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  • View in gallery

    La Plata basin and its subbasins: Upper Paraná (UP), Middle Paraná (MP), Lower Paraná (LP), Paraguay (PY), and Uruguay (UY). The three streamflow gauging stations are shown in the map: Paso de los Libres for the Uruguay River, Bermejo for the Paraguay River, and Corrientes for the Paraná and Paraguay Rivers. A network of rain gauges is represented by small dots. Orography is shaded

  • View in gallery

    Mean annual cycle of river discharge for (a) the La Plata basin and (b) the three main tributaries: Paraná, Paraguay and Uruguay. Units are m3 s−1

  • View in gallery

    The mean annual cycle of river discharge plus the five largest and five lowest historical river discharges (open circles) and their averages (bar): (a) La Plata, (b) Paraná, (c) Uruguay, and (d) Paraguay Rivers (as in Fig. 2). Some circles are superimposed and not visible. Units are m3 s−1

  • View in gallery

    As Fig. 3, but for the Mississippi River

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    CMAP annual mean precipitation (mm) for the La Plata basin

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    CMAP seasonal mean precipitation (mm day−1) for the La Plata basin: (a) SON (spring), (b) DJF (summer), (c) MAM (autumn), and (d) JJA (winter). [The dotted line in (d) is the 0.5 mm day−1 contour]

  • View in gallery

    (a) Annual cycle of La Plata basin area-averaged CMAP precipitation; (b) annual cycle of precipitation over the monsoon region inside the La Plata basin; (c) annual cycle of precipitation over the region with a secondary maximum of precipitation within the basin; and (d) annual cycle of CMAP precipitation averaged for the band between 60° and 50°W as a function of latitude. The annual cycle of gridded rain gauge precipitation (Willmott and Matsuura 2001) is superimposed in panels (b) and (c). Units are mm day−1

  • View in gallery

    Vertically integrated moisture flux estimated from NCEP–NCAR global reanalyses for (a) austral winter and (b) austral summer, and (c) their difference; (d) transient contribution to the total moisture flux during winter. Units are kg m−1 s−1, and values larger than 100 kg m−1 s−1 are shaded. The two parallel solid lines east of the Andes represent the core of the low-level jet as discussed in the text. AB and CD are transects used to show cross sections of moisture flux in Fig. 9

  • View in gallery

    Cross sections of the meridional component of moisture flux estimated from NCEP–NCAR reanalyses at 30°N (Great Plains LLJ) during (a) JJA and (b) DJF; cross sections of moisture flux for the AB and CD transects (South American LLJ) are presented in (c) for austral winter and (d) austral summer, respectively. Units are g kg−1 m s−1

  • View in gallery

    Annual cycle of the vertically integrated moisture flux along the low-level jet as a function of latitude (see Fig. 8 for details). Units are kg m−1 s−1

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    Time series for the period 1979–2000 (discriminating the annual march) of (a) vertically integrated moisture flux for the average band 57.5–62.5°W, 20°S and (b) La Plata basin area-averaged CMAP precipitation. Horizontal dotted lines represent the three El Niño events that lasted into the following calendar year

  • View in gallery

    Time series of river discharge for the period 1910–2000 (discriminating the annual march) for the (a) La Plata, (b) Paraná, (c) Paraguay, and (d) Uruguay Rivers.

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The Hydrologic Cycle of the La Plata Basin in South America

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  • 1 Department of Meteorology, University of Maryland at College Park, College Park, Maryland
  • 2 Department of Atmospheric and Oceanic Sciences, University of Buenos Aires, Buenos Aires, Argentina
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Abstract

The main components of the hydrologic cycle of the La Plata basin in southeastern South America are investigated using a combination of observations, satellite products, and National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) global reanalyses. La Plata basin is second only to the Amazon basin in South America in river discharge and size and plays a critical role in the economies of the region. It is a primary factor in energy production, water resources, transportation, agriculture, and livestock.

Of particular interest was the evaluation of the annual cycle of the hydrologic cycle components. The La Plata annual-mean river discharge is about 21 000 m3 s−1, and the amplitude of its mean annual cycle is small: it is slightly larger during late summer, but continues with large volumes even during winter. The reason for this is that different precipitation regimes over different locations contribute to the total river discharge. One regime is found toward the northern boundary, where precipitation peaks during summer in association with the southernmost extension of the monsoon system. A second one is found over the central part of the basin, where precipitation peaks at different times in the seasonal cycle. Further analysis of the main tributaries of La Plata (Paraná, Uruguay, and Paraguay) reveals that each has a well-defined annual cycle but with different phases that can be traced primarily to each basin's physiography and precipitation regime.

Interannual and interdecadal variability of the basin's precipitation is amplified in the variability of streamflow by a factor of 2, implying a high sensitivity of the hydrologic system to climate changes like those observed in the last few decades. This becomes more important when considering the large variability of streamflow: for example, the historical maxima of river discharge during the year following the onset of El Niño can triple the typical mean river discharge.

A crucial component of the atmospheric water cycle, the low-level jet east of the Andes, supplies moisture from tropical South America to La Plata basin throughout the year. In lower latitudes, the jet has the greatest intensity during summer, but south of about 15°S there is a phase shift and the largest moisture fluxes are found during winter and spring. This is an uncommon feature not observed in other regions like the Great Plains of the United States, where the low-level jet develops only during the warm season.

Corresponding author address: Ernesto Hugo Berbery, Dept. of Meteorology, 3427 Computer and Space Sciences Bldg., University of Maryland at College Park, College Park, MD 20742-2425. Email: berbery@atmos.umd.edu

Abstract

The main components of the hydrologic cycle of the La Plata basin in southeastern South America are investigated using a combination of observations, satellite products, and National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) global reanalyses. La Plata basin is second only to the Amazon basin in South America in river discharge and size and plays a critical role in the economies of the region. It is a primary factor in energy production, water resources, transportation, agriculture, and livestock.

Of particular interest was the evaluation of the annual cycle of the hydrologic cycle components. The La Plata annual-mean river discharge is about 21 000 m3 s−1, and the amplitude of its mean annual cycle is small: it is slightly larger during late summer, but continues with large volumes even during winter. The reason for this is that different precipitation regimes over different locations contribute to the total river discharge. One regime is found toward the northern boundary, where precipitation peaks during summer in association with the southernmost extension of the monsoon system. A second one is found over the central part of the basin, where precipitation peaks at different times in the seasonal cycle. Further analysis of the main tributaries of La Plata (Paraná, Uruguay, and Paraguay) reveals that each has a well-defined annual cycle but with different phases that can be traced primarily to each basin's physiography and precipitation regime.

