Physical oceanography of the Río de la Plata Estuary, Argentina

~

Pergamon

ContinentalShelf Research, Vol. 17, No. 7, pp. 727-742, 1997 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain P l h S0278-4343(96)00061-1 0278-4343/97 $17.00 + 0.00

Physical oceanography of the Rio de la Plata Estuary, Argentina 1 RAIJL A. G U E R R E R O , * t E D U A R D O M. ACHA,t* MARIANA B. FRAMI/qAN~ and CARLOS A. LASTA* (Received 2 February 1996; accepted 30 April 1996)

Abstract--The Rfo de la Plata drains the second largest basin of South America. It flows into the Atlantic Ocean generating an estuarine system of about 35,000 km 2 , with only 5-15 m water depth. On the basis of temperature and salinity data from the last 29 years, property seasonality at the estuary were studied. Surface salinity distribution is controlled by the balance between onshore and offshore winds, the river discharge and the Coriolis force. As a result of the combined effects of these forces, two periods were observed for the salinity distribution. Fall-winter is characterized by a balance between onshore and offshore winds and a maximum in the continental drainage, generating a main NNE drift of the estuarine waters along the Uruguayan coast. During springsummer, onshore winds become dominant and a minimum in the runoff is observed, resulting in Ekman surface drift advecting freshwater southwards along the Argentine coast. Consequently, shelf waters penetrate the estuary up to Punta del Este (Uruguay). Bottom salinity distribution does not exhibit seasonality, because the shelf water intrusion is controlled by bathymetry. The temperature field, driven by the atmospheric cycle, presents a warm and a cold season. Homogeneity in the vertical as well as horizontal scales characterizes these periods. High-resolution CTD data were arranged into salinity sections to describe the river-ocean interaction. A salt wedge is a quasi-permanent feature for the central and southern sectors of the estuary, defining an area of strong vertical stratification. However, the salinity stratification is destroyed by moderate to strong onshore winds. © 1997 Elsevier Science Ltd

INTRODUCTION The Rio de la Plata drains the second largest basin of South America. It flows into the Atlantic Ocean at 36°S and 56°W, with a total average discharge of 20,000-25,000 m 3 s-1 (Urien, 1967; Comisi6n Administradora del Rio de la Plata, 1989). The river has a funnel shape 320 km in length, oriented northwest to southeast, with an open mouth of 230 km along the line Punta Rasa-Punta del Este. A comprehensive description of this riverine system has been presented by Framifian and Brown (1996). The compilation and analysis of all the available temperature and salinity measurements

*Instituto Nacional de Investigaci6n y Desarrollo Pesquero, Paseo Victoria Ocampo No. 1, 7600 Mar dcl Plata, Argentina. t D t o . Ciencias Marinas, U N M D P , Funes y Pefia, 7600 Mar del Plata, Argentina. ~Servicio de Hidrograffa Naval, Montes de Oca 2124, 1271 Buenos Aires, Argentina. iContribution IN1DEP no. 979. 727

