Project LA 26 1

PROJECT LA 26: PROGRESS REPORT

ABSTRACT The mean water level of the Rio de la Plata responds to mean sea level, wind field and discharge of its tributaries. Models HADCM3, ECHAM4/OPYC3, CSIRO-mk2 and GFDL-R30 represent adequately the mean annual SLP fields and most of their annual cycle. Therefore, SRS A2 scenarios of these models were used to develop SLP and wind scenarios until the year 2100. A regional trend in SLP and wind was detected in the observed fields (1950/2000) and in GCM SLP outputs 1960/2000). This trend can be summarized as a southward shift of the western border of the South Atlantic High that persists during the XXI century in the four models. Digitized information of geology and geomorphology of the coastal areas of the Argentine was completed, while topography is being improved using a GP system. Social vulnerability indicators were developed for the administrative units affected by future floods under an extreme climate change scenario. The future RP water level is being studied with two hydrodynamic. One of them is completely validated and ready for the development of future scenario. It continue the inter-consultation process with qualified stakeholders and a workshop was organized to present the Project and discuss with them the pertinence of the Project objectives to social and governmental demands.

Report on the Tasks Scheduled for the July-December 2002 period Task numbers according to the working plan. 10. Photo interpretation of dry, normal and flood events The objective of this task was to contribute to the construction of topographic maps in the Samborobóm bay since available altitude information had a resolution of only 2.5-m. According to the IPCC (2001) predicted sea level rise, this value is not enough to precisely define areas that could be affected by future marine ingressions. This task was changed to an analysis of Landsat images to determine lines of equal height according to the boundary of water. As explained in the Second Report, this task will complement measures to be taken with a high resolution GPS. Results indicate that along the coast in most of the bay, the altitude of the land after the first one or two kilometers from the coast is about 50 to 100 cm over the actual river level. 11. Data bases construction in a GIS environment. Point 3 of the Joint

Document with LA 32 This is a continuous activity in the Project including outputs of the tasks. This activity continued in two ARC VIEW systems. Social, economic, and geological information is being merged with basic cartography and satellite information 12. Topographic measurements and fieldwork will be carried out to produce

detailed altitude level maps of coastal areas subject to possible floods During the previous six-month period, topographic maps of the coastal area and its zone of influence between the southern limit of the Rio de la Plata and the Paraná delta were digitized from 1:50.000 IGM maps. Interpolation was made when necessary using the geomorphology maps. In this period the same work was done with the San Antonio Cap area because is under the influence of the Río de la Plata being a transition zone with the marine environment.

Project LA 26 2

These maps resolution are being upgraded with field measurements taken with a GPS system. The field activity in the northern part of the Great Buenos Aires City was completed. The GPS was acquired with collateral funds in last March. 13. Election of future climate scenarios in cooperation with Project LA32 Point 6 of the joined document As expressed in the first report, the mean water level of the Rio de la Plata (RP) estuary responds to the wind field and to the discharge of its tributaries. Therefore, the variables required from climate scenarios are surface mean winds over the RP and over the outer adjacent ocean and precipitation over the RP basin. Surface winds are not a standard output of the GCM scenarios. However, this variable is strongly coupled with sea surface pressure (SLP) fields. Consequently, wind scenarios are being developed through SLP scenarios. In the Second Report was described that four experiments (HADCM3, CSIRO-mk2, ECHAM4, GFDL-R30) simulate adequately the mean regional SLP field. Therefore, we elected these models for future scenarios. As a first step, we are using the SRS A2 scenarios up to the year 2100. Surface wind scenarios As explained in the Second Report, it is required daily data. While the daily data from GCM experiments are obtained, an alternative approach was undertaken that is based on downscaling from daily SLP using NCEP/NCAR data. Thus, a regression model for regional surface daily wind was constructed, and its results are being evaluated. Future wind scenarios will be constructed using a perturbation technique that accounts for the mean SLP GCM scenarios. Other direct approach is reported in the next-semester tasks. A regional trend in winds and SLP was detected both in data and in GCM outputs. A paper is being prepared in this subject. A resume is presented in Annex 1 Precipitation scenarios The ETA model is being run with high resolution over South America for the 1990-1999 period. The intention is assessing if the underestimation of precipitation is caused by the low resolution of the GCM models. This activity will take the whole year 2003. January results indicate a good mean precipitation over most of the Plata basin, except in the SACZ area. Other alternative for precipitation scenarios is being reported under next semester tasks. In the meantime, it was recommended to the AIACC Projects dealing with agriculture (La 27 and LA 29) to use the HADCM3 scenarios with some sort of correction accounting for systematic errors. 19. Writing of a paper manuscript on the study of weather storms over the region embracing the Rio de la Plata using NCAR/NCEP reanalysis for the period 1950-2000 A manuscript on this subject entitled: ¨ Surface level atmospheric conditions associated to extreme surge tides in the Rio de la Plata estuary ¨ was prepared by Gustavo Escobar, Walter M. Vargas and Susana A. Bischoff was submitted to the International Journal of Climatology. A copy was sent with the Second Report 21. Writing of a paper manuscript on extreme streamflow events in the Paraná and Uruguay rivers The paper ¨ The greatest discharge events in the Paraná River and their climate forcing¨ by Inés Camilloni and Vicente Barros was published by the Journal of Hydrology, 278 (2003) 94-106. It was submitted in the previous semester, and therefore its copy was sent with the Second Report The manuscript ¨ The largest floods in the Uruguay River and their climate forcing ¨ by Inés Camilloni and Ruben Caffera was submitted to the J. of Hydrometeorology in May 2003. Copy of the paper in Annex 2.

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22. Consultation about demands and communication of Project partial results with stakeholders through mail and email (March 2003) This task was almost completed in 2002 but continued with a consultation to a greater group of stakeholders. In March 5, 2003, a workshop was held to present the Project results to stakeholders and to receive from them suggestions and demands. There were about forty participants from 30 governmental and non-governmental organizations. In the same workshop, it was also presented results from the University of Buenos Aires Project on Floods on the Paraná and Uruguay Rivers that provide the collateral funds to this Project. The report of the workshop is in Spanish in the web page of the Project http://www-atmo.at.fcen.uba.ar/~lcr/proyectos/aiacc/. The English version of this document is being prepared and will be attached in the next report. 23. Geomorphology description and study of the evolution of the coastal area

and construction of geomorphology maps In the Second Report was reported the geological and geo-morphological maps of the coastal area of Samborombón Bay, which included the digitization of the different units and their lithological features represented in the same format as the topographic maps. In this semester, it was completed the respective two maps of the area Paraná Delta – Punta Piedras. Therefore, all the Argentine coast of the Rio de la Plata was completed. The geo-morphological maps in scale 1:40.000 were made base upon satellite images, airplane photography and field activities. This map interpretation allows identifying the areas affected by regular astronomical tides and storm tides and wetlands. The geological map was based upon previous bibliography and field tasks as well as with the use of airplane photography and Landsat 5TM (Thematic Mapper) images This map allows through its interpretation to extract the necessary data to estimate coastal erosion. 25. Develop of a Social Vulnerability Index to flooding in a GIS environment.

Point 3 of the Joint Document with LA 32 Number of people living in the area of possible affectation Last semester we arrive to a preliminary figure for the AMBA Sector (Census 1991). Now we complete the elaboration from the INDEC census with the number of inhabitants for the rest of the area, and a more detailed design for the 5 meters topographic line. The population that can be affected by a probable sea level rise for climatic change in the area of the Project is near 1.500.000 inhabitants. Social Vulnerability Index The social vulnerability index developed during 2002 was improved by the inclusion of indicators of production and consume. The indicators have been clustered into sub-indexes: demographic indicators, conditions of life indicators and work/ production/ consumption indicators The demographic sub index includes the following indicators: total population, population's density, index of potential dependence: the young, and index of potential dependence: the elderly. The administrative units that have a higher demographic vulnerability sub index are Buenos Aires City, General Sarmiento and Vicente López. The conditions of life sub index includes the following indicators: population’s percentage in homes with unsatisfied basic needs (NBI), percentage of homes with boss woman, total rate of infantile mortality, and population’s percentage without access to health services. The administrative units that have the higher conditions of life vulnerability sub index are San Fernando, Tigre, General Sarmiento and Castelli. The work, production, consumption sub index includes the following indicators: unemployment rate, Aggregated gross product, registered cars rate (inhabitants / car rate), and percentage of workers without provisional taxes. The administrative units that have the higher

Project LA 26 4

conditions of vulnerability according to this sub index are Partido de la Costa, General Sarmiento and Esteban Echeverría. The last steep in this task is to develop a comprehensive social vulnerability index and to apply it to the administrative units. 27. Selection of sea level scenarios according to TAR IPCC This task was started in this semester. It will be considered not only the mean sea global rise scenarios, but also the predicted regional increase in the South Atlantic. The main effort was in developing the mean sea level with respect to the reference geode in the domain of the hydrological models being used in the Project and for different seasons. These levels are being calculated from the TOPEX set of data for the 1990` decade. It was found substantial differences in height both in space and along the year. Once this task will be completed, it results will be used as baseline to add the regional increments as predicted by coupled GCMs. The outcome of this step will be the input for the hydrological model. 28. Validation of the ETA model and the hydrodynamic models The use of the ETA model to develop surface winds was abandoned, at least momentarily, since calibration from NCEP/NCAR surface winds were satisfactory. The calibration process of model RPP-2D, through successive stages (purely astronomical tide, normal tide condition, and storm tide condition), was explained in the previous Report. Preliminary, and encouraging, results were shown. The calibration is now completed. The following improvements were made:

i. Bathymetric information was rebuilt based on more accurate data sources.

ii. The astronomical tide boundary condition at the southern border was redefined by using results from a global tidal model.

iii. The adjustment of wind fields from NCEP/NCAR was refined through detailed experimentation.