Interannual and interdecadal variability of the basin's precipitation is amplified in the variability of streamflow by a factor of 2, implying a high sensitivity of the hydrologic system to climate changes like those observed in the last few decades. This becomes more important when considering the large variability of streamflow: for example, the historical maxima of river discharge during the year following the onset of El Niño can triple the typical mean river discharge.

A crucial component of the atmospheric water cycle, the low-level jet east of the Andes, supplies moisture from tropical South America to La Plata basin throughout the year. In lower latitudes, the jet has the greatest intensity during summer, but south of about 15°S there is a phase shift and the largest moisture fluxes are found during winter and spring. This is an uncommon feature not observed in other regions like the Great Plains of the United States, where the low-level jet develops only during the warm season.

Corresponding author address: Ernesto Hugo Berbery, Dept. of Meteorology, 3427 Computer and Space Sciences Bldg., University of Maryland at College Park, College Park, MD 20742-2425. Email: berbery@atmos.umd.edu

1. Introduction

The La Plata basin covers about 3.2 × 106 km2 and spreads over five South American countries; approximately 46% of its surface is in Brazil, 30% in Argentina, 13% in Paraguay, 7% in Bolivia, and 4% in Uruguay. By far, the main tributaries of La Plata River are the Paraná and Uruguay Rivers. Another important river is the Paraguay, a tributary of the Paraná River. The water resources of the basin sustain one of the most densely populated regions of South America, where harvests and livestock are among the region's most important assets. In addition, several hydroelectric plants provide energy to the region, including the world's largest to date, Itaipú, over the Paraná River. Not less important, transportation has also greatly increased in recent years due to the integration of the regional economies, and the rivers are used as natural waterways. The hydrologic cycle of the basin is therefore a subject of interest not only for physical reasons but also for practical ones.

Of all the components of the hydrologic cycle, river discharge arguably has been the more discussed. Basic statistics of river discharge and precipitation at selected stations show significant differences between the subbasins and in some cases even within subbasins (García and Vargas 1996). The discharges of different tributaries show trends and changes, some of them related to the construction of dams and others that could be attributed to climate variability (García and Vargas 1998; Genta et al. 1998; Camilloni and Barros 2000). The interannual variability of river discharge for some subbasins of the La Plata basin has received further attention, and linkages to sea surface temperatures (SSTs) in the Pacific and Atlantic Oceans have been found (Mechoso and Pérez Iribarren 1992; Robertson and Mechoso 1998; Camilloni and Barros 2000). Construction of dams in some of the rivers and the consequent streamflow regulation have led to changes in the annual cycle of river discharge, as will be discussed later.

Studies of precipitation variability have been hampered by the lack of an extensive observational network. Nevertheless, there are some results, mainly in connection with the El Niño–Southern Oscillation (ENSO), that show that there is a considerable signal in the interannual variability of the precipitation over the La Plata basin (Aceituno 1988; Ropelewski and Halpert 1987, 1989; Kiladis and Diaz 1989). This signal varies along each of the ENSO phases but is particularly strong during the spring. Studies focused on the response to ENSO in smaller areas within La Plata basin show no evidence of a signal in rainfall during midsummer (Rao and Hada 1990; Grimm et al. 1998; Pisciottano et al. 1994; Grimm et al. 2000), but in late summer and autumn there is again a strong correlation between SSTs at El Niño regions 3 and 3.4, and outgoing longwave radiation (OLR) over the Upper and Middle Paraná (Camilloni and Barros 2000).

There are also links between SST anomalies in the nearby Atlantic Ocean and precipitation anomalies in the La Plata basin. Diaz et al. (1998) found that precipitation in Uruguay and southern Brazil are positively correlated with SST anomalies in the southwestern tropical Atlantic during spring and summer. La Plata basin precipitation south of 25°S is positively correlated with the SSTs near the South Atlantic convergence zone (SACZ); on the other hand, precipitation near the Paraná headwaters is negatively correlated with the SSTs in the western South Atlantic (Barros et al. 2000b; see also Nogués-Paegle and Mo 1997, 2002; Doyle and Barros 2002). Robertson and Mechoso (2000) and Robertson et al. (2002, manuscript submitted to J. Climate), however, suggest that SST anomalies are driven by atmospheric anomalies, thus questioning the predictive values of these relationships.

Lower frequency variability in annual rainfall has been documented as well. Barros et al. (2000a) observed an important positive trend in precipitation, south of 25°S, while Camilloni and Castañeda (2000) found a positive trend in the autumn precipitation over the Upper and Middle Paraná after 1980. Zhou and Lau (2001) found that the variability in interannual timescales is associated with ENSO, while that in decadal timescales is associated with cross-equatorial SST gradients in the Atlantic and Pacific.

Attempts at computing the atmospheric component of the hydrologic cycle of the La Plata basin have proved to be complex and more difficult to address: significant differences, even for the mean values, were obtained when moisture flux convergence was estimated from global analyses of two forecast centers, which Wang and Paegle (1996) attribute to uncertainties in the wind analyses. According to Min and Schubert (1997), estimates from global reanalyses also depict significant differences, and a similar result was obtained by Higgins et al. (1996) for the Mississippi River basin; these results suggest that the problem goes beyond the need for improvement in the data assimilation techniques. A possible reason is that some features of the circulation associated with moisture flux and its convergence are of a mesoscale nature; therefore, both spatial and temporal resolutions are crucial for describing quantitatively the moisture transports carried out by low-level jets (LLJs) into basin domains (Berbery and Rasmusson 1999; Berbery and Collini 2000). First, global reanalyses do not resolve mesoscale features like LLJs and their narrow core, or the sharp lateral gradients at and near them; instead, global reanalyses tend to represent LLJs as regions of intense winds. Second, the inadequacy of temporal resolution in global models may also affect the estimates of moisture flux convergence. Last, global models have difficulties in representing adequately the diurnal cycle of precipitation, thus possibly misrepresenting the physical mechanisms associated with it (Betts et al. 1998). Regional model products are better prepared to resolve these processes, as discussed in Berbery et al. (1996) and in the above references, but typically these products are not available for long periods.

