M.B. Framiiian . M.P. Etala . E.M. Acha . R. A. Guerrero· C.A. Lasta . O.B. Brown
zontal extent on the inner-shelf offshore the estuary as in December 1989 (Plate 8.2a).
This temperature minimum has the surface characteristics of coastal upwelling. However,
the in situ observations available are not appropriate to describe the water column
during these events, so the characterization of this phenomenon is incomplete. Between
Punta del Este and Cabo Polonio the coast has approximately an orientation of 60°;
southward to northwestward winds are upwelling favourable. The wind field statistics
from the weather station Ponton Pnicticos Recalada, located in the estuary (Fig. 8.1),
for the period 1981-1990 (SMN 1992) shows that in January and December such winds
are predominant, with 71 % occurrence, and 69% in February. During these three months,
westward winds are predominant (26%, 22% and 27% of the total distribution for January, February and December, respectively) followed by the southwestward winds. Wind
time series recorded at several weather stations in the area during December 1989 to
January 1990 are presented (Plate 8.2). Weather stations Punta Brava (Montevideo),
Laguna del Sauce (located east of Punta del Este) and La Paloma (c. Santa Maria) are
located on the Uruguayan coast (Fig. 8.1). The records consist of four observations per
day; direction is from an 8-point wind rose at P.P. Recalada station and a 16-point rose
at the other stations. The reference system is rotated 30° counterclockwise and vectors
point in the downwind direction. Upwelling favourable winds on the P. del Este-C.
Polonio coast are represented on this reference system to the left of page. In weather
station P. Recalada 78% of the observations are southward to northwestward winds; 85%
at Punta Brava, 58% at Laguna del Sauce and 78% on the northern coast at La Paloma.
The analysis shows that during December and January the wind field is favourable
for driving coastal upwelling between P. del Este and C. Polonio. But, the shallowness
of the area and the homogeneity observed in the water column in the estuary and inner
shelf during summer (Fig. 8.13) suggest a more complex picture. Strong onshore
advection of cool water from deeper layers on the shelf is necessary to generate the
temperature gradient observed at the surface in satellite images. Further studies are
necessary to fully understand the phenomenon. Bottom topography and shelf circulation might play an important role in the process.
8.6
Turbidity Front
In the transition zone between the upper and lower estuary, the processes associated
with the interaction of the fresh river water, the saline shelf water, and tidal-stirring
generate a turbidity front. The turbidity maximum is clearly defined in the NOAAAVHRR visible channel as a strong gradient in reflectance and a sharp change in water color (Plate 8.3a). Framiftan and Brown (1996) used these characteristics to determine the position of the front; results of their study are reviewed here.
Framiftan and Brown (1996) used a four-year span of NOAA-AVHRR daily images,
from September 1986 to August 1990. Channels 1 and 2 (visible and near-infrared) were
used to digitise the fronts, and channel 4 and sea surface temperature were used as
complementary information for cloud detection. From a total of 2578 images, 1274 daytime images allowed determination of 333 positions of the front (Plate 8.3b). This information served to estimate the distribution of the frontal density, a probalistic measure of frontal occurrence in a given area (for a full description of the data and methodology see Framiftan and Brown (1996).
zontal extent on the inner-shelf offshore the estuary as in December 1989 (Plate 8.2a).
This temperature minimum has the surface characteristics of coastal upwelling. However,
the in situ observations available are not appropriate to describe the water column
during these events, so the characterization of this phenomenon is incomplete. Between
Punta del Este and Cabo Polonio the coast has approximately an orientation of 60°;
southward to northwestward winds are upwelling favourable. The wind field statistics
from the weather station Ponton Pnicticos Recalada, located in the estuary (Fig. 8.1),
for the period 1981-1990 (SMN 1992) shows that in January and December such winds
are predominant, with 71 % occurrence, and 69% in February. During these three months,
westward winds are predominant (26%, 22% and 27% of the total distribution for January, February and December, respectively) followed by the southwestward winds. Wind
time series recorded at several weather stations in the area during December 1989 to
January 1990 are presented (Plate 8.2). Weather stations Punta Brava (Montevideo),
Laguna del Sauce (located east of Punta del Este) and La Paloma (c. Santa Maria) are
located on the Uruguayan coast (Fig. 8.1). The records consist of four observations per
day; direction is from an 8-point wind rose at P.P. Recalada station and a 16-point rose
at the other stations. The reference system is rotated 30° counterclockwise and vectors
point in the downwind direction. Upwelling favourable winds on the P. del Este-C.
Polonio coast are represented on this reference system to the left of page. In weather
station P. Recalada 78% of the observations are southward to northwestward winds; 85%
at Punta Brava, 58% at Laguna del Sauce and 78% on the northern coast at La Paloma.
The analysis shows that during December and January the wind field is favourable
for driving coastal upwelling between P. del Este and C. Polonio. But, the shallowness
of the area and the homogeneity observed in the water column in the estuary and inner
shelf during summer (Fig. 8.13) suggest a more complex picture. Strong onshore
advection of cool water from deeper layers on the shelf is necessary to generate the
temperature gradient observed at the surface in satellite images. Further studies are
necessary to fully understand the phenomenon. Bottom topography and shelf circulation might play an important role in the process.
8.6
Turbidity Front
In the transition zone between the upper and lower estuary, the processes associated
with the interaction of the fresh river water, the saline shelf water, and tidal-stirring
generate a turbidity front. The turbidity maximum is clearly defined in the NOAAAVHRR visible channel as a strong gradient in reflectance and a sharp change in water color (Plate 8.3a). Framiftan and Brown (1996) used these characteristics to determine the position of the front; results of their study are reviewed here.
Framiftan and Brown (1996) used a four-year span of NOAA-AVHRR daily images,
from September 1986 to August 1990. Channels 1 and 2 (visible and near-infrared) were
used to digitise the fronts, and channel 4 and sea surface temperature were used as
complementary information for cloud detection. From a total of 2578 images, 1274 daytime images allowed determination of 333 positions of the front (Plate 8.3b). This information served to estimate the distribution of the frontal density, a probalistic measure of frontal occurrence in a given area (for a full description of the data and methodology see Framiftan and Brown (1996).