The Near-Inertial Response to Hurricane Ivan

of a thesis at the University of Miami Thesis supervised by Dr. Lynn Shay No. of pages in text. (95) As part of a US Naval Research Laboratory Experiment (Teague et al., 2007), fourteen Acoustic Doppler Current Profilers (ADCPs) were deployed on the northern rim of the DeSoto Canyon in 2004, as well as four Aanderaa current meters placed at approximately 950 m depth. Preceding placement of this moored array, dozens of conductivity, temperature, and depth probes (CTDs) were ship-cast in the surrounding area. At 0000 UTC on September 16, 2004, hurricane Ivan passed directly over the mooring array, leaving all fourteen moorings intact. Unprecedented measurements of currents in three dimensions and bottom temperature and pressure were recorded during storm passage. Using the HYbrid Coordinate Ocean Model (HYCOM) and the Navy Coupled Ocean Data Assimilation (NCODA) system, background Loop Current features and geostrophically balanced eddies were included in the simulation of Ivan’s response at the fourteen ADCP moorings. With such a large span of data, spatial and temporal resolution of features is possible at the array site. A comprehensive study is conducted of the near-inertial dynamics left behind in Ivan’s wake. Vertical and horizontal wave characteristics such as phase, wavelength, and frequency are determined for the observed and simulated currents. Near-inertial motions were separated into barotropic and baroclinic modes, and rotary kinetic energy of these components was examined. The importance of individual modes was determined through a least squares fit of modal amplitudes to the observed currents. Modal contribution to shear at the mixed-layer base (MLB) was examined in light of rotary kinetic energy output at the same depth. Background relative vorticity was calculated to determine if it affects frequencies and length scales of near-inertial motions. Surface heights from altimetry and depth-averaged mass divergence calculated from HYCOM simulations were used to further understand the barotropic mode and its interactions with the depthindependent current and baroclinic modal response to Ivan. The baroclinic near-inertial amplitude at 50 m is 70 to 80 cm s, and the depthaveraged (barotropic) amplitude is 6 to 8 cm s, consistent with previous studies. Oscillations similar to the ADCP-measured barotropic response are recorded by the Aanderaa current meters at a depth of 950 m. Altimetry and HYCOM output suggest a free surface depression of approximately 15 cm from Ivan and a sinusoidally oscillating depth-averaged mass divergence at near-inertial frequencies. A wavelength of 229 km was calculated for mode one, and carrier frequencies of 1.08f to 1.20f (where f is the local Coriolis parameter) were observed for surface-intensified flows induced by a hurricane. The presence of background relative vorticity may have shifted near-inertial frequencies. The calculation of clockwise (CW) rotary kinetic energy for HYCOM output and ADCP observations indicated that HYCOM slightly overestimated response amplitudes and kinetic energies. An analysis of vertical shear indicated that mode two is responsible for mixing in the upper thermocline according to ADCP data (which does not include the mixed-layer (OML)). By contrast, an examination of HYCOM simulations (which include the OML) indicated that mode three controls shear at the MLB, and hence lowers the Richardson number below the critical value required for vertical mixing. Acknowledgments The author would like to thank the BP Gulf of Mexico Research Institute through the Deep C Consortium at Florida State University for providing funding for this research, Dr. George Halliwell at NOAA AOML for providing HYCOM output, and Mr. William Teague at the Department of Oceanography at the Naval Research Laboratory located at Stennis Space Center for providing ADCP, CTD, and Aanderaa data. Dr. Halliwell also provided invaluable advice as part of the author’s thesis committee, as did the committee chair Dr. Lynn Shay and Dr. Kevin Leaman. Much needed support and teaching were also provided by Dr. Benjamin Jaimes and Ms. Jodi Brewster.