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, 3 (11), eaao0609

The Mediterranean Overflow in the Gulf of Cadiz: A Rugged Journey


The Mediterranean Overflow in the Gulf of Cadiz: A Rugged Journey

Ricardo F Sánchez-Leal et al. Sci Adv.


The pathways and transformations of dense water overflows, which depend on small-scale interactions between flow dynamics and erosional-depositional processes, are a central piece in the ocean's large-scale circulation. A novel, high-resolution current and hydrographic data set highlights the intricate pathway travelled by the saline Mediterranean Overflow as it enters the Atlantic. Interaction with the topography constraints its spreading. Over the initial 200 km west of the Gibraltar gateway, distinct channels separate the initial gravity current into several plunging branches depth-sorted by density. Shallow branches follow the upper slope and eventually detach as buoyant plumes. Deeper branches occupy mid slope channels and coalesce upon reaching a diapiric ridge. A still deeper branch, guided by a lower channel wall marked by transverse furrows, experiences small-scale overflows which travel downslope to settle at mid-depths. The Mediterranean salt flux into the Atlantic has implications for the buoyancy balance in the North Atlantic. Observations on how this flux enters at different depth levels are key to accurately measuring and understanding the role of Mediterranean Outflow in future climate scenarios.


Fig. 1
Fig. 1. MOW in the eastern GoC.
(A) A three-dimensional (3D) view of the 35.90 isohaline as a buoyant, salty plume abutting the slope. (B) As in (A) but for the 36.30 isohaline and the along-slope undercurrent. (C) Shaded relief map of the GoC seafloor bathymetry with near-bottom instantaneous velocity vectors (black arrows) over salinity (color shades). The 35.9 and 36.3 salinity contours are included for visual reference. Colored dots indicate the approximate pathways of the historical upper and lower cores (see Fig. 2). White open arrows depict branching of the historical cores. Numbered squares indicate inset areas for the following figures. CDR and BH stand for the Cadiz Diapiric Ridge and Basement High, respectively. (D) Vertically integrated mean MOW volume transport vectors across a number of sections (T1 to T18, from east to west; all transects quoted throughout the text refer to this numbering). Colored bands outline the density-sorted MOW branches as described in the text. Black open arrows depict branching of the historical cores. The MOW plume expands as it stretches westward (A). West of 7.0°W, it separates from the seafloor to travel at mid-depths (800 to 1400 m). Bathymetric features influence salinities, velocities, and integrated transport (C), highlighting the role of the small-scale channels on transverse dispersion of saline water. Note that whereas discrete, near-bottom velocity observations appear to vary smoothly across the overflow (C), the vertically integrated transport within the MOW layer at (D) suggests individual branches that are forged upstream (D).
Fig. 2
Fig. 2. Upper and lower MOW evolution downstream of Spartel.
Representation of dynamical, hydrological, and geometrical properties versus distance (lower axis) and longitude (upper axis). (A) Dynamical properties. From top to bottom: Reference location of the upper and lower MOW boundaries at each section in Fig. 1D; bulk Froude number (Frb); entrainment velocity (w*) calculated from salt (dashed line) and volume flux (solid line) conservation from T1 to T14; outflow (QMOW, crosses), pure MOW (Qmed, triangles), and entrainment (Qent, stars) transport across each section (the entrainment transport was estimated by subtracting Qmed series from the integrated outflow transport QMOW; see fig. S8 for details); and MOW speed near the bottom (solid) and at the velocity maximum (dashed). (B) Hydrological properties. From top to bottom: Salinity loss rate, reduced gravity [g′, the acceleration of gravity (g) experienced by a parcel of density ρ1 immersed in an ambient fluid of density ρ0; g′ = g1 − ρ0)/ρ0], potential density anomaly (σθ), MOW salinity, and ENACW salinity. Except for the latter (calculated at the ENACW salinity minimum), all curves show values near the seafloor. (C) Geometrical properties. From top to bottom: Normalized MOW width/QMOW ratio across T1 to T14 (MOW width was taken as the distance between the inner and outer 36.3 isohalines near the bottom), MOW width, plunge rate, and MOW depth. Properties were either collocated at the (upper and lower) pathways outlined in Fig. 1C or averaged across the sections indicated in Fig. 1D. In all panels, lines marked with dots indicate values following the upper MOW (the lower MOW for lines without markers). Lines marked with crosses indicate values at each section in Fig. 1D. Note that this data set does not show the LC past T14.
