Spatio-temporal variability of turbid freshwater plumes in the Inner Sea of Chiloé, northern Patagonia

Abstract

Northern Patagonia is characterized by multiple rivers that discharge considerable amounts of freshwater into the coastal ocean, forming large river plumes that influence hydrographic and ecological processes. In this study, we use satellite ocean color data from MODIS (Moderate Resolution Imaging Spectroradiometer) to characterize the seasonal and interannual variability of turbid freshwater plumes in the inner waters of northern Patagonia for the first time, with a focus on the connections to river discharge and large-scale climatic variability. The turbidity signal from the surface reflectance product centered at 645 nm, Rrs(645), correlates well with peaks in river discharge data and surface salinity minima from boat-based profiles and a surface buoy, validating its use for the identification of turbid river plumes in the region. The seasonal climatology of Rrs(645) showed the presence of large river plumes throughout the year, with variability associated mainly to that of river discharge. Analysis of Rrs(645) fields under low and high discharge conditions allowed for the identification of a threshold value to delineate plume fronts and determine the probability of plume occurrence. EOF analysis reveals the dominant modes of plume variability, associated to turbidity differences between the coastal margin and deeper waters and to in-phase variability of large river plumes throughout the study area. The largest plume event occurred in year 2008, during negative (cold) phases of ENSO (La Niña) and PDO but a positive phase of SAM. The severe drought event of 2016 that occurred during positives phase of ENSO (El Niño), PDO and SAM is well captured by the EOF analysis and is characterized by large negative anomalies in the Rrs(645) signal. In general, complex co-variations are observed between monthly anomalies of Rrs(645) and the ENSO, PDO and SAM indices, highlighting the heterogeneity of climatic regulation in the region.