Model RPP-2D was first exploited to analyze sensitivity of system response to probable changes in the forcings: sea level rise, change in predominant seasonal wind pattern and tributary discharge increase. More information on this task is in Annex 3.

31. Development of strong waves scenarios This task was completed in the past semester. In Annex 4 is attached a copy of the manuscript ¨ Impact of a possible regional wind change on the wave climate of the Rio de la Plata ¨ by Walter Dragani and Silvia Romero that is being prepared for submission to the Int. J. of Climatology. 35. Engineering implications and prospective of the evolution of coastal area. This includes both the new coastal geography arising from sea level rise and the advancement of the Paraná Delta front This task was initiated collecting data from different institutions, both public and private that have large infrastructure on the coast. In addition, soil uses, goods and infrastructure localized in the study area are been survey, in order to asses the impact of present floods and the future impact caused by climatic change.

Travel activities V. Barros made a short trip (one-day) to present the Project activities in a workshop held in Mar del Plata to discuss the future Argentine satellites. The Co-PI M. Caffera made one short trip to Buenos Aires to work with I. Camilloni and V. Barros on the extreme discharges of the Uruguay River.

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Dr Kokot and Dr. Codignotto made a short trip to the Samborombón bay area to make field- work. Dr Barros and Dr Kokot made a short trip to Santa Fe to discuss a cooperative agreement with the Faculty of Water Resources of the University of Litoral. This agreement will favor the Project with the help of experienced professionals on the use of GPS Madeleine Renom from Uruguay made two trips to Buenos Aires to work on climate scenarios with I. Camilloni and to attend a course on the South American Climate at the University of Buenos Aires. She was also funded by the LA 32 Project.

Description of Difficulties Encountered and Lessons Learned Task 12 Because of the lack of available high-resolution altitude maps, the topography was digitized with a resolution of 1.25 m. This is not enough for the Project requirements and therefore, field measurements are being taken with a GPS. Task 13 Other problem is the availability of daily data from GCM experiment scenarios. We are trying of get this data by ourselves, but help from AIACC would help.

Description of Tasks to be performed in the Next Eight-Month Period Tasks numbers as in the working plan. It is indicated when the task was already initiated as reported in part B.

11. Data base construction in a GIS environment. Point 3 of the Joint Document

LA 32 (Initiated). This will be a continuous activity during the Project duration, including outputs of the different tasks. 12. Topographic measurements and fieldwork will be carried out to produce

detailed altitude level maps of coastal areas subject to possible floods (Initiated).

Fieldwork with a GPS will be conducted to improve the actual topography resolution 13. Election of future climate scenarios in cooperation with Project L32

(Initiated) Wind: If available daily GCM outputs, it will be used to generate daily winds from daily SLP, to be used as input in the hydrodynamic model Precipitation: Sensitivity analysis indicates that only the greatest and exceptional discharges have some influence on the Rio de la Plata level. On top of that, Task 20 showed that these exceptional discharges are closely related to El Niño events. Therefore, a shortcut could be taken, to estimate the frequency of the exceptional discharges if the GCMs properly represent the ENSO phases. 16. Development of future scenarios for the Paraná delta growth (Initiated). The past evolution of the Delta during the last 300 years is being completed. Future evolution of the Delta will depend on many variables; one of the most important is the future level of waters

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in the inner estuary of the Rio de la Plata. Therefore, scenarios will be constructed after tasks 32 and 33 will be well completed. 25. Develop a Social Vulnerability Index to flooding in a GIS environment. Point 3 of the Joint Document (Initiated) The remained activity is to construct a comprehensive social vulnerability index and to apply it to the administrative units. 27. Selection of sea level scenarios according to TAR IPCC (Initiated) 32. Development of mean and extreme scenarios of water level with

hydrodynamic models 33. Development of extreme scenarios of water level with hydrodynamic models

under different storm weather scenarios 34. Writing a manuscript on the future mean and extreme scenarios of water

level 35. Engineering implications and prospective of the evolution of coastal area. This includes both the new coastal geography arising from sea level rise and the advancement of the Paraná Delta front (Initiated) 36. Identification of critical zones of social vulnerability to flooding 41. Joint workshop with Project LA 32 as explained in Point 11 of the joint document 43. Integrated analysis of the lower basin and mouth of the Santa Lucía using the Samborombóm experience as explained in point 8 of the joint document 45. Indicator analysis and social vulnerability diagnosis

Anticipated Difficulties in the Next Eight-Month Period Task 13 The use of daily data from GCM scenarios can be delayed because of the difficulty to get these data and because of the large consume of computer time involved

Connections with the National Communication The plans for the Second Communication are being made in the Foreign Affairs Ministry by Dr. Estrada Oyuela. Recently, it was also incorporated the Secretary of Environment. Vicente Barros and Walter Vargas participate of informative meetings at the Foreign Ministry were the first steps of the planning activity are reported to key experts. There is no yet a formal commitments, but we learnt in the informative meetings that it will be a selection of groups of national experts for each vulnerability activity based on their qualifications.

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Annex 1 CLIMATE SCENARIOS Sea level pressure (SLP) fields trends were analyzed considering SRES-A2 GCM outputs and NCEP/NCAR reanalyzes. This analysis was performed considering only the four GCMs that best agree with the NCEP/NCAR reanalyzes for the region delimited by 20°S-47.5°S and 67.5°W-45°W. These models and the available periods are presented in Table 1.

Model Period HADCM3 (1) 1951-2099 CSIRO-Mk2 (2) 1951-2099 ECHAM4/OPYC3 (3) 1991-2099 GFDL-R30 (4) 1961-2099

Table 1. GCMs with best agreement with NCEP reanalysis in Southeastern South America. Available periods are included. Rotated principal component analysis (PCA) was applied to the mean seasonal SLP fields of the reanalyzes and GCMs outputs of models (1), (2) and (4) for 1961-2000. Model (3) was excluded from this analysis because data start in 1990. The two first PCs of the reanalyzes data (Figure 1) explain more than 90% of the total variance and represent correctly the seasonal variability. These components also show a shift to the south of the axis of maximum South Atlantic SLP. The principal modes of variability and the southward displacement of the South Atlantic high observed

Figure 1. Principal components of seasonal SLP NCEP reanalysis

in the reanalysis data are also observed in the PCs of the GCMs outputs. This is confirmed in Table 2 where the spatial correlation between the first two PCs derived from the reanalysis SLP data and the GCMs outputs are presented.

Model Spatial correlation PC 1 PC 2

HADCM3 0.90 0.85 CSIRO-Mk2 0.89 0.73 GFDL-R30 0.88 0.77

Table 2. Spatial correlation coefficients between the first two PCs obtained for the NCEP-NCAR SLP data and GCMs outputs. The PCA was also performed for the seasonal SLP outputs of the four models for the complete periods indicated in Table 1. The first two PCs for the four GCM SLP outputs are quite similar and show a future scenario with a positive trend in PC2 that represent a southward displacement of the South Atlantic high. As example, Figures 2 a and b show

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Figure 2a. Principal components of SLP outputs for GFDL-R30 (1961-2099). the first two PCs for the SLP outputs of GFDL-R30 and the decadal explained variance by both PCs. The displacement of the South Atlantic high implies an increase in the frequency of easterly winds that could be responsible for the rise of the Río de la Plata level in Buenos Aires.

Figure 2b. Decadal explained variance by the two first principal components of SLP of GFDL-R30 outputs (1961-2099). M1 correspond to PC1 and M2 to PC2. Table 3 presents the historical and future scenarios of meridional SLP gradient over the Río de la Plata according to the NCEP-NCAR reanalysis and the GCMs indicated in Table 1. This variable is proportional to the wind intensity and results show that although there are discrepancies among the NCEP reanalysis and the different GCMs outputs, all of them agree with the fact that an increase in frequency of easterly winds will be also accompanied by an increase in their intensities.

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Period 1950-59 1960-69 1970-79 1980-89 1990-99 2000-2050 2050-2099 NCEP 1.74 2.22 2.10 3.00 2.52 - - HADCM3 1.94 1.88 2.20 2.36 2.16 2.76 3.88 CSIRO-Mk2 - 0.74 0.42 0.74 0.72 1.02 1.34 ECHAM4 - - - - 4.18 4.78 5.26 GFDL-R30 - 0.62 0.82 1.10 1.00 1.28 1.76 Table 3. Meridional annual SLP gradients over the Río de la Plata according to NCEP-NCAR reanalysis and GCMs outputs.

Annex 3

HYDRODYNAMIC MODEL RPP-2D

Hydrodynamic model RPP-2D, already introduced in the previous Report, was taken as the basic model to make experimentation. As already explained RPP-2D, based on software HIDROBID II, is a 2.5 km x 2.5 km resolution 2D-Horizontal model, which includes the Río de la Plata and an extended area of its maritime front (Figure 1). Its performance was shown to be comparable to more sophisticated, but much higher time-computer demanding, 3D models, like HANSOM-CIMA.

