-generated near-inertial oscillations (NIOs) by the island chain of Palau. Surface velocity measurements from a 10-month observational study show far-field near-inertial currents are intermittent, energetic, and clockwise rotational, suggestive of wind-generated NIOs. Closer to topography, near-inertial currents become more rectilinear, with enhanced energy at the northern and southern tips compared to along the north-south oriented coastline of the elongated island chain. A cluster of moorings at the northern tip of Palau is used to estimate vorticity, which increases as near-inertial current speeds increase. Here, we explore the observed behavior of NIOs near island topography using a 1.5-layer reduced gravity model with realizations using idealized (a circle, an ellipse, and two ellipses) and realistic (reflective of Palau) land masks. In all model runs, NIOs are excited by clockwise rotational winds at the inertial frequency. Due to the rotary nature of NIOs, there is a clockwise rotation of currents around the island (regardless of island geometry) that is associated with a clockwise phase propagation of the interface displacement, which oscillates at the inertial frequency with a wavelength equal to the circumference of the topography. Simulations using realistic and idealized land shapes suggest that the observed energy enhancement at the island tips is due to island blocking and flow acceleration of wind-driven NIOs. The realistic model run captures broad features of the observed spatial variability of near-inertial kinetic energy. The simulations also demonstrate how irrotational NIOs can contribute to vorticity generation in the areas of enhanced flows, suggesting that NIOs contribute to island wake dynamics.