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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