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taken in this estuary constitutes the basis for this paper, in which the general patterns of temperature and salinity distributions are presented. Our results show that the seasonality of salinity distribution is forced by a balance between offshore and onshore winds and, to a lesser extent, by freshwater drainage. Two periods with different discharge patterns are defined for the Rio de la Plata Estuary. From April to August offshore winds almost neutralize onshore winds, and continental drainage reaches a maximum. In this situation, a main NNE drift of estuarine waters along the Uruguayan coast is observed. From October to February, onshore winds become dominant along with a minimum in the continental drainage; under these forcing conditions fresher waters show a southern extension on the Argentine coast, and shelf waters penetrate up to Punta del Este (Uruguay), constraining the NNE drift. The temperature distribution has a warm season (December-March) and a cold season (June-September). Within each period, estuarine waters are almost vertically and horizontally isothermal. The mean temperature difference between both seasons is about 10°C. The river-ocean interaction is described by the synoptic scale, based on the vertical structure of salinity. Under calm winds, freshwater overlying sea water defines a salt wedge with a typical horizontal scale of 100 km. Vertical salinity gradients ranging from 3.9 to 15.8 sum -1 were measured in the highly stratified water column. Nevertheless, it was observed that moderate to strong winds erode the halocline, mixing the water column from surface to bottom. DATA AND METHODS The study area is presented in Fig. 1. Due to the shallowness of the estuary, the property distributions are affected by wind, in addition to river discharge. Statistical winds from the period 1970-80 (Servicio Meteorol6gico Nacional, 1982), taken at Pont6n Pr~icticos Recalada (35°10'S, 56°15'W), were employed to analyze seasonality [Fig. 2(a) and (b)]. Discharge data of the last decade (provided by the Instituto Nacional de Ciencia y T6cnica Hidricas, Argentina) were employed to estimate water flow of the Rio de la Plata [Fig. 2(c)]. This time series shows an abnormal runoff from September 1982 to August 1983, when the average discharge increased 50-70% over the historical value [Fig. 2(c)]. This event, which was produced by anomalous high precipitation that occurred in the drainage basin, was probably connected to the most intense "El Nifio Southern Oscilation" (ENSO) of the century, which occurred in 1982-83 off the Pacific coast of South America (Jordan, 1985). This atypical period was not considered for seasonal discharge analysis. The data base includes data from a total of 1600 hydrographic stations collected during the last 29 years (Table 1). Salinity is reported as salinity units (su), because the historical data employed were not converted to the 1978 Practical Salinity Scale (psu). However, salinity sections employed to study vertical stratification are solely based on recent CTD data, and their values are reported as psu. Salinity data from December 1982 to December 1983 were not included for the general description of the salinity field, because they show extremely low values connected to the anomalously high riverine runoff mentioned above. The wind data were grouped into onshore and offshore winds, according to the line perpendicular to the major axis of the river. The onshore winds are composed of winds from NE, E, SE and S which act to pile up river water. Winds from N, NW, W and SW, that

Physicaloceanographyof the Rio de la Plata Estuary

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increase seaward drainage, are grouped as offshore winds. The effect of wind direction on water height, observed by Balay (1961) at Buenos Aires harbor, supports our classification based on the effect of the wind on the estuary. Due to the antagonistic effect of winds from the sea and the continent, monthly frequency analyses were performed and are presented as the ratio of onshore to offshore winds [Fig. 2(a)]. The average wind speed for each sector was also computed [Fig. 2(b)]. Data of riverine discharge during 1980-90 are presented as monthly average+standard deviation [Fig. 2(c)]. To study the seasonal distribution of salinity, two periods were defined on the basis of forcing cycles: from April to August, characterized by high river discharge, the lowest frequency of onshore winds and the highest average speed of offshore winds; and from October to February, when river discharge tends to be minimum and onshore winds dominate both in frequency and intensity. These periods are named herein fall-winter and spring-summer, respectively. March and September were considered transitional months. Two periods were chosen to study seasonal distribution in temperature. They were defined by arranging information in a 1-year cycle with monthly resolution. No differences

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were detected among surface and bottom cycles, and so both data sets were jointly analyzed to study periodicity. To delimit warm and cold seasons a least-square fitting was p e r f o r m e d over the data set, resulting in a fifth-order polynomial. Four months around the m a x i m u m and minimum values of this fitting were considered for each season (Fig. 3). D a t a from D e c e m b e r to March represent the w a r m period while J u n e - S e p t e m b e r characterize the cold season. The thermal cycle presented here is restricted to the estuarine region. T e m p e r a t u r e information from stations deeper than 30 m were not considered in the fitting.

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Physical oceanography of the Rio de la Plata Estuary

Table 1. Sourcesand characteristics of the information employed Cruise Pesc uerfa I Pese uerfa I1 Pesc uer/a Ill Peso uer/a IV Pesc uerfa V Pesc uerfa VII Peso uer/a VIII Pes~ uerfa IX Merluza I Anchofta II Merluza 11 Anchoita III Merluza III Ancho/ta IV CC-03/81 ECRLP-8101 ECRLP-8103 ECRLP-8104 ECRLP-8109 CC-14/81 ECRLP-8206 Lamatra 8219 EH-114/82 CC-04/82 EH-02/85 OB-03/86 BS-01/88 BS-02/88 BS-03/88 EH-01/88 BS-01/90 BS-01/91 OB-03/91 OB-07/91 BS-03/91 BS-01/92 EH-06/92 EH-08/92 Ot3-05/93 EH-09/93 OB-11/93 OB-12/93 EH-05/94 EH-08/95