Figure 1 Calculation domain of model RPP-2D

FINAL CALIBRATION OF MODEL RPP-2D

The calibration process of model RPP-2D, through successive stages (purely astronomical tide, normal tide condition, and storm tide condition), was explained in the previous Report. Preliminary, and encouraging, results were shown.

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The calibration is now completed. The following improvements were made:

1. Bathymetric information was rebuilt based on more accurate data sources.

2. The astronomical tide boundary condition at the southern border was redefined by using results from a global tidal model.

3. The adjustment of wind fields from NCEP/NCAR was refined through detailed experimentation.

As an illustration, Figure 2 shows the comparison between water level Tide Table data and model results for a pure astronomical tide scenario (only the oscillatory component is presented); the agreement is considered quite satisfactory. The comparison for a storm event is presented in Figure 3, showing a good agreement taking into account the relatively poor wind information. In Figure 4 velocity records and predictions are compared, showing an acceptable degree of agreement for such a sensitive quantity.

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Figure 4 Comparison between recorded and calculated flow velocities, for a normal tide scenario. Component South-North Period 27/ Aug/ 96 –01/ Sep/ 96 (station 30-519)

PRELIMINARY MODEL EXPLOITATION

Model RPP-2D was first exploited to analyze sensitivity of system response to probable changes in the forcings: sea level rise, change in predominant seasonal wind pattern and tributary discharge increase. The results for Buenos Aires are summarized in Figure 5. It is observed that the system is practically blocked to sea level rise; the change in wind pattern could produce mean water

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level increases up to about 10 cm; the discharge increase could lead to a mean level rise above 20 cm.

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MG: Martín García; BA: Buenos Aires; MO: Montevideo; SC: San Clemente; MP: Mar del Plata.

The largest floods in the Uruguay river and

their climate forcing

Inés A. Camilloni 1 Departamento de Ciencias de la Atmósfera y los Océanos.

Universidad de Buenos Aires Centro de Investigaciones del Mar y la Atmósfera (CIMA/CONICET-UBA)

Buenos Aires, Argentina

and

Ruben M. Caffera Unidad Docente Meteorología. Facultad de Ciencias. Universidad de la República

Montevideo, Uruguay

Mayo 2003

1 Corresponding author address:

Inés A. Camilloni Departamento de Ciencias de la Atmósfera y los Océanos Ciudad Universitaria. Pabellón 2. Piso 2. (1428) Buenos Aires. Argentina. Tel. (5411) 4576-3398 Fax (5411) 4576-3356/3364 ext. 12 e-mail: inesc@at.fcen.uba.ar

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Abstract

The Uruguay river is the second tributary in importance of the La Plata river with a basin that

includes a vast region of Southeastern South America. The objective of this article is to examine

the largest discharge events in the Uruguay river and to explore their atmospheric forcing.

Extreme daily Uruguay river levels registered during the austral warm season (October to

March) are related to intense rainfall in the upper basin particularly in the period -12 to -9 days

before the date of maximum level at the Salto Grande gauging station while those occurred

during the austral cold semester (April to September) are mostly due to large rainfall amounts

registered over and upstream Salto Grande in two separate periods before the flooding dates (-12

to -9 and -4 to -1 days).

The composites of the low-level wind fields for the warm season events show a northwestern

flow over Bolivia and Paraguay that converges with a weak northern component over the

Uruguay basin during the 12 days before the flooding date. The composites for the cold season

events show an intense northwesterly flow over the same region that continues over southeastern

South America and penetrates in the Atlantic ocean for the days -12 to -9 and -4 to -1 in

accordance with the two heavy rainfall periods identified over the Uruguay basin. The

circulation pattern during the -8 to -5 period shows northern flow and weak convergence over

the basin.

The enhanced precipitation due to moisture flux convergence in the South American low-level

jet (SALLJ) region could be responsible for almost half of the largest floods of the Uruguay

river at Salto Grande. For both the warm and cold seasons, the SALLJ occurrence largest

frequencies are observed during the periods -12 to -9 and -4 to -1 before the flooding dates with

a major decrease in the frequency during the intermediate period of the cold season events.

In most cases, the extreme daily Uruguay river levels induced extraordinary discharges at the

monthly scale. The largest monthly discharge anomalies at the Paso de los Libres gauging

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station occurred mostly during warm ENSO phases explained by the large positive rainfall

anomalies observed in the region during these events.

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1. Introduction

The Uruguay river is the second tributary in importance of the La Plata river with a basin of

365,000 km2 that includes parts of Brazil, Uruguay and Argentina and a mean flow of about

4500 m3/s. It begins near the Atlantic Ocean at a height of approximately 1800 m above sea

level and ends at the La Plata river (Figure 1). The major tributary of the Uruguay is the Negro

river, which originates in Brazil and crosses Uruguay with a northeast to west direction.

During recent years, many authors have explored different features of the hydrology of the

Uruguay basin. García and Vargas (1996, 1998) investigated the spatial variability of average

monthly and yearly flows at selected gauging stations in the river. They also searched for

significant changes in the streamflow trends, and found a positive trend since 1970 and changes

around the years 1917-1918 and 1943. Genta et al (1998) identified long-term trends in the

monthly streamflow records of four major rivers in Southeastern South America including the

Uruguay river that were consistent with a significant decrease in the amplitude of the seasonal

cycle. Robertson and Mechoso (1998) analyzed the interannual and decadal cycles in river flows

of Southern South America. They focused the analysis on annual mean streamflows and found

interannual cycles (at around 3.5, 6 and 9 years) and a nonlinear trend. Other studies were

focused on the relationship between river discharges and climatic forcing. For instance,

Mechoso and Perez Iribarren (1992) analyzed the relationship between the Southern Oscillation

(SO) and the Uruguay and Negro rivers. They found that streamflows in both rivers present a

tendency to be below average in years with high SO index and a tendency to be slightly above

average in El Niño-Southern Oscillation (ENSO) years. Robertson and Mechoso (1998)

demonstrated that the 3.5 years cycle in the Uruguay and Negro flow series is associated to an

ENSO-like pattern in the Pacific sea surface temperature anomalies with above-average

streamflow coinciding with a warm phase in the eastern equatorial Pacific. Bischoff et al. (2000)

studied the variability of the Uruguay river flows and the influence of the ENSO phenomenon.

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They found significant differences in the river flow between warm and cold events in the

equatorial Pacific during November, December, July and August. Recently, Krepper et al.

(2003) analyzed the river flow and the precipitation in the Uruguay river basin and its

subcatchments in order to address how the signals of the subcatchments relate to the overall

basin signal. They found that the around 6-year and the 3.5-year ENSO signals are the two

statistically significant peaks for the river flow and that both signals are present only in the

precipitation of the upper two-thirds of the basin.

However, despite the aforementioned contributions, there is a lack of a description of the largest

floods of the Uruguay river and their causes. A better understanding of the climate forcing of

these events can help to prevent future disasters and huge economic loss. The aim of this paper

is to examine the extreme discharge events in the Uruguay river for the period 1950-2000 and to

explore their atmospheric forcing. The paper is structured as follows. Section 2 summarizes the

main regional climate features and the datasets are described in section 3. Section 4 discusses

the main hydrological features of the Uruguay river and the differences with the regime of the

Paraná river. Section 5 discusses the climatic forcing of the extreme discharge events in the

Uruguay river and the relation between these extreme events and the SALLJ. Finally, section 6

examines the relation between the largest monthly discharge anomalies and ENSO and section 7

summarizes results and conclusions.

2. Regional climate background

Northern Argentina, Paraguay, Uruguay and southern Brazil experience different types of

transient disturbances. Cold frontal passages are common transient weather events over the

continent and convective activity is triggered as they pass over northern Argentina, Paraguay,

Uruguay and southern Brazil. Oliveira (1986) performed monthly statistics of frontal

penetrations and their effect on the convective activity over subtropical South America. The

6

frontal passages are well spread over all seasons and although their convective activity is very

low during the winter months, they are responsible for a large part of the rainfall. Southeastern

South America also experiences the effects of sudden development of mesoscale convective

complexes (MCCs) especially in the period November-April (Velasco and Fritsch, 1987). These

complexes have a life cycle of less than a day (Satyamurty et al., 1998).

An important feature of the South American climate is that as the Atlantic subtropical high

migrates southward in early summer, convection develops along the South Atlantic

Convergence Zone (SACZ). This band of intense convective activity emanates from the

Amazon region extending from the tropical South America southeastward into the South

Atlantic Ocean (Kodama, 1992; Figueroa et al., 1995; Nogués-Paegle and Mo, 1997; Liebmann

et al., 1999; Barros et al., 2000). Nogués-Paegle and Mo (1997) found evidence of a seesaw

pattern in the convection over the SACZ with each phase lasting no more than 10 days. This

pattern indicates that the intensification (weakening) of the SACZ is associated with rainfall

deficit (abundance) over the subtropical plains of South America. Doyle and Barros (2002)

found that the midsummer interannual variability of the low-level tropospheric circulation and

of the precipitation field in subtropical South America is associated to the sea surface

temperature (SST) anomalies in the western subtropical South Atlantic Ocean. Composites

corresponding to extreme SSTs in the area 20ºS-30ºS and 30ºW-50ºW show two different low-

level circulation and precipitation patterns. The composite of cases with positive SST anomalies

shows two regional maximum in the precipitation field with one of them centered at about 30ºS

and 55ºW and the other one located along the continental extension of a southwardly displaced

SACZ.

