  Citation: da Silva, D.V.; Oleinik, P.H.; Costi, J.; de Paula Kirinus, E.; Marques, W.C.; Moller, O.O. Variability of the Spreading of the Patos Lagoon Plume Using Numerical Drifters. Coasts 2022, 2, 51–69. https://doi.org/10.3390/ coasts2020004 Academic Editor: Gregorio Iglesias Rodriguez Received: 18 February 2022 Accepted: 30 March 2022 Published: 6 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Variability of the Spreading of the Patos Lagoon Plume Using Numerical Drifters Douglas Vieira da Silva 1, * , Phelype Haron Oleinik 2 , Juliana Costi 3 , Eduardo de Paula Kirinus 4 , Wiliam Correa Marques 3 and Osmar Olinto Moller 5 1 Institute of Astronomy, Geophysics and Atmospheric Sciences, Sao Paulo University, Sao Paulo 05508-090, Brazil 2 Escola de Engenharia, Campus Carreiros, Universidade Federal do Rio Grande, Rio Grande 96203-900, Brazil; phe.h.o1@gmail.com 3 Institute of Mathematics, Statistics and Physics, Universidade Federal do Rio Grande, Rio Grande 96203-900, Brazil; ju.costi@gmail.com (J.C.); wilianmarques47@gmail.com (W.C.M.) 4 Laboratório de Infraestrutura de Transporte e Obras Portuárias, Campus Pontal do Paraná, Universidade Federal do Paraná, Pontal do Parana 80060-000, Brazil; ekirinus@gmail.com 5 Instituto de Oceanografia, Universidade Federal do Rio Grande, Rio Grande 96203-900, Brazil; dfsomj@furg.br * Correspondence: douglascpe@gmail.com Abstract: The Patos Lagoon coastal plume is a small-scale outflow that is strongly controlled by meteorological tides. However, the riverine discharge of the lagoon is subject to high decadal variability. Hence, the discharge amount alters the scale of this coastal plume and its effects over the inner shelf environment. This study uses hydrodynamic simulations and a Lagrangian model to estimate the spreading of the plume under two different discharge conditions.Through scale parameters, we characterized the contrasts of the plume structure between high discharge and low discharge conditions. During a strong discharge regime, the width and thickness of the plume are enhanced, and the inertial processes increase against the frictional effect of the wind. The consequences of these differences include higher values of alongshore and cross-shore spreading of the drifters for the strong discharge regime. These findings indicate that under similar wind conditions, different amounts of riverine discharges alter the extent to which the material delivered by the plume can spread over the inner continental shelf. Keywords: coastal plume; hydrodynamic; radial spreading; Patos Lagoon coastal plume 1. Introduction A considerable amount of the global precipitation over the continents is delivered by rivers to the oceans through coastal plumes [1]. The transport and mixing of the material delivered by rivers to the deep ocean is strongly influenced by the plume structure [2]. Coastal plumes have diverse scales and structures; large-scale plumes can extend beyond the continental shelf and affect mesoscale processes due to superficial stratification [3], while small scale plumes have a significant role on the transport of sediments and organic matter at the inner shelf and coastal zones due to mixing and straining processes [4–6]. The diversity of coastal plumes is more extensively studied by the use of numerical simulations, thus enabling the characterization of the main physical forcings of plume dynamics [7]. In the inner shelf environment, the variability of the spreading of small coastal plumes is subjected to ambient currents, tides, Coriolis force, wind and river discharge [8,9]. Winds and Coriolis parameter are major mechanisms on the far-field (inner and outer shelf) [10], while in the near-field (adjacent region to the river mouth), buoyancy and tides are more important [11]. The near-field has a supercritical flow condition and is where the bulk turbulent dissipation and dilution of the plume occurs [12,13]. Turbulent dissipation is linked to the lateral spreading of plumes, consequently modifying plume Coasts 2022, 2, 51–69. https://doi.org/10.3390/coasts2020004 https://www.mdpi.com/journal/coasts