Institution SHN-FAO SHN-FAO SHN-FAO SHN-FAO SHN-FAO SHN-FAO SHN-FAO SHN-FAO SEAG-FAO SEAG-FAO SEAG-FAO SEAG-FAO SEAG-FAO SEAG-FAO INIDEP SOHMA-SHN SOHMA-SHN SOHMA-SHN SOHMA-SHN INIDEP SOHMA-SHN INAPE INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP INIDEP

Date VIII '66 XlI '66 II '67 VI '67 IX '67 II '68 V '68 VIII '68 VIII '69 V '70 VII '70 VIII '70 VIII '70 XI '70 II1 '81 III '81 V '81 VI '81 IX '81 IX '81 IlI '82 VIII '82 X '82 X1 '82 V '85 V '86 II '88 Ill '88 X '88 XI '88 X '90 IV '91 VI '91 XI '91 XII '91 V '92 VIII '92 XI '92 IV '93 VII '93 X '93 XI '93 V '94 XI '95

Sampl. Dens. Sampl. Device 8 7 7 6 5 6 13 12 18 23 14 20 14 14 22 30 17 96 20 27 16 19 20 43 23 22 63 59 55 52 74 67 44 99 81 52 36 24 34 37 12 31 32 30

Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles Bottles TS TS TS Bottles TS TS CTD CTD TS TS CTD CTD CTD CTD CTD CTD CTD CTD

Sampl. Design Transects Transects Transects Transects Transects Transects Transects Transects Systematic Systematic Systematic Systematic Systematic Systematic Random Transects Transects Transects Systematic Random Systematic Systematic Random Random Random Random Systematic Systematic Systematic Random Systematic Systematic Transects Transects Systematic Systematic Transects Systematic Transects Random Systematic Transects Transects Transects

Depth (m) 21-2040 16-143 14-183 20-97 22-770 17-205 15-143 14-259 25-200 7-1700 50-240 11-1350 54-142 9-900 7-50 5-25 8-28 19-30 7-26 4-33 7-28 18-1700 10-125 5-24 6-40 43-365 2-8 2-8 2-8 8-37 1-7 2-7 13-28 9-40 2-8 2-8 4-32 9-29 4-42 5-47 10-900 4-62 4-73 4-25

Sampl. Dens. = sampling density, expressed as number of stations/10,000 kin2; ECRLP = Estudio de la Contaminaci6n del Rio de la Plata; TS = thermosalinometer; CTD = conductivity-temperature-depth profiler; SHN = Servicio de Hidrograffa Naval (Argentina); FAO = Food and Agriculture Organization (UNESCO); SEAG = Secretarfa de Estado de Agricultura y Ganaderfa (Argentina); INIDEP = Instituto Nacional de lnvestigaci6n y Desarrollo Pesquero (Argentina); SOHMA = Servicio de Oceanograf/a (Uruguay); INAPE = Instituto Nacional de Pesca (Uruguay).

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Periodicity observed in salinity and temperature do not agree due to their different forcing, e.g. winds and water discharge for salinity and sea-air heat flux for temperature. Contour lines for bottom and surface temperature and salinity were made for the respective periods, including the adjacent shelf area to show boundary conditions. For those stations located offshore of the estuary, temperature at 30 m was considered as the bottom values. This depth was chosen to avoid false horizontal gradients at the intersection of the seasonal thermocline with the bottom, and considering that the depth of the mixed layer at the shelf surrounding the estuary is 30-40 m (Servicio de Hidrografia Naval, 1973). As stated by Kjerfve (1989), in shallow estuaries salinity alone determines the water density. Consequently, vertical stratification in the estuary of the Rio de la Plata is adequately interpreted using salinity sections. Cruises using high-resolution sampling, representatives of the fall-winter (August 1992) and the spring-summer (November 1995) patterns, were chosen to characterize the vertical structure. Three transects with a common origin (35°00'S, 57°00'W) portraying the whole estuarine system are shown for each cruise: a northern one along the Oriental Channel; a central leg following the bisecting line of the river; and the third one to the South, along the Argentine coast. Moderate to strong winds can alter property distributions at the synoptic scale. Data from a third cruise (April 1993) were employed to describe the vertical salinity distribution under moderate to strong wind conditions, in contrast to the calm situation present in fallwinter.