Earlier work describing the moisture transports as estimated from global analyses or reanalyses can be found in Rao et al. (1996) and Labraga et al. (2000). In both studies the relation between moisture flux—in particular its transient component—and precipitation, was discussed. The La Plata basin is supplied moisture from lower latitudes by low-level flow from the Tropics and subtropics, where a poleward LLJ east of the Andes is frequently embedded (Paegle 1998; Berbery and Collini 2000). Observational evidence of the LLJ was presented by Virji (1981) from satellite estimates of low-level winds. Numerical modeling evidence has been more frequent (see, e.g., Berri and Inzunza 1993; Berbery and Collini 2000, and references therein). This LLJ resembles in some ways the Great Plains LLJ east of the Rockies that supplies moisture to the Mississippi River basin; nevertheless, some differences will be pointed out in this article. At present, least known are the contributions of moisture directly from the Atlantic Ocean.

The objectives of this article are to document the properties of the main components of the La Plata basin's hydrologic cycle and, to a limited extent, to contrast them with those of the Mississippi basin. Such a comparison is of interest in view of the intense research that has been devoted to the latter in the framework of the Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project (GCIP); it is expected that the experience gained in studies of the Mississippi's hydrologic cycle could greatly help to understand better that of La Plata. On one hand, both basins are about the same size, and lie east of mountain chains: the Andes for La Plata and the Rocky Mountains for the Mississippi; and both have significant input of moisture from lower latitudes by an LLJ east of those mountains. On the other hand, the atmospheric components of the corresponding hydrologic cycles appear to have distinct features, as the Great Plains LLJ has marked differences with its South American counterpart, both in structure and seasonal cycle. This will be discussed further in section 5.

The focus here will be on the large-scale aspects of the hydrologic cycle; thus, also due to the already discussed limitations in global analyses, no attempt will be made to compute a balance of the different components. Rather, the annual cycle will be investigated as a basic state around which other variabilities can be diagnosed. Purportedly, some components of the hydrologic cycle, like evaporation fields, have been left out of the analysis. Evaporation could be shown from the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) global reanalysis, but this is one of the least reliable variables in that dataset because it depends largely on the model's parameterizations and therefore cannot be considered an observed variable (Kalnay et al. 1996). Alternatively, estimates of evaporation as a residual of the atmospheric water vapor budget are also subject to uncertainties due to the difficulties of estimating the moisture flux convergence, as discussed earlier. Section 2 describes the physiography of the basin and presents the datasets employed for this analysis; the annual cycles of river discharge and its monthly extreme events are discussed in section 3; and precipitation is addressed in section 4. Large-scale moisture flux is analyzed in section 5; interannual variability is discussed in section 6; and, finally, section 7 summarizes and discusses the main results.

The Variability of the American Monsoon Systems (VAMOS) panel of the Climate Variability and Predictability Program (CLIVAR) of the World Climate Research Programme (WCRP) has appointed the La Plata basin Scientific Study Group to identify critical climatological and hydrological problems of the basin. This article is the result of contributions of the authors to that study group.

2. Background information

a. Physiography of the region

The La Plata basin covers subtropical and midlatitude areas of South America [see Fig. 1; also, a hydrologic map that includes the rivers can be found at the American Association for the Advancement of Science Web site (http://www.aaas.org/international/edehn/edehn/platahydr3_02.jpg)]. It lies between the Andes Mountains to the west, and the Brazilian Plateau and “Sierra del Mar” to the northeast and east. The basin consists basically of three large subbasins, corresponding to the Paraná, Paraguay, and Uruguay Rivers.

The Paraná River basin makes up about half of the La Plata basin area. It is usually partitioned in three subbasins, the Upper, Middle, and Lower Paraná. Most of the Paraná River streamflow comes from the upper and middle parts, with a small contribution from the lower portion. High streamflows in the Middle Paraná subbasin produce extended floods over large areas of the Lower Paraná subbasin, even without a significant local contribution. The Lower Paraná also receives streamflow from the Paraguay River and, together with the Uruguay River, forms the La Plata River.

The Paraguay River basin is mostly a large plain of slightly over 1 000 000 km2 and, with few exceptions, has a small and uniform slope (Tossini 1959). The elevation of the Paraguay basin rarely exceeds 70 m above sea level, and its gradient is typically less than 1.5 cm km−1. About 100 000 km2 of the Paraguay basin are covered by a vast swamp called the Pantanal, which has distinct seasonal changes. During the dry season, the swamps shrink to patches of marshy land, and the river courses become visible. With the onset of the rains in spring, the region starts to inundate from the north around February, and as late as June to the south, when it achieves its maximum extent (Hamilton et al. 1996). Therefore, the Pantanal can regulate the Paraguay streamflow so that a different annual cycle of river discharge is found north and south of it (this behavior is sometimes affected by interannual variability that makes the flooded area persist from one year to the next).

The Uruguay River basin is a smaller subbasin, with an area of about 0.365 × 106 km2, and is distinct from the above basins: it has a varied relief along its 1500-km course, with many small valleys and short water courses. Because the longitudinal gradient of the basin is small, while the transverse section of the basin is comparatively narrow, the lag between river discharge and rainfall is small (Tossini 1959).

b. Datasets

Monthly values of river discharge for the major La Plata subbasins for 1910–2000 (except for the Paraguay River, which was only available until August 1997) were obtained from the Argentine Department of Hydrology. The Paraná River discharge was measured at Corrientes (27.58°S, 58.49°W); the Uruguay River discharge was measured at Paso de los Libres (29.6°S, 57.1°W), and the Paraguay River discharge was gauged at Puerto Bermejo (27.33°S, 58.50°W), near the confluence with the Paraná River (see locations in Fig. 1). Measurements at Corrientes include the contributions of the Upper and Middle Paraná, but also that of the Paraguay. Therefore, the discharge of the Paraná River alone was obtained by simple subtraction of the Paraguay discharge at Puerto Bermejo. Consequently, the time series of Paraná River discharge (subtracting the Paraguay's contribution) also ends in August 1997. On the other hand, the La Plata River discharge, which is the sum of the streamflow at Corrientes (Paraná plus Paraguay) and the Uruguay contribution, was computed until the end of 2000.