Fig. 3
Fig. 3. Leaving the Strait.
(A) Inset 1 from Fig. 1C. Small-scale bathymetric features guide the MOW as it passes Spartel (indicated with white open arrows). These include the MR, a straight, sloping channel, and a relatively rough BH dissected by two deep zonal gorges. The MR causes an initial division into coherent, separate flows across the northern (NS) and southern (SS) Spartel channels. Instantaneous velocity observations indicate less temporal variability for the southern flow than for the northern one, which experiences tidally forced current reversals. Past the ridge, both components appear to coalesce with the northern flow overriding the southern flow (fig. S4). The BH diverts as much as 70% of the transport through a southern channel. A maximum bottom velocity for the entire data set (1.82 m s−1) was observed here (35.77°N/6.38°W) during the 201306I3S cruise (table S1). (B) Mean velocities (m s−1; negative for westward flow) across T1 [red line in (A)]. The color scale is saturated beyond −0.8 m s−1, and additional velocity contours (white lines) are included every 0.1 m s−1. Magenta contours indicate tangential velocity every 0.05 m s−1 (solid for southward and dashed for northward). Ticks at the top axis indicate the location of individual observations. The ridge separates two MOW forks residing at different depths. The more energetic, southern flow banks against the southern channel slope. (C) Gradient Richardson number (Rig) across T1. Rig provides insight into the magnitude and vertical location of Kelvin-Helmholtz (K-H) instabilities. Despite the high density gradient and high static stability, the lowest Rig values occur around the MOW-ENACW interface. Black contours indicate potential density anomaly (σθ) every 0.25 kg m−3. Red contours indicate the SD of velocities in (B) (m s−1). After removal of tidal currents, variability remains relatively low everywhere except at the southern channel density interface, where high variability likely reflects seasonal forcing (6). (D) Mean salinity across T1. The color scale is saturated beyond 36.8, and additional contours are included every 0.25. Below the surface layer, the 36.3 isohaline approximately defines the interface between ENACW (with salinities down to 36.0) and MOW > 38.25.
Fig. 4
Fig. 4. Asymmetric curving.
(A) Inset 2 of Fig. 1C. Individual MOW branches are labeled M1 to M5. Beyond the BH, the shallower M3 to M5 flows occupy curved (clockwise) channels, whereas the deeper M1 and M2 flows continue westward, and the overall MOW width expands by a factor of 2. Further downstream, M1 traces a gentle clockwise arc, whereas M2 courses northwestward along a nearly straight path. Both currents meet at 35.90°N/6.80°W. (B) As in Fig. 3B but for T4 [red line in (A)]. The MOW extends below two AI jets. It features diverging cores travelling along the slope at the base of the water column. Maximum velocities are constrained by the channel morphology at the southern border. (C) As in Fig. 3C but for T4. Strong vertical shears at the top of the MOW over M1 to M3 (and also at T3; fig. S5) exceed stratification (as measured by N2) and bring Rig close to the K-H instability limit. Increased likelihood of K-H instabilities, and hence vertical mixing, is higher east of 6.75°W (as shown in terms of Frb in fig. S7). (D) As in Fig. 3D but for T4. Across-stream salinity structure below the 36.3 surface reveals the multicored MOW structure.
Fig. 5
Fig. 5. Consolidated MOW branches.
(A). Inset 3 of Fig. 1C. The CDR and the Gusano, Cadiz, and Huelva channels are labeled. Fs1 to Fs3 indicate the transverse furrows. The UC and LC are also outlined with dashed arrows. (B) As in Fig. 3B but for T7 [red line in (A)]. Individual MOW branches identified in (A) occupy every slope channel and the transverse furrow Fs3. (C) As in Fig. 3C but for T7. (D) As in Fig. 3D but for T7.

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