RESULTS Horizontal distribution o f salinity

Surface salinity distribution for the fall-winter period is presented in Fig. 4(a). An area of high horizontal gradients is observed between the 10 and 25 su isolines, bounded by an inner and an outer relaxed gradient bands. The inner band shows the upriver marine influence and it is defined by the isolines of 5-10 su. The outer zone shows a northeastward discharge along the Uruguayan coast, with salinity between 25 and 30 su. A low salinity

Physical oceanography of the Rfo de la Plata Estuary 57

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Fig. 4. Fall-winter salinity distributions for (a) surface and (b) bottom layers. Dots indicate station locations.

core, lower than 25 su, is centered in Cabo Polonio. S a m b o r o m b 6 n Bay remains h o m o g e n e o u s as a whole, in the range 10-15 su. The b o t t o m salinity field for the same period shows for the range 10-25 su higher horizontal gradients than the surface one [Fig. 4(b)]. Salinity from 20 to 25 su closely follows the b o t t o m topography. As in the surface distribution, horizontal gradients beyond the 25 isohaline decrease offshore. The isohaline of 30 su to the northeast of Cabo Santa Marfa constitutes the b o t t o m signal of the low salinity core centered in Cabo Polonio. In the surface distribution for the spring-summer period, the low salinity signal to the

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Spring-summer salinity distributions for (a) surface and (b) bottom layers. Dots indicate station locations.

northeast is also observed (Fig. 5(a)]. However, riverine discharge along the Uruguayan coast is obstructed by a high salinity tongue at Punta del Este. As a consequence, an increased southward discharge may be seen along the Argentine coast, defining a relaxed gradient extension compared to the Uruguayan regime. Samboromb6n Bay also presents for this period a dominant homogeneous pattern, with salinity values slightly higher than the fall-winter period. The bottom salinity for spring-summer closely resembles the fall-winter distribution [Fig. 5(b)]. The 20 su isoline is the one that more closely follows bottom topography. The salty water intrusion at the south coast shows more upriver penetration than during the fall-winter period.

Physical oceanography of the Rfo de la Plata Estuary

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Horizontal distribution of temperature Surface horizontal distribution for the summer period shows near homogeneity within the estuary, and a weak thermal gradient (21-19°C) denotes at the south the boundary with the continental shelf regime [Fig. 6(a)]. The bottom distribution presents a similar pattern, with a more extended penetration of cold waters upriver, at this southern boundary [Fig. 6(b)]. The estuarine zone is defined by a nearly constant thermal field between 20 and 22°C. A weak vertical stratification of about half a degree was calculated from the synoptic observations. The shallow waters of Samboromb6n Bay are warmer

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Fig. 7. Cold season temperature distributions for (a) surface and (b) bottom layers. For those stations located at the deeper area, the 30 m temperature data were used as bottom values. Dots indicate station locations.

and totally mixed. The outer zone, corresponding to the continental shelf regime, shows warmer waters to the northeast for the surface and bottom levels. The winter temperature field (Fig. 7), unlike the summer one, does not show the boundary between estuarine and continental shelf waters. Temperature values, ranging from 10 to 12°C, show horizontal homogeneity at the surface and bottom. The vertical stratification calculated from the synoptic observations is similar to that of the warm period, but with a negative sign, showing cooler riverine waters laying over warmer shelf waters.

737

Physical oceanography of the Rfo de la Plata Estuary

Vertical distribution of salinity The fall-winter synoptic sections shown in Fig. 8 were taken under average 2.7 m soffshore winds. During this mild wind situation, high vertical stratification was present at the three transects [Fig. 8(a), (b) and (c)]. A steady state is reached between a fresher surface water layer overlying a saline bottom layer. Vertical gradients ranging between 4.6 and 15.8 psu m -1 were measured. The depth of the low salinity layer remains almost constant along each section, and so a salt wedge is formed between the halocline and the rising bottom. The offshore extension of the estuarine waters is larger in the northern section. The 30 psu isohaline reaches the surface at 240 km from the origin, at this section. The same isohaline reaches the surface at 130 and 150 km in the central and southern sections, respectively. The spring-summer synoptic sections shown in Fig. 9 were taken under average onshore winds of 7.4 m s ~. No stratification was observed in the northern section, where the waters of 30 psu or less extend over 100 km. The central and southern sections present vertical gradients ranging between 3.9 and 8.3 psu m -1. Estuarine waters present the larger extension southward, where the 30 psu isohaline reaches the surface beyond 275 km. In the central section, the low salinity influence at the surface is 100 km long. Frequent meteorological events in this area characterized by strong winds over 13 m sfrom the southeast are called "sudestadas" (Balay, 1958, 1961), which are responsible for disastrous flooding in the littoral of the Rio de la Plata. During a cruise performed in May 1993, a condition close to a "sudestada" was present. Onshore winds (mainly SE and ESE) with intensity of 10-14 m s - t blew for 60 h, forcing up estuarine waters upstream. Salinity

