Synoptic features associated with critical water level rises in the Río de la Plata

Transcription

1 Meteorol. Appl. 12, (2005) doi:10:1017/s Synoptic features associated with critical water level rises in the Río de la Plata A. P. Alessandro Department of Atmospheric and Oceanic Sciences, The University of Buenos Aires, Argentina This study aims to describe the synoptic features that caused the water level in the Río de la Plata estuary to rise above critical levels between December 1989 and December Floods in the estuary can seriously affect the land beside the river from Punta Indio (35.22 S, W) to the Paranádelta, including the lowlands of Buenos Aires City. Surface pressure patterns associated with floods in the Río de la Plata estuary were obtained by using Quartimax rotated Tmode Principal Component Analysis (PCA) of 1000 hpa geopotential heights. The PCA patterns and 1000 hpa composite and anomaly fields show two main causes for overflows in the Río de la Plata estuary. First, the presence of a surface anticyclone, located south of Buenos Aires province and over northern Patagonia; and, second, cyclogenesis over northeastern Argentina or over Uruguay. The two synoptic features are often present simultaneously. Two representative points were selected in the area under study: one over the continent at the Aeroparque meteorological station (34.34 S, W) and another over the ocean between 36 S, 56 W and 36 S, 50 W. Predominant 1000 hpa wind directions associated with overflows were SE at the former location and SSW at the latter. Based on the analysis of the obtained fields, the relationship between the estuary overflow and blocking situations and/or positive pressure anomalies over southern South America and adjacent seas was studied. The zonal circulation index (I), used to detect blocking actions, was found to be useful for identifying synoptic situations related to the estuary swelling. The probability of water level rises in the Río de la Plata with a blocking or I > 0at70 W is 0.48, 0.72, 0.78 and 0.73, for summer, autumn, winter and spring, respectively. When I > 0 is observed over the Atlantic at 40 W the probability of flooding is 0.16 for the whole year, but it decreases to in autumn, winter and spring. These results and weather charts from different numerical prediction models show that alerts of possible Río de la Plata estuary overflow can be released five days in advance. 1. Introduction The Paraná and the Uruguay rivers run into the Río de la Plata (RP) estuary, forming the Río de la Plata basin. The ocean RP estuary border, or mouth of the RP, is defined as a line that joins Punta Piedras (36.5 S, 57.5 W) in Argentina and Punta del Este (34.58 S, W) in Uruguay. Water level in the RP estuary depends on several factors: (a) The sea level of the South Atlantic Ocean at the mouth of the RP, depending mainly on lunar semidiurnal tides that travel along the estuary from its mouth to the Paraná delta in 12 hours. Normally, the tide is about 0.5 m at the port of Buenos Aires. (b) The South Atlantic sea level at the mouth of the R P, which depends on surface winds blowing over a vast ocean area. (c) The Coriolis force that deflects moving waters left from wind direction (Ekman 1905), so that northerly winds cause water levels to drop whereas it rises with southerly winds. (d) The surface stress of winds blowing directly over the RP waters. As the RP estuary is oriented in a WNW ESE direction, northwesterly winds drain waters toward the ocean whereas southeasterly winds block their normal flow. This is why persistent and strong southeasterly winds can cause critical water level rises in the RP. Another factor is the discharge of the Paraná and the Uruguay rivers. Although their contributions to the RP 123

