Complexity of near-bottom ocean currents in the northern Gulf of Mexico from a sea-bed ADCP

Abstract

The oceanography of the northern Gulf of Mexico is complex. The Loop Current dominates the eastern half of the Gulf and a-periodically shed anti-cyclonic Loop Eddies that move into the western Gulf of Mexico. Wind-generated inertial waves move throughout the water column, carrying surface energy to depth. Stronger inertial currents associated with storm events, including cold front passages. Energy from tropical cyclone winds has been shown to impact near-bottom currents in water nearly two kilometers deep. Topographic Rossby Waves (TRWs), apparently generated by interactions associated with Loop Current and Loop Eddy processes, travel through the northern Gulf of Mexico and accelerate near-bottom currents. Current profile data from a bottom-mounted acoustic Doppler current profiler (ADCP) at 1300 meters depth in Green Canyon Block 645, south of the Atchafalaya Bay in Louisiana, are analyzed to investigate these phenomena. The seabed mounted 75 kHz ADCP was installed in June 2005 in response to a Notice to Lessees by the Bureau of Ocean Energy Management to collect data to insure the safety of drilling and production platforms. The innovative design, supplying power and recovering data from the ADCP via a tether, allows almost continuous receipt of data. The 75 kHz ADCP collects data in 20 meter bins to 500 meters above the ocean bottom at 20 minute intervals. As with all current profile data required by the NTL, the data are transmitted to the National Data Buoy Center (NDBC) where they are quality controlled and transmitted to the world's numerical prediction centers. Copies of the raw binary data and the quality controlled data are maintained at NDBC. Statistical and spectral analysis indicates a complex current structure temporally and vertically. Yearly averaged currents are approximately 5 cm/s from May 2005 through May 2010. At 1200 meters, the yearly averaged data range between 3.8 and 5.0 cm/s. The spectral analysis indicates that energy at 1000 and 1200 meter depths are dominated by processes near the inertial frequency (26.08 hours) and in the 17–20 day band that is related to Topographic Rossby Waves (TRWs). At 1000 meters, the energy in the two bands is approximately equal. Energy in the TRW frequency band exceeds that in the inertial frequency band at 1200 meters depth. Although the inertial currents are obvious in the data record, they weaken with depth. Individual time series are investigated to determine the impact of Loop Current Eddies and Gulf of Mexico hurricanes Katrina, Rita, Gustav, and Ike. Periodic currents at approximately 20-day intervals are obvious in the time series data following Loop Eddy generation. A common Loop Eddy path following eddy shedding is near the Green Canyon 645 site. Time series records show intense near-bottom currents. The highest speed currents at this location, exceeding 35 cm/s are associated with the passage of nearby Loop Eddies. Hurricanes also pass near the Green Canyon site and the near bottom currents respond on a delayed basis.