Near-inertial ocean response to tropical cyclone forcing on the Australian North-West Shelf

The Regional Ocean Modeling System (ROMS) was applied to the Australian North-West Shelf (NWS) to hindcast the ocean response to four intense historical tropical cyclones (TCs). While the four cyclones had very different trajectories across the NWS, all passed within 150 km of a long-term vertical mooring located on the continental shelf in 125 m depth. The observed ocean response at this relatively shallow, Southern Hemisphere shelf site was characterized by the development of a peak in the counter-clockwise (CCW) near-inertial kinetic energy, mixed layer deepening, and subsequent restratification. Strong near-inertial isotherm oscillations were also observed following two of the cyclones. ROMS reproduced these features and also showed that the peak in the near-inertial CCW kinetic energy was observed on the left side of each cyclone trajectory. The time rate of change of near-inertial kinetic energy depended strongly on the storm Rossby number, i.e., defined based on the storm speed, the storm length scale, and the Coriolis frequency. The shallow water depth on the NWS resulted in first, a more rapid decay of near-inertial oscillations than in the deep ocean, and second a generation efficiency (the ratio of near-inertial power to the rate of wind work) of up to 10%, smaller than found for cyclones propagating across deeper water. The total energy put into near-inertial motions is nevertheless large compared to the background tidal energy. The rapid decay of near-inertial motions emphasizes the importance of frictional effects in characterizing the response to cyclone forcing in shallow seas.