Abstract
Information about near-surface currents provided in near-real-time is required in a variety of applications. However, direct measurements of near-surface currents are scarce. In the Gulf of Mexico, the oil industry has been collecting current profiles from deepwater rigs, platforms, and moorings in accordance with BSEE Notices to Lessees (NTLs) since 2005. On the rigs and some platforms, these profiles are usually collected with long-range ADCPs positioned beneath the rig/platform keel, so the measured current profiles typically extend from 70-80m below the surface to as deep as 1000m. Unfortunately, only a few stations are equipped to provide good-quality near surface current data. This study assesses the validity and limitations of some simplistic current profile extrapolation algorithms, including approximation of the near surface current by "slab" flow and profile extrapolation algorithms derived from the shape of the measured subsurface profile or from a generic current profile shape.
The analysis is based on data collected by a long-term mooring anchored in approximately 1400m of water in the north central Gulf of Mexico (Green Canyon 782, NDBC ID 42369) and on a year-long data set from a mooring deployed in Keathly Canyon in approximately 1900m of water. The available data include periods of Loop Current Eddies (LCEs), weak cold-core eddies, relatively strong flows characterized by a subsurface current intensification, and high-amplitude inertial oscillations.
Velocity profiles were mapped onto fixed depth levels. The subsurface section of the profiles was used to extrapolate mappings from 80-100m to the near-surface. Our analysis shows that, in the winter and in the absence of an LCE, currents in the upper 80m of the water column were quasi-homogeneous. Although current speed near the surface is likely to be higher than current speed at 80m, there is a 90% probability that this difference does not exceed 10cm/s and a slab flow extrapolation is reasonable. When an LCE is present, the accuracy of extrapolation is better if the shape of the near-surface current profile is based on deeper observations. This is also true for the stratified summer water column. The uncertainties of profile extrapolation are quantified for each data set and for each flow regime.
