![]() ![]() These high current fluctuations near the bottom provide an erosive environment that leads to resuspension of bottom sediment, as clearly seen in near-bottom turbidity measurements. Observations from the outer part of the upper section of the canyon, just north of the Berlengas Islands, revealed the downward inten- sification of tidal current fluctuations, which reached about 50 cm s -1 near the bottom (1700-m depth) during the spring tides, with the total current (residual plus tidal) attaining up to 70 cm s -1. HERMES observations revealed that these processes contribute at some locations along Nazaré Canyon to the development of very high bottom currents, which have a profound effect on sediment and faunal distributions. The canyon axis slope, however, is subcritical to these frequencies, allowing up-canyon propa- gation of internal-tide energy. The steep walls of the upper section of Nazaré Canyon are supercritical to incident internal waves at a semi-diurnal (M2) period, leading to reflection toward the bottom of internal tides generated at the rim. Like most submarine canyons, Nazaré serves as a trap for internal waves, and particularly internal tides. This generation mechanism is particularly effective at the canyon rim, where critical conditions for internal wave reflection occur. The interaction of the barotropic tide with the canyon topography, in the presence of water-column stratification, leads to the generation of internal waves of tidal period (internal tides) that radiate from the generation point, propagating the tidal energy up- and down-canyon. The exter- nal (barotropic) tide off the western Portuguese coast is dominated by the lunar semi-diurnal (M2) constituent and modulated by the solar semi-diurnal (S2) constituent with a clear spring- neap tide cycle. In addition to residual circulation, tidal currents are important in Nazaré Canyon, where they maintain a high-energy environment near the bottom at several locations. During the same period, a persistent offshore (down- canyon) residual flow was observed at 1700-m depth, close to the bottom, at the outer part of the upper section of the canyon (personal observations of authors Oliveira and Vitorino). Close to the Vitório tributary, near the bottom (700-m depth), a persistent onshore (up-canyon) flow was observed for at least one year. Although only weak and variable flows were measured at the depths of the MOW cores, observations revealed sustained and relatively strong (10–20 cm s -1 ) residual flows at the upper and lower boundaries of the MOW. High-salinity MOW bod- ies were found inside the upper section of Nazaré Canyon out to about 20 km from shore, where the canyon floor is 600-m deep but the nearby shelf is only 100-m deep. Off Nazaré, the main core of MOW occurs at 1200-m depth, with a second (less-saline but warmer) core observed at about 700-m depth. Off the western Portuguese coast, the MOW is observed as a tongue of high-salinity water that follows the continental slope and extends to depths between 600 m and 1500 m. Other forcing mechanisms, such as the interaction of the north-flowing Mediterranean Outflow Water (MOW) with the canyon, play a role in controlling circulation dynamics. Below 300-m depth, flow inside the canyon is independent of wind-forcing conditions and shelf circulation. During cessation of northerly winds, or during periods of predominant southerly winds that promote downwelling on the shelf, the residual flow near or just above the canyon rim depth is down-canyon. During strong northerly wind events, an onshore (up-canyon) flow up to 10 cm s -1 has been observed inside the canyon, with upwelling occurring near the canyon head. Northerly winds promote upwelling conditions and southward flow across the shelf. Within the canyon to depths of about 300 m, the residual currents correlate well with the winds affecting the western Portuguese margin. ![]() the canyon substantially disturbs the predominantly north-south slope- parallel circulation.
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