2 A. P. Alessandro Argentina Rio Uruguay Rio Parana Aeroparque Palermo RP Punta Piedras Uruguay Punta del Este A Atlantic Ocean B Figure 1. The Río de la Plata and adjacent Atlantic Ocean. water level have not yet been studied, the effect is assumed to be much smaller than that of the astronomic tide and wind stress and is usually not considered in the calculations. Since the input of the astronomic tide to the water level in the RP is at most 1.56 m (Tide Section of the Servicio de Hidrografía Naval), and the levels recorded at Palermo station often exceed this level, the winds that blow over vast areas within the estuary and nearby ocean within the estuary have an important effect on water levels. The aim of this study is to describe synoptic situations associated with the RP overflows that affect the lowlands of Buenos Aires City and flood the Argentine banks of the estuary (estuary width: 35 km) from Punta Indio to the Paraná delta (see Figure 1). A relevant synoptic situation in the study area is the rapid development of intense cyclones that frequently occur in the latitude band S and east of 60 W (Carleton 1979; Gan & Rao 1991). This phenomenon is known as Ciclogénesis del Litoral Fluvial Argentino and takes place mostly in spring and winter (Seluchi 1993). These depressions give rise to heavy rainfall and strong southeasterly winds, which often cause flooding in the RP, a phenomenon locally known as Sudestada. According to Ciappesoni & Salio (1997), Sudestadas in general can develop under three distinct synoptic situations. They studied the cases in which the RP levels were higher than 2.5 m at the hydrometric station of Buenos Aires (Palermo) from 1990 to Apart from the cyclogenesis, they identified two other predominant synoptic situations in which a Sudestada can occur. One is the presence of a quasistationary surface anticyclone over southern Buenos Aires province and northern Patagonia and the other is the passage of a northerly cold front. Ciappesoni & Salio (1997) report that the end of a Sudestada can be predicted from the moment 124 the southern anticyclone stops intensifying at the stations of Viedma (40.51 S, W) and Mar del Plata (37.56 S, W). Likewise, the evolution of a potential Sudestada can be followed by observing positive geopotential height anomalies at 200, 500 and 1000 hpa at about 50 S. Two days before a Sudestada takes place, positive anomalies are observed between 95 and 85 W, one day before at W and between 75 and 68 W on the day of the Sudestada. When Seluchi (1993) analysed the 1980s cyclogenesis over northeastern Argentina and Uruguay, he observed that the 1000 hpa pressure anomaly field on the day of cyclogenesis shows cyclonic anomalies leeward of the Andes. He also found positive anomalies south of 50 S and east of 70 W, with similar positions at 500 hpa. He suggested that in some cases this phenomenon could be related to blocking situations. This situation was also studied by Hessling (1993), Grandoso & Nuñez (1955) and, indirectly, by Berbery & Lozano (1991). Their results indicate that the presence of highpressure systems at latitudes higher than normal is connected to possible blocking situations. In a previous paper, Alessandro (2003a) studied blocking episodes and defined them as the persistence of positive pressure anomalies over the southern South Pacific and southern South Atlantic for more than five days. To identify blocking episodes at 100 W, 70 W and 40 W between December 1989 and December 1998, the 500 hpa zonal circulation index (I) was adopted. The highest positive and more persistent Ivalues were observed at 100 W diminishing slightly at 70 W and dropping to almost 50% at 40 W. In addition, Alessandro (2003b) conducted using the Iindex a statistical study of the recurrence and influence of blocking episodes on temperature and precipitation in Argentina for the same period. Like the papers previously mentioned, this paper aims to study the relationship between overflows in the RP and blocking situations and positive anomalies over southern South America using index I.

