Cross-shore exchanges imposed by an upwelling filament

Abstract

Upwelling filaments are mesoscale structures of cold water that stretch seaward in a tongue-like shape with origin in the coastal upwelling zone. Filaments represent preferred pathways for the exchange of water, dissolved and particulate matter from the productive shelf region towards the oligotrophic offshore regions. Upwelling filaments are a common feature of the western margin of the Iberian Peninsula. The culminating point of SW Iberia, the Cape São Vicente, is the root of a recurrent well developed filament observed in the satellite imagery during the upwelling season. The Cape São Vicente filament was intensively investigated through remote sensing and in situ multidisciplinary observations, carried out during an upwelling favorable wind relaxation event, but just after a relatively intense upwelling period. The upwelling signal was still present and the associated filament, although not fully developed, was evident in the satellite sea surface temperature field. A total of 42 Rosette+CTD casts up to 400 m depth were distributed on an almost regular grid of 15 km mean spacing, with reduced spacing close to the fronts. Ten standard levels were selected for water collection. The sampling was guided by satellite SST imagery transmitted to the ship in near-real time. The parameters sampled during the sea campaign comprised temperature, salinity, chlorophyll a (Chl a), dissolved oxygen, nutrients (nitrates, phosphates and silicates) and metals (cadmium, lead and zinc), along with the velocity field sampled along the ship track through a hull-mounted 38 kHz RDI ADCP and on board meteorological variables. Two transects crossing perpendicularly the filament jet were selected for analysis in this study. The filament transported 0.9 Sv of coastal water offshore. A shallow return flow transporting 0.4 Sv was observed in the southern flank. Vertical velocities of about 15 m/day were found inside the filament. Although this filament may be considered small, the cross-shore transport of Chl a was estimated as 18 tons/day in the root of the filament, where Chl a concentration was higher. The fluxes of the three nutrients (nitrate, phosphate, and silicate) were weak in the top layers due to their consumption by the growing phytoplankton, increasing with depth. The estimated seaward nutrient mean transports were about 3500 tons/day for nitrates, 450 tons/day for phosphates and 4000 tons/day for silicates. It must be pointed out that the transport of nutrients corresponds to a residual after consumption by phytoplankton. Our observations show a high increase of the export of metals inside the filament core relatively to the surrounding waters, particularly offshore. The estimated offshore transports range from 18 to 26 tons/day of zinc, 17 to 31 kg/day of cadmium and 83 to 282 kg/day of lead, depending of the filament cross section. The survey sampled the Cape São Vicente filament just after an upwelling event, but under relaxed winds. In consequence, the filament was not in it most developed stage. Considering the periods of strong upwelling events and the extent of their duration along the year, the amounts of exported matter must be hugely increased and responsible for the high productivity of the waters, showing the vital importance of the upwelling filaments to understand the functioning of the regional ecosystem.