Patos lagoon outflow within the Río de La Plata plume using an airborne salinity mapper: observing an embedded plume

Abstract

Major river systems discharging into continental shelf waters frequently form buoyant coastal currents that propagate along the continental shelf in the direction of coastal trapped wave propagation(with the coast on the right/left, in the northern/southern hemisphere). The combined flow of the Uruguay and Parana´ Rivers, which discharges freshwater into the Rı´o de la Plata estuary (Lat. 361S), often gives rise to a buoyant coastal current (the ‘Plata plume’) that extends northward along the continental shelf off Uruguay and Southern Brazil. Depending upon the prevailing rainfall,wind and tidal conditions, the Patos/Mirim Lagoon complex (Lat. 321S) may also produce a freshwater outflow plume that expands across the inner continental shelf. Under these circumstances the Patos outflow plume can be embedded in temperature, salinity and current fields that are strongly influenced by the larger Plata plume. The purpose of this paper is to present observations of such an embedded plume structure and to determine the dynamical characteristics of the ambient and embedded plumes. We describe selected results of coincident airborne remote sensing and shipboard in-situ surveys of the salinity distribution and extent of the Plata and Patos/Mirim Lagoon plumes conducted under contrasting winter (2003) and summer (2004) conditions. The surveys were carried out in the context of a comprehensive multi-disciplinary study of the Plata plume and its response to prevailing seasonal weather conditions. The objective was to map the surface salinity distribution of the Plata plume at synoptic scales under representative winter and summer conditions. Additionally, the airborne survey included finer-scale mapping of specific features including the Rı´o de Plata estuarine front and the Patos Lagoon plume, with the objective of determining the distribution and behavior of the plumes in the estuaries and on the continental shelf. The airborne survey was conducted with an aircraft carrying an infrared and microwave radiometer system, the Naval Research Laboratory’s (NRL) Salinity, Temperature and Roughness Remote Scanner (STARRS). A series of broadscale flights over the continental shelf off Argentina, Uruguay and Brazil were made using STARRS to determine the spatial extent of the Plata plume, and detailed mapping flights were undertaken in the vicinity of Rı´o de la Plata Estuary and Patos/Mirim outflows to observe associated frontal features. The results of the airborne surveys were compared with shipboard hydrographic data obtained from a conductivity, temperature and depth (CTD) profiler. The combined ship and aircraft data were used to estimate parameters of dynamical classification schemes. These schemes were used to characterize the gross behavior and dynamics of the ambient Plata plume and embedded Patos plume. The Plata plume was highly asymmetric with along-shelf development towards the north and it behaved dynamically like a buoyant coastal boundary current, with an approximately geostrophic across-shelf momentum balance. The Patos plume, on the other hand, maintained its integrity as a relatively symmetric, ageostrophic, frictionally dominated plume with significant across-shelf, and modest along-shelf, development.