CHAPTER 8 .. Physical Characteristics and Processes of the Rio de la Plata Estuary
175
The bottom salinity has a smaller seasonal variation than the surface. The general
pattern is similar in both seasons, with isohalines mainly following the bottom topography. Fresher water is associated to shoals and more saline water is found in the
deeper channels. Similar effects of bottom topography on the transverse salinity distribution has been studied in Delaware and Chesapeake Bay (Wong 1994; ValleLevinson and Lwiza 1995; Valle-Levinson and O'DonneI1997). The importance of the
shallow banks is suggested by the locus of the 25 su isohaline, especially during fall/
winter; the influence of the deepest Oriental Channel along the Uruguayan coast is
clearly seen in the position of the 30-32 su isohalines. Another difference between the
two seasons is that during fall/winter a stronger gradient along the bottom occurs at
the northern end of Samboromb6n Bay, where the Maritime Channel borders the central shallow area of the bay. In spring/summer, the 25 and 30 su isohalines are located
at greater depths in the bay and the channel. This difference could be associated ~ith
an increase in the freshwater flow through the southern part of the estuary during
spring/summer compared to fall/winter.
Estuarine-shelf interaction processes are the responses to tidal, buoyancy and atmospheric forces, and the relative significance of these forces varies in time and space
(Beardsley and Boicourt 1981; Wiseman 1986; Simpson 1997). Local and remote forcing mechanisms are important, and the response of the estuary and the buoyant plume
occurs over a wide range of time-scales. Although the lack of long time-series of currents and physical properties in the Rio de la Plata limits a complete analysis of the
exchange processes, the characteristics of the salinity fields (Fig. 8.7) allows a qualitative explanation of the seasonal response to winds and continental runoff.
In fall/winter, the lower salinity values observed on the shelf northeast of the estuary can be explained as the result of an anticyclonic turn of the buoyancy flow after
leaving the estuary. The position of the 25 su isohaline parallel to the northern coast
suggests the existence of a coastal current. During this season, the river discharge is
maximum, the offshore wind frequency almost equals the occurrence of onshore
winds, downwelling favourable winds have their maximum mean speed and are more
frequent than upwelling-favourable winds. Under this condition, buoyancy dominates
and a river-forced plume (Garvine 1987; Chao and Boicourt 1986; Chao 1988a) flowing to the left of the estuary is observed. Modelling and observational studies show
that downwelling-favourable winds narrow the plume and reinforce the coastal jet,
while upwelling-favourable winds spread the buoyancy flow offshore and tend to arrest the alongshore current (Chao 1988b; Miinchow and Garvine 1993; Kourafalou
et al. 1996; Fong et al. 1997). These effects of the alongshore winds may explain some
of the characteristics observed, such as the better definition of the coastal current in
the fall/winter. During spring/summer, river discharge is minimum, and onshore and
upwelling-favourable winds are predominant. No coastal current is observed (Fig. 8.7b),
and the offshore position of 25 isohaline in the central and southern part of the estuary might indicate increased offshore flow at the surface, both characteristics that can
be explained by the action of upwelling-favourable wind. In the longitudinal direction, onshore (offshore) winds has been shown to reduce (increase) the estuarine circulation (Hansen and Rattray 1965; van der Kreeke and Robaczewska 1989; Geyer 1997).
Most of the studies assume lateral homogeneity, in rectangular channels or small estuaries. The effect of longitudinal winds on the cross-section characteristics of circulation and properties was analysed in modelling studies, for example, by Chao (1988b)
175
The bottom salinity has a smaller seasonal variation than the surface. The general
pattern is similar in both seasons, with isohalines mainly following the bottom topography. Fresher water is associated to shoals and more saline water is found in the
deeper channels. Similar effects of bottom topography on the transverse salinity distribution has been studied in Delaware and Chesapeake Bay (Wong 1994; ValleLevinson and Lwiza 1995; Valle-Levinson and O'DonneI1997). The importance of the
shallow banks is suggested by the locus of the 25 su isohaline, especially during fall/
winter; the influence of the deepest Oriental Channel along the Uruguayan coast is
clearly seen in the position of the 30-32 su isohalines. Another difference between the
two seasons is that during fall/winter a stronger gradient along the bottom occurs at
the northern end of Samboromb6n Bay, where the Maritime Channel borders the central shallow area of the bay. In spring/summer, the 25 and 30 su isohalines are located
at greater depths in the bay and the channel. This difference could be associated ~ith
an increase in the freshwater flow through the southern part of the estuary during
spring/summer compared to fall/winter.
Estuarine-shelf interaction processes are the responses to tidal, buoyancy and atmospheric forces, and the relative significance of these forces varies in time and space
(Beardsley and Boicourt 1981; Wiseman 1986; Simpson 1997). Local and remote forcing mechanisms are important, and the response of the estuary and the buoyant plume
occurs over a wide range of time-scales. Although the lack of long time-series of currents and physical properties in the Rio de la Plata limits a complete analysis of the
exchange processes, the characteristics of the salinity fields (Fig. 8.7) allows a qualitative explanation of the seasonal response to winds and continental runoff.
In fall/winter, the lower salinity values observed on the shelf northeast of the estuary can be explained as the result of an anticyclonic turn of the buoyancy flow after
leaving the estuary. The position of the 25 su isohaline parallel to the northern coast
suggests the existence of a coastal current. During this season, the river discharge is
maximum, the offshore wind frequency almost equals the occurrence of onshore
winds, downwelling favourable winds have their maximum mean speed and are more
frequent than upwelling-favourable winds. Under this condition, buoyancy dominates
and a river-forced plume (Garvine 1987; Chao and Boicourt 1986; Chao 1988a) flowing to the left of the estuary is observed. Modelling and observational studies show
that downwelling-favourable winds narrow the plume and reinforce the coastal jet,
while upwelling-favourable winds spread the buoyancy flow offshore and tend to arrest the alongshore current (Chao 1988b; Miinchow and Garvine 1993; Kourafalou
et al. 1996; Fong et al. 1997). These effects of the alongshore winds may explain some
of the characteristics observed, such as the better definition of the coastal current in
the fall/winter. During spring/summer, river discharge is minimum, and onshore and
upwelling-favourable winds are predominant. No coastal current is observed (Fig. 8.7b),
and the offshore position of 25 isohaline in the central and southern part of the estuary might indicate increased offshore flow at the surface, both characteristics that can
be explained by the action of upwelling-favourable wind. In the longitudinal direction, onshore (offshore) winds has been shown to reduce (increase) the estuarine circulation (Hansen and Rattray 1965; van der Kreeke and Robaczewska 1989; Geyer 1997).
Most of the studies assume lateral homogeneity, in rectangular channels or small estuaries. The effect of longitudinal winds on the cross-section characteristics of circulation and properties was analysed in modelling studies, for example, by Chao (1988b)