Deep flow variability in a deeply incised Mediterranean submarine valley (Blanes canyon)

Abstract

Deep flow variability in Blanes canyon is analyzed by means of a unique dataset in which the canyon was instrumented at its axis and at the two canyon walls, from March 2003 to July 2004. The mean intensity of deep currents range between 3.5cms−1 (at the canyon axis, 1500m) and 5.4cms−1 (upstream canyon wall, 900m). A wavelet analysis shows that the energy is concentrated in the 2–30day band, and it is mainly associated to single (not periodical, not wave-related) events, uncoupled at the different canyon sites. An empirical orthogonal function analysis shows two main patterns of current variability explaining 65% of the total variance. The first mode represents intensifications of the typical along-bathymetry pattern of currents over the canyon. The second mode corresponds to near-bottom upwelling events along the canyon axis with water exiting the canyon through the canyon head. After discussing possible forcing mechanisms, it is here suggested that such deep upwelling events are associated to offshore displacements of the Northern Current. Namely, the interaction of the current with the particular shape of the canyon topography at large depths (>1800m) would be responsible for the observed deep upwelling events.