The Lower Paraná's contribution to the total river discharge is small and poorly measured. From streamflow measurements in Chapetón, at 31.66°S, and those of the main tributaries south of this point, this contribution can be estimated to be no more than 2500 m3 s−1 (Argentine Secretary of Energy 1994); for these reasons it is not included here. A discussion of this dataset can be found in García and Vargas (1998). The river discharge for the Mississippi River consists of a dataset of monthly measurements of river discharge at Vicksburg, covering the period 1932–98. This dataset is discussed in detail in Ropelewski and Yarosh (1998).

A dataset of satellite estimates of precipitation for the period 1979–2000 was employed. The dataset is called Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and is discussed by Xie and Arkin (1997). The precipitation estimates on a 2.5° × 2.5° latitude–longitude grid are the result of merging several satellite products and observations, including the infrared-based Geostationary Operational Environmental Satellite (GOES) Precipitation Index (GPI), Outgoing longwave radiation–based Precipitation Index (OPI), and microwave measurements. The CMAP version employed here does not include the NCEP–NCAR reanalysis precipitation forecasts. The quality of this dataset will be discussed in section 4, associated with Fig. 7. Precipitation data before 1979 was needed to assess low-frequency changes over La Plata basin (section 6). In this case monthly rain gauge records were used (see distribution of stations in Fig. 1). The Argentine National Meteorological Service and the National Direction of Meteorology of Paraguay provided the records from their respective countries. Most of the series for Brazil were obtained from the Institute of Agricultural Research of Rio Grande do Sul and from Brazil's National Agency of Electricity. Additional records for Brazil and Uruguay were obtained from the Climate Data Center (CDC) in Boulder, Colorado.

Mean daily NCEP–NCAR global reanalysis fields for the 1979–2000 period, available on a 2.5° × 2.5° latitude–longitude grid and at 17 pressure levels for winds and 8 levels for moisture were used to compute the vertically integrated moisture flux. Ideally, the vertical integrations should be performed in the model's sigma coordinates, but a comparison of the two estimates for a shorter period revealed no visible differences.

3. Annual cycle of river discharge

a. La Plata basin and subbasins

The La Plata River discharge (the combined discharge of the Paraná/Paraguay and Uruguay Rivers), presented in Fig. 2a, is fairly uniform throughout the year, with a mean annual value of about 21 000 m3 s−1, and a slight discharge increase from February to July (late austral summer into winter). However, the analysis of the discharge for the three main tributaries (Fig. 2b) shows that, in fact, each has a well-defined seasonal cycle but that their peaks are out of phase. The upper and middle portions of the Paraná River have a maximum in late austral summer. The Uruguay River has the largest discharge (although of smaller magnitude) between June and November, and the Paraguay River is rather smooth, with a maximum during austral winter. The annual cycles of river discharge can be related to the different climate regimes for each basin and to the different responses to precipitation, as will be discussed in section 4.

Figure 3 presents, in addition to the mean seasonal cycle of river discharge, the five highest and five lowest historical monthly discharges, and their average. Figure 3a suggests that flooding in the La Plata basin may occur at any time of the year, with greater probability during austral winter. According to Fig. 3a, the average of the five highest historical maxima of monthly river discharge for the La Plata basin is as high as 2.5 times the time-mean river discharge, but individual cases can be higher, particularly during winter. The more outstanding maxima of the twentieth century occurred during the austral winter of the year following the onset of El Niño (EN+1) as long as the SST anomalies persisted and remained large during the EN+1 austral autumn and early winter (Camilloni and Barros 2000). In those cases, the largest contribution to the total discharge was from the Paraná River (Fig. 3b). This happened during the recent 1983, 1992, and 1998 events, but also in 1905 (not included in our record). In the case of the Lower Paraná, floods generally occur in connection with El Niño events in winter, but also in late summer (February–March), regardless of El Niño conditions (Camilloni and Barros 2000). The Uruguay River discharge (Fig. 3c), while usually small in comparison to that of the Paraná River, in some cases can achieve flows that are as high as the Paraná mean discharge. Finally, the Paraguay River discharge (Fig. 3d) shows that historical minimum/maximum deviations are comparatively smaller than those for the other two subbasins. This is because the Pantanal naturally modulates the discharge, preventing extreme values.

The average of the five historically lowest monthly river discharges can be interpreted as a potential for hydrologic drought in the basin. Figure 3a shows that minima in the La Plata basin during the first half of the year are about one-half the mean values, while toward the second half of the year the minima are about one-third the mean values. In terms of individual rivers, minima in the Paraná River (Fig. 3b) are about one-half the mean values for most of the year. The most striking case is that of the Uruguay River (Fig. 3c), where historical minima can be one order of magnitude smaller than the mean values at any time of the year. The behavior of the Paraguay River during spring is similar. Section 6 will discuss interannual variability in a more general sense.

One word of caution is necessary regarding the annual cycle, in particular for the Paraná River, since the construction of several dams in recent years has contributed to changes in the timing of river discharge. Camilloni and Barros (2000) showed that there was a change in the river discharge annual cycle of the Middle and Lower Paraná after the early 1980s with respect to the earlier period of 1930–80. After 1984 the annual cycle of river discharge presents a smaller range, with a reduction of the austral summer maximum and an increment of the winter discharge. This is consistent with two facts: first, the dams over the Upper and Middle Paraná basin that were built during the 1970s and early 1980s have a large storage capacity that amounts to a total of about 200 000 km3; second, due to the discharge regime with a maximum in summer and smaller discharges during winter and early spring, the dams' management of water is such that it tends to retain water in summer to release it later during the winter and the spring. Therefore, it is very likely that the construction of the total storage of the dams' reservoirs and the subsequent water management lead to a lower range in the annual cycle of the river. Moreover, and because of the unchanged contribution of the Paraguay River, in the case of the Lower Paraná the maxima is now in June/July instead of February as before. However, other factors cannot be disregarded as contributors to these changes in the annual cycle: an important positive trend both in rainfall and OLR over the Upper and Middle Paraná basins during autumn has been shown by Camilloni and Castañeda (2000), and the change in use of soil—less native vegetation and more cultivated land—may be playing a role in the altered runoff conditions.

b. Comparison to the Mississippi basin

The Mississippi River discharge (Fig. 4) has a marked seasonal cycle with a maximum of about 25 000 m3 s−1 during spring (March–April–May) and a minimum of about 8000 m3 s−1 at the end of the boreal summer and autumn (see also Ropelewski and Yarosh 1998). The potential for flooding of the Mississippi basin, as revealed from the historical maxima, also has a well-defined annual cycle, which is in phase with the mean evolution. The magnitudes of the historical maxima are about double the mean values during spring. The large regulation of the water streams may play a role in decreasing the dispersion of the historical maxima, which is less pronounced than in La Plata basin. As a result, and despite both basins having similar general topography with large flood plains, the consequences of flooding in the La Plata basin may be far more severe than those in the Mississippi basin.