Another of the key atmospheric features that contributes to the precipitation over Southern

South America is a northwesterly low-level jet that is situated east of the Andes mountains and

transports atmospheric moisture from the Amazon basin toward subtropical South America. The

7

South American low-level jet (SALLJ) is particularly pronounced over Bolivia, extending

southeastward to the La Plata basin comprising Paraguay, southeast Brazil, Uruguay and

northern Argentina. The low-level jet provides the moisture transport for the formation of clouds

(Berri and Inzunza, 1993) and precipitation in MCCs that generally moves eastward from their

source region in northern Argentina and Paraguay to affect southwestern Brazil and Uruguay

with intense rainfall (Satyamurty et al., 1998). The observational evidence of the SALLJ is

scarce because of the poor radiosonde coverage. Because of this, current understanding of the

SALLJ is founded on diverse basis, including operational analyses (Nogués-Paegle, 1981;

Sughara, 1994; Wang and Paegle, 1996), NCEP/NCAR reanalyses (Douglas et al., 1999;

Marengo and Soares, 2002), high resolution regional models (Douglas et al., 1999) or

considering special observation projects like the Pan American Climate Studies (PACS)

Sounding Network (SONET) during a two-months period of 1998 (Douglas et al., 1999) or the

Wet Season Atmospheric Mesoscale Campaign (WETAMC) of January-April 1999 conducted

in the southwest corner of the Amazon basin (Marengo et al., 2002). The SALLJ occurs all year

long but 75% of the cases during 1950-2000 were detected during the austral warm season

November-February at the core region near Santa Cruz (17.76ºS, 63.15ºW) while at the exit

region near Mariscal Estigarribia (22.02°S, 60.6ºW) the jet is more frequent during the austral

winter and spring seasons (Marengo and Soares, 2002). According to Berbery and Barros

(2002), the moisture flux over South America during the austral summer has a southward

maximum between 900 and 950 hPa and a somewhat higher maximum at 850 hPa during

winter. The winter SALLJ is probably the responsible for providing moisture and heat ahead of

frontal zones and cyclonic systems that produce much of the winter precipitation (Vera et al.,

2002).

During the austral summer, the meridional transport of tropical air into midlatitudes is linked to

the Chaco Low located approximately at 25ºS and 65ºW. During this season, this system

8

intensifies due to positive net radiation favoring the transport of tropical air masses to the south

and the presence of strong convection activity fed by moisture from tropical regions (Seluchi

and Marengo, 2000). The SALLJ displays a maximum near Santa Cruz and in some occasions a

secondary maximum is found in southern latitudes called Chaco jet events. During the Chaco

events the SALLJ penetrates downwind up to 30ºS and exhibits a strong moisture flux

convergence and enhanced precipitation over Paraguay, northeastern Argentina, southeastern

Brazil and Uruguay (Nicolini and Saulo, 2000, Nicolini et al, 2002). Likewise, Nicolini and

Saulo (2000) showed that in those cases the composite fields of the 850 hPa wind and sea level

pressure show an intensification of the Chaco thermal-orographic low extending farther south up

to Uruguay.

3. Data

Daily Uruguay river levels at the Salto Grande monitoring station (see Figure 1) were obtained

from the National Direction of Hydrology of Uruguay for the period 1950-2000. In order to

evaluate the possible effect of anthropogenic management of the river flows immediately

upstream from Salto Grande, daily Uruguay river levels at the Argentine gauging station Paso de

los Libres (Figure 1) were also considered for the available period (1997-98). Monthly

streamflows at Paso de los Libres for 1909-2000 were analyzed to explore the relation between

the largest monthly discharges of the Uruguay river and ENSO events. The Argentine

Department of Water Resources provided these data.

The data used to examine the atmospheric conditions during the major discharge events of the

Uruguay river were the 0000, 0600, 1200 and 1800 UTC and daily averaged gridded reanalyses

from the National Centers for Environmental Prediction-National Center for Atmospheric

Research (NCEP-NCAR) (Kalnay et al., 1996) with a horizontal resolution of 2.5º latitude by

2.5º longitude. Daily averages of the National Oceanic and Atmospheric Administration

9

(NOAA) satellite outgoing longwave radiation (OLR) were used as a precipitation proxy to

validate with independent data results obtained with the NCEP-NCAR precipitation reanalyses.

4. Major discharge events

The Paraná and Uruguay rivers are the main tributaries of the La Plata river. Therefore the

comparison between their regimes is important due to the different time scales associated to

their extreme events. The greatest discharge events in the Paraná river present a longer time

scale than in the Uruguay and consequently they have been studied on a monthly basis

(Camilloni and Barros, 2003). The time scale of the greatest discharge events in the Uruguay

river have approximately the same duration of the atmospheric perturbations (3-10 days) that

induce the associated extreme rainfall events. This is due to the steep slope (~0.086 m/km for

the Uruguay river around Salto Grande in comparison to ~0.036 m/km of the Paraná river in the

proximity of Corrientes) and the narrow transverse section that favor the runoff (Tossini, 1959).

The differences between the daily regimes of both rivers can be visualized in Figure 2 where the

daily river level values for 1997-98 at Corrientes in the Paraná river (Figure 1) and Paso de los

Libres in the Uruguay River are presented. Although both rivers exhibit the same low frequency

modulation, the Paraná river presents lower interdiurnal variability. Figure 3 also provides an

insight of the differences between both rivers as presents the spectral densities of the monthly

discharge anomalies at Corrientes and Paso de los Libres for 1946-2000. The largest spectral

densities of the Paraná river are found for periods of 4.5 months or longer and are almost null

for periods shorter than 3 months. In the case of the Uruguay river, large spectral densities are

also observed for the shortest periods. Figures 2 and 3 indicate that the greatest discharge events

in the Uruguay river are mainly noticeable in a daily scale and eventually in a monthly scale as

it will be discussed in section 6.

10

The critical Uruguay river level for flooding at the Salto Grande monitoring station established

by the National Direction of Hydrology of Uruguay is 8 meters. Table 1 lists all the events

during 1950-2000 with the eighteen highest river levels that in all cases were higher than 14

meters. The dates presented in this table were selected as the ones during the flood event with

maximum height. Most of the extreme events presented in the table lasted for less than a week

with the exception of the events occurred in 1983 and 1998 simultaneously with the strongest El

Niño events of the century.

The monitoring station Salto Grande is located downstream the Salto Grande dam shared by

Uruguay and Argentina. In order to identify the possible effect of anthropogenic regulation of

the discharges by the dam after 1980 when Salto Grande started to operate, Figure 4 presents a

comparison between the daily river levels at the monitoring station Paso de los Libres upstream

the Salto Grande dam and the Salto Grande values for a flooding period of 1997-98. Although it

is possible to appreciate a slight modulation in the river flows at Salto Grande, the regulation

activities at the dam are relatively small and do not hide the flood tide. Figure 4 also allows

identifying that the lags between the beginning of the increase of the river level at Paso de los

Libres and the date of maximum level at Salto Grande were of no more than 12 days.

Therefore, to analyze the largest river levels of the Uruguay at Salto Grande, only the daily

atmospheric fields of different variables up to 12 days before the flooding date will be studied.

Table 1 shows that although the number of extreme events occurred during the warm semester

(nine) is the same than during the cold season, the highest river level values are mostly observed

during the cold semester. In fact, 8 out of the 10 highest river levels occurred between April and

September.

11

5. Climate forcing of the major discharge events

a. Rainfall, OLR and low-level atmospheric circulation

In order to explore the climatic forcing of the flood events included in Table 1, they were

divided in two groups of nine events each according to the month of occurrence, in warm

season events (occurred during the austral spring and summer from October to March) and cold

season events (occurred during the austral autumn and winter from April to September). Figures

5 to 7 present the composites for the warm and cold seasons events of the values of different

meteorological variables for the averages of three four-day periods comprising the days -12 to

-9, -8 to -5 and -4 to -1 prior the selected flooding dates presented in Table 1.

Figure 5 presents the daily mean accumulated precipitation for the composites defined

previously. Rainfall patterns show a quite different spatial distribution between the warm and

cold season events. For the events occurred during the austral spring and summer, daily rainfall

values are between 6 and 12 mm day-1 upstream Paso de los Libres in the period between -12 to

-5 days before the maximum river level at Salto Grande with a maximum in the period -12 to -9.

The four days immediately before the flooding date of the warm season events show lower daily

rainfall values in the basin than the previous eight days although a high precipitation rate

continues to the north outside the Uruguay basin. During the cold events, the largest

precipitation values are observed in a southernmost position than during the warm events

although they show a similar intensity during the -12 to -9 and -4 to -1 days. The rainfall

amounts are quite small during the period -8 to -5 indicating that the extreme discharge events in

Salto Grande could be caused by two wet periods with an intermediate one probably associated

to a cold frontal passage. This is confirmed when inspecting individual cases (not shown).

