Belmiro M. Castro

Annual Cycle and Variability of the North Brazil Current

Abstract Current meter observations from an array of three subsurface moorings located on the Brazil continental slope near 4°N are used to describe the annual cycle and low-frequency variability of the North Brazil Current (NBC). The moored array was deployed from September 1989 to January 1991, with further extension of the shallowest mooring, located over the 500-m isobath near the axis of the NBC, through September 1991. Moored current measurements were also obtained over the adjacent shelf for a limited time between February and May 1990. The NBC has a large annual cycle at this latitude, ranging from a maximum transport of 36 Sv (Sv ≡ 106 m3 s−1) in July–August to a minimum of 13 Sv in April–May, with an annual mean transport of approximately 26 Sv. The mean transport is dominated by flow in the upper 150 m, and the seasonal cycle is contained almost entirely in the top 300 m. Transport over the continental shelf is 3–5 Sv and appears to be fairly constant throughout the year, based on the available...

Physical oceanography of the Amazon shelf

Abstract The Amazon shelf is subject to energetic forcing from a number of different sources, including near-resonant semi-diurnal tides, large buoyancy flux from the Amazon River discharge, wind stress from the northeasterly tradewinds and strong along-shelf flow associated with the North Brazil Current. Although the large volume of river discharge produces a pronounced salinity anomaly, the water motions on the shelf are dominated by the other forcing factors. Tidal velocities of up to 200 cm s−1 are generally oriented in the cross-shelf direction. Tide-induced mixing influences the position and structure of the bottom salinity front that separates the well-mixed nearshore region from the stratified plume. High concentrations of suspended sediment trapped along the frontal zone increase the stability of the tidal boundary layer and thus reduce the bottom stress. At subtidal frequencies, motion is primarily along-shelf toward the northwest, both in the plume and in the ambient, high-salinity water of the outer-shelf. The plume is generally 5–10 m thick, with a salinity of 20–30 psu. The along-shelf velocity within the plume varies as a function of the along-shelf wind stress. This variability results in large temporal variations in plume structure and freshwater content on the shelf. The net northwestward motion of the Amazon plume and of the ambient shelf water appears to be the result of a large-scale pressure gradient associated with the North Brazil Current system.

The M2 tide on the Amazon Shelf

As part of A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS), moored and shipboard current measurements made over the Amazon shelf during 1990–1991 have been analyzed to determine the dominant semidiurnal tidal constituent, the M2. These results have been combined with coastal sea level data from within the Amazon and Para Rivers, the adjacent shelf, and with satellite-derived tidal elevation data from off the shelf to provide a more complete description of the M2 tide in this complex river/shelf system. Near the Amazon River mouth the M2 tide propagates across the shelf and through the mouth as a damped progressive wave, with its amplitude decreasing and phase increasing upriver. Over the adjacent shelf north of Cabo Norte, the M2 tide approaches a damped standing wave, with large amplitudes (greater than 1.5 m) near the coast due to near resonance within the coastal embayment formed by the Cabo Norte shoal to the south and Cabo Cassipore to the north. The observed M2 tidal currents are nearly rectilinear and oriented primarily across the local isobaths. Comparisons between tidal observations in both the North Channel and the Cabo Norte-Cabo Cassipore embayment and a simple variable-width channel tidal model indicate that (1) most of the M2 tidal energy dissipation occurs over the mid- and inner shelf (in water depths less than 20 m) and (2) fluid muds found there cause a significant reduction (of order 50%) in the effective bottom friction felt by the M2 tide. The approximate resonant period of the Cabo Norte-Cabo Cassipore embayment is 11.9 hours, and at resonance the average energy dissipation per forcing period is roughly 2.2 times the average mechanical energy in the embayment. This damping rate is large enough that the tidal amplification is rather insensitive to forcing frequency, so that the response of the embayment to forcing over the semidiurnal band should be essentially the same. The vertical structure of the M2 tidal current is examined at one outer shelf site located in 65-m water depth. The observed semimajor axis increases logarithmically with height above bottom within the lowest 1–2 m and reaches a maximum in excess of 0.5 m/s at approximately 11 m above bottom. The mean ellipticity is small (less than 0.1) and positive, indicating clockwise rotation of a nearly rectilinear current, and the semimajor axis is oriented within 10° of the local cross-isobath direction. The M2 phase increases with height above bottom, with flood at the bottom leading flood at the surface by about 1 hour. A simple, local homogeneous tidal model with time- and space-dependent eddy viscosity simulates the observed near-bottom velocity reasonably well, however, the model suggests that stratification above the lowest few meters may significantly affect the tidal boundary layer structure at this site. The M2 energy flux onto the Amazon shelf and into the Amazon and Para Rivers has been estimated using current and surface elevation data and the best fit variable-width channel model results. The net M2 energy flux into the mouths of the Amazon and Para Rivers is 0.47×1010W and 0.19×1010W, respectively. A net M2 energy flux of about 3.3×1010W occurs onto the shelf between the North Channel of the Amazon River and Cabo Cassipore. This stretch of the Amazon shelf accounts for about 1.3% of the global dissipation of the M2 tide.

