Marcus Silva

Seasonal changes in the mixed and barrier layers in the western Equatorial Atlantic

Climate is closely related to the dynamics of the surface layer of the tropical Atlantic and the exchange between this latter and the atmosphere, and wearther forecasting will improve with increasing understanding of the processes that govern the relative distribution of thermodynamic properties of the water column. This paper focuses on the isolation of warm surface waters from the cold ones of the deep ocean by a salinity induced barrier layer (BL) in the western equatorial Atlantic (3oS-7oN; 40o-52oW), based on 487 CTD profiles (REVIZEE - 1995-2001). The main process contributing to the seasonal BL formation is the discharge of low salinity waters from the Amazon river. During boreal late winter/spring (Mar-May; high river discharge), deeper isothermal (ZT) and mixed layers (ZM) prevail and the formation of a 16m-thick BL was clearly determined the formation of a salt-induced marked pycnocline within a deeper isothermal layer. However, during the boreal autumn (Oct-Dec; low river discharge), density stratification was mainly determined by temperature distribution (ZM m ZT; BLT = ZM - ZT m 0). There was no clear register of a BL on the Amazon shelf, but a maximum BL (40 m) formed near the shelf break at 45°W.

Tidal Turbulence and Eddy-Viscosity in Coastal Waters at Northeastern Brazil

Abstract Vertical distribution of Turbulent Kinetic Energy (TKE) at coastal NE-Brazilian waters was evaluated using a combination of field data and numerical modelling. Microestructure temperature profiles obtained with a SCAMP probe were used to estimate the rate of TKE dissipation at coastal waters off Suape Harbour. SCAMP measurements and field cinematic information were then used as input data to a turbulence model and numerical results used to obtain simple eddy-viscosity formulations as a function of low-order parameters associated to measured characteristics of velocity profiles. Stronger TKE dissipation rates at Suape waters showed to be associated to surface and bottom boundary layers during spring tides, with values ranging between 5 × 10−7 m2s−3 ≤ ϵ ≤ 3 × 10−6 m2s−3. Less accentuated dissipation rates were found at the interior regions of the flow, with 5 × 10−8 m2s−3 ≤ ϵ ≤ 2 × 10−7 m2s−3. Numerical results indicated that energy balances immediately near boundaries are mainly driven by an equilibrium between production and dissipation of TKE, reflecting classical log-layer behaviours near a wall. Diffusion of TKE acts as an important process for vertical energy distribution all over the interior flow depth at Suape area. Vertical distributions of TKE point out highest energy intensities at central part of the flows, which is reflected at the eddy-viscosity profiles. Outside bottom and surface regions the turbulence production induced by velocity rotation is predominant. Model results and a performed scale analysis indicate the maximum eddy-viscosity for each situation is vt = O((Δθ)5/2), where Δθ(rad) is the global current field rotation observed along water depth. This analysis is expanded to find a simple bilinear expression for eddy-viscosity along water depth.

Modeling Subsurface Gas Release in Tropical and Shallow Waters: Comparison with Field Experiments off Brazil's Northeast Coast

ABSTRACT Despite the fast growth of underwater oil and gas exploration in low latitude regions, very few experimental data acquisition and modeling studies involving gas release in tropical and shallow waters are found in literature. In this article, a dataset of geophysical and gas release measurements obtained from an in situ experiment conducted off the northeastern Brazilian coast are used as a baseline for evaluating the GASOCEAN blowout model. Hydrological and hydrodynamic data were collected for distinct seafloor gas plume releases (3000 to 9000 L/hr) during neap/spring tides of summer/dry and winter/wet periods. Simulation results indicate that the gas plume is horizontally displaced by the horizontal current as it rises through seawater column. The extreme situation provided a critical radius (maximum horizontal displacement) from the gas release source of 35.2 m. The comparison between the measured and the calculated data showed that the model satisfactorily represented the main features of the gas release, such as the displacement (11.6–35.2 m), diameter (1.2–2.8 m), and ascending time (1.1–1.6 min) of the plumes. Although the mean plume widths have the same order of magnitude between the measurements and the calculations, improvements may enhance the models performance during the earlier plume development.