A study of tropical cyclone influence on the generation of internal tides

Abstract

An investigation is made into modifications of the semidiurnal internal tide on the Western Australian Shelf by a passing tropical cyclone. Current mooring observations are presented, taken during the passage of Tropical Cyclone Bobby (1995) over these mooring locations. The observations reveal large vertical excursions at near‐inertial frequency of the isotherm during and following the cyclone's passage. This coincides with diminishing semidiurnal baroclinic tides. To gain dynamical insight into the observations, a fully three‐dimensional, free surface, nonlinear, hydrostatic model is applied to the Western Australian Shelf. The model is initialized with realistic stratification and forced by representative tides and cyclone winds. The wind‐forcing is derived from an idealized analytical cyclone model prescribed from observed cyclone track and central pressure data, and the tidal forcing allows for the generation of internal tides. The cyclone modifies stratification by turbulent mixing, upwelling/downwelling and density advection up or down the shelf slope. This results in significant changes to internal tide characteristic paths and hence the internal tide generation process on the continental slope. Mixing is enhanced over the shelf due to shallow topography with respect to the shelf slope region creating a strong density front at the shelf edge. Observations and modeling show that the water column response to the cyclone passage is most abrupt in shallow water at 125 m, then at 300 m depth. The model reproduces the main features of the observed inertial oscillations concomitant with the dampening of M2 internal tides after the passage of the cyclone.