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738

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Fig. 9. Salinity sections (November 1995) characterizingthe southern riverine discharge along the Argentine coast. The location of the transects [(a) northern transect; (b) central transect; (c) southern transect] is shown in the upper right panel. Vertical dots indicate CTD observations.

sections in Fig. 10 correspond to this strong onshore wind condition. No stratification is observed in the estuary, except for the short wedge in the northern section [Fig. 10(a)], where vertical gradients were weakened to 2.6 psu m -1. At the central and southern sections, the halocline has been broken by wind-induced vertical mixing, and a homogeneous layer of average salinity resulted. At the northern and central sections, waters with salinity higher than 25 psu occupy the outer half, while fresher waters occupy most of the southern section.

DISCUSSION For large estuaries, when time periods longer than the tidal cycle are considered, geometry and wind stress effects become important (Officer, 1992). The Rio de la Plata flowing into the wide Argentine continental shelf forms an extended coastal plain estuary, strongly influenced by energy exchanges with the atmosphere. Seasonal changes in the mean salinity surface distribution may be explained by the wind and the continental runoff patterns. Nevertheless, the Rfo de la Plata does not present strong wet and dry seasons like other large rivers such as the Amazon (Gibbs, 1970) or the Zaire (Eisma and van Bennekom, 1978), both located in the tropical zone, or the Yangtse river, whose basin is under the monsoonal regime (Beardsley et al., 1983). Due to the weak runoff seasonality, and the spatial scale of the estuary, it is expected that winds play a dominant role affecting salinity distribution in the Rio de la Plata. It has been assumed that the Rfo de la Plata mainly discharges along the Uruguayan

Physical oceanographyof the Rio de la Plata Estuary 50

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coast, solely driven by the Coriolis force (Brandhorst and Castello, 1971; Castello and Mfiller, 1977; Lusquifios and Figueroa, 1985). However, synoptic situations showing southern discharge were frequently observed (Servicio de Hidrograffa Naval 1968; Brandhorst 1971), and Carreto (1981) suggested its seasonal occurrence during springtime. From December to January, Framifian (1993) using infrared temperature images observed an upwelling on the Uruguayan coast• This upwelling, probably induced by NNE winds, would contribute to the presence of salty water off Punta del Este, here shown for spring-summer. Also, a storm surge model forced by easterly winds (O'Connor, 1991) predicts a current up river along the Uruguayan coast and discharge along the Argentine side. The above observations at a synoptic scale, reporting a two-way discharging pattern for the Rio de la Plata, may be explained by the forcing cycles presented in this work. During the fall-winter season, riverine discharge is maximum, and the offshore winds almost neutralize onshore winds. Consequently, without wind stress the continental runoff is solely affected by Coriolis force, making river water turn to the left and discharging along the Uruguayan coast, as shown in Figs 4(a) and 8. This is the most commonly described pattern in the literature• For the spring-summer period, onshore winds overcome offshore winds, tending to reverse river drainage. In this case, Ekman drift forces waters southward, and under the constraint of the coastline, generates a fresh water southern extension along the Argentine coast [Figs 5(a) and 9)]. As river waters drift southward, shelf waters penetrate up to the Uruguayan coast, generating a seawater intrusion off Punta del Este [Figs 5(a) and 9(a)]. A salinity time series taken at the shore of Punta del Este from August

