Freshwater input into the coastal ocean and its relation with the salinity distribution off austral Chile (35–55°S)

Abstract

The southern coast of Chile is characterized by high discharge of freshwater into the South Eastern Pacific Ocean. The latitudinal distributions of the freshwater input through rivers and continental precipitation are here discussed in relation with the salinity distribution found in the coastal zone. All river basins that discharge into the Pacific Ocean between 35°S and the southernmost part of Chile (ca. 55°S) were selected in order to estimate the meridional distribution of the freshwater input off central-south Chile. Maximum values of the mean annual river discharge were found at 42°S, 46°S and 50°S with volumes of 2470, 3480 and 3344 m 3 s −1, respectively. The total amount of freshwater obtained between 35°S and 55°S is 27.8×10 3 m 3 s −1, using the river basin fluxes, and 33.5×10 3 m 3 s −1, using precipitated water. The seasonal mean salinity anomaly shows basically negative values in summer and positive values in winter, indicating less (more) salty water in the warm (cold) period. Possible explanations for that are discussed in terms of seasonal variations of river flow, wind stress and solar radiation. Objective salinity fields were also used to estimate the effect of the river discharge on the coastal ocean through the fraction of freshwater ( K f ) in the marine system, calculated with different column thickness. Maximum values of K f are located at 44°S, and from 50°S to 54°S, and a minimum K f is observed at 49°S. This last latitude corresponds to a zone of no significant freshwater discharge. Although the correlation between mean annual freshwater input and salinity at three selected distances offshore (75, 30, and 10 km) are small, their sign seems to express the physics of the relation. The absolute correlation values increase towards the shore. In the case of the mean annual salinity distributed at 75 km offshore, significant correlation coefficients were found at 10 and 50 m depth, while at 10 km offshore a significant correlation was found at all considered depths, except at the surface. The correlation coefficients of the monthly mean freshwater input and salinity (both averaged to one degree of latitude), calculated at certain latitudes, show negative correlation values in the latitude range of 38–42°S, although they are not significant except for the case of 40°S.