Energy balance and time-scales of mixing and stratification in the Jaboatão estuary, NE-Brazil

Abstract

2CNPq fellow . Abstract: Estuarine systems undergo different physical processes that simultaneously control their stratification and mixing dynamics. This energy balance determines both, the estuarine hydrodynamics and the dynamics of water. This article presents a quantitative and comparative analyses between the forces maintaining stratification (surface heating; rainfall precipitation; and differential advection of the longitudinal density gradient due to the vertical velocity field) and those responsible for the vertical mixing (mechanical stirring of botiom tidal stress; mechanical stirring ofsurface wind stress; and surface evaporation) in the lower estuary ofthe Jaboatão River (JE), Pernambuco, NE-Brazil. The energy available to mix the water column at thé lower Jaboatão was 2.2 and 2.0-fold greater than that available to promote stratification, during the dry and rainy seasons, respectively. The botiom shear turbulence caused by the tides was the major source of energy for the vertical mixing. A theoretical analyses revealed that the turbulence decay time-scale in both seasons was much greater (~29min; rainy=25 min) than the stratification time-scale (dry=8 min; rainy=7 min) and than the slack water time-scale (15 min). Thus the estuary was vertically well-mixed even during slack water periods during both, dry and rainy seasons. Theoretical results were confirmed by field data and are in agreement with earlier numerical simulations. · Resumo:Sistemas estuarinos estão sujeitos à influência simultânea de prQCessos físicos que controlam tanto sua dinâmica de estratificação quanto de mistura. Este balanço energético determina seu comportamento hidrodinâmico e a dinâmica das propriedades da água. O presente artigo apresenta uma análise quantitativa e comparativa das forças de manutenção da estratificação (aquecimento superficial; precipitação pluviométrica e advecção diferencial do gradiente longitudinal da densidade devida ao campo vertical da velocidade), e daquelas responsáveis pela mistura vertical (turbulência de fimdo devido às marés; turbulência de superfície devida à ação dos ventos e evaporação superficial) no baixo estuário do Rio Jaboatão (JE), Pernambuco, NEBrasil. A quantidade de energia disponível para misturar a coluna d'água foi 2,2 e 2 vezes maior que aquela disponível para manter a estratificação durante as estações seca e chuvosa, respectivamente. O atrito turbulento de fundo pelas marés foi a principal fonte de energia do sistema para a mistura da coluna d'água. Uma análise teórica revelou que a escala de tempo para o decaimento turbulento nas duas estações sazonais foi superior (seca=29 min; chuvosa=25min) que a escala de tempo para estratificação (seca=8min; chuvosa=7 min) e que a escala de tempo dos estofos de maré (15 min). Assim, o estuário apresentou-se verticalmente bem misturado mesmo durante os estofos de preamar e baixa-mar tanto na estação seca quanto chuvosa. Os resultados teóricos foram confirmados pelos dados de campo e estão em concordância com aqueles obtidos em estudos pré...

Comparative analysis of rating curve and ADP estimates of instantaneous water discharge through estuaries in two contrasting Brazilian rivers

“…The tidal co‐oscillation at the estuaries' mouths is classified as meso‐tidal and semi‐diurnal according to its height and periodicity, respectively, ranging between 1·1–1·4 and 1·8–2·0 m in neap and spring‐tide periods, for the Contas and Jaguaribe Rivers, respectively (Dias, 2007; Dias et al , 2007). Many studies conducted in tropical estuaries (Cunha, 1982; Medeiros and Kjerfve, 1993; Araujo, et al , 1999; Miranda et al , 2005) observed a higher tidal influence during dry period, where the estuaries were controlled for a semi‐diurnal barotropic tide. Nicolite et al (2009) observed that during lower discharges water level variation in the estuarine channel is due to the co‐oscillation of the astronomical tides and/or meteorological tides in the estuary of the Paraiba do Sul River.…”

Circulation and salt balance in the São Francisco river Estuary (NE/Brazil)

“…For example, Araujo et al (1999) termed the major source of energy for vertical mixing in the Joboatão river estuary (latitude 8º 14'S) being the bottom shear turbulence caused by tidal currents. Medeiros and Kjerfve (1993) observed the fresh-water input and the gravitational circulation in the Itamaraca estuary (7º 34'S) as the principal factor in determining the internal mixing.…”

Interactions of river discharge and tidal modulation in a tropical estuary, NE Brazil

“…An analysis on the processes and time scales controlling the dynamics of the lower Joboata˜o river estuary (latitude 8°14¢S), NE-Brazil, was made by Araujo et al (1999). A quantitative and comparative study on the forces maintaining stratification (solar heating, rainfall, longitudinal density gradient) and those responsible for the vertical mixing (mechanical stirring of tidal bottom stress, wind stress and evaporation) were studied.…”