These results are consistent with the OLR composites presented in Figure 6. Considering that

OLR values less than 240 Wm-2 are indicative of convective activity in the region (Kousky,

1988; Barros et al., 2000), the figure shows that the main convective centers over the Uruguay

12

basin are located in a southernmost position and are more intense during the cold events than

during the warm ones. Figures 5 and 6 show a different temporal and spatial distribution of

convection and rainfall over the Uruguay basin between the warm and cold events during the

days previous to the flooding date. Consequently, floods registered during the warm semester

(October to March) are due to intense rainfall in the upper basin particularly in the period -12 to

-9 days before the flood date while those occurred during the cold one (April to September) are

mostly due to the large rainfall amounts registered over and near upstream Salto Grande in two

separate periods (-12 to -9 and -4 to -1).

Figure 7 shows the four-day composites of the 925 and 850 hPa wind fields for the warm and

cold events respectively. The different tropospheric levels considered for both periods were

selected according to the results of Berbery and Barros (2002) showing that the maximum

moisture flux over South America during the austral summer has a maximum between 900 and

950 hPa and a higher maximum at 850 hPa during winter. The composite for the warm season

events show a northwestern flow over Bolivia and Paraguay that converges with a weak

northern component over the Uruguay basin during the first two four-day periods. The

composites for the cold events show an intense northwesterly flow over Bolivia and Paraguay

that continues over northern Argentina and southern Brazil and penetrates in the Atlantic ocean

for the periods -12 to -9 and -4 to -1. This circulation pattern is probably associated to warm and

humid advection and it is in accordance with the two heavy rainfall periods over the Uruguay

basin identified in Figure 5. The circulation pattern during the -8 to -5 period is somewhat

different with a predominantly northern flow and weak convergence over the Uruguay basin.

b. Relation between extreme discharge events and the SALLJ

The SALLJ has been identified as a key factor in the water vapor transport and the triggering of

organized convection associated to enhanced precipitation over Southeastern South America.

13

This section explores the occurrence of SALLJ events up to 12 days before the flooding dates

indicated in Table 1.

Although Bonner's (1968) criterion for low-level jet identification is based on observations, his

methodology was adapted to NCEP reanalyses for studies to identify the SALLJ (Marengo et al,

2002). In this study, the conditions that must be satisfied to identify a day with SALLJ are: (a)

meridional winds must be more predominant than zonal winds (v>u) immediately east of the

Andes, (b) the meridional component must be southward (v<0), (c) wind speed must exceed

12m/s in some pressure level between surface and 850 hPa, and (d) vertical wind shear between

850 and 700 hPa (for the austral autumn and winter cases) or between 925 and 700 hPa (for the

austral spring and summer cases) must be at least 6 m/s. These conditions must be observed at

least at one of the four daily individual reanalysis data (0000, 0600, 1200 and 1800 UTC).

The SALLJ was identified in 42.6% of the days among the 12 days before the flooding dates

occurred during the austral autumn and winter. This percentage is slightly higher than the 40.7%

obtained for the spring and summer cases. Consequently, almost half of the largest discharge

events of the Uruguay river at Salto Grande could be related to enhanced precipitation due to

moisture flux convergence in the SALLJ exit region. Table 2 summarizes the occurrence of

SALLJ during the warm and cold seasons for three four-day periods before the extreme

discharge dates at Salto Grande indicated in Table 1. Both seasons show the largest frequencies

of occurrence of SALLJ during the periods -12 to -9 and -4 to -1. During the cold season events,

these frequencies are more than twice the frequency of the intermediate period in accordance

with the two heavy rainfall periods identified in Figure 5.

6. Relations between the largest monthly discharge events and ENSO

The largest monthly discharge anomalies of the Uuruguay river for 1909-2000 at Paso de los

Libres the gauging station were selected following the criterion that the discharge anomalies

14

were larger than at least three times the monthly standard deviation for the respective month.

The eighteen events that satisfied this condition are listed in Table 3. The magnitude of these

extraordinary discharges minimizes the possible impact of water management by upstream

dams.

Comparison between the maximum daily Uruguay river level at Salto Grande (Table 1) and the

extreme monthly discharge anomalies at Paso de los Libres (Table 3) indicates that 10 out of the

18 extreme daily values resulted in extraordinary discharges at the monthly scale. Out of the

other 8 extreme daily cases, six occurred during months that rank among the thirty-five largest

monthly discharge anomalies of the period with values higher than twice the corresponding

standard deviation for that month while the two remaining daily cases also show positive

anomalies at the monthly scale.

The cases listed in Table 3 constitute events that must necessarily have been caused by

considerable precipitation anomalies. The Uruguay basin is part of a region that has a strong

precipitation signal during ENSO events (Kousky et al., 1984; Ropelewski and Halpert, 1987,

1996; Kiladis and Díaz, 1989, Pisciottano et al., 1994) and therefore it can be expected a

hydrological response to ENSO. To facilitate the discussion of this aspect, Table 3 includes a

classification of the events according to the season and ENSO phase and the SST anomalies in

the El Niño-3 region. The starting and end months of ENSO events that occurred after 1950

were defined according to Trenberth (1997) and the ENSO years before 1950 were taken from

Kiladis and Diaz (1989). There is a clear relationship between ENSO phases and the major

discharge anomalies in the Uruguay river. In Paso de los Libres, 13 out of 18 occurred during El

Niño events. In addition, none of them occurred during La Niña phase. Table 3 shows that

although the largest monthly discharge anomalies at Paso de los Libres occurred mostly during a

warm ENSO phase, there is not a dominant season since there are 5 cases during the austral

autumn (March to May) of the year following the onset of an El Niño event (autumn +), 4

15

during the austral spring 0 (September to November), 3 during the austral summer + (December

to February) and only 1 during the austral winter+ (June to August). This result shows a

different response to ENSO events between the Uruguay and the Paraná basins as the analysis of

the largest monthly discharge anomalies of the Paraná river at Corrientes shows that the top

discharges occurred during the austral autumn + (Camilloni and Barros, 2003).

Figure 8 presents the seasonal rainfall anomalies in Southeastern South America for the El Niño

phases with monthly extreme discharge events in the Uruguay basin considering all the El Niño

events occurred after 1950. All seasons show positive rainfall anomalies over the Uruguay basin

which are statistically significant at the 95% level for three of the four analyzed phases. The

largest significant anomalies occurred during autumn + in the upper part of the basin at about

25ºS. During spring 0 and summer +, the largest anomalies occurred to the north of 30ºS

upstream from Salto Grande. This figure suggests that the extreme monthly discharges at Paso

de los Libres are a consequence of large rainfall anomalies to the north of that station with a

relatively small contribution of local rainfall events.

7. Concluding remarks

The Uruguay basin is part of a region in Southeastern South America that has a strong

precipitation signal during ENSO events. The hydrological response to ENSO is evident as most

of the largest monthly discharge events of the Uruguay river at Paso de los Libres gauging

station occurred during the El Niño phase. Nevertheless, the analysis of the individual extreme

discharge events on daily basis show that these floods last between 3 and 10 days and

consequently it is necessary to study their climate features in the synoptic scale.

The number of extreme events occurred at the Salto Grande gauging station at the daily scale

during the warm semester (October to March) are the same than during the cold season (April to

September). However, the highest river levels are mostly observed during the cold semester.

16

Floods registered during the warm season are due to intense rainfall in the upper basin

particularly in the period -12 to -9 days before the flooding date, consequently the hydrological

forecast of these events is possible many days in advance. The largest discharges occurred

during the cold semester are mostly due to the large rainfall amounts registered over and

upstream from Salto Grande in two separate periods (-12 to -9 and -4 to -1 days before the

flooding date) and the hydrological prediction must necessarily depend on an appropriated

weather forecast.

Almost half of the largest discharge events of the Uruguay river at Salto Grande could be related

to enhanced precipitation due to moisture flux convergence in the SALLJ region. The frequency

of occurrence of SALLJ during the 12 days before the flooding date of the cold season extreme

events is slightly larger than during the warm semester. This result shows the relevance of the

cold season SALLJ in the contribution to moisture transport and atmospheric convection and

also indicates a departure from the behavior of the North American LLJ that is only present and

exhibits large moisture fluxes during the warm season (Berbery and Barros 2002).

Acknowledgements

This paper was funded by the University of Buenos Aires under its Program of Strategic

Projects; by the AIACC, a Project of the GEF implemented by the UNEP and co-executed by

START and the TWAS and by IAI CRN-55 (PROSUR).

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Captions

Figure 1. Uruguay and Paraná basins and location of gauging stations (1: Salto Grande, 2: Paso

de los Libres, 3: Corrientes).

Figure 2. Daily river levels at Corrientes (Paraná river) and Paso de los Libres (Uruguay river)

for 1997-98.

Figure 3. Spectral densities of the monthly discharge anomalies at Corrientes (Paraná river) and

Paso de los Libres (Uruguay river) for 1946-2000.

Figure 4. Daily Uruguay river levels at Salto Grande and Paso de los Libres for a period

between December 1997 and June 1998.

Figure 5. Composites of daily mean accumulated precipitation for the averages of three four-

day periods comprising the days -12 to -9, -8 to -5 and -4 to -1 prior the selected flooding dates

for the warm (left)and cold (right) season events.

Figure 6. As fig. 5, for OLR.

Figure 7. As fig. 5, for 925 hPa wind (left) and 850 hPa wind (right).

