Observation of ocean current response to 1998 Hurricane Georges in the Gulf of Mexico

摘要

The ocean current response to a hurricane on the shelf-break is examined. The study area is the DeSoto Canyon in the northeast Gulf of Mexico, and the event is the passage of 1998 Hurricane Georges with a maximum wind speed of 49 m/s. The data sets used for analy- sis consist of the mooring data taken by the Field Program of the DeSoto Canyon Eddy Intrusion Study, and simultaneous winds ob- served by NOAA (National Oceanic and Atmospheric Administration) Moored Buoy 42040. Time-depth ocean current energy density images derived from the observed data show that the ocean currents respond almost immediately to the hurricane with important differ- ences on and off the shelf. On the shelf, in the shallow water of 100 m, the disturbance penetrates rapidly downward to the bottom and forms two energy peaks, the major peak is located in the mixed layer and the secondary one in the lower layer. The response dissipates quickly after external forcing disappears. Off the shelf, in the deep water, the major disturbance energy seems to be trapped in the mixed layer with a trailing oscillation; although the disturbance signals may still be observed at the depths of 500 and 1 290 m. Verti- cal dispersion analysis reveals that the near-initial wave packet generated off the shelf consists of two modes. One is a barotropic wave mode characterized by a fast decay rate of velocity amplitude of 0.020 s-1, and the other is baroclinic wave mode characterized by a slow decay rate of 0.006 9 s-1. The band-pass-filtering and empirical function techniques are employed to the frequency analysis. The results indicate that all frequencies shift above the local inertial frequency. On the shelf, the average frequency is 1.04f in the mixed layer, close to the diagnosed frequency of the first baroclinic mode, and the average frequency increases to 1.07f in the thermocline. Off the shelf, all frequencies are a little smaller than the diagnosed frequency of the first mode. The average frequency decreases from 1.035f in the mixed layer to 1.02f in the thermocline, implying a trend for the shift in frequency of the oscillations towards f with the depth.