3 2. Data and methodology Daily maximum RP water levels from the Palermo tide station in Buenos Aires provided by the Tide Section of the Servicio de Hidrografía Naval (SHN, Naval Hydrographic Service) for the period were used in this study. At this hydrometric station, astronomic tides do not exceed 1.56 m, and therefore water levels higher than 2 metres are assumed to be caused by wind stress. The days that were included in the present study are those in which maximum water levels recorded at the Palermo hydrometric station were equal to or greater than 2.30 m. (h 0 ), a compromise value between 2 m and 2.50 m, which are the established critical levels at the SHN and the Servicio Meteorológico Nacional (SMN, Argentine Weather Service) respectively. Water levels higher than critical may cause floods in the lowlying areas of Buenos Aires City owing to overflow in the R P. In the period analysed, there were more than 145 days when RP water levels (h) were higher than h 0. In order to identify the synoptic situation associated with overflow events, reanalysed daily 500 and 1000 hpa geopotential height fields at 12 UTC were obtained from SMN. The 1000 hpa daily synoptic features associated with these overflow cases were analysed by applying Principal Component Analysis (PCA) with a Tmode input matrix (Green 1978; Richman 1986), as the Tmode yields more appropriate and useful spatial results for our study. The unrotated solution was first tried. However, the Varimax (v) and Quartimax (q) rotations were applied in order to maximise the physical interpretation of the resulting components (Richman 1986). There are different criteria (Richman et al. 1992) that can be used to identify the number of principal components (PC) to be retained, i.e. to distinguish significant PCs from random noise. One is Kaiser s criterion (1958), according to which PCs with eigenvalues greater than one are significant and thus retained. The Lev (logeigenvalue) diagram is another criterion in which the logeigenvalues are plotted against the eigenvectors number. The significance threshold appears as a straight line at the tail, which is attributed to eigenvalues representing noise. The North et al. (1982) test was also applied. Although this test estimates eigenvalue errors (δλ = λ(2/n)), it can be used if the information contained in the PCs is assumed to be random from the moment when the eigenvectors degenerate. Finally, Varimax and Quartimax rotations were performed successively on PC sets to which components were added one by one. Variances were calculated at each step, and then compared. The number of representative components was set on that PC after which Critical water level rises in the Río de la Plata the explained variances were approximately equal to those obtained at previous steps. The component loadings of the different patterns were analysed to assess whether the map patterns are representative of real synoptic situations, which occurs when loading values are approximately equal to one (Harman 1976; Cattel 1978). In order to find a possible relationship between blocking and positive anomalies over southern Pacific and Atlantic oceans, the circulation index I was defined as: I = U(30 S) U(60 S) 2U(45 S) where U is the zonal component of wind speed at 500 hpa in metres per second. The index was calculated at 100 W, 70 W, and 40 W. According to this index, a blocking situation is defined as one in which I remains positive for at least five consecutive days at any of the selected longitudes. 3. Results 3.1. Composite and anomaly fields To study the mean prevailing weather situation for the days when RP estuary water levels (h) were higher than h 0, the composite daily fields at 1000 hpa (Figure 2a) and the anomalies at 1000 hpa and 500 hpa (Figure 2b and 2c, respectively) were calculated from 1989 to The incursion of highpressure systems over the continent is observed in Figure 2a. The circulation over the Atlantic gives rise to pronounced southerly and southeasterly flows north of 40 S. Figure 2b shows the intensification of surface highpressure systems centred over Patagonia represented by a positive anomaly. This anomaly is shifted southwestwards at 500 hpa (Figure 2c). In order to find the position of this positive anomaly in different seasons, anomalies of seasonal mean 1000 hpa fields were plotted in Figure 3. In winter, the anomaly is located over the southern Pacific, whereas in summer it shifts to the north and is situated over the Atlantic to the east of Patagonia with an intensity of 50 mgp, about half its intensity in winter. In spring and autumn, the positive anomaly shows intermediate intensity and position. Consequently, one can deduce that the mean positions of high pressure systems shift southward from their normal mean positions (Alessandro, 1995) and that the position of the positive anomaly might involve blocking action in the southern Pacific and over southern Argentina Wind analysis Given the important effect of both wind speed and direction on the RP water level, 1000 hpa wind 125

4 A. P. Alessandro (a) (b) (c) Figure 2. (a) Composite daily field associated with RP swelling at 1000 hpa; (b) mean daily anomaly field at 1000 hpa; and (c) mean daily anomaly field at 500 hpa in gpm. components at 12 UTC were analysed for days in which h > h 0. To determine the study area, two representative points were selected: one over the continent at the Aeroparque meteorological station (34.34 S, W) and another over the ocean as a mean between 36 S, 56 W and 36 S, 50 W (points A and B in Figure 1). The mean vector (Vmc) of zonal (U) and meridional (V) wind components, the mean vector of wind speed (Vme) and the number of days (Ncd) for different directions at Aeroparque station and over the ocean area are displayed in Tables 1 and 2 respectively. The most frequent wind directions at both reference points are SE and SSW, although at the ocean reference point SE winds are less frequent and SSW winds are more frequent when compared to those obtained at Aeroparque. Such winds cause water to accumulate at the mouth of the RP with a consequent water level Table 1. Mean speed values (Vmc) of U (zonal) and V (meridional) wind components, mean vector wind speed (Vme) and number of days (Nca) for each wind direction at Aeroparque station. Vmc U < 0 V< 0 U< 0 V> 0 U> 0 V> 0 U> 0 V< 0 U= 0 V< 0 U< 0 V= Vme fcd Table 2. Mean speed values (Vmc) of U (zonal) and V (meridional) wind components, mean vector wind speed (Vme) and number of days (Nca) for each wind direction calculated between 36 o S 56 o Wand36 o S 50 o W. Vmc U < 0 V< 0 U< 0 V> 0 U> 0 V> 0 U> 0 V< 0 U> 0 V= 0 U= 0 V> 0 U< 0 V= Vme fcd

