RL Mello† and LB de Miranda

Journal of Coastal Research
553 - 555
SI 39
ICS 2004 (Proceedings)
Brazil
ISSN 0749-0208
Circulation and Hidrography of SantoAmaro Bight, Guarujá (SP), Brazil
R. L. Mello† and L. B. de Miranda‡
† Oceanography Institute
University of São Paulo, São
Paulo
05508-900, Brazil
[email protected]
‡ Oceanography Institute
University of São Paulo, São Paulo
05508-900, Brazil
[email protected]
ABSTRACT
MELLO, R. L. and MIRANDA, L. B., 2006. Circulation and hidrography of Santo Amaro Bight, Guarujá (SP),
Brazil. Journal of Coastal Research, SI 39 (Proceedings of the 8th International Coastal Symposium), 553 - 555.
Itajaí, SC, Brazil, ISSN 0749-0208.
This work describes spatial and time changes of hydrographic properties, water mass formation and set up the main
circulation characteristics in the Santo Amaro Bight. Three cruises with 13 hydrographic stations and three current
meter moorings have been undertaken (between austral winter/1997 and autumn/1998). The longitudinal current
component does not indicate the predominant direction, however, the transversal component presented residual
movement towards the coast, with the possibility of sewage plume of the outfall reaching the coast in a tidal time
scale. Hydrographic characteristics were strongly influenced by meteorological systems. South Atlantic Central
Water mass was contributing indirectly to the formation of the Coastal Water mass.
ADDITIONAL INDEX WORDS: Bight, circulation, hydrographic properties.
INTRODUCTION
The Santo Amaro Bight (SAB) is a transitional coastal zone
on the southeast continental shelf located south of the Santo
Amaro Island (ISA) (Figure 1). The Bight is delimited by the
Munduba (MP) and Santo Amaro (SAP) Points, located to the
east and west of the Bight, respectively. The Moela Island (MI),
southeast of the MP, is an obstacle to the free passage of coastal
currents along the Bight mouth where the channel between the
ISA and the island enhances the current. The distance between
the MP and the SAP is approximately 10 km and the mean depth
of the SAB is almost 8.m. The submarine outfall of Guarujá,
located in the eastern region of the SAB, was projected to
promote a 1:180 primary dilution, through diffusers which are
located 4500 m from the beach and at a depth of 14 m. The
treatment station removes 98% of coliforms and the domestic
effluents promote little toxity to the marine environment
(RACHID, 2002). The SAB has free connection to the
continental shelf which widens almost 230 km offshore; on the
continental shelf the isobaths are oriented approximately along
the coastline and the shelf break is located at a depth of 180 m.
Due to small river discharged and runoff, the small density
gradient and the geometry of the region, we may hypothesize
that the SAB is forced mainly by winds and tides. These forces
are functions of space and time acting simultaneously on the
coastal region.
From wind fields obtained at the National Centers for
Environmental Prediction National Center for Atmospheric
Research (NCEP-NCAR) Reanalysis, it was possible to
conclude that the oceanographic experiment of October/1997
was made under the cold fronts influence. A tidal analysis was
made with hourly values for 20 years (1971-1990) of tide gauge
located in the port of Santos channel, lat. 23º57'18''S; long.
046º18'36''W. Results of this analysis, performed with the
PACMARE software (FRANCO, 2000), were accepted to
represent the main characteristics of the tidal forcing of the SAB
and were used to preview the tidal conditions in simultaneous of
currents measurements. The shape number (Nf), introduced by
A. Courtier in 1938 (DEFANT, 1960), was 0.30 and was obtained
with the amplitudes of the diurnal and semidiurnal main
1
http://www.cdc.noaa.gov
Figure 1. The Santo Amaro Bight (SAB), limited by Munduba
(MP) and the Santo Amaro (SAP) Points. Current moorings (x),
hydrographic stations ( ), tide gauge station ( ) and Moela
Island (MI) are also indicated in the map.
components, which indicates a mixed tide with semidiurnal
predominance.
The goal of this work is to describe spatial and time changes
in hydrographic properties, water mass formation and main
circulation characteristics in the Santo Amaro Bight (SAB),
Guarujá, SP.
METHODOLOGY
The nearly synoptic hydrographic data (salinity, temperature
and pressure/depth) was spatially sampled in the water column
by 13 stations (Figure 1) using a Sea Bird CTD. These data were
obtained during three cruises in austral spring (October/1997)
and summer (March/1998).