Figure 8. Rainfall anomalies in Southeastern South America during El Niño phases with

extreme monthly discharges at Paso de los Libres (significant anomalies at the 95% level are

shaded).

21

Table 1. Largest Uruguay river daily levels at Salto Grande during 1950-2000.

Table 2. Relative frequency (%) of occurrence of SALLJ for three four-day periods before the

flooding dates indicated in Table 1.

Table 3. Extreme monthly discharge anomalies of the Uruguay river at Paso de los Libres.

ENSO phase and El Niño-3 SST anomalies are also included.

22

Date Height (m)

16 April 1959 20.18

25 July 1983 17.85

16 April 1986 17.71

7 June 1992 17.71

19 June 1972 17.16

8 May 1983 16.91

10 January 1998 16.70

8 September 1972 16.43

26 April 1998 16.26

20 October 1997 16.01

17 November 1982 16.01

25 April 1987 15.91

13 November 1997 15.76

9 March 1998 15.59

18 October 1979 15.46

13 February 1998 15.33

9 November 1979 15.04

30 October 1954 14.63

Table 1. Largest Uruguay river daily levels at Salto Grande during 1950-2000.

23

period before the flooding date

-12 to -9 days -8 to -5 days -4 to -1 days

Warm season events 41.7 33.3 47.2

Cold season events 50.0 22.2 55.6

Table 2. Relative frequency (%) of occurrence of SALLJ for three four-day periods before the

flooding dates indicated in Table 1.

24

Date of maximum monthly

discharge anomaly

Discharge anomaly (m3/s) SST Niño 3 anomaly (ºC)

May 1941 (Autumn +) 17338 1.88

July 1983 (Winter +) 15838 1.04

November 1997 (Spring 0) 15592 3.59

May 1983 (Autumn +) 15196 1.91

October 1929 (Neutral) 15129 0.53

November 1982 (Spring 0) 14689 2.66

February 1998 (Summer +) 13615 2.57

October 1997 (Spring 0) 12946 3.29

June 1990 (Neutral) 12633 0.08

April 1998 (Autumn +) 11932 1.68

September 1928 (Neutral) 11591 0.07

September 1972 (Spring 0) 11301 1.52

April 1932 (Neutral) 9926 0.50

March 1998 (Autumn +) 9410 2.05

March 1983 (Autumn +) 9267 2.01

August 1983 (Neutral) 8894 0.92

January 1998 (Summer +) 8703 3.32

January 1912 (Summer +) 7692 1.39

Table 3. Extreme monthly discharge anomalies of the Uruguay river at Paso de los Libres.

ENSO phase and El Niño-3 SST anomalies are also included.

25

Figure 1. Uruguay and Paraná basins and location of gauging stations (1: Salto Grande, 2: Paso

de los Libres, 3: Corrientes).

ATLANTIC OCEAN

1

2

3

26

Figure 2. Daily river levels at Corrientes (Paraná river) and Paso de los Libres (Uruguay river)

for 1997-98.

0

2

4

6

8

10

12

14

01/0

1/97

01/0

2/97

01/0

3/97

01/0

4/97

01/0

5/97

01/0

6/97

01/0

7/97

01/0

8/97

01/0

9/97

01/1

0/97

01/1

1/97

01/1

2/97

01/0

1/98

01/0

2/98

01/0

3/98

01/0

4/98

01/0

5/98

01/0

6/98

01/0

7/98

01/0

8/98

01/0

9/98

01/1

0/98

01/1

1/98

01/1

2/98

leve

l (m

)

Corrientes (Paraná River) Paso de los Libres (Uruguay River)

27

Figure 3. Spectral densities of the monthly discharge anomalies at Corrientes (Paraná river) and

Paso de los Libres (Uruguay river) for 1946-2000.

period (months)

Spe

ctra

l den

sity

0

1

2

3

4

5

6

7

2 3 4 5 6 7 8 9 10 11 12

Corrientes (Paraná River)Paso de los Libres (Uruguay River)

period (months)

0

1

2

3

4

5

2.0 2.5 3.0 3.5 4.0 4.5 5.0

28

Figure 4. Daily Uruguay river levels at Salto Grande and Paso de los Libres for a period

between December 1997 and June 1998.

leve

l (m

)

0

2

4

6

8

10

12

14

16

18

20

19-D

ec97

24-D

ec97

29-D

ec97

3-Ja

n98

8-Ja

n98

13-J

an98

18-J

an98

23-J

an98

28-J

an98

3-F

eb98

8-F

eb98

13-F

eb98

18-F

eb98

23-F

eb98

28-F

eb98

5-M

ar98

9-M

ar98

14-M

ar98

19-M

ar98

24-M

ar98

29-M

ar98

3-A

pr98

8-A

pr98

13-A

pr98

18-A

pr98

23-A

pr98

28-A

pr98

3-M

ay98

8-M

ay98

13-M

ay98

18-M

ay98

23-M

ay98

28-M

ay98

2-Ju

n98

Paso de los LibresSalto Grande

29

Figure 5. Composites of daily mean accumulated precipitation for the averages of three four-day periods comprising the days -12 to -9, -8 to -5 and -4 to -1 prior the selected flooding dates for the warm (left)and cold (right) season events.

30

Figure 6. As fig. 5, for OLR.

31

Figure 7. As fig. 5, for 925 hPa wind (left) and 850 hPa wind (right).

32

Figure 8. Rainfall anomalies in Southeastern South America during El Niño phases with

extreme monthly discharges at Paso de los Libres (significant anomalies at the 95% level are

shaded).

ANNEX 4

Title: Impact of a possible local wind change on the wave climate in the upper Río de

la Plata.

Walter C. Dragani1,2,3 and Silvia I. Romero1,2

1Departamento Oceanografía, Servicio de Hidrografía Naval and ESCM-INUN, Av. Montes de Oca 2124 (C1270ABV) Ciudad Autónoma de Buenos Aires, Argentina. 2Departamento Ciencias de la Atmósfera y los Océanos, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, (1428) Ciudad Universitaria, Pabellón II, 2do. piso. Ciudad Autónoma de Buenos Aires, Argentina. 3CONICET, Consejo Nacional de Investigaciones Científicas y Técnicas. Corresponding author: Walter César Dragani Depto. Oceanografía Servicio de Hidrografía Naval Av. Montes de Oca 2124 (C1270ABV) Ciudad Autónoma de Buenos Aires Argentina. e-mail: dragani@hidro.gov.ar TE: 54 11 4301 0061/67 int. 4050 Fax: 54 11 4301 0061/67 int. 4046

1

ABSTRACT The aim of the present work is to give a quantitative approximation of the changes in the

wave climate in the upper Río de la Plata (RDP) considering a possible future change in

local winds. First, sea and swell climate in the outer RDP were statistically analysed from

direct observations obtained at the outer RDP. Secondly, propagation and transformation

of sea and sewll, from the outer RDP throughout the intermediate and upper regions, were

computed and analysed. It was obtained that heights of waves propagating from the outer

to the upper RDP were attenuated 94.9% by refraction, shoaling and friction effects.

Consequently, it was shown that predominant wave climate in the upper RDP can only be

described considering wind waves locally generated (sea). Present wave climate

(directional wave heights and periods) was estimated by hindcasting methodology based

on ten-year statistics of winds measured at “Aeroparque Jorge Newery” meteorological

station. Then, a possible future scenario was sketched for which wind frequencies and

intensities for the East directions were 30% and 10% higher than the present values,

respectively. Our results show that mean East wave height and mean total wave height will

increase within the upper RDP. Present mean East wave heights (0.90 m) will increase by

0.12 m (13%) and their frequencies will increase by 30% (from 18.4 to 23.9%).

Consequently, the coast of Buenos Aires City will be more frequently exposed to wave

effects, therefore giving rise to intensified associated littoral processes. The mean period

for East waves will not change significantly (less than 4%, from 5.3 to 5.5 s). Results

obtained in this paper are a first approximation to the problem suggesting that wave

climate within the upper RDP is very sensitive to a slight change in the wind field.

2

1. INTRODUCTION

The Río de la Plata (RDP) is a shallow and extensive estuary located on the eastern coast

of South America at approximately 35º S (Figure 1). It has a NW—SE general orientation

and it is formed by the confluence of two of the most important rivers of South America:

the Paraná and Uruguay rivers. The estuary has a funnel shape approximately 300 Km long

that narrows from 220 Km at its mouth to 40 Km at its upper end. The RDP can be divided

into three regions: upper, with an averaged depth of less than 5 m; intermediate, 5-10 m

deep, occupied by several shallow sand banks, and an outer region with depths ranging

from 10 to 20m. The main characteristics of this estuary are its shallowness and its

increasing width (Balay, 1961). The RDP substantially contributes to the nutrient,

sediment, carbon and fresh water budgets of the South Atlantic Ocean (Framiñan et al.,

1999). It affects the hydrography of the adjacent continental shelf, impacts on important

coastal fisheries, and influences coastal dynamics up to more than 400 Km north on the

Brazilian shelf (Campos et al., 1999; Piola et al., 2000). The Río de la Plata has huge social

and economical importance for the countries on its shores, Argentina and Uruguay. The

most important cities and harbors, including both capitals (Buenos Aires and Montevideo

respectively) and many of the industrial centers and resorts of these countries are placed on

its margins. The estuary is also the most important source of fresh water for million of

inhabitants and it is an area of spawning and nursery for many coastal species (Cousseau,

1985; Boschi, 1988).

