New oceanographic observations of the Gulf of Mexico deep waters

Lugo-Fernández, Alexis, Minerals Management Service, New Orleans, LA; Peter Hamilton, Science Applications International Corporation, Raleigh, NC; W. R. Johnson, Minerals Management Service, Herndon, VA

Abstract

The deep water oceanography of the Gulf of Mexico is being recognized as much more dynamic than expected. Available data and modeling results suggest that in the simplest terms, the Gulf of Mexico behaves as a two-layer system with waters above 1,000 m uncoupled from the lower layer. The Loop Current (LC) dominates circulation in the upper layer, Loop Current rings (LCR), smaller scale eddies, and wind. While basic flows associated with the LC, LCR, and winds are relatively well understood, smaller eddies are less understood. Recent data show rich fields of eddies having sizes of 30-150 km that significantly influence the LCRs and currents near the shelf edge. More recently, the oil and gas industry has been concerned with jet-like currents lasting about one day and reaching maximum speeds of 100 cm/s at 100-300 m. These currents, however, are highly infrequent and thus, hard to study. The Minerals Management Service is trying to unravel the mechanisms responsible for their existence.

Circulation in the lower layer is much less understood. Oceanographers measured near-bottom currents of #19 cm/s at depths of ~3,000 m in the early 1970's. A decade ago, currents of ~40 cm/s were reported as vertically unchanged at depths of 1,000 to 3,000 m, but showing near-bottom intensification (Hamilton 1990). These observations were explained as topographic Rossby waves (TRWs) with periods of 20-30 days, spatial scales of 100-200 km, traveling westward at about 9 km/day. Recent current measurements south of the Mississippi Delta in 2,000 m reveal even stronger speeds (70-90 cm/s) 11 m above the bottom. These flows exhibited TRWs characteristics; however, the periods were only ~10 days with spatial scales of 70 km. The vertical shear in the lower 1,000 m was small suggesting weak near-bottom trapping. Industry has recently made similar observations of speeds and vertical structures. In this lower layer, models suggest the presence of deep cyclone-anticyclone pairs that move westward and interact with the bottom topography creating intense flows. Model results also show a band of high kinetic energy between the 1,000 and 3,000 m, isobaths indicative of TRWs.

Aside from these measured and modeled results, direct observations of large furrows, presently active, suggest the presence of strong (~100 cm/s) near-bottom currents. In addition, lack of sediment cover on bottom rocks and high rates of sediment movement suggest the presence of strong currents near the region of the Sigsbee escarpment. The role of the escarpment's steep slope in the generation of large amplitude TRWs is at present unknown. It still needs to be established whether the observed strong currents are the cause of the large furrows.