The currents time series data were measured in three
moorings located along the submarine outfall and at 4500, 2800
and 1100.m from the beach (Figure 1). In each mooring, two
current meters (SensorData, model SD6000) were installed to
measure the intensity and direction of the current and its
temperature, with a sampling rate of 0.5 h; these instruments
were approximately 2 m below the surface and above the
............
Journal of Coastal Research, Special Issue 39, 2006
554
Mello and Miranda
bottom. The instruments located 1100 m from the beach
sampled data in the summer (January/March) and autumn
(April/June) of 1998. The mooring measurements 2800 and
4500 m away from the beach were made in the winter
(July/August) of 1997.
The basic specifications of the CTD used in hydrographic
cruises are ±0.001ºC, ±0.0001 S/m and 0.015% of the full scale,
for the temperature, conductivity and pressure, respectively.
The current meters used in the moorings had a precision of ±0.5
cm/s and ±7.5º for speed and direction and ±0.1ºC for the
temperature sensor.
The velocity vectors were decomposed in longitudinal (v)
and transversal (u) components in relation to the local system of
Cartesian coordinates: Oxyz. The Oy axis was oriented parallel
to the coastline. In such case, the positive values of the vcomponent (v>0) indicate movements towards NE (from
Munduba to Santo Amaro Points), and positive values of the ucomponent (u>0) indicate off shore movements. The axis Oz
was oriented downwards.
RESULTS AND DISCUSSION
The v- and u-components time series, near surface and
bottom, presented oscillations of high and low frequencies.
Low frequency current vectors stickplots are presented in
Figure 2. The vertical axis indicates movements towards NE
(component v) and the horizontal axis indicates transversal
movements (component u).
The longitudinal component (v) did not have a predominant
direction; its highest intensities varied from 25.5 to 31.2 cm/s,
and from 28.5 to 34.7 cm/s on the surface and the bottom,
respectively. The movements were more intense in the vicinities
of the bottom than the surface. The u-component presented
predominantly converging movements towards the coastline
(u<0) during the observations in summer, autumn (mooring at
1100 m) and winter (moorings at 2800 and 4500 m), with the
highest intensities varying from 9.8 to 18.2 cm/s in the surface
and from 16.7 to 29.8 cm/s near the bottom. Transversal
components, with convergent and divergent movements
towards the coastline and significant intensities (average of
22.0 cm/s), were also been observed at a depth of 20 m around
the Moela Island, in the period of winter-spring transition
(MOREIRA VALENTE et al., 2001). The residual mean values
computed from the surface and bottom time series were
approximately 3.0 e 3.6 cm/s in module, respectively, close to
the values simulated with the hydrodynamic model of HARARI
et al. (2000).
Considering the results of spectral analysis of the u and vcomponents, from the moorings at 1100, 2800 and 4500 m from
the beach, it was possible to observe that the circulation inside
the SAB was influenced by high and low frequencies due to tide
and meteorological forcing. In general, the energy spectrum of
these components was very similar and increased off shore.
This effect was caused by the energy dissipation due to the
bottom friction and shoreline. Moreover, the v-component on
the surface presents higher values in the coherence spectra in
the moorings at 4500 and 2800 m, in relation to the ucomponent, suggesting that the geometry of the region has little
influence in the longitudinal motions. From the analysis of the
tide autospectrum and the coherence spectrum of the crossed
correlation of tide time series and velocity components, it was
possible to see that the tidal forcing signal is present mainly in
diurnal and semidiurnal frequencies, with the semidiurnal tide
being the main forcing mechanism; this mechanism also had the
greatest contribution to the oscillations of the u-component,
except for the mooring at 4500 m in winter, where the
longitudinal component (v) presented greater coherence in the
cross-correlation with the tide. The tide diurnal component
presented low coherence, being some times below of the
significance limit.
The T-S volumetric statistical analysis distribution indicated
a greater volume (>60%) of water mass, in October of 1997 and
March of 1998, occurred in the salinity and temperature
intervals of 34.2 - 35.4 and 21.5 - 23.ºC. However, in summer
(March/1998), the distribution was well uniform, with
predominant salinity interval from 35.0 to 35.4, associated to
temperatures varying between 24.5 and 28.ºC.