Water level fluctuations in the RDP are the sum of three main constituents: tides, storm

surges and wind waves. Tides in the RDP are mixed with both semi-diurnal and diurnal

constituents present. They have a maximum range of 1.08 m in the upper RDP, on the

3

coast of Buenos Aires and they are somewhat smaller to the north, along the Uruguayan

coast being minimum in Punta del Este (0.33 m) (SHN, 2003). D’Onofrio et al. (1999)

estimated return periods of extreme water levels for some vulnerable low-lying areas of

Buenos Aires. Their analysis combines the probability density functions of tides and

surges. They obtained for example that the last 89-yr. maximum (4.44 m) has a recurrence

of approximately 265 yr. Fiore, et al. (2001) carried out a statistical analysis of surges in

Buenos Aires based on hourly residuals for the period 1905-2000. They obtained positive

trends for positive and negative surges and explained them as a consequence of a mean sea

level rise rather than an increase in the intensity of the storm surges.

There are no direct measurements of waves in the RDP except for a single 5-year record of

wave data gathered in the outer RDP by means of a directional wave recorder Datawell

Waverider. Based on these data, Anschütz (2000) showed that wave climate in the outer

RDP is a combination of swell (wave generated far away, not related to local wind) and sea

(wave generated by local wind).

The analysis of the waverider data revealed predominant heights between 0.5 to 1.5 m.

When sea prevailed, periods were between 4 and 6 s, when swell prevailed they were

between 10 to 12 s. In the upper RDP, wind waves have never been neither measured nor

modelled, consequently, a realistic wind wave climate in this area is not available at the

present.

Escobar et al (2003) showed that the western border of the South Atlantic High is slowly

moving southward. They applied Principal Component’s methodology to a set of monthly

mean sea level pressure data from NCEP-NCAR reanalysis (1951/2000) in a domain

limited by 25° and 45° S and 67.5° and 45°W. For the last decades their results show a

slight trend in the western border of the South Atlantic semi-permanent high-pressure

4

system to move toward the south. This displacement has produced increased frequencies in

east winds over the RDP Estuary.

The aim of the present work is to give a quantitative approximation of the changes in the

wave climate in the upper RDP considering a possible future change in local winds. The

first step was to analyse if waves observed in the outer RDP, could propagate toward the

Northwest and reach the upper region. When waves did not reach this zone, wave climate

in the upper RDP could only be represented by sea. Our hypothesis is that in this case,

slight changes both in frequency and intensity of local winds, as the ones Escobar et al ’s

results suggest, should affect the wave climate within the upper RDP. This fact has not

been addressed up to the present. In this paper, present and future mean directional wave

heights are estimated and compared for the upper RDP. Finally, changes in mean height,

period and some energetic considerations are studied and possible impacts on littoral

processes and sediment transport are briefly discussed.

2. WAVE CLIMATE IN THE UPPER RDP

Under this section, the wave climate in the upper region of the RDP, estimated from

climatic wind data (Section 2.3), is presented. First, sea and swell climate in the outer

RDP are statistically analysed from direct observations (Section 2.1). Secondly, we

studied the propagation and transformation of sea and swell from the outer RDP

throughout the intermediate and upper regions (Section 2.2) in order to assess if outer

waves can reach the upper RDP.

5

2.1. Sea and swell climate in the outer RDP

A single series of 11,297 records, gathered from June 1996 to November 2001 was

obtained with a directional wave recorder Datawell Waverider in the outer RDP at

latitude 35° 40’ S and longitude 55° 50’ W (Figure 1). The instrument was programmed

to measure 20-minute sea level records, every 2 hours 40 minutes, with a 0.5 s sampling

interval. The wave parameters analyzed in this paper are the energy-based wave height

HMO (computed as four times the squared root of the 20-minute record total variance)

and the peak spectral period TP. Directional bi-dimensional distributions (HMO - Tp)

were made using the waverider data. Table 1 shows the number of occurrences for the

eight directions analysed. SE direction, followed by E and S are the main directions of

propagation with 41%, 28% and 14% of occurrences, respectively. Frequencies for the

rest of the directions are equal or lower than 5%. The bi-dimensional distribution for the

direction with largest number of occurrences, i. e. SE, is shown in Figure 2. Two areas

with very high number of events around periods of 10 s and heights of 0.8 m (swell)

and periods of 5 s and heights of 1.25 m (sea) can be clearly identified.

Considering the general orientation (NW-SE) of the RDP and its very shallow waters,

only those waves propagating from the Southeast could reach the upper RDP. Thus,

swell and sea propagating from the outer to the upper RDP would be the long and short

swell, respectively, within the intermediate and upper regions. Refraction, shoaling and

bottom friction transformations of long and short waves propagating throughout the

intermediate and upper RDP are considered in the next section.

6

2.2. Swell climate in the upper RDP

Wave propagation and transformation from the outer RDP towards the coast of Buenos

Aires were analyzed by means of a numerical model of wave transformation (Dragani

and Mazio, 1991). The wave number, a parameter which must satisfy the modified

dispersion relation at any point (Watanabe, 1982), was computed considering realistic

bathymetry and stationary but spatially variable current field corresponding to flood and

ebb conditions. Refraction, shoaling and friction effects along the wave ray were

computed based on a bathymetric grid of 1 Km spatial resolution obtained from the

nautical charts SHN (1999a) and SHN (1999b). In our model the refraction coefficient

was computed by means of the classical methodology given by Griswold (1963). The

shoaling coefficient was based on wave celerity at any point by applying the linear

wave-theory. Bottom friction removes energy from waves by interaction between

oscillatory wave-induced currents on the seabed, and the sediment and bed forms.

Friction effects always diminish the wave height along the ray. In order to obtain the

rate at which energy was removed from waves we adopted the formulation given by

Putnam and Johnson (1942). The friction coefficient adopted was 0.005 suggested by

CARP (1992) as the most adequate value for the RDP. Energy dissipation was

computed along the ray using an integral expression given by Vincent and Carrie

(1988).

We herein assumed that waves propagate as swell from the outer RDP towards the

upper RDP under non-locally generated wave conditions. Two different swell

conditions were analysed: short swell, associated to sea in the outer RDP (T < 6 sec)

and long swell, associated to swell in the outer RDP (T > 6 sec). Refraction diagrams

7

were built using initial heights of 1.25m in all cases, though similar conclusions can be

obtained using different heights within the observed range. Results show that shallow

water effects (especially refraction) are less evident over short swell, thus being the

most likely one to reach the intermediate and upper RDP. Outputs from our

transformation model corresponding to short swell coming from the Southeast are

shown in Figure 3. The figure depicts a refraction diagram for twenty rays directed

towards the NW from the outer RDP. An initial separation of 3 km was adopted

between rays.

Shallow waters and banks were identified as areas where wave braking (crossing rays:

caustics) occurred. Caustics seem to have an important role in wave propagation and

transformation towards the upper RDP thus producing the consequent wave attenuation.

Figure 3 also shows that rays strongly divert at the intermediate RDP producing two

areas of caustics: one located between Punta Piedras and Atalaya, (near the coast of

Buenos Aires) and another Southeast of Colonia (near the coast of Uruguay). In order to

analyse the wave transformation between the outer and upper regions, an additional ray

able to reach the upper RDP was computed (dashed line in Figure 3). The refraction

(KR), shoaling (KS) and friction (KF) coefficients computed for the end point of the ray,

within the upper RDP were 0.057, 1.034 and 0.859, respectively. The transformation

coefficient, computed as the product of the three aforementioned coefficients, was

0.051. Therefore, heights of waves propagating from the outer to the upper RDP were

attenuated 94.9% by refraction, shoaling and friction effects. Consequently,

predominant wave climate in the upper RDP can only be described considering wind

waves locally generated (sea).

8

2.3. Sea climate in the upper RDP

The Wave Hindcasting Method (CERC, 1984) and the improvements given by CERC,

(2002) were applied at a coastal point near Buenos Aires City (Figure 1). The

methodology was applied in shallow waters, off the surf zone, where local depth is of

approximately 2 m. Monthly wind statistics (SMN, 1992) were obtained from hourly

data gathered at “Aeroparque Jorge Newery” meteorological station, located on the

coast of Buenos Aires City (Figure 1). Based on these data we estimated mean wave

heights within the upper RDP. For the SE and NW directions (approximately parallel to

the coastline) wave parameters might have been slightly overestimated because narrow

fetch conditions were not considered. Directional mean winds used in the wave

hindcasting are presented in Table 2. Table 2 shows that the East direction presents the

highest frequency (18.4% of cases), N, NE, SE and S directions present similar

frequencies (ranging from 11.5 to 15.8%) and SW, W, NW directions and calms are the

least frequent ones, with values ranging from 6.5 to 7.8%. Directional mean wind

intensity (from 4.6 to 6.1 ms-1) is quite uniform for all directions.

Fetch-limited conditions have been considered herein given the nature of the

meteorological data available and considering the geographical characteristics of the

upper RDP. Under these conditions, the supposition is made that winds have been

blowing long enough for wave heights to reach equilibrium at the end of the fetch. The

parameters required for hindcasting are fetch and wind speed, the latter being

representative of the average value over the fetch. The wave parameters computed are

the energy-based wave height and the peak spectral period.

