Modeling near-inertial waves in Lake Superior

Abstract

Numerical models were used to investigate basic properties of near-inertial waves in large lakes. The Regional Oceanographic Modeling System (ROMS) was used to run a hierarchy of models to investigate the role wind stress (idealized and semi-realistic), thermal structure (isothermal and two layer), and bathymetry (flat bottom and spatially varying) have on near-inertial waves in lakes. The currents produced by idealized forcing were compared with observations from the Lake Superior mooring array. The inertial response was found to be very sensitive to the duration of the impulse. Idealized wind stress acting for half an inertial period puts a substantial amount of energy into the inertial band of frequencies and little energy into other frequencies.The first models considered were flat bottom square basins with closed boundaries and a two layer thermal structure. The inertial kinetic energy was weakest near the shore and was greatest near the center of the basin. Very little inertial kinetic energy was put into currents when using a uniform thermal structure. Modeling with periodic boundary conditions produced pure inertial oscillations in the upper mixed layer. No thermocline displacements were observed when using periodic boundary conditions, which shows that a transport divergence is necessary to initiate internal waves.Lake Superior was modeled during the period of July 1, 2011 to September 19, 2011 and output from the model was compared with observations from the Lake Superior mooring array. A slow, approximately 30 day, counterclockwise rotation in the direction of wave propagation was observed in the model, which corroborates previous observations made in Lake Superior. Modeling suggests that near-inertial surface kinetic energy is enhanced over the Superior shoal, possibly due to a convergence of waves atop the shoal. The magnitude of the modeled currents agreed well with observations but phase did not. In order to accurately model near-inertial events in large lakes a higher resolution wind field may be needed and surface heat fluxes need to be included in the model.