5 Critical water level rises in the Río de la Plata 70 W (a) (b) (c) (d) Figure 3. Mean seasonal fields at 1000 hpa for (a) winter, (b) spring, (c) summer, and (d) autumn. rise within the estuary. During 82% of the critical days, a southerly wind component existed simultaneously at both selected points. The low frequency of critical days with a northerly component is due to water level rise taking place before or after 12 UTC. At both points and for all the flood days, the average wind speed is 5.8 m/s and the mean direction is SSE, which agrees with the synoptic situation shown in Figure 2a. It is worth noting that the prevailing wind direction on all days, regardless of flood occurrence, is NNE at both points Description of synoptic situations that cause water level rises in the Río de la Plata using principal components Since the theoretical PC patterns can be associated with two possible real modes direct (positive) and inverse (negative) Table 3 shows the explained variance for each of them, Var () and Var( ), respectively, as well as the overall (Var (t)) and cumulative variances (Var(ac)) for each PC. Table 3 shows the explained variance for the first six unrotated PCs (A, B, C, D, E and F), the explained variance for the first six PCs after applying v (Varimax) (Av, Bv, Cv, Dv, Ev and Fv) and q rotations (Quartimax) (Aq, Bq, Cq, Dq, Eq and Fq) respectively. The first PC (A) explains the highest variance and its associated pattern represents the mean field (Figure 1a). Except for the third PC (C), all the unrotated PCs represent real situations associated with overflows in the RP. To distinguish significant PCs from random noise, Kaiser s criterion was applied. According to this criterion, the first eleven PCs might be significant, as their eigenvalues are greater than one (Table 4). However, in the Lev diagram (Figure 4) points from the second PC are arranged in a straight line, which means that only the first two PCs are significant. After applying the North et al. (1982) test, the eigenvalue errors revealed that the selective cut could not be made until between the seventh and eighth PCs. These tests yield different results and thus make it difficult to make an objective decision on the number of components to retain. Given the abrupt drop in the explained variance after the first PC and the low values of the variance of the following PCs and taking into account the results of the performed tests, the first six PCs were considered significant and were retained for further analysis. This decision was confirmed by the 127

6 A. P. Alessandro Table 3. Explained variance for the first six unrotated PCs in the direct and inverse modes: Var(), Var( ) respectively, total variance Var(t) and cumulative variance Var(ac), Varimax (v) rotated Tmode PCs and qrotated (q) Tmode PCs. Pattern PCs Var () Var ( ) Var (t) Var (ac) A B C D E F Av Bv Cv Dv Ev Fv Aq Bq Cq Dq Eq Fq log(eigenvalue) Figure 4. Lev diagram eigenvector order comparison of the explained variances resulting from the test for successive rotations (Section 2 above), as after the sixth PC there were no significant differences between them. The third unrotated PC as well as the third and fourth vrotated PCs show northwesterly winds over the RP that produce water level drops in the estuary. The q rotated direct and inverse modes were analysed for each pattern, and it was found that a mode was associated with 1000 hpa geopotential height synoptic situations producing overflow when its explained variance was more than double the explained variance of the other. So, only the results of the qrotation were adopted in this paper. Patterns Aq, Bq, Cq, Dq, Eq and Fq, which correspond to the first six qrotated PCs, explain 89.1% of the variance and are plotted in Figure 5. Of the 17.6% variance that is not explained by the first PC, 4.7% corresponds to cases in which water levels (h) were higher than h 0 before or after 12 UTC, since water levels were available hourly and geopotential height charts were available only for 12 UTC. The most important feature of inverse mode pattern Aq is a maximum negative centre associated with high pressure systems, which is similar to the 1000 hpa mean field, but the high is shifted southward from its mean position. Inverse mode pattern Bq also has a maximum associated with a high pressure system at about 80 W that gives rise to strong southerly winds over the area under study, while in the direct mode of pattern Cq, associated with the third PC, the system is displaced eastward over the Atlantic. A cyclonic circulation over Uruguay is also associated with this pattern. This cyclone is displaced northward in the fourth PC (pattern Dq) and intensified in patterns Eq and Fq (fifth and sixth PCs, respectively). A consideration of component loading values makes it possible to see whether the pattern depicted by the PCs represents real synoptic situations. Loading values of qrotated pattern Aq are displayed in Figure 6. These loading values are over 0.80 in 77% of critical days and their structure is similar to that of unrotated pattern A, as shown by the correlation coefficient of 0.99 for the two patterns. The remaining patterns are much less representative given that most of the loading values are between 0.2 and 0.2. Figure 7 shows some 1000 hpa geopotential height charts for critical days that correspond to the highest component loadings of all patterns. There is a great similarity between the synoptic situation in the upperleft corner of Figure 7 and the pattern of the first PC, with a factor loading of The map for 30 April 1998 is similar to the pattern associated with the third PC with a factor loading of On 7 June 1997 and 26 February 1990, real weather situations are slightly different from the patterns associated with the fifth and sixth PCs because of the low loadings, i.e and 0.56 respectively. The 1000 hpa geopotential heights on critical days can be represented by a linear combination of the first six PCs, although the first PC synthesises the structure that is common to almost all the real situations. Two prevailing causes of floods in the RP were identified from the principal component analysis. There is a surface anticyclone over the south of Buenos Aires Table 4. Eigenvalues. N Eig