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1991 to March 1993 (M6ndez et al., 1993) supports the periodic surface shelf water intrusion. This time series shows a low salinity phase for June-October and a high salinity signal for December-May, in close agreement with our results. It is also important to consider the boundary conditions imposed by the shelf water movements that force the estuarine waters by lateral shear. Surface Ekman transport at the coast, analyzed with a two-month time resolution, have a southward drift from August to March, and a northward drift from April to July (Bakun and Parrish, 1991). The first situation will contribute to maintaining the diluted southern lobe during spring-summer, while the fall-winter condition supports the NNE river discharge inferred from the salinity distributions. The presence of the low salinity signal eastward of Punta del Este, during the springsummer period, could be fed by higher frequency periodic connections, as those seen in the time series presented by M6ndez et al. (1993). Another possibility is a southward coastal drift of freshwater along the Brazilian and Uruguayan coasts, originated in Lagoa dos Patos (32°S), a coastal lagoon with a net average outflow of 8600 m 3 s -I during 200 days a year (Herz and Mascarenhas, 1993). Hubold (1980) observed for August-November a low salinity tongue that originated in Lagoa dos Patos extended to the south up to 35°S. This tongue is maintained by the prevailing winds for springsummer (Pereira, 1989; Bakun and Parrish, 1991). Moreover, an analytical sea current model forced by winds and density distribution (Pereira, 1989) shows the southwestward current forced by easterly winds during the spring-summer period, feeding the low salinity tongue. At the estuary, bottom salinity distributions for both periods are very similar [Figs 4(b) and 5(b)]. However, a slight seasonality may be observed for salinity higher than 30 su, showing fresher waters northeastward of Punta del Este for fall-winter. The faint seasonality of bottom salinity patterns reveals that the effect of wind stress and riverine discharge at mean scales may only be seen in the upper water layer. The presence of a salt wedge is a quasi-permanent feature along the central and southern sectors. The upstream reach of the seawater intrusion seems to be controlled by topography. Under this condition a frontal line is defined at the bottom, following the 10 m isobath. This salt wedge can be broken by synoptic winds above 10 m s -I , blowing longer than 12 h. Restoring of the salt wedge is forced by the strong horizontal pressure gradients due to density differences between shelf waters, the wind-mixed estuarine waters and the newly added freshwater. The atmospheric thermal cycle is responsible for the temperature differences between summer and winter, which is typical of the temperate zone. Despite the strong vertical stratification induced by salinity on the density field, the temperature distribution in the estuary remains almost vertically homogeneous. Probably the waters of the mixed layer at the shelf reach a thermal equilibrium with the atmosphere before flowing upstream underneath the riverine waters. The similar temperature values of the continental waters and the mixed layer shelf waters explain the quasi-thermal vertical homogeneity in the estuary. This pattern is observed for the warm and cold seasons defined here. The transitional periods (cooling and warming phases) are expected to present a horizontal thermal contrast between estuarine and shelf waters, induced by the different thermal inertia between both regimes, and consequently a thermocline at the estuary should be expected. Synoptic information supporting this inference has been presented by Nagy et al. (1987).

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CONCLUSIONS Surface salinity patterns are controlled by the wind field and, to a lesser extent, by riverine discharge. Both forces act upon the upper layer. Diluted shelf waters occupy the bottom layer, since this distribution is mainly controlled by topography. Two periods were defined as a result of forcing cycles acting on salinity: fail-winter (April-August), characterized by the lowest wind influence and a maximum in riverine runoff; and spring-summer (October-February), characterized by predominantly onshore winds and minimum riverine runoff, Fall-winter surface salinity pattern shows a NNE drift of estuarine waters along the Uruguayan coast and salty water off Cabo San Antonio (Argentina). This pattern changes for the spring-summer period, characterized by a southern low salinity tongue along Cabo San Antonio and a shelf water penetration up to Punta del Este (Uruguay). No seasonality was observed in the bottom layer. The salt wedge is a quasi-permanent feature at the estuary, defining an area of strong vertical stratification. This vertical structure may be destroyed by moderate to strong winds. Mean temperature field remains almost homogeneous for the warm (DecemberMarch) and cold (June-September) periods, both in the vertical and horizontal scales, as a consequence of the minimum temperature differences between shelf surface and estuarine waters. Acknowledgements--We are grateful to M.S. Andr6s Rivas, Lic. Alberto Piola and Lic. Alejandro Bianchi, who reviewed the manuscript and made many valuable suggestions.

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