Unlike La Plata basin, whose summer precipitation is at least in part associated with a monsoon system, the Mississippi River owes its river discharge to snow melting in the northern parts of the basin, thus the springtime maximum (Guetter and Georgakakos 1993). Precipitation associated with convective activity that develops over the U.S. Great Plains also helps increase the river discharge during spring. On the other hand, the minimum during late summer and autumn occurs when the contribution from ice melting has decreased significantly. An additional factor in the reduced river discharge is the development of the North American monsoon system: with its onset over northwestern Mexico (usually at the beginning of July), there is an associated decrease of precipitation over the Great Plains (Higgins et al. 1997; Barlow et al. 1998).

4. Precipitation regimes

The annual mean precipitation over the La Plata basin (Fig. 5), while showing an west–east gradient, has two maxima: one toward the northern boundary and the second one over the central region of the basin. These two centers are the result of different precipitation regimes that can be distinguished in the seasonal means of precipitation presented in Figs. 6a–d. The northern region has the largest precipitation during summer (Fig. 6b), which is related to the southernmost extension of the monsoon system (Horel et al. 1989; Zhou and Lau 1998). On the other hand, the spatial maximum over the central region of the basin is present during all seasons (Figs. 6a–d): During the warm season (October–April), mesoscale convective complexes (MCCs) are frequent and account for a large part of the total precipitation (Velasco and Fritsch 1987; Laing and Fritsch 2000). During the cold season, the most relevant forcing is due to transient activity, which accounts for much of the total precipitation of southeastern South America (Vera et al. 2002).

Figure 7a presents the annual cycle of area-averaged precipitation over La Plata basin: largest precipitation of about 5.5 mm day−1 during the warm season is the result of the predominant effect of the monsoon regime, which is presented in Fig. 7b. In this case the summertime maximum achieves values close to 9 mm day−1. This well-defined annual cycle can be contrasted with that in the middle of the basin (Fig. 7c), which is markedly irregular with only hints of larger precipitation during late summer/autumn (February–May) and spring (September–October). The transition between the two precipitation regimes is better depicted in Fig. 7d, which presents the annual cycle of precipitation averaged for the longitude band between 60° and 50°W, as a function of latitude. The summertime regime, associated with the South American monsoon system, is observed as far south as 20°S, while farther south no unique seasonal maximum is found, suggesting that different mechanisms, other than the monsoon forcing, are acting.

The reliability of the CMAP dataset was examined by comparing it to a new dataset of observed precipitation interpolated to a regular 0.5° × 0.5° latitude–longitude grid (Willmott and Matsuura 2001). The annual cycle for the two points representing the two precipitation regimes in La Plata basin (see Figs. 7b,c) show that the two estimates are close and have a similar annual cycle. However, CMAP underestimates the precipitation during the warm season, probably due to its coarse resolution, which cannot adequately resolve convective precipitation, and slightly overestimates it during the cold season.

At this point, the annual cycle of precipitation can be associated with that of river discharge. Because of their steep slopes, most of the Upper and Middle Paraná basins and, similarly, the Iguazú basin (not discussed here) have a relatively fast runoff as compared to the Paraguay and Lower Paraná basins. Therefore, the streamflow at the exit of the Middle Paraná course has a lag time of approximately 1–2 months with respect to the spatially averaged rainfall over each of the subbasins of the Iguazú and the Upper and Middle Paraná Rivers.

The late summer maximum of river discharge for the Paraná River is consistent with the summertime maximum of the monsoon precipitation over the northern part of the basin (Fig. 7b). As a consequence, the streamflow of the Upper Paraná at Jupiá (20°S) has a pronounced annual cycle ranging from about 10 000 m3 s−1 in February to little more than 3000 m3 s−1 in September. However, at 25°S the annual cycle of rainfall is small, and in the case of the Iguazú basin (not shown, but see Figs. 7c,d) it presents a double maximum in October and January. Consequently, the Middle Paraná streamflow at Posadas (27°S) varies only from 16 700 m3 s−1 in February to 9000 m3 s−1 in August. These numbers indicate that the relative amplitude of the annual cycle decreases with increasing latitude from a factor of more than 3 at 20°S to less than 2 at 27°S. This relative amplitude is further reduced in the Lower Paraná after receiving the contribution of the Paraguay, which peaks in winter due to the great lag time with rainfall, as discussed in the next paragraph. In this way the relative amplitude of the annual cycle at 31.66°S (Chapetón) is very small: the mean streamflow varies from almost 17 000 m3 s−1 in September to 20 000 m3 s−1 in February. This cycle was modified after 1980, due to the management of the water by the dams over the Upper and Middle Paraná. Its amplitude was reduced, with a more even distribution of river discharge throughout the year (see also Camilloni and Barros 2000).

The annual cycle of river discharge for the Uruguay River (which has a secondary maximum in June and a primary maximum in spring, separated by a relative minimum in August) can be associated with the precipitation regime over the upper part of the basin (Fig. 7c) that shows maxima in autumn and spring. Again, there is a fast response to precipitation because of the physiography of the basin (see section 2a). Finally, the precipitation over the Paraguay basin reveals two maxima (Fig. 5): the first one due to the monsoon (northern part) and the second one over eastern Paraguay/northeastern Argentina. However, the annual cycle of river discharge is smooth and depicts only one maximum (in June, austral winter). The reason is that the Pantanal, whose storage capacity may vary by one order of magnitude (Hamilton et al. 1996), can smother the signal within the basin. Cross correlations indicate that convection over the Upper Paraguay basin leads the river streamflow at the exit of the basin by 5–10 months (Camilloni and Barros 2000; Almeida and Barros 1998).

5. Large-scale moisture flux

Global reanalyses are not the best datasets with which examine a phenomenon like the LLJ, which has mesoscale characteristics (probably it would be more adequate to refer to it as a region of enhanced winds). In general, the products of mesoscale regional models like the Eta Model (e.g., Berbery and Collini 2000) are better for the analysis of LLJs. On the other hand, some useful information regarding the transports of moisture can still be extracted from global reanalyses. For example, the features discussed in this section (Figs. 8–10) were obtained first from a two-year dataset of regional Eta Model forecasts developed at the University of Maryland, and only then we looked at the capability of the reanalysis to represent them climatologically.

