Water-Column Inertial and Sub-Inertial Oceanic Response to Hurricane Isaac in the Deepwater Gulf of Mexico



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Tropical Storm Isaac entered the Gulf of Mexico on 27 August 2012 and strengthened to become a Category 1 hurricane shortly before making landfall in southern Louisiana. Hurricane Isaac approached Southwest Pass near the mouth of the Mississippi River on August 29, 2012 at 00:00 UTC. The center of the storm then moved westward before making landfall eight hours later at Port Fourchon, LA. On August 28, 2012 at approximately 18:00 UTC, Hurricane Isaac passed directly over the center of a mooring array in a northwesterly trajectory. As part of the Gulf Integrated Spill Research Program, six deepwater moorings featuring upward-looking 75 kHz Acoustic Doppler Current Profilers were deployed in water depths between 836 m and 1690 m in the Mississippi Fan region of the northern Gulf of Mexico in July 2012. Each of the six moorings featured 3 Aanderaa RCM current meters. One current meter was located near the bottom, the next positioned approximately 180 m shallower, then the last positioned an additional 200 m shallower. One mooring featured four InterOcean S4A current meters positioned 100 m apart between 790 m and 1090 m. Maximum current speeds of 41.3 cm/s at 100 m, 35.5 cm/s at 300 m, and 32.7 cm/s at 500 m depth were observed during the passage of the Hurricane Isaac. Maximum bottom current speeds measured from Aanderaa RCMs ranged between 16.1 cm/s at 1645 m depth and 34.0 cm/s at 1020 m depth. Inertial band oscillations (1/2-2 days) are seen to 800 m depths, with energy propagation speeds on the order of 30 m/day vertically and 5.7 km/day horizontally. A blue shift in the effective frequency to 1.11f is observed in the near-inertial band in the wake of Hurricane Isaac. Wavelet analyses of the time-series records indicate two subinertial oscillations (2-5 days and 5-12 days) initiated throughout the water-column at the time of the storms closest approach that persist for approximately one week. Each sub-inertial band response was fundamentally different from the near-inertial response to Hurricane Isaac and showed a strong barotropic response.