Shelf-Slope Exchanges in the Northern Gulf of Mexico
Current measurements have and are currently been made from an extensive array of moorings deployed by the Minerals Management Service (MMS) funded LATEX A and ongoing De Soto Canyon studies. These measurements at the shelf break, both east and west of the Mississippi delta, have shown that:
The LATEX C and Gulfcet I programs (MMS) made extensive aircraft and ship-based surveys, respectively, of the northern Gulf of Mexico slope between the 200 and 2000 m isobaths over a three year period. These surveys showed that slope was dominated by cyclonic and anticyclonic eddies of diameters between ~40 and 150 km. On the lower slope large anticyclones (~200 to 300 km in diameter) that had been shed from the Loop Current occasionally intruded over the 2000 m isobath. However, except near the delta, Loop Current eddies rarely influence directly currents over the shelf break. Consequences of this eddy field on the slope is that shelf-slope exchanges often occur as offshore or onshore jets between eddies rotating in opposite directions. These jets have limited spatial alongshore scales, and can occur at any position on the northern slope. The eddies generally occupy the upper layer which in the case of cyclones can be 800 m or deeper. Therefore, the offshore jets usually occupy the whole of the water column at the shelf break. These eddy generated jets seem to be the prime mechanism for shelf-slope exchange and these events can be quite long lasting (~months). The existence of a complex eddy field over the slope also creates pathways between the deep Gulf basin and the shelf and vice-versa. Many drifter tracks have shown the effectiveness of the eddy field in transporting water parcels to and from the shelf and deep Gulf basin waters. The SCULP-I (MMS) program deployed a total of 370 drifters on the Louisiana shelf between October 1993 and October 1994. Of these 370 drifters, 107 left the shelf sometime during their 100 to 120 day lifetimes. The drifters that left the shelf were usually entrained by shelf break eddies and transported to the offshore regions in a relatively random manner with only the narrow, northeast Mexican shelf, being favored as an export site. This is because of the prevalence of offshore, oppositely rotating paired eddies adjacent to this slope that arise from the topographic interaction of old Loop Current eddies with the north-south directed Mexican slope.
The region east of the delta had some different characteristics but was still dominated by small scale warm and cold eddies. The upper slope region was dominated by a two layer flow with eastward and westward components above and below about 150 to 200 m depth, respectively. This jet appears to be driven by lower slope anticyclonic eddies. Eddy-driven upwelling in the De Soto canyon is quite common, particularly in the energetic winter months. The dynamics of this process are not yet understood. The eddy field between an extended Loop Current and the Northeast Gulf shelf is often effective in moving filaments of warm Loop Current derived water towards the shelf break. The generation and life histories of slope eddies have not yet been described because of the lack of high resolution databases. It is clear that they can be long lasting and can be moved around by larger scale eddies such as Loop Current anticyclones. The ultimate source of eddy vorticity is the Loop Current and its periodic shedding of large anti-cyclones.
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