The vertically integrated moisture flux for the austral winter and summer are presented in Figs. 8a,b. During winter, the largest magnitude of moisture flux is found near the Tropics associated with easterly winds, and toward the extratropics associated with westerly winds. During summer (Fig. 8b), the same basic structure of tropical easterly/extratropical westerly moisture flux is also found, although the intensification of the Atlantic subtropical anticyclone is such that the moisture flux associated with its western boundary is much more evident.

East of the Andes, large northwesterly moisture flux toward the La Plata basin is found during the two seasons. The horizontal structure is not identical, though; during the warm season, the largest magnitudes are found near the tropical latitudes and during winter the maximum is observed toward the south, closer to and over the northwestern part of the basin. A lateral shift toward the east can also be noticed during summer, which implies that the monsoon region to the north of the La Plata basin may receive moisture directly from the warm and moist flow from the tropical continent.

The presence of large vertically integrated moisture flux east of the Andes in summer and winter indicates a departure from the behavior of the North American Great Plains moisture fluxes, which are largest during the warm season only. This is more evident in Fig. 9, which shows the cross sections along 30°N and the transects AB and CD (transects perpendicular to the LLJ east of the Andes; see Fig. 8). Transect CD is shifted 5° to the north in comparison to AB to capture better the core of the summer jet. Despite the coarse resolution of the global reanalyses, the structure of the strong southerly moisture flux in Fig. 9a represents the well-known Great Plains LLJ east of the Rockies during summer, which decays and disappears during winter (Fig. 9b). The moisture flux across transect CD during austral summer (Fig. 9d) is similar to that over the U. S. Great Plains, with the maximum (southeastward, in this case) flux between 900 and 950 hPa. The positive values toward the northeast represent the moisture flux from the Atlantic Ocean due to the trade winds.

The cross section of moisture flux during austral winter (transect AB, Fig. 9c) shows a remarkable feature: strong southeastward moisture flux with the structure of an LLJ, with its core at 850 hPa. The winter jet is slightly smaller in magnitude than the typical summer LLJ, and it has no equivalent over the Great Plains. This structure is responsible for the large vertically integrated moisture flux east of the Andes during winter shown in Fig. 8a. (Note that the height of the maximum moisture flux is somewhat lower than the height of the maximum in wind because of the weighing effect of specific humidity that is largest at lower levels.) It could be argued that the different annual cycle of the jets is due to their different latitudinal location, but different tests show that, unlike the Andes case, the North American LLJ cannot be found at any latitude during winter.

Due to the relevance of this winter jet for the atmospheric water cycle in the La Plata basin, we further explore its structure. Figure 8c presents the winter-to-summer changes in the vertically integrated moisture flux: the subtropical anticyclones over the Pacific and Atlantic Oceans are stronger in summer, consistent with the arguments presented by Hoskins (1996). Additionally, northerly flux across the equator is larger also in summer, probably due to shifts in the trade winds. The eastward arrows over the Amazon basin actually reflect a reduction in the intensity of the easterly moisture flux during summer. Focusing on the region east of the Andes, this figure suggests that from the Tropics to about 15°S the southeasterly moisture flux is larger during summer, but further south it is largest during the cloud season. This can be seen better in Fig. 10, which presents the mean annual cycle of moisture flux along the channel depicted in Fig. 8. The annual cycle is repeated twice to facilitate the analysis. The tropical region north of about 15°S has a well-defined austral summer maximum of southeastward moisture flux and of the opposite direction during austral winter. At about 15°S the circulation regime changes significantly, with features better defined at about 20°S; here, the largest southeastward flux is found from April to November, with the maximum in October (austral winter and spring).

The relation between the winter LLJ and the precipitation in the La Plata basin is not clear yet. However, the exit region of this jet is located in an area of increased transient activity, as shown in Fig. 8d. Transients are a small fraction of the total moisture flux, but over land they achieve large values precisely over La Plata basin. Thus, the LLJ seems to provide a continuous supply of moisture and heat ahead of frontal zones and cyclonic systems that produce much of the winter precipitation (Vera et al. 2002). In addition, the transient components of the flow may be particularly relevant in the case of the contribution of moisture from the Atlantic Ocean to rainfall over La Plata basin (not discussed here). Additional information about the relative contributions of the stationary and transient fluxes of moisture can be found in Labraga et al. (2000).

6. Interannual variability

The interannual variability of river discharge was discussed in section 3 in terms of “potential for floods or drought.” Here, it is further analyzed in association with moisture flux and precipitation variability. The time evolution of the southward moisture flux east of the Andes (Fig. 11a) reveals low-frequency variability, and the figure supports the hypothesis that El Niño events favor a more intense LLJ. The three more relevant El Niño events of the last few years (those that persisted into the following year) show an increased southward component of moisture flux. In particular, the recent El Niño of 1997/98 is a remarkable case, in which the moisture flux was the largest of the 22-yr period included in this study; the moisture flux peaked in November of 1997 with values estimated from the reanalysis of about −285 kg m−1 s−1 (this value is 180% larger, in magnitude, than the 22-yr mean values for November). The increased moisture flux into La Plata basin during El Niño springs is consistent with the larger observed precipitation (e.g., Ropelewski and Halpert 1987; Grimm et al. 2000), although this is not as clear in Fig. 11b, due to significant regional variability within the basin. Note, however, that large moisture flux is not limited to El Niño events, as there are other periods when it is intense as well.

