Coastal upwelling process on a continental shelf limited by submarine canyons, Concepción, central Chile

Abstract

[1] The subinertial flow on the continental shelf off Concepción (central Chile) and its relation with the local wind-forcing and the topography are studied. Emphasis is placed on the three-dimensional aspects and the effect of the topography on the classical twodimensional approach to the coastal upwelling. Time series coming from 18 current meters installed on six moorings during March-April 1994 were analyzed. Hydrographic data were collected during two hydrographic cruises (6-7 March and 30-31 March). Complementary information includes wind data and sea level. Empirical orthogonal functions using all series showed two principal subinertial flow patterns. The first mode corresponds to an alongshore flow responding to the intensification of southwesterly wind on a 3-to 10-day scale. The second mode is dominated by the cross-shore fluctuations at the shelf break. The importance of the local wind-forcing appears clearly when comparing the theoretical and observed Ekman transport in the north and central shelf (R = 0.8). Near the bottom a mean southward flow was found with a clear southward intensification induced by the bathymetry and probably by a meridional pressure gradient. This bottom flow modifies the typical coastal upwelling pattern through an offshore Ekman transport in the surface and bottom layers. This implies that the compensation flow for the coastal divergence comes from intermediate waters that arrive onto the shelf through ageostrophic process related with the submarine canyons located toward the north and south of the shelf.

Larval fish assemblages off central Chile upwelling ecosystem

“…Franco-Gordo et al (2002) described a spatially homogeneous larval fish assemblage when advective processes dominated the system off the Pacific coast of Mexico. In our case, the studied area has a considerable variability in its topography, with a continental shelf limited by two submarine canyons which directly affect the coastal circulation (Figueroa & Moffatt 2000, Sobarzo & Djurfeldt 2004, Sobarzo et al 2007, Morales et al 2007) and indirectly may influence the spatial structure of larval fish assemblages off Talcahuano area (see Castillo et al 1991). In this sense, over the Oregon shelf it has been described that during upwelling season the Astoria Canyon causes currents to flow landward (Hickey 1997), carrying and concentrating normally offshore organisms closer to shore (Bosley et al 2004, Parnell et al 2008.…”

“…1) is located at the coastal edge of the northward-flowing Humboldt Current system off central Chile, and is bounded by the Itata Canyon to the north and the Biobio Canyon to the south. The continental shelf narrows from 60 km in the north to 25 km in the south over an alongshore expanse of 70 km (Sobarzo & Djurfeldt 2004). On this shelf, there are several embayments utilized as spawning and nursery zone by several marine fishes (Castillo et al 1991, Landaeta & Castro 2006a, two important rivers (Biobio and Itata), and one prominent point (Punta Lavapié) that is an important coastal upwelling site in central Chile.…”

Chemolithoautotrophic production mediating the cycling of the greenhouse gases N<sub>2</sub>O and CH<sub>4</sub> in an upwelling ecosystem

“…The seasonal intensification of the south and southwest winds brings Equatorial Subsurface Water (ESSW), with high NO − 3 and low O 2 concentrations, to the surface (Sobarzo and Djurfeldt, 2004). The width of the continental shelf provides a large surface area for material exchange and benthic-pelagic coupling in subsurface waters that can reach the surface via upwelling processes.…”

Vertical segregation among pathways mediating nitrogen loss (N<sub>2</sub> and N<sub>2</sub>O production) across the oxygen gradient in a coastal upwelling ecosystem

“…Blanco et al, 2001), which transports nutrient-rich, high-saline, and O 2 -depleted Equatorial Subsurface Waters -ESSWs (Strub et al, 1998). During austral spring-summer (SeptemberApril), a marked upwelling-favourable period driven by persistent southwesterly winds, ESSWs arrive at the coastal area off central Chile, impinging on one of the widest continental shelves in Chile (Sobarzo and Djurfeldt, 2004;Sobarzo et al, 2007). Throughout the upwelling season high primary production rates (Daneri et al, 2000) occur along with a large downward flux of organic matter, leading to an increase in respiration rates at depth (Montero et al, 2007).…”