Turbulence Measurements in a Gulf of Mexico Warm-Core Ring

Oceanographic measurements were made in a Loop Current Eddy in the Gulf of Mexico to characterize the turbulence associated with these eddies. Measurements were made within the eddy, and across the strong frontal boundary delineating the eddy from the surrounding waters. The survey was conducted August 23–30, 2003, from the R/V Pelican. The towed vehicle, the TOMI, was equipped with a special 300 kHz acoustic Doppler current profiler (Medusa ADCP) that had its four beams directed fore, port, starboard and down. The along-beam velocities resolved structures with wavelengths of 4 to 60 m. The vehicle also carried shear probes for measuring velocity fluctuations in the dissipation range (0.5 to 100 cycles per meter), and other environmental sensors for measuring temperature, salinity, depth and vehicle orientation. Ship equipment included a 75 and 300 kHz hull-mounted ADCP, CTD, and meteorological sensors. Tows were conducted at 25, 50, 100 and 150 m depths around the northern edge of the Loop Eddy in currents of up to 1.7 m s⁻¹. Turbulence was detected with the shear probes, but mostly in the 130–150 m depth range around the local salinity maxima. The level of turbulence is weak and it is distributed intermittently in both space and time. The most energetic events of turbulence have eddy scales of at most 4 meters and velocity scales of only 1 cm s⁻¹. The typical and average values are more than 10 times smaller. The concurrent measurements of velocity with the Medusa ADCP did not reveal any signals significantly larger than the noise level of this instrumentation, namely 2 cm s⁻¹. Overlap averaging of the forward directed beam reduced the noise level to 0.5 cm s⁻¹ but still failed to reveal real environmental signals. This “null-result” is consistent with the simultaneous measurements taken with the shear probe. These low levels of turbulence are also consistent with reports of measurements in the Gulf Stream, the Florida Current, and a Gulf Stream Warm-Core Ring. Funding was provided by the DeepStar oil industry research consortium. Complete details of the program are provided in Reference [6].