Interannual and interdecadal variability of river discharge can be inferred from Fig. 12. Of particular interest for this study is the increased river discharge toward the latter half of the period, which is noticeable in all three tributaries (and their sum, the La Plata River). The streamflow of the main rivers on La Plata basin, and the La Plata River itself, have strong interannual and interdecadal variability forced by the climatic variability (Robertson and Mechoso 1998; Camilloni and Barros 2000). In support of this point, three cases (one case study and two “climatological” cases) corresponding to different timescale variability are presented in Table 1, which shows the river discharge of the La Plata River as well as the basin-averaged rainfall rates. The first case is an example of extreme year-to-year variability of the hydrologic cycle (focused on 1998 and 1999). The second case is a generalization of the first example, as it contrasts composites of El Niño and La Niña for 1951–99. El Niño and La Niña periods until 1996 are defined following Trenberth (1997), and the Climate Diagnostics Center afterward. The third case assesses the changes in the hydrologic cycle between two 20-yr periods (1951–70 and 1980–99) that are illustrative of a low-frequency variability or trend. Precipitation for 1951–90 was taken from the rain gauge network depicted in Fig. 1, and CMAP was used for 1991–99. The table also includes an estimate of evaporation plus infiltration rate, which was calculated as the difference between rainfall and streamflow.

a. First case

In 1997, an El Niño event began and continued into the first part of 1998; it was accompanied by large streamflow in the Paraná River during 1998 (as mentioned earlier, this happened in all El Niño cases that persisted into the autumn of the following year). El Niño was followed by La Niña conditions during 1999, which were accompanied by negative rainfall anomalies over most of the basin, resulting in a precipitation difference of about 23% (Table 1). Changes in streamflow were also observed in association with the changes in precipitation. The mean streamflow of the Uruguay River at Paso de Los Libres in 1998 was 9533 m3 s−1 and only 3305 m3 s−1 in 1999 (about one-third the value during El Niño). The Paraná River at Corrientes registered, in those same years, mean flows of 27;th127 and 17;th137 m3 s−1 respectively, which implies a difference of about 36%. The overall combined effect resulted in a 44% change of the La Plata River discharge.

b. Second case

The composite of El Niño events shows larger precipitation and streamflow than the composite of La Niña events (precipitation is 7% larger; streamfunction is 17% larger; evaporation and infiltration are the least affected, with changes of only 3%). Although the magnitude of the changes is smaller compared to the other cases, the amplification of the streamflow signal is still noticed. Notably, cold events (La Niña) are not associated with droughts or even with a significant reduction of the streamflow. The reason is that the reduced precipitation occurs toward the south of the basin and outside the areas that feed the streamflow of the main rivers (Grimm et al. 2000).

c. Third case

According to Table 1, from 1951–70 to 1981–99, the precipitation increased by about 16%. Since 1950 there was a considerable change in the use of the soil all over the considered basins, with a notorious increment of agriculture at the expense of natural vegetation (Tucci and Clarke 1998). This aspect may have contributed to the observed streamflow changes (see Fig. 12), in addition to the effect of rainfall variation. The mean annual river flows of the two 20-yr periods are believed to be largely unaffected by the increment of the dams' storage, as changes in the evaporation rate over the basin are relatively small due to the comparatively limited surface of the reservoirs. As in the case of the year-to-year variability, the interdecadal variability of streamflow is also large (a 35% increase between 1951–70 and 1981–99). This increase is found in each of the major rivers: the Uruguay experienced an increase of 32%, the Paraná (excluding the contribution of the Paraguay River) had an increase of 31%, and, finally, the Paraguay's increase in river discharge was about 45%. This also indicates that the amplification of the precipitation signals in the river flow at the interdecadal scale is, at least in part, due to causes other than the change in the soil use.

As stated, in all cases the variability in precipitation is considerably amplified in the corresponding river streamflow. In any given year (or long period) a significant amount of the water precipitated over the basin is either evaporated or infiltrated in a way that does not runoff to the river, at least upstream of the two locations where streamflow is measured. In the examples of Table 1, the evaporated plus infiltrated fraction accounts for approximately 70% of the precipitated water, but this fraction is larger during relatively dry years and smaller during the rainy ones. On the other hand, although the streamflow accounts only for approximately 30% of the precipitated water, its interannual or interdecadal variability is larger (in absolute values) than the evaporation plus infiltrated water, and, consequently, its relative variability is even larger. These examples indicate that the balance between precipitation, streamflow, evaporation, and infiltration are such that extreme interannual variability in precipitation is mostly translated to the river discharge while only a small fraction of it is converted in evaporation or infiltration.

The relative changes in streamflow in all examples are considerably large, about 17% as the average in the El Niño/La Niña composites, but as large as 44% for the 1998/99 case, and 35% between the two 20-yr periods. These relative changes are more impressive when they refer to the streamflow of one of the largest rivers of the world. At the same time, these examples show the great vulnerability of the streamflows of the major rivers within the La Plata basin to climate variability. This becomes even more important because of the large interannual and interdecadal variability already observed in the region's precipitation during the twentieth century.

7. Summary and discussion

The large-scale aspects of the hydrologic cycle of the La Plata basin in South America were examined with a special interest in river discharge, precipitation, moisture flux, their annual cycle, variability, and linkages. River discharge of La Plata has a small-amplitude mean annual cycle, due to the different precipitation regimes present in the basin throughout the year. The maximum discharge tends to occur in late austral summer and autumn, as a result of the more dominant effect of summertime precipitation. The minimum takes place during spring and early summer. At subbasin scales, the annual cycle of river discharge is determined by the physical characteristics of the subbasin and the primary precipitation regime on the location. The upper and middle portions of the Paraná River are most influenced by the summer monsoon regime; thus, the river has a maximum discharge in late summer. The annual cycle of precipitation over the Uruguay River basin has two maxima, one in late autumn and the second one in spring, and, consistently, the river discharge is largest in winter and spring. The smooth annual cycle of the Paraguay River discharge, with a maximum in winter, is the result of the Pantanal, a large wetland that naturally regulates the river discharge; although the maximum precipitation tends to occur during summer, river discharge lags it by about half a year.

The potential for flooding of the La Plata River, measured as the historically largest river discharges for a given month, is present at any time of the year, with a peak during austral winter. The largest contribution during flood episodes comes from the Paraná River. Taken individually, both the Paraná and the Uruguay Rivers can at least triple the mean river discharge during flood events, while the Paraguay does not show peaks as extreme, due to the Pantanal's regulatory influence. In each tributary, the minimum river discharges do not depict much dispersion, but, notably, the Uruguay River throughout the year and the Paraguay River during spring are the most affected since minima can be about one order of magnitude smaller than the mean values for the given month. This interannual variability is of significant importance, not only because of the damage caused to settlements and rural production by floods, but also because of the losses, produced by ebbs, to navigation and to the energy sector.

