Simulations and predictions of the tidal and general circulations in the South and Tropical Atlantic: high resolution grids in the Brazilian shelves

Joseph Harari 1 & Ricardo de Camargo 2 & Carlos Augusto de Sampaio França 3

1 ; 2 ; 3

1,3 Institute of Oceanography
2 Institute of Astronomy, Geophysics and Atmospheric Sciences
University of Sao Paulo - SP - Brazil



A three-dimensional hydrodynamical numerical model with sigma vertical coordinates (Blumberg & Mellor, 1987) was implemented to the South and Tropical Atlantic Ocean, from 85°S to 30°N and from 70°W to 25°E, in a grid of 191 x 231 points, with resolution 0.5° x 0.5° and 16 sigma levels (Ezer & Mellor, 1994, 1997, 2000). The model is being used for operational and scientific purposes and allows the hydrodynamics detailing of any sub-domain through grid nesting, especially continental shelves or coastal regions.
The model may be processed considering tidal, winds and thermodynamics forcings, separately or combined, based on: 1) the tidal potential in each grid point and tide oscillations at the borders; 2) surface winds in the model area and variations of the mean sea level at the boundaries; and 3) flows of heat and salt through the sea surface or, alternatively, temperature and salinity (TS) values at the grid surface, besides vertical profiles of TS at the open boundaries of the grid.


The hydrodynamical numerical model has been used with the following objectives:

    reproduction of the tidal circulation;
    studies of the seasonal variability of the general circulation;

    simulations of situations of interest (typical or extreme) and operational forecasts;

    processing of high resolution nested grids, to represent the transition of the circulation characteristics from the deep ocean to shelf areas.

Model runs and results

The bathymetry was extracted of Databases (ETOPO2 and GEBCO) and smoothed with 2D five points filter, being set to minimum values of 10 m and maximum of 5500 m; the Levitus et al (1998) annual climatologies of TS were prescribed at the open boundaries and the correspondent monthly climatologies considered for surface forcing and 3D relaxing of model results; mean sea level oscillations at the grid limits were estimated from outputs of global climatological experiments of OCCAM - Ocean Circulation and Climate Advanced Modelling (Webb et al, 1998). Appart from these mean annual and seasonal forcings, other two forcings are representative of specific periods of model integration: wind stresses at the surface, given by the NCEP / NCAR reanalysis, and tidal oscillations at the open boundaries, composed by using the harmonic constants generated by the AG95.1 global tidal model (Andersen, 1995).

Fig. 1 - Instantaneous model results - surface currents (m/s) and elevations (shaded, in m) - for the South and Tropical Atlantic.

Present publication is concerned with the processing of high resolution grids, to represent the circulation characteristics in shelf areas, with emphasis in the Brazilian coast, where grid spacings progressively decrease from 30' to 15', 10' and 5', in a process where the finer grids have boundary conditions specified by the coarser ones.

Figures 1 to 3 show instantaneous model results with tidal, winds and thermodynamic forcings, on 10:00 GMT 03 August 2005, for (1) the principal grid (30' spacing) and two nestings (5' spacing) of the following Brazilian coastal areas and neighbour deep oceans - E, NE and N (Fig. 2) and S, SE and E (Fig. 3). Figures 4 (a, b, c) show three other nestigns (5' spacing), with grid axes oriented parallel and perpendicular to the coast line, in norhtern, eastern and southern Brazil, respectively.

Fig. 2 - Instantaneous model results - surface currents (m/s) and elevations (shaded, in m) - for the nesting covering E, NE and N Brazil, and adjacent deep ocean.

The model has reproduced fairly well the principal features of the tidal and general circulations, as known from independent measurements (satellite altimetry and "in situ" data) and other large scale models: for the whole model area, the intense currents at the Equatorial region are predominant, enhanced by the tidal currents in the North Brazilian shelf (Fig. 1); besides the strong tidal currents at the Amazon Riven Mouth, the North Brazil Current Retroflexion is evident in Fig. 2; and the turning of the South Equatorial Current originating the Brazil Current is clearly shown in Fig. 3. More details of the shelf circulations are presented on Fig. 4, such as the huge tidal elevation at the Amazon river mouth (Fig. 4a), discontinuities in the Brazil Current (Fig. 4b) and the Brazil – Malvinas Confluence (Fig. 4c).

Fig. 3 - Instantaneous model results - surface currents (m/s) and elevations (shaded, in m) - for the nesting covering S, SE and E Brazil, and adjacent deep ocean.

The model results have been evaluated through time series analyses, applied to surface elevations and currents at several levels, as well as depth-mean currents and transports, in each grid point. Long term model runs have provided series that include seasonal variations of the general circulation, which are especially important in the equatorial area and the Brazil – Malvinas Confluence.

In the next stage of researches, the inter-annual variability of the circulation will be evaluated, as done by Ezer (1999) for the North Atlantic, considering as model inputs the winds and temperature values at the surface generated by re-analyses of global operational meteorological models relative to long periods of time, of the order of decades (from 1980 to 2005).

In all the numerical experiments that have been performed, the quality of the simulations are evaluated, by comparing the hydrodynamic fields produced by model runs with direct measurements (tide gauges and current meters) and remote sensing (altimeter satellites), including coastal observations.







Fig. 4 - Instantaneous results of the model for nested grids with axes oriented parallel and perpendicular to the coast line, in the Northern (a), Eastern (b) and Southern (c) Brazilian shelves.

Operational predictions of the ocean circulation in the South and Tropical Atlantic, as given by the model outputs, are regularly available on: and


Andersen, O. B. 1995. Global Ocean Tides from ERS1 and TOPEX/POSEIDON Altimetry. J. Geophys. Res. 100, p. 25,249-25,259.

Blumberg, A. F.; Mellor, G. L. 1987 A description of a three-dimensional coastal ocean circulation model. In: Heaps, N. S. (Ed.), Three-dimensional Coastal Ocean Models, vol. 4, American Geophysical Union, Washington D. C., pp. 1 -16.

Ezer,T. 1999. Decadal variabilities of the upper layers of the subtropical North Atlantic: An ocean model study. J. Phys. Oceanogr. 29 (12), 3111-3124.

Ezer,T.; Mellor, G. L. 1994. Diagnostic and prognostic calculations of the North Atlantic circulation and sea level using a sigma coordinate ocean model. J. Geophys. Res. 99, p. 14,159 – 14,171.

Ezer,T.; Mellor, G. L. 1997. Simulations of the Atlantic Ocean with a free surface sigma coordinate ocean model. J. Geophys. Res. 102(C7), 15,647-15,657.

Ezer, T.; Mellor, G. L. 2000. Sensitivity studies with the North Atlantic sigma coordinate Princeton Ocean Model. Dynamics of Atmospheres and Oceans 32 (2000), 185 – 208.

Levitus, S.; Conkright, M. E.; Boyer, T. P.; O’Brien, T.; Antonov, J.; Stephens, C.; Stathoplos, L.; Johnson, D.; Gelfeld, R. 1998. World Ocean Database 1998, vol. 1, Introduction (CD-ROMs), NOAA Atlas NESDIS 18, U. S. Govt. Print. Off., Washington, D. C. , 346 pp.

Webb, D. J.; Cuevas, B. A.; Coward, A. C. 1998. The first main run of the OCCAM global ocean model. Southampton Oceanography Center, Internal Report 34, 43 pp.