14 research outputs found
Numerical study of circulation on the inner Amazon Shelf
Author Posting. © Springer, 2008. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Ocean Dynamics 58 (2008): 187-198, doi:10.1007/s10236-008-0139-4.We studied the circulation on the coastal
domain of the Amazon Shelf by applying the hydrodynamic
module of the Estuarine and Coastal Ocean
Model and Sediment Transport - ECOMSED. The first
barotropic experiment aimed to explain the major bathymetric
effects on tides and those generated by anisotropy
in sediment distribution. We analyzed the continental
shelf response of barotropic tides under realistic bottom
stress parametrization (Cd), considering sediment granulometry
obtained from a faciologic map, where river
mud deposits and reworked sediments areas are well distinguished,
among others classes of sediments. Very low
Cd values were set in the fluid mud regions off the Amapa
coast (1.0 10-4 ), in contrast to values around 3:5 10-3
for coarser sediment regions off the Para coast. Three-dimensional
experiments represented the Amazon River
discharge and trade winds, combined to barotropic tide
influences and induced vertical mixing. The quasi-resonant
response of the Amazon Shelf to the M2 tide act on
the local hydrodynamics by increasing tidal admittance,
along with tidal forcing at the shelf break and extensive
fluid mud regions. Harmonic analysis of modeled
currents agreed well with analysis of the AMASSEDS
observational data set. Tidal-induced vertical shear provided
strong homogenization of threshold waters, which
are subject to a kind of hydraulic control due to the topographic
steepness. Ahead of the hydraulic jump, the
low-salinity plume is disconnected from the bottom and
acquires negative vorticity, turning southeastward. Tides
act as a generator mechanism and topography, via hydraulic
control, as a maintainer mechanism for the low-salinity
frontal zone positioning. Tidally induced southeastward
plume fate is overwhelmed by northwestward
trade winds so that, along with background circulation,
probably play the most important role on the plume fate
and variability over the Amazon Shelf
Surface effects of bottom-generated turbulence in a shallow tidal sea
Turbulence in shelf seas strongly affects the spread of pollution (such as oil spills) as well as the distribution of sediment and phytoplankton blooms. Turbulence is known to be generated intermittently close to the sea bed, but little is known of its evolution through the water column, or to what extent it affects the surface. Here we present observations of the surface effects of bottom-generated turbulence in a tidally influenced and well mixed region of the North Sea, as derived from acoustic and visual images. Although the sea bed in the area is flat, we find that at any one time, 20–30% of the water surface is affected by boils—circular regions of local upwelling—of diameter 0.90.2 times the water depth. The signature of individual boils persists for at least 7 minutes and, in accordance with laboratory and numerical studies, shows the appearance of eddies. The boils contribute to the replacement of surface waters from depth in unstratified waters, and may therefore enhance the fluxes of gases between atmosphere and ocean