172
M.B. Framiftan . M.P. Etala . E.M. Acha . R. A. Guerrero· C.A. Lasta . O.B. Brown
Over the past six years, several cruises have been conducted in the lower estuary
and the adjacent shelf as part of an ongoing inter-disciplinary project to study the
coastal ecosystem. During these cruises, continuous profiles of conductivity and temperature (CTD) were obtained, together with biological and chemical samples. Comparison of the resultant salinity distribution with the wind and river flow conditions
suggests the importance of these two forces for seasonal distributions. The seasonal
cycle of the forcing, and the response of the salinity and temperature distributions to
it, was investigated by Guerrero et al. (1997). Here, results from cruises conducted during 1994, 1995 and 1996, are added to the original database. Following the seasonal
scheme proposed by Guerrero et al. (1997), new surface and bottom salinity and temperature fields are obtained. The new information improves the definition of salinity
field features especially in the outer estuary. The analysis of the horizontal salinity fields
(and temperature fields in the next section) obtained with this expanded database, a
review of the results obtained by Guerrero et al. (1997) for the vertical salinity sections,
and the analysis of the vertical stratification are presented in the following sections.
8.4.2
Seasonal Distribution
The effects of two forces, wind and river discharge, was considered when studying the
seasonal variability of the salinity field. Guerrero et al. (1997) considered river and
longitudinal wind forcing; here the study is expanded to include the effect of alongshore winds in the analysis. The coast in the Rio de la Plata region has approximately
a general orientation of 35° from the north. The wind field was divided into onshore
(southwestward to northward winds) and offshore (northeastward to southward) relative to a perpendicular to the river axis. In the alongshore direction, the winds were
divided into downwelling-favourable (northwestward to eastward) and upwelling-favourable (southeastward to westward). Monthly frequencies and mean speeds for each
group were computed based on 10 year monthly statistics of mean wind speed and
direction (expressed as frequency of occurrence from an 8-point wind rose) at weather
station Ponton Pnicticos Recalada (SMN 1982). The monthly distribution of the ratio
frequency of onshore to frequency of offshore winds, and the ratio frequency of upwelling-favourable to frequency of downwelling-favourable winds are shown (Fig. 8.6a),
and the average speed for each group is presented in Fig. 8.6b. Monthly mean transports of the Rio de la Plata (Fig. 8.6c) were estimated from discharge data of the two
major tributaries, Parana and Uruguay Rivers. The period of September 1982 to August 1983 was excluded in the seasonal analysis of the river discharge cycle, because it
presents highly anomalous values. Data from cruises during this period were also set
aside (for a brief discussion see Guerrero et al. (1997)). Based on the monthly distribution of the wind and river discharge, two seasons were defined to study the salinity
distribution. The period from April to August, fall/winter, characterized by large river
flow, minimum frequency of onshore winds, maximum frequency of downwellingfavourable winds and the highest average speed of offshore and downwelling-favourable winds; and October to February, spring/summer, with minimum discharge and
predominance of onshore and upwelling-favourable winds. March and September are
considered periods of transition.
M.B. Framiftan . M.P. Etala . E.M. Acha . R. A. Guerrero· C.A. Lasta . O.B. Brown
Over the past six years, several cruises have been conducted in the lower estuary
and the adjacent shelf as part of an ongoing inter-disciplinary project to study the
coastal ecosystem. During these cruises, continuous profiles of conductivity and temperature (CTD) were obtained, together with biological and chemical samples. Comparison of the resultant salinity distribution with the wind and river flow conditions
suggests the importance of these two forces for seasonal distributions. The seasonal
cycle of the forcing, and the response of the salinity and temperature distributions to
it, was investigated by Guerrero et al. (1997). Here, results from cruises conducted during 1994, 1995 and 1996, are added to the original database. Following the seasonal
scheme proposed by Guerrero et al. (1997), new surface and bottom salinity and temperature fields are obtained. The new information improves the definition of salinity
field features especially in the outer estuary. The analysis of the horizontal salinity fields
(and temperature fields in the next section) obtained with this expanded database, a
review of the results obtained by Guerrero et al. (1997) for the vertical salinity sections,
and the analysis of the vertical stratification are presented in the following sections.
8.4.2
Seasonal Distribution
The effects of two forces, wind and river discharge, was considered when studying the
seasonal variability of the salinity field. Guerrero et al. (1997) considered river and
longitudinal wind forcing; here the study is expanded to include the effect of alongshore winds in the analysis. The coast in the Rio de la Plata region has approximately
a general orientation of 35° from the north. The wind field was divided into onshore
(southwestward to northward winds) and offshore (northeastward to southward) relative to a perpendicular to the river axis. In the alongshore direction, the winds were
divided into downwelling-favourable (northwestward to eastward) and upwelling-favourable (southeastward to westward). Monthly frequencies and mean speeds for each
group were computed based on 10 year monthly statistics of mean wind speed and
direction (expressed as frequency of occurrence from an 8-point wind rose) at weather
station Ponton Pnicticos Recalada (SMN 1982). The monthly distribution of the ratio
frequency of onshore to frequency of offshore winds, and the ratio frequency of upwelling-favourable to frequency of downwelling-favourable winds are shown (Fig. 8.6a),
and the average speed for each group is presented in Fig. 8.6b. Monthly mean transports of the Rio de la Plata (Fig. 8.6c) were estimated from discharge data of the two
major tributaries, Parana and Uruguay Rivers. The period of September 1982 to August 1983 was excluded in the seasonal analysis of the river discharge cycle, because it
presents highly anomalous values. Data from cruises during this period were also set
aside (for a brief discussion see Guerrero et al. (1997)). Based on the monthly distribution of the wind and river discharge, two seasons were defined to study the salinity
distribution. The period from April to August, fall/winter, characterized by large river
flow, minimum frequency of onshore winds, maximum frequency of downwellingfavourable winds and the highest average speed of offshore and downwelling-favourable winds; and October to February, spring/summer, with minimum discharge and
predominance of onshore and upwelling-favourable winds. March and September are
considered periods of transition.