9

In shallow waters the deep-water methodology is applicable CERC (2002). Shallow-

water formulae are quite close to the ones of deep-water wave growth for the same wind

speeds, up to a point where an asymptotic depth-dependent wave height is attained. In

light of this evidence it is convenient to disregard bottom friction effects on wave

growth in shallow waters (CERC, 2002). Evidence from Bouws et al. (1985) also

indicates that wave spectra for shallow waters do not seem to have a noticeable

dependence on variations in bottom sediments. Consequently, it is recommended that

deep-water wave growth formulae be used for all depths, with the constraint that no

peak spectral period can grow past a limiting value TP,MAX as shown by Vincent (1985)

given by:

where h is the mean depth and g is the acceleration due to gravity.

Table 2 presents mean wind intensity, frequency, and fetch for the eight analysed

directions and the heights (HMO) and periods (TP) estimated by hindcasting. For each

analysed direction, the fetch was measured as the distance from the hindcasting point to

the coast (Figure 1), except for the case of the Southeast direction where a 150 Km-

fetch was adopted beginning at the imaginary line that joins Punta Piedras and

Montevideo. The largest heights and the longest periods resulted for the Southeast 1.22

m and 5.5 s and for the East 0.9 m and 5.3 s, respectively. The shortest heights and

periods resulted for the West with 0.12 m and 1.4 s, for the Southwest with 0.13 m and

1.4 s and for the South with 0.15 m and 1.4 s.

)1(78.9, gh

T MAXP =

10

3. Present and future mean wave conditions in the upper RDP

3.1. Present mean wave conditions

Directional mean heights and periods presented in Table 2 were used to compute the

non-directional mean height MOH and period PT at the upper RDP near the coast of

Buenos Aires City by means of:

where i is an index related to wind directions (i=1 is North, i=2 is Northeast, and so on)

and iMOH , , iPT , and if are the heights, periods and frequencies shown in Table 2. The

mean height and period obtained from equations (2) and (3) are 0.52 m and 3.3 s,

respectively. Both values should only be considered as a rough approximation of the

mean wave condition within the upper RDP, near Buenos Aires City.

3.1. Future mean wave conditions

Experiments with GCM run with SRS A2 scenarios indicate that observed southward

shift of the western border of the South Atlantic High will continue along the XXI

century and consequently, both, wind frequency and intensity for the East direction

would increase in the future (Barros 2003). In the present study we analyze a possible

)2(8

1,∑

=

=i

iiMOMO fHH

)3(8

1,∑

=

=i

iiPP fTT

11

scenario with an increase of 30% and 10% respectively in the east wind frequency and

intensity. Likewise, in this hypothetical scenario it is assumed a decrease of the same

magnitude would occur in frequency for the West direction. In order to assess the wave

climate sensitivity under a slight but probable wind climatic change the aforementioned

suggested values were adopted and a hypothetical future scenario was sketched.

Considering these changes, future East wind frequencies and mean intensity for the East

direction would increase by 5.5% and 0.49 ms-1, becoming 23.9% and 5.4 ms-1,

respectively. The total decrease in frequencies for the West directions (NW, W and SW)

would be 5.5%, compensating the predicted increase in the East directions. Mean height

and period for the East direction resulted 1.02 m and 5.5 s, computed by the

methodology explained in Section 2.3 based on the suggested future intensity.

Replacing the estimated future figures for the East direction in equations (2) and (3), the

future mean wave condition would be described by a mean height of 0.59 m and a mean

period of 3.6 s.

4. Summary and Discussion

It was shown in section 2.2 that both short and long swell coming from the outer RDP

are strongly attenuated by refraction and bottom friction within the intermediate RDP

and hence the predominant wave climate in the upper RDP can only be described

considering wind waves locally generated (sea). Present wave climate (directional wave

heights and periods) was estimated using the hindcasting methodology given by CERC

(1984, 2002) based on ten-year statistics of winds measured at “Aeroparque Jorge

Newery” meteorological station (SMN, 1992). Next, a possible future scenario was

12

sketched for which wind frequencies and intensities for the East directions were 30%

and 10% higher than the present values, respectively. Wind frequencies for the West

directions were lower in this scenario in proportion to the East increases, meaning that

the rest of the directions kept unchanged. This hypothetical future scenario has been

supported by recent preliminary results of GCM scenarios (Barros 2003). Our results

show that, mean East wave height and mean total wave height will increase within the

upper RDP. Present mean East wave heights (0.90 m) will increase by 0.12 m (13%)

and their frequencies will increase by 30% (from 18.4 to 23.9%). Consequently, the

coast of Buenos Aires City will be more frequently exposed to height wave effects,

therefore giving rise to intensified associated littoral processes. Given the predominant

orientation of the coast of Buenos Aires (NW-SE) north-westward long-shore current

and along shore sediment transport will be increased producing both accretion of

sediments upstream the structures and erosion of the shore downstream the coastal

emplacements. The mean period for East waves will not change significantly (less than

4%, from 5.3 to 5.5 s).

In addition we estimated a difference of +0.07 m (from 0.52 to 0.59 m), between

present and future total (non-directional) mean wave height, that means, an increase of

13%. The mean period for the total mean wave will be 9% greater (0.3 s) than the

present value (from 3.3 to 3.6 s). Differences between present and future heights will

have significant increases in the density of energy due to the quadratic relation between

wave energy and wave height. Total average energy per unity surface area is

proportional to H2MO (Dean and Dalrymple, 1984). The quotient between the future and

the present total average energy is 1.29. This means that, an increase of 29% could

occur under this future scenario of climatic change. An increase in the mean wave

13

energy would implicate a higher capability of water to maintain sediments suspended

within the water column and thus an increase in the suspended sediment transport which

could modify the sediment transport dynamics and would affect the deposition rates.

Results obtained in this paper are a first approximation to the problem suggesting that

wave climate within the upper RDP is very sensitive to a slight change in the wind field.

In consequence, major efforts should be directed to evaluate possible future changes

considering different scenarios. Numerical modelling including wave generation and

transformation, realistic bathimetry, tides and currents would be a very powerful tool in

order to assess possible changes in the wave climate under hypothetical atmospheric

future conditions resulting from specific climatic studies.

Although there are many uncertainties in our predictions, the potential impacts of future

changes in the wave climate need to be assessed. Likewise, decision-makers should

thoroughly consider the possible impacts on the coast of Buenos Aires City.

Acknowledgements: This paper was funded by the AIACC, a Project of the GEF

implemented by the UNEP and co-executed by START and the TWAS.

14

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SHN, 2003, Tablas de Marea, Publicación H-610, Servicio de Hidrografia Naval,

Armada Argentina, Buenos Aires.

SMN, 1992, Estadisticas Climatologicas 1981 –1990, Serie B, Nro. 37, Servicio

Meteorológico Nacional, Fuerza Aérea Argentina, Buenos Aires.

Vincent, C. ,1985. Depth-controled wave height. Journal of Waterway, Port, Coastal

and Ocean Eng., 111, 3, 459-475.

Vincent, C. and A. Carrie, 1988, Evaluation of an energy-propagation wave refraction

model. Continental Shelf Research, 8(3), 287-305.

Watanabe, A., 1982, Numerical modelling of nearshore currents and beach deformation,

Coastal Engineering in Japan, 25, 147-161.

17

Figure captions:

Figure 1. Río de la Plata Estuary. Hindcasting point and directional wave recorder

location are indicated. Depth contours in meters are depicted.

Figure 2. Bidimensional distribution of heights and periods. Mean sea and swell

characteristics are indicated.

Figure 3. Refraction diagram corresponding to wave coming from Southeast.

Refraction, shoaling and dissipation coefficients are indicated. An intermediate ray

(dashed line) reaching the upper RDP is included.

Table captions:

Table 1. Number of events for each of the eight analysed wave directions at the outer

Río de la Plata estuary.

Table 2. Directional mean wind and probability of occurrence obtained from data

gathered at “Aeroparque Jorge Newery” Meteorological Station located on the coast

of Buenos Aires City. Also included are directional fetch, heights and periods

obtained by hindcasting.

18

Direction Number of events

Percentage %

N 278 2 NE 237 2 E 3161 28

SE 4646 41 S 1607 14

SW 539 5 W 420 4

NW 409 4

Table 1. Number of events for each of the eight analyzed wave directions at the outer RDP.

Index Direction Frequency ( %)

Mean wind (m s-1)

Fetch (Km)

HMO

(m) TP

(sec.) 1 N 15.8 4.9 36 0.49 3.5 2 NE 11.5 4.6 45 0.50 3.7 3 E 18.4 4.9 125 0.90 5.3 4 SE 12.2 5.8 150 1.22 5.5 5 S 13.7 6.1 2 0.15 1.4 6 SW 7.5 5.5 2 0.13 1.4 7 W 7.8 5.2 2 0.12 1.4 8 NW 6.6 5.2 22 0.41 3.1 9 Calms 6.5 - - - -

Table 2. Directional mean wind and probability of occurrence obtained from data gathered at “Aeroparque Jorge Newery” Meteorological Station located on the coast of Buenos Aires City. Also included are directional fetch, heights and periods obtained by

hindcasting.

19

Figure 1

20

Figure 2

21

Figure 3

22

23

24

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