Energy Balance and Mixing Timescales in a Stirring Tropical Estuary, Itamaracá, Brazil

Abstract

Stratification and mixing dynamics in estuaries are controlled by different physical processes, which determine estuarine hydrodynamics and the transport of plankton, nutrients, and organic compounds. In this paper, we present an investigation of features that control the dynamics of the Itamaracá estuary, Pernambuco (northeastern Brazil) through quantitative and comparative analyses between stratification processes (surface heating, rainfall precipitation, and differential advection of the longitudinal density gradient resulting from the vertical velocity structure) and mixing processes (stirring of bottom tidal stress, stirring of surface wind stress, and surface evaporation). Thermodynamic and cinematic field data were obtained during rainy and dry seasons. Results indicate that mixing intensity in the water column was about one and a half to twice as high as the potential capacity of external forcing in promoting vertical stratification. The bottom tidal stirring was the most important cause of vertical mixing. A theoretical analysis showed that, during the dry season, the stratification timescale (30 min) was greater than the turbulence decay timescale (9–10 min) and also greater than the slack water timescale. During the rainy season, the stratification timescale (7 min) had the same order of magnitude as the turbulence decay timescale (9–10 min). The Itamaracá estuary is a vertically well-mixed/weakly-stratified system during both periods (dry and rainy), except during rainy slack water periods, when stratification can be expected. These theoretical results are in agreement with previous field data and recent works involving numerical simulations at the Santa Cruz Channel.