A modeling study of Patos lagoon (Brazil) flow response to idealized wind and river discharge: dynamical analysis

Abstract

A B S T R A C TA three-dimensional numerical model is used to investigate the Patos Lagoon flow response to idealized northeasterly winds and river inflow. North of Ponta da Feitoria, in the central lagoon, the main balance in the alongshore direction at steady state is between the surface stress and the pressure gradient, with a small contribution from the bottom stress. The alongshore variability in the pressure gradient is balanced by the Coriolis acceleration. In the cross-shore direction, the balance is nearly geostrophic, with deviations from that balance in the shallow regions, where the bottom stress term makes a significant contribution. Close to headlands, nonlinear terms are also important, leading to an additional increase in the cross-shore pressure gradient. Relaxation events were found to be important for seiche generation in the central lagoon. Close to the lagoon's mouth, nonlinear terms were found to be the major terms balancing the pressure gradient generated in response to the wind forcing. After the wind relaxation, the pressure gradient in the estuarine region is reversed, driving a landward flow. River inflow acts to dampen out the generation of seiches following wind relaxations. A river inflow Q = 2000 m 3 s -1 was found to be sufficient to shut down the landward flow generated following relaxation events. R E S U M OUm modelo numérico tridimensional é utilizado para investigar a circulação na Lagoa dos Patos em resposta aos ventos e à descarga fluvial. Ao norte da Ponta da Feitoria, o principal balanço na direção longitudinal é entre o estresse superficial e o gradiente de pressão, com pequena contribuição do estresse de fundo. A variabilidade longitudinal no gradiente de pressão é balanceada pela aceleração de Coriolis. Na direção transversal à lagoa, o balanço é aproximadamente geostrófico, com desvios deste balanço nas regiões rasas, onde o estresse de fundo contribui significamente. Os termos não lineares são importantes próximo aos cabos, causando um aumento adicional no gradiente de pressão na direção transversal. Eventos de relaxação são significativos na geração de seiches na parte central da lagoa. Próximo à desembocadura da lagoa, os termos não lineares são os principais termos contrapondo-se ao gradiente de pressão gerado pela ação dos ventos. O gradiente de pressão é revertido após a relaxação dos ventos, causando a entrada de água na lagoa. A descarga fluvial atenua a geração de seiches após a relaxação. Valores médios de descarga (Q = 2000 m 3 s -1 ) são suficientes para impedir a entrada de água na lagoa após a relaxação do vento.

The Exchange Processes in the Patos Lagoon Estuarine Channel, Brazil

“…This process results from the competition between flood currents driven by remote wind effects and the gravitational circulation modulated by the intensity of the freshwater discharge. These results corroborate the previous studies done by Möller et al [5], Castelão and Möller [24], Fernandes et al [8]. These authors have demonstrated the combination of local and remote wind action controlling the exchange process between the Patos Lagoon and adjacent coast, during low river discharge conditions, in synoptic time scales from 3 to 17 days.…”

Straining and advection contributions to the mixing process of the Patos Lagoon coastal plume, Brazil

“…Several studies using time series analysis and numerical simulations showed that mixing and exchange processes during low and mean discharge conditions of the Patos lagoon circulation pattern were controlled by wind effects at synoptic times scales that were coincident with the passage of meteorological systems every 3 to 17 days [ Möller , 1996; Möller et al , 1996, 2001; Fernandes , 2001; Fernandes et al , 2002, 2004, 2005; Castelão and Möller , 2006; W. C. Marques et al, The exchange processes in the Patos Lagoon estuarine channel, Brazil, submitted to Estuaries and Coasts , 2010]. Marques et al [2009] observed a similar pattern of migration southwestward/northeastward of the Patos Lagoon coastal plume according with the passage of meteorological systems over the area.…”

Numerical modeling of the Patos Lagoon coastal plume, Brazil

“…Several studies using time series analysis and numerical simulations (Mö ller, 1996;Mö ller et al, 1996Mö ller et al, , 2001Fernandes, 2001;Fernandes et al, 2002Fernandes et al, , 2004Castelão and Mö ller, 2006) have shown that the Patos lagoon circulation pattern itself, the mixing and exchange process during low and mean discharge conditions are controlled by the wind effect at synoptic times scales coincident with the passage of meteorological systems every 3-17 days.…”