Wind‐forced sea level variability on the southeast Brazilian shelf

Coastal sea level data collected along the southeast coast of Brazil, from Cabo Frio to Cabo de Santa Marta, for the period August–November 1978 were examined, together with meteorological data for evidence of wind-forced, low-frequency fluctuations. A dominant 9- to 12-day perturbation in sea level, atmospheric pressure, and wind propagated equatorward from Cananeia and appeared to propagate poleward between Cananeia and the southernmost station. A 6- to 7-day perturbation in sea level and wind propagated equatorward from the southernmost station, in agreement with the movement of cold fronts through the area. Sea level fluctuations in the central and northern portions of the bight for both 9- to 12- and 6- to 7-day bands were better correlated with winds located southward and earlier in time than with the local wind at the time of sea level measurement. Modeling of sea level response at Ubatuba for a period of 90 days, using the theory of barotropic continental shelf waves generated by wind forcing, presented results that were similar to the observed response.

Circulation and mixing due to tidal forcing in the Bertioga Channel, São Paulo, Brazil

Bertioga Channel is a partially mixed (type 2) tidal estuary on the coastal plain of São Paulo, Brazil. Hourly current and salinity measurements during neap and spring tides in July 1991 yielded information about the physical structure of the system. Peak along-channel velocities varied from 40 cm s−1 to 60 cm s−1 during flood tides and from 70 cm s−1 to 100 cm s−1 during ebb tides. Net vertical velocity profiles indicate that the net current reverses directions at a depth of 2.5–3.0 m in the halocline. Due to appreciable fortnightly tidal modulation, the estuary alternates from being highly stratified (type 2b) during neap tides, with advection and diffusion contributing equally to the net upstream salt flux, to being moderately stratified (type 2a) during spring tides, when 90% of the net upstream salt transport is the result of effective tidal diffusion. Decomposition of the salt flux indicates that the relative contribution to the upstream salt transport by gravitational circulation shear is greater than the oscillatory tidal flux by a factor of 2.6 during neap tides. The oscillatory tidal flux is generated by the correlation of the tidal components of the u-velocity and salinity and is responsible for approximately the same amount of upstream salt transport, during neap and spring tides. However, during spring tides, this oscillatory term is greater than the other salt flux terms by a factor of 1.4. The total salt transport, through a unit width of the section perpendicular to the flow, was within 2% of the sum of the seven major decomposed, advective and dispersive terms. On the assumption that the Bertioga Channel is laterally homogeneous, the results also indicate that the estuary is not in steady state with respect to salt flux.

Internal Tides in the Southwestern Atlantic off Brazil: Observations and Numerical Modeling

Abstract Data collected from moored instruments, deployed over the southeastern Brazilian continental shelf during the summer and winter months of 2001, show internal tide activity near the shelf break. To help to elucidate the observations, a fully three-dimensional nonlinear primitive equation model is applied to simulate the regional barotropic and baroclinic tides. Two semidiurnal (M2 and S2) and two diurnal (K1 and O1) tidal frequencies are considered. Tidal surface elevations are relatively small over the whole modeled area, reaching maximum values of about 0.40 m for M2 and 0.11 m for O1. Comparison between observed and computed tide elevation and Greenwich phase shows reasonable agreement. When the baroclinic response of the model is investigated, stratification is prescribed using summer and winter climatology data of potential density. In this case, the model response to summer and winter stratifications is very similar and internal tides are generated over the shelf break and slope, with vertic...

