Off the Bay of Santos, at the positions (Lat. 24° 20' S, Long. 046° 23' W) and (Lat. 24° 15' S, Long. 046° 01' W), two Acoustic Doppler Current Profilers - ADCP´s were moored, profiling the currents in water columns with depths of 32 and 52 meters (stations cos32 and cos52 - Figure 1). The sampling interval was 30 minutes and the vertical resolution of 1 meter. In both stations the first level of observations was 5 m and the last one 7 m above the bottom. The currents sampling was part of a survey to infer the hydrodynamic characteristics of this area, as a support for companies that use the Port of Santos.

Fig. 1 - Bathymetry (m) and positions of ADCP moorings.

Figures 2 and 3 present the time series of the current components (EW and NS), at the surface and bottom of stations cos32 and cos52.

Fig. 2 - Time series of current components at the surface and bottom, in cos32.

Fig. 3 - Time series of current components at the surface and bottom, in cos52.

The time series have high frequency oscillations due to the tides and low frequency oscillations forced by winds (associated to cold fronts in the area). In station cos32, the EW component at the surface has values between -70 and 50 cm/s and the NS between -40 and 40 cm/s; both components have the tidal signals, but the low frequency oscillations are found in the EW component only. These characteristics are also found in the time series relative to 25 m depth.

In station cos52, the surface currents are much stronger than those of cos32, with extreme values of -100 and 160 cm/s for the EW component; again there are lower values for the NS component. But, at the surface of cos52 station, the low frequency oscillations also appear in the NS current component. The currents decay with depth and present much lower values at 45 m: the EW component varies between -50 and 50 cm/s and the low frequency oscillations of the NS component almost disappear.
 
 

The spectra of amplitudes of the EW current components are shown on Figures 4 and 5. Obviously the characteristics of the currents described above are present in the spectra, but with more details. The most striking effect here is the huge decay of the diurnal current components, in both stations, but especially in cos52: in this position, at the surface, except the low frequency oscillations, the diurnal oscillation is the biggest component, with almost 12 cm/s in amplitude; however, at 45 m this oscillation does not exist. On the other hand, the semi-diurnal components present a small rate of decay along the vertical. This expressive decay of the diurnal components and much smaller one of the semi-diurnals was also observed in other current time series of this shelf, such as the ones of Mesquita & Harari (2000, 2001).
 
 

Fig. 4 - Spectra of amplitudes of EW current components at the surface and bottom, in cos32.
 
 

Fig. 5 - Spectra of amplitudes of EW current components at the surface and bottom, in cos52.

The data analysis showed above is part of a complete survey performed in the shelf area, which included meteorological and sea level observations. The set of measurements is being used for calibrating a hydrodynamical numerical model and will give support to many important researches actually in progress.

REFERENCES

MESQUITA, A. R. & HARARI, J. - 2000 - "On the sea level network and circulation in the southeastern Brazilian coast" - In: "Ocean circulation science derived from the Atlantic, Indian and Arctic sea level networks" - International Oceanographic Commission (IOC) - Workshop Report n° 171, p. 34 - 51.

MESQUITA, A. R. & HARARI, J. - 2001 - "On the harmonic constants and tidal ellipses of currents of the South-eastern Brazilian shelf" - Afro-America Gloss News, vol. 4 (1), 12 p.

ACKNOWLEDGMENTS

We are grateful to FUNDESPA - Fundação de Estudos e Pesquisas Aquáticas (Foundation for Aquatic Studies and Researches) for providing the equipments and data (Project Assessment of dredging the Channel of Access to the Port of Santos - area of Cubatao - 2001 / 2002).