Dr. Lara-Peña, Carlos

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.

The variability of thermal fronts in eastern ocean boundaries has received increased attention because of being active regions of vertical fluxes of tracers and biological activity. Sea Surface Temperature (SST) images from three distinct satellite products are used to identify areas with enhanced surface thermal gradients (i.e. SST fronts) in the coastal ocean off central–southern Chile. The main objective is to evaluate their use in the study of SST frontal variability in a river-influenced continental shelf. In contrast with previous studies focused on the mesoscale structure of the upwelling front, this study highlights the importance of using high spatial resolution (i.e. 1 km) satellite products to resolve the small-scale surface thermal gradients on a relatively narrow continental shelf impacted by freshwater river outflows. An improved approach, merging Moderate Resolution Imaging Spectroradiometer (MODIS) imagery from Aqua and Terra satellites (MODIS-AT), increases the percentage of available SST data for the computation of SST gradients and frontal probability over these shelf waters. Overall, SST data from all three sources (Pathfinder, Geostationary Operational Environmental Satellite (GOES), and MODIS) resolve the major mesoscale frontal features along the offshore limit of the continental shelf. However, MODIS-AT considerably improves the detection of SST fronts over the continental shelf, especially during winter and spring when river outflows are important on the dynamics of coastal flows. A case study off the Itata River mouth reveals high spatio-temporal variability of thermal fronts over the continental shelf, which is not well detected from GOES and Pathfinder data. The analysis of MODIS-AT images is highly recommended for studies of thermal fronts over shelf waters. In contrast, the use of GOES imagery improves the monitoring of the mesoscale frontal activity farther offshore.