Observational/Numerical Study of the Upper Ocean Response to Hurricanes

摘要

Time evolution and the vertical structure of ocean currents are examined using moored current meter observations from hurricane Frederic in 1979 and data acquired by Airborne eXpendable Current Profilers (AXCP) in hurricane Norbert in 1984. Energetic near-inertial oscillations excited by the passage of these hurricanes have frequencies that are shifted by 1-20 % above the local inertial frequency. These oscillations, evident in the upper 1000 m of the water column, are primarily due to excitation of low-order vertical modes. The first 5 free vertical modes are calculated from the Brunt-Vaisala frequency and the Sturm-Liouville problem. Horizontal velocity eigenfunctions for a flat and a sloping bottom are fit to the demodulated amplitudes observed in Frederic. In the wake of the hurricane, the time evolution of the depth-averaged component plus the first two baroclinic modes explain about 60% and 68% of the near-inertial current variance using the flat- and sloping-bottom models, respectively. Since the AXCP observations were acquired in the high wind regime, the near-inertial response for the 3-dimensional velocities is simulated by projecting a hurricane-like wind stress field onto the first 5 baroclinic modes. Divergence and curl of the wind stress are also convolved with the Green's function for each baroclinic mode. In Norbert the sum of the first 4 near-inertial modes explains over 70% of the observed current variance on the right side of the storm track.