Synoptic Flow Variability in a River-Influenced Inner Shelf off Central Chile

Abstract

The response of inner shelf circulation and bottom temperature variability to synoptic wind forcing and freshwater outflow is evaluated in an area with a wide continental shelf off central Chile. This forced circulation, with a strong seasonal evolution from upwelling- to downwelling-favorable conditions, is a key process modulating the exchange of water properties in a coastal zone characterized by multiple river outflows and high biological productivity. Ocean currents on the inner shelf (34 m depth) off the Itata River mouth were obtained and analyzed for a nine-month record (December 2008 to September 2009). The synoptic wind and current variability was defined between 2 and 16 days (0.02 to 0.0026 cph). The subinertial coastal circulation within the inner shelf off Itata River was dominated by the along-shelf flow, while cross-shelf flows driven by along-shelf winds were substantially reduced. The alongshore synoptic currents showed two distinct modal structures: (i) A mean two-layer flow field during upwelling-favorable wind stress with northward (southward) flow at the surface (bottom), and (ii) a mean southward flow through the entire water column during downwelling-favorable wind stress periods, which were intensified during peak river discharge events in winter. Calculations of the wind index clarified the relative importance of wind versus buoyancy forcing on the alongshore flow. The wind forcing dominated during summer when the river discharge was minimum, whereas the relative importance of the freshwater outflow from the Itata River became dominant in winter (May-August) when the buoyancy forcing, in conjunction with southward wind events, modulated strong barotropic southward flows. The change in the long-term regime of coastal winds and river discharges, with a dominance of upwelling winds and reduced river outflows in the last decade, is discussed in the context of observations of thinner river plumes under frequent upwelling conditions off central Chile.