Comparisons of the scatter T-S diagram for different seasons
indicated that during summer, the water column was well
stratified, with the images of the (T,S) points converging to the
characteristic signal of the South Atlantic Central Water
(SACW). From previous works (MELLO, 2003; Miranda et al.
2003), it is known that a great portion of the continental shelf
bottom waters during the summer may be classified as SACW.
However, this water mass does not penetrate effectively in the
SAB, having an indirect influence in Coastal Water (CW)
formation. The hydrographic characteristics varied very little
spatially in the SAB and the properties (S,T,st) had not varied
more than 6% in relation to the average in all the region, in the
sampling periods of October and March.
CONCLUSIONS
The u- and v- components presented high and low
frequencies oscillations forced mainly by tide and synoptic
wind, with time scales of hours and days, respectively. The
oscillations of low frequency had presented average intensity in
module of 3 cm/s and the residual (high frequencies) intensities
of 5 cm/s. Time series of v-velocity component on the surface
and the bottom layers had indicated movements from Munduba
Point to Santo Amaro Point and vice-versa, with highest
intensities of 31.2 and 34.7 cm/s. However, the u-velocity
component presented predominantly movements towards the
coast, with highest intensities of 18.2 and 29.9 cm/s, for surface
and near bottom, respectively. The intensities of these
components are lesser than those movements measured
offshore (HARARI et al., 2001), indicating the SAB's water mass
is advected by relatively slow motions. The shearing stress of
these components was small, contributing very little to the
vertical mixture process.
Our investigation indicates that the circulation intensities in
the SAB were lesser than that resulted of measurements made
and analyzed by other researchers nearby the Moela Island
(MOREIRA VALENTE et al., 2001) and around the Laje de Santos
Marine State Park (HARARI et al. 2001). The filtered highest
intensities of the transversal component (u) near the bottom had
decreased (in module) towards the beach from 20.0 to 10.0
cm/s. Even so, the velocities observed in summer (time of
intense tourist activities) were small, with a convergent
movement to the beach in the bottom layers reaching values as
high as 10.0 cm/s. The advective effect of this movement, on the
effluent plume discharge by the diffusers system of the Guarujá
submarine outfall, may transport sewage concentrations which
may reach the beach in approximately 13 hours.
Spectrum analysis indicates that v-component has greater
energy on the surface layers in comparison of u-component,
while on the bottom layers the u and v-components have
approximately equal energies. From the power spectrum
analysis it was seen that the energy increases with the distance
from the coast. These components have similar power spectrum
with energies forced by meteorological events and tide. The
main frequencies found in the spectrum were diurnal and
semidiurnal, with the last component being the most important.
Scatter T-S diagrams showed small stratification of the water
column during October and great stratification in March. The
meteorological effects are important agents for the
hydrographic properties distribution; the SW winds of cold
fronts become water column more homogeneous through the
increase of the vertical mixture and the presence of winds of NE
with moderate intensities (~ 10 m/s) amplifies the SACW
bottom intrusion.
Through the termohaline horizontal structures and the T-S
statistic-volumetric analysis, it was observed during the
summer (March) the occurrence of salinity reduction on the
surface, due to an increased runoff. The termohaline horizontal
structures indicated small stratification; therefore the
hydrographic properties presented small space variation.
Journal of Coastal Research, Special Issue 39, 2006
Circulation and Hidrography
555
Figure 2. Stickplot of the low frequency velocity variability for currents at 1100, 2800 and 4500 m from beach, in the surface and near the
bottom (from Mello, 2003).
Considering that the experiments analyzed in this work
covered only three sazonal short period experiments, theirs
results on the oceanographical characteristics of the SAB are
only partial. More systematic experimental efforts in time and
space must be scheduled in the future, for the best knowledge of
this important region used intensively for tourist and urban
activities and is the receptacle of effluent domestic thrown
through the submarine outfall.
ACKNOWLEDGEMENTS
We are grateful to the Fundação de Estudos e Pesquisas
Aquáticas (FUNDESPA) and the Companhia de Saneamento
Básico do Estado de São Paulo (SABESP) to enable the use of
the oceanographic data set. We are also grateful to the M. Sc.
and Research Productivity scholarships of the Conselho
Nacional de Desenvolvimento Cientifico e Tecnológico
(CNPq) and FUNDESPA. English revision by Ana Claudia de
Paula was very much appreciated.
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Journal of Coastal Research, Special Issue 39, 2006