SS: Metocean: Measurements and Modeling Measurements of Topographic Rossby Waves along the Sigsbee Escarpment

Abstract

Abstract Observations of strong currents with wave-like behavior have been documented from specific sites near the sea floor along the Sigsbee Escarpment (SE) in the northern Gulf of Mexico. The strongest of these currents reached 100 cm/s, with currents exceeding 40 cm/s observed multiple times. These currents tend to occur when the Loop Current or its separated eddies are in the vicinity, typically to the east of the SE. The currents are oriented dominantly along isobaths with fluctuations of roughly 10-day periods, are decoupled from the upper ocean flow, and appear intensified over the lower part of the water column. These traits are generally consistent with Topographic Rossby Waves. Observational and numerical modeling efforts were funded in 2008 by DeepStar to better understand these events and their dynamics. This paper summarizes the observations, as well as numerical model hindcasts and process studies. Several events of moderately intense currents with quasi-periodic fluctuations were observed in early 2009 while the Loop Current was positioned to the east of the SE. Results show that the model hindcasts are capable of replicating many of the characteristics of the deep currents along the SE. Analysis of the model data show that the energy for these strong deep currents along the SE may originate to the east over the Mississippi Fan under the strongest part of the Loop Current when it is fully extended over the region to the east of the SE. Introduction The Sigsbee Escarpment (SE) (Figure 1) runs through the Walker Ridge and Atwater Valley blocks where many new discoveries are being made and developed (e.g. Chevron's Jack/St. Malo and Big Foot, and BPs Atlantis). While the covers a relatively few number of blocks, pipelines of blocks developed further south can be affected by flow dynamics along the SE. Strong near-bottom currents, with instantaneous speeds exceeding 100 cm/s, have been observed along the SE. Such strong currents can cause costly delays in drilling and present a major engineering challenge to the design of production risers and pipelines. Despite their importance, only a handful of strong (> 50 cm/s) events have been measured to date, and no well validated numerical model has previously been developed. To help fill this gap, DeepStar funded two efforts in 2008: deployment of four moorings for one year along the SE and development of a numerical model. In addition to the DeepStar measurements, Chevron also had overlapping measurements at two other sites along the Escarpment thus providing an antenna that has provided more detail than any previous measurement program. This paper summarizes the results of the combined measurements and modeling. Observations along the upper portion of the SE beginning in September, 1999 (Hamilton and Lugo-Fernandez, 2001), showed strong currents with fluctuations resembling those associated with Topographic Rossby Waves (TRWs) near the 2000 m bottom with speeds estimated from the low-pass filtered time series in excess of 85 cm s-1 (and over 100 cm s-1 in the unfiltered data record shown for the MMS I2 mooring in Nowlin et al. (2001)) (Figure 2). Additional direct measurements of strong currents along the SE have been reported, with an extensive data set presented by Donohue et al. (2006). During this project, currents of just over 50 cm/s were measured near the bottom. In July, 2005, ROV measurements conducted by Chevron at the Big Foot prospect location along the SE captured very strong speeds near the bottom (Cooper and Stear, 2009). It is interesting that each of the documented strong events occurred when a strong loop current eddy (LCE) or meander of the Loop Current (LC) was in the vicinity, particularly with the SE near its western edge.