208
Gerardo M.E. Perillo· M. Cintia Piccolo
Wind effect on the estuarine circulation is observed at the level waves, storm surges
and subtidal sea level variations. At high frequencies, the wind produces two types of
waves a) wind waves and b) interaction waves. The former are normally small waves
of about 5-10 cm in height and 1-3 m in wavelength found in the channels and on the
tidal flats as they are covered by water. The later are waves formed by the interaction
of the incoming tide and the wind blowing from the Nand NW direction. These are
extremely steep waves, up to 1.5 m high, with wavelengths of the order of 10-20 m.
Although wind waves may be locally important for navigation of small boats, the
main effect of the wind is found over the tidal wave at lower frequencies. Comparison
of the predicted astronomical tide with the actual tidal records shows large differences
both in height and time. In general, winds blowing from the NW sector produce a set
down on the water level by pushing the surface water out or preventing the tidal penetration. SE winds generate the opposite effect. However, the predominant winds from
NW and N produce the greatest variations by:
.
a advancing the time of the low water,
b delaying the time of high water, and
c reducing the predicted water levels.
Perillo and Piccolo (1991) have analysed the deviations of two years of hourly simultaneous records of tide both at Ingeniero White and the Oceanographic Tower
(Fig. 9.1) from the predicted astronomical tides for both sites. They found 24 cases in
which the deviations were larger than 2 m with maximum of -4.01 and 2.39 m at
Ingeniero White, whereas at the Oceanographic Tower these maxima were -1.51 and
1.87 m. In both cases the maximum negative values coincide with winds blowing from
the NW and the maximum positive with winds from the SW. The latter are very intensive winds that blow normally after a storm and their effect is stronger (as in these
cases) when they coincide with high tide. The cross effect on the estuary is to induce
a set up on the northern coast.
Piccolo and Perillo (1989) studied the low-frequency sea level response to wind also
for the same stations. Spectral analysis indicated that the long term changes (time
scales >10 days) prevail. For shorter scales energy peaks are maximum at around 3 day
periods. In both cases, corresponding to the passage of fronts over the area. Although
both stations showed similar responses to wind forcing, wind effect is greater in the
inner estuary.
9.4.3
Salinity and Temperature
The analysis of historical data compiled by Perillo et al. (1987) from several stations
located along the axis of the Principal Channel of the Bahia Blanca Estuary presents a
mean annual surficial temperature of 13 0 C varying from 21.6 0 C in summer to 8.5 0 C
in winter (Piccolo et al. 1987). Average temperature in all seasons is slightly higher at
the head. Figure 9.7a shows the along channel distribution of the mean temperature
for the four seasons. The distribution of mean surface salinity (with a corresponding
correlation with density) shows an exponential growth from the head (Sauce Chico
discharge) up to the middle reach of the estuary (Fig. 9.7b). In the middle reach, the
Gerardo M.E. Perillo· M. Cintia Piccolo
Wind effect on the estuarine circulation is observed at the level waves, storm surges
and subtidal sea level variations. At high frequencies, the wind produces two types of
waves a) wind waves and b) interaction waves. The former are normally small waves
of about 5-10 cm in height and 1-3 m in wavelength found in the channels and on the
tidal flats as they are covered by water. The later are waves formed by the interaction
of the incoming tide and the wind blowing from the Nand NW direction. These are
extremely steep waves, up to 1.5 m high, with wavelengths of the order of 10-20 m.
Although wind waves may be locally important for navigation of small boats, the
main effect of the wind is found over the tidal wave at lower frequencies. Comparison
of the predicted astronomical tide with the actual tidal records shows large differences
both in height and time. In general, winds blowing from the NW sector produce a set
down on the water level by pushing the surface water out or preventing the tidal penetration. SE winds generate the opposite effect. However, the predominant winds from
NW and N produce the greatest variations by:
.
a advancing the time of the low water,
b delaying the time of high water, and
c reducing the predicted water levels.
Perillo and Piccolo (1991) have analysed the deviations of two years of hourly simultaneous records of tide both at Ingeniero White and the Oceanographic Tower
(Fig. 9.1) from the predicted astronomical tides for both sites. They found 24 cases in
which the deviations were larger than 2 m with maximum of -4.01 and 2.39 m at
Ingeniero White, whereas at the Oceanographic Tower these maxima were -1.51 and
1.87 m. In both cases the maximum negative values coincide with winds blowing from
the NW and the maximum positive with winds from the SW. The latter are very intensive winds that blow normally after a storm and their effect is stronger (as in these
cases) when they coincide with high tide. The cross effect on the estuary is to induce
a set up on the northern coast.
Piccolo and Perillo (1989) studied the low-frequency sea level response to wind also
for the same stations. Spectral analysis indicated that the long term changes (time
scales >10 days) prevail. For shorter scales energy peaks are maximum at around 3 day
periods. In both cases, corresponding to the passage of fronts over the area. Although
both stations showed similar responses to wind forcing, wind effect is greater in the
inner estuary.
9.4.3
Salinity and Temperature
The analysis of historical data compiled by Perillo et al. (1987) from several stations
located along the axis of the Principal Channel of the Bahia Blanca Estuary presents a
mean annual surficial temperature of 13 0 C varying from 21.6 0 C in summer to 8.5 0 C
in winter (Piccolo et al. 1987). Average temperature in all seasons is slightly higher at
the head. Figure 9.7a shows the along channel distribution of the mean temperature
for the four seasons. The distribution of mean surface salinity (with a corresponding
correlation with density) shows an exponential growth from the head (Sauce Chico
discharge) up to the middle reach of the estuary (Fig. 9.7b). In the middle reach, the