Upwelling of subsurface water into the rim of the Biobio submarine canyon as a response to surface winds

The submarine canyon of the Biobio River (36 degrees 50'S; 73 degrees '15'W) is located in the vicinity of a zone of intense seasonal upwelling off Central Chile. This zone conforms a coastal ecosystem that produces more than 50% of the Chilean fish catch and approximately 4% of the world fish captures. As the small- and mesoscale oceanographic processes associated to this canyon are practically unknown. it has nor been demonstrated whether the interaction of the canyon with the coastal ecosystem contributes to the high biological productivity. Using horizontal currents and hydrographic variables measured in and around this canyon, this study shows the first evidences of the rise of water through the canyon of the Biobio River. Water velocities were measured using three moorings, deployed near the head and the flanks of the canyon in two periods of the year (March-April and August 1991). During the first period, when the wind conditions were relatively constant, both sides of the canyon had different circulation patterns. The kinetic energy of bottom currents in the northern side, which is associated to the continental shelf, was at least one order of magnitude larger than in the southern side, which is associated to interior waters. During the second period a storm of northerly winds, non-favorable for upwelling, generated a mixing of the water column down to 50 m, approximately. Two days after the beginning of the storm the water column under 60 m depth cooled down. This cooling responded mainly to the northerly and coastward currents that were generated almost at the end of the storm. Cooling was more evident at the northern side of the canyon, with cooling rates of 0.5 degreesC/day. Ascent rates of 0.02 cm/s for the 11 degreesC isotherm and 0.03 cm/s for the 10.5 degreesC isotherm were estimated. The divergence of the currents, calculated at 40 m depth, also indicates the rise of cold waters through the canyon during and after the storm. (C) 2001 Elsevier Science Ltd. All rights reserved.