Hydrodynamics and water properties at the entrance of Araçá Bay, Brazil

Current meter and hydrographic properties at the entrance of the Araçá Bay (AB), an intertidal flat adjacent to the São Sebastião channel, were collected between July 2013 and February 2014. These data sets show two different hydrographic periods, marked by a sharp change in the temperature and salinity values, clearly caused by the arrival of the South Atlantic Central Water (SACW). The first period is characterized by small variabilities on both properties with the dominance of coastal water, with relatively low salinity values. The second period shows a strong increase in the average salinity values and a much larger variability of temperature. This change in the hydrographic characteristics seems to be caused by anomalous winds, capable of displacing the SACW toward the coast. On the other hand, current meter data shows that the dynamics is mainly driven by the large-scale wind and is not impacted by the arrival of the SACW. Also, the currents are dominated by the barotropic mode, independently of the stratification differences that are observed between the beginning and end of the observational period.

A Multidisciplinary Amazon Shelf SEDiment Study (AmasSeds): Physical Oceanography Moored Array Component

Abstract : A Multidisciplinary Amazon Shelf SEDiment Study (AmasSeds) is a cooperative research program by geological, chemical, physical, and biological oceanographers from Brazil and the United States to study sedimentary processes occurring over the continental shelf near the mouth of the Amazon River. The physical oceanography component of AmasSeds included a moored array deployed on the continental shelf approximately 300km northwest of the Amazon River mouth near 3.5 deg N. The moored array consisted of a cross-shelf transect of three mooring sites located on the 18-m, 65-m, and 103-m isobaths. The moored array was deployed for approximately 4 months, from early February, 1990 to mid-June, 1990, obtaining time series measurements of current, temperature, conductivity, and wind. This report describes the physical oceanography moored array component and provides a statistical and graphical summary of the moored observations.... moored oceanographic observations, Amazon River/ North Brazil Continental Shelf, AmasSeds(A Multidisciplinary Amazon Shelf SEDiment Study).

Methods for estimating the velocities of the Brazil Current in the pre-salt reservoir area off southeast Brazil (23 ∘ S–26 ∘ S)

The Brazil Current (BC) is likely the least observed and investigated subtropical western boundary current in the world. This study proposes a simple and systematic methodology to estimate quasi-synoptic cross-sectional speeds of the BC within the Santos Basin (23∘ S–26∘ S) based on the dynamic method using several combinations of data: Conductivity, temperature, and depth (CTD), temperature profiles, CTD and vessel-mounted Acoustic Doppler Current Profiler (VMADCP), and temperature profiles and VMADCP. All of the geostrophic estimates agree well with lowered Acoustic Doppler Current Profiler (LADCP) velocity observations and yield volume transports of -5.56 ±1.31 and 2.50 ±1.01 Sv for the BC and the Intermediate Western Boundary Current (IWBC), respectively. The LADCP data revealed that the BC flows southwestward and is ∼100 km wide, 500 m deep, and has a volume transport of approximately -5.75 ±1.53 Sv and a maximum speed of 0.59 m s−1. Underneath the BC, the IWBC flows northeastward and has a vertical extent of approximately 1,300 m, a width of ∼60 km, a maximum velocity of ∼0.22 m s−1, and a volume transport of 4.11 ± 2.01 Sv. Our analysis indicates that in the absence of the observed velocities, the isopycnal (σ0) of 26.82 kg m−3 (∼500 dbar) is an adequate level of no motion for use in geostrophic calculations. Additionally, a simple linear relationship between the temperature and the specific volume anomaly can be used for a reliable first estimate of the BC-IWBC system in temperature-only transects.

Environmental Assessment of Admiralty Bay, King George Island, Antarctica

Monitoring has been developed by Antarctic Programmes such as Antarctic New Zealand (www.antarcticanz.govt.nz) and the US Antarctic Programme (USAP) (http://www.usap.gov) considering expansion of human activities and inevitable environmental.