Synoptic Observations of Near-Inertial Motions in an Enclosed Basin

Abstract

Near-inertial motions are common in the coastal ocean, producing significant currents, isopycnal displacements, and turbulent mixing. Unknown fractions of near-inertial energy are locally dissipated in the mixed layer and converted to offshore propagating internal waves along the coast. Here, we examine near-inertial motions from July to October 2017 at 10 moorings in Lake Superior, which provides a natural laboratory for the coastal ocean. The lake has an approximate two-layer structure and is dominated by near-inertial currents that reach 0.50 m $s^{-1}$ and isopycnal displacements that reach 10 m. Average mode-1 near-inertial kinetic energy (KE) and available potential energy (APE) are 320 J $m^{-2}$ and 10 J $m^{-2}$, respectively. KE is inhibited near the coast and APE has no basin-wide structure. Velocity is separated into a basin-averaged inertial oscillation (IO) and a near inertial wave (NIW) residual. A slab model explains 87% of the IO variance, while the NIW field exhibits 5 W $m^{-1}$ offshore energy fluxes along the coasts, a group speed of 0.1 m $s^{-1}$, and a wavelength of 60 km. The IOs and NIWs contain 200 J $m^{-2}$ and 120 J $m^{-2}$, respectively. We determine that 1.0 mW $m^{-2}$ of wind work goes into to IOs, and 60% of this power is locally dissipated, while the other 40% is converted to NIWs at the coasts. IOs are found to dissipate more rapidly than NIWs (4.4 vs. 7.2 days residence time). NIWs are hypothesized to be important for catalyzing shear instabilities that drive turbulence.