Topographic control on the nascent Mediterranean outflow
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Data collected during a 12-day cruise in July 2009 served to examine the structure of the nascent Mediterranean Outflow Water (MOW) immediately west of the Espartel Sill, the westernmost sill in the Strait of Gibraltar. The MOW is characterized by high salinities (>37.0 and reaching 38.3) and high velocities (exceeding 1 m s−1 at 100 m above the seafloor), and follows a submerged valley along a 30 km stretch, the natural western extension of the strait. It is approx. 150 m thick and 10 km wide, and experiences a substantial drop from 420 to 530 m over a distance of some 3 km between two relatively flat regions. Measurements indicate that the nascent MOW behaves as a gravity current with nearly maximal traveling speed; if this condition is maintained, then the maximum MOW velocity would decrease slowly with distance from the Espartel Sill, remaining significantly high until the gravity current excess density is only a small fraction of its original value. The sharp pycnocline between the Mediterranean and the overlying North Atlantic Central waters is dynamically unstable, particularly where the flow interacts with the 100 m decrease in bottom depth. Here, subcritical gradient Richardson numbers coincide with the development of large interfacial undulations and billows. The very energetic downslope flow is likely responsible for the development of a narrow V-shaped channel downstream of the seafloor drop along the axis of the submerged valley, this probably being the very first erosional scour produced by the nascent MOW. The coincidence of subcritical gradient Richardson numbers with relatively high turbidity values above the channel flanks suggests it may be undergoing upstream erosion.
The authors are deeply grateful to all aboard the R/V García del Cid, in particular Captain Eduardo Otal and technicians José Antonio Pozo, Maribel Lloret and Joaquín Salvador, as well as Mikhail Emelianov and Sam Kelly for their help in gathering data during the cruise. The authors are also very grateful to Gemma Ercilla and two anonymous reviewers for many useful comments. Funding for this work comes from the Ministerio de Ciencia e Innovación, Spain, through project MOC2 (CTM2008-06438-C02-01), project INGRES (CTM2006-02326), and two complementary actions (CTM2008-03422-E/MAR and CTM2009-05810-E/MAR); funding also comes from the US National Science Foundation (grants OCE-0825287 and OCE-0825297).
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