OCEANOGRAPHIC CHARACTERISTICS OF BAIA DE TODOS OS SANTOS,BRAZIL by klutzfu58

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									          OCEANOGRAPHIC CHARACTERISTICS OF BAIA DE TODOS OS
          SANTOS,BRAZIL: CIRCULATION, SEASONAL VARIATIONS AND
                  INTERACTIONS WITH THE COASTAL ZONE

                          Mauro Cirano and Guilherme C. Lessa
             Centro de Pesquisa em Geologia e Geofísica, Instituto de Geociências
              Universidade Federal da Bahia, Campus Ondina, Salvador, Brasil

         Baía de Todos os Santos (BTS – Figure 1) is a large coastal bay, situated at 12°50' S
and 38°38' W, that borders part of the third major city in Brazil, Salvador, with a population
of over 3 million people and the largest petrochemical complex in the southern hemisphere.
The bay has an area of 1223 km2 and an approximate maximum width (west-east axis) and
length (north-south axis) of 32 and 50 km, respectively. Despite its socio-economic relevance,
there is a lack of good quality oceanographic data, and a comprehensive overview of the
oceanographic characteristics of the bay is yet to be done. In the year of 1998 the State
Government launched a large-scale water quality project in the bay, mooring 33 instruments
in 20 stations (Figure 1) to sample the velocity field, temperature, salinity and pressure for 15
days in the summer and winter. It also performed synoptic hydrographic cruises, measuring
salinity and temperature in the water column during complete tidal cycles, besides installing
four wind gauge stations. The main goal of this article is to present the results of a
comprehensive analysis of this data set and to determine what are the leading driving forces
that regulate the water circulation in the bay and in the neighboring coastal region.
         Winds inside the bay during the summer had a maximum speeds of 10.3 m/s, and
followed a well established daily pattern of strong sea breezes during the day and calm land
breezes at night. Strong afternoon winds attained maximum speeds at around 14:30 hs and
tend to arrive from around 100°. Mean wind speed was 3,2 m/s, with a unimodal distribution
centered at 2,3 m/s. No well defined pattern is observed during the winter, when the average
wind speed was higher (4 m/s) and with bimodal distribution (2,1 and 5,0 m/s). The strongest
winds tend to arrive from 160°, although with a more scattered distribution (between 120° and
200°).
         The tides are semidiurnal, with the diurnal components accounting for 7% to 11% of
the total tidal amplitude, with slightly larger values during summer. The amplitude of the M2
constituent varies between 0.66 m and 0,68 m in the ocean, but increases significantly up the
bay, where it reaches 1,06 m at station #18. Along with the tidal range, tidal asymmetry also
becomes more important landwards, as it is indicated by the M4/M2 ratio rising gradually
along the stations from 0.015 in the ocean to 0.09 at station #18.
                   Figure 1 – Study area and position of the oceanographic stations


       The M2 phase angle gradually increases from 90° in the ocean to 109° at station #18,
implying a time lag of 40 minutes between the tides in the ocean and in the inland most
station. In reality, measured high-tide time lags between stations #13 and #18 varied between
45 minutes and 1,5 hours, with larger lags during the summer spring tides. Larger spring-tide
time lags are ascribed to increased tidal asymmetries, more pronounced in the summer. The
astronomical tides explained between 98% and 99% of the observed tidal variation. The low-
frequency oscillations (maximum of 0.19 m) were in phase in all stations during the winter,
but curiously up to 180° out of phase inside and outside the bay during the summer.
       The currents inside the BTS are clearly tidally driven. The correlation coefficient and
the percentage of variance explanation between the predicted and the observed time series
show values above 0.90 and 85%, respectively. Outside the bay the correlation coefficients
and the levels of variance explanation decrease with increasing distance from the bay mouth,
until the tidal signature is obscured at station #14. Maximum velocities occur at the two
entrances of the bay, the Salvador Channel (station 8) and Itaparica Channel (station 17),
where the M2 component has magnitude of 65 cm/s close to the surface and 50 cm/s,
respectively. In most of the stations the M2 component of the currents ranged from 16 to 28
cm/s. Outside the bay the tidal currents are important only at stations 9 and 10. In the latter, a
constriction imposed by an ebb-tidal delta causes the M2 component to reaches values of
more than 60 cm/s. Tidal ellipses are generally oriented along the main channels, and
eccentricity is small.
        The residual circulation inside the bay (Figure 2), calculated for 15 days, was similar
in both campaigns, with velocities not exceeding 5 cm/s (equals 65 km of travelling in 15
days). At the central part of the bay (station 5), surface currents flow to the northwest whereas
the bottom currents flow to the southeast, suggesting gravitation circulation. In inner shelf, the
residual circulation is clearly seasonal (except for station 10) (Figure 2). During summer, the
predominant easterlies appear to drive southwest currents of up to 15 cm/s near the surface,
also generating a significant velocity shear in the water column. During winter, the
predominant winds are from southeast, and the associated currents flow to the northeast with
magnitudes comparable to those in the summer.




Figure 2 – Residual velocities in the summer and winter. Thick line vectors represent either a single
current meter or the near surface current meter, whereas the lighter vector represents the near bottom
instrument.


        The average temperature gradient in the summer was up to 3°C between the warmest
surface waters at station 1 (T = 29.9°C) and the relatively colder surface waters at stations 9-
12. In the water column the temperature gradient is in the order of 10-1°C, and gradients of
more than 1°C are only observed in the oceanic stations. The bay is dominated by oceanic
water, with an average salinities higher than 36.4. Exception was the region close to
Paraguaçu Channel (station 4). Outside the bay the average salinity was always above 37.
Worth mentioning is the above-average temperature and salinity at station 3, which suggests
that evaporation rates are higher than precipitation. Water properties during the winter were
quite distinct from those at summer, which is ascribed to increasing local precipitation.
Horizontal average salinity gradients on the surface were up to 4.2 between station 4 and the
ocean. The vertical salinity gradients are also more pronounced, with differences of more than
1°C in various stations inside the bay.
       According to the oceanographic characteristics study region presents two different
scenarios, both in terms of water properties and circulation. Inside the bay, the circulation is
primarily driven by tides and the patterns, as expected, do not differ from summer to winter.
Outside the bay the influence of tidal circulation is diminished and shelf dynamics, apparently
driven by the wind, plays an important role. If water circulation inside the bay is not altered
by the seasons, the same cannot be said about the water properties, which show increased
dilution of seawater during the winter.

ACKNOLEDGEMENT
       The authors thank the Project PETRORISCO (FINEP / REDE 05 / CTPETRO /
CNPq) for financial support for availability of basic infrastructure necessary for
accomplishment of this research.

								
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