Interannual and longer time variability of the components of the hydrologic cycle reveal a high vulnerability of the region to increased precipitation. Evidence was presented that small changes in precipitation are amplified in the streamflow signal. The ratio between streamflow and basin-averaged precipitation changes is slightly more than 2; that is, for every 1% change in precipitation there was a change slightly larger than 2% in streamflow. This sensitivity was reasonably stable whether interpreted over two consecutive years (1998 and 1999), interannual variability (El Niño/La Niña composites), or even decadal changes (1951–70 vs 1980–99). The interdecadal changes could be attributed in part to the changes of soil/vegetation; however, this effect is almost negligible when two consecutive years are considered, and, therefore, in this case it is plausible that almost all the river flow variability is forced by the precipitation variability alone.

Overall, the La Plata basin has a large annual mean river discharge that exceeds by about 25% that of the Mississippi River. Despite the similarities in size, extensive plains, and location with respect to large mountain barriers (the Andes and the Rockies, respectively), there are well-defined differences that become readily evident from the analysis: the Mississippi has a large-amplitude annual cycle of river discharge, with a maximum in spring and a minimum in autumn. Its greatest flooding risk is also during the boreal spring, in phase with the mean annual cycle. The main reason for the differences between the basins can again be traced to the precipitation regimes, since the Mississippi streamflow is in large part the result of snow accumulation during winter and melting during spring.

A remarkable difference between the La Plata and Mississippi basins refers to the moisture transports: while the Great Plains LLJ is well known to be a warm season phenomenon, all evidence suggests that the LLJ east of the Andes is largest during austral summer only in the tropical region north of 15°S. South of this latitude, it is present throughout the year with largest values during the cold season and spring. The wintertime maximum of moisture flux has a somewhat different vertical structure, with the core located at a higher elevation than that during the warm season (850 vs 925 hPa); still, the total supply of moisture to the basin during winter and summer remains at about the same magnitude. Large-scale patterns seem to affect the magnitude of the LLJ east of the Andes, which gains intensity during El Niño events.

A consistent description of the river discharge and precipitation over the La Plata basin has been offered, and the structure and annual cycle of the LLJ east of the Andes that supplies moisture to the basin were examined. However, the reasons for the existence of the LLJ throughout the year and its differences with the Great Plain LLJ are not understood yet. Field experiments like the one formulated with VAMOS guidance should help clarify these issues.

Acknowledgments

The authors are thankful for comments received from C. Roberto Mechoso and the three anonymous reviewers whose helpful comments served to clarify several parts of this article. They also acknowledge N. O. García and E. Yarosh for providing the river discharge datasets of the La Plata and Mississippi basins, respectively. NCEP–NCAR reanalyses were downloaded from the Climate Diagnostics Center in Boulder, Colorado. This work was supported by NOAA Grants NA76GP0479 (PACS) and NA16GP1479 (GAPP), and the UBA Program 15982/00; interactions between the authors were supported by IAI Grant CRN-55 (PROSUR).

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Fig. 1.
Fig. 1.

La Plata basin and its subbasins: Upper Paraná (UP), Middle Paraná (MP), Lower Paraná (LP), Paraguay (PY), and Uruguay (UY). The three streamflow gauging stations are shown in the map: Paso de los Libres for the Uruguay River, Bermejo for the Paraguay River, and Corrientes for the Paraná and Paraguay Rivers. A network of rain gauges is represented by small dots. Orography is shaded

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 2.
Fig. 2.

Mean annual cycle of river discharge for (a) the La Plata basin and (b) the three main tributaries: Paraná, Paraguay and Uruguay. Units are m3 s−1

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 3.
Fig. 3.

The mean annual cycle of river discharge plus the five largest and five lowest historical river discharges (open circles) and their averages (bar): (a) La Plata, (b) Paraná, (c) Uruguay, and (d) Paraguay Rivers (as in Fig. 2). Some circles are superimposed and not visible. Units are m3 s−1

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 4.
Fig. 4.

As Fig. 3, but for the Mississippi River

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 5.
Fig. 5.

CMAP annual mean precipitation (mm) for the La Plata basin

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 6.
Fig. 6.

CMAP seasonal mean precipitation (mm day−1) for the La Plata basin: (a) SON (spring), (b) DJF (summer), (c) MAM (autumn), and (d) JJA (winter). [The dotted line in (d) is the 0.5 mm day−1 contour]

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 7.
Fig. 7.

(a) Annual cycle of La Plata basin area-averaged CMAP precipitation; (b) annual cycle of precipitation over the monsoon region inside the La Plata basin; (c) annual cycle of precipitation over the region with a secondary maximum of precipitation within the basin; and (d) annual cycle of CMAP precipitation averaged for the band between 60° and 50°W as a function of latitude. The annual cycle of gridded rain gauge precipitation (Willmott and Matsuura 2001) is superimposed in panels (b) and (c). Units are mm day−1

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 8.
Fig. 8.

Vertically integrated moisture flux estimated from NCEP–NCAR global reanalyses for (a) austral winter and (b) austral summer, and (c) their difference; (d) transient contribution to the total moisture flux during winter. Units are kg m−1 s−1, and values larger than 100 kg m−1 s−1 are shaded. The two parallel solid lines east of the Andes represent the core of the low-level jet as discussed in the text. AB and CD are transects used to show cross sections of moisture flux in Fig. 9

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 9.
Fig. 9.

Cross sections of the meridional component of moisture flux estimated from NCEP–NCAR reanalyses at 30°N (Great Plains LLJ) during (a) JJA and (b) DJF; cross sections of moisture flux for the AB and CD transects (South American LLJ) are presented in (c) for austral winter and (d) austral summer, respectively. Units are g kg−1 m s−1

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 10.
Fig. 10.

Annual cycle of the vertically integrated moisture flux along the low-level jet as a function of latitude (see Fig. 8 for details). Units are kg m−1 s−1

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 11.
Fig. 11.

Time series for the period 1979–2000 (discriminating the annual march) of (a) vertically integrated moisture flux for the average band 57.5–62.5°W, 20°S and (b) La Plata basin area-averaged CMAP precipitation. Horizontal dotted lines represent the three El Niño events that lasted into the following calendar year

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Fig. 12.
Fig. 12.

Time series of river discharge for the period 1910–2000 (discriminating the annual march) for the (a) La Plata, (b) Paraná, (c) Paraguay, and (d) Uruguay Rivers.

Citation: Journal of Hydrometeorology 3, 6; 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Table 1. 

Basin averaged rainfall rates and river discharge for the La Plata River corresponding to different timescale variability

Table 1. 
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