7 Critical water level rises in the Río de la Plata Aq Bq Cq Dq 70 W Eq Fq Figure 5. Patterns associated to the first six Quartimax (q) rotated PCs factor loadings days Figure 6. Factor loadings of pattern Aq (first qrotated PC). 129

8 A. P. Alessandro 50S (a) (b) 70 W (c) (d) 70 W (e) (f) Figure 7. Geopotential height fields (mgp) at 1000 hpa for 4 April 1996, 20 February 1993, 30 April 1998, 9 January 1995, 7 June 1997 and 26 February 1990 corresponding respectively to the 1st, 2nd, 3rd, 4th, 5th and 6th PCs. province and northern Patagonia and, furthermore, cyclogenesis over northeastern Argentina and Uruguay. In accordance with the mean field, the most frequent is the first of the described causes. Nonetheless, both synoptic features are often present simultaneously Relationship between swelling and the circulation index (I) The Student s t test was applied to assess whether the series of daily indices I calculated at 100 W, 70 W 130 and 40 W for the days with flooding are significantly different from indices computed on days without floods. Table 5 shows the mean values of I and the standard deviation σ obtained for the three analysed longitudes and the two samples, i.e. days with floods (h h 0 ) and days without floods (h < h 0 ). At the 95% confidence level, the samples at 70 W and 40 Ware statistically different as their t values lie beyond the acceptance interval for the null hypothesis, ± Positive values of t indicate that overflows take place when I > 0, as happens in every case at 70 W. These

9 Critical water level rises in the Río de la Plata Table 5. Mean values (I m) and standard deviations (σ )of the circulation index I for critical days (h 2.3 m: RP estuary overflow, h: water height level 2.3 m assumed as critical value) and for normal days (h < 2.3 m). h 2.3 m h 2.3 m Longitudes Im σ Im σ T 100 W W W Table 6. Mean value (I m) of I during blocking occurrence on critical and normal days. Longitudes h 2.3 m h < 2.3 m I 100 W W W results show that index I can identify overflow cases when estimated at 70 W and 40 W. At the latter longitude, statistic T < 0 implies that if I values are positive one can expect a lower than normal overflow probability. Mean index I was estimated for cases in which floods took place on a day when a blocking action was apparent (I > 0 on five consecutive days) at 100 W (B100), 70 W (B70) or 40 W (B40). Mean I was also calculated for days with a blocking action but without floods (Table 6). Differences between the mean indices ( I) follow a similar relationship to that of the t values previously found; that is, the greatest difference between mean values is obtained at 70 W. In order to evaluate the influence of the circulation index on days with floods, the frequency was calculated of critical days (Cd) in the RP for those days when a blocking action was present (I > 0) and those days when I < 0 at 100 W, 70 Wor40 W (Table 7). Results show that only 17.2% of floods take place on days with negative indices; thus, overflow in the RP is highly likely to occur when high pressure systems are located at higher than normal latitudes. The number of cases with h > h 0 and B100, B70 and B40 was counted to assess how the longitudinal position of any blocking action affects overflows. The same was done for cases with h > h 0 and individual positive indices I100, I70, I40 (Table 8). The sum of frequencies exceeds 120 because the counting was carried out independently; for example, if on a day with overflow a blocking situation was registered at 100 W and I > 0at40 W, they were recorded as separate cases. The number of overflows increases with positive circulation index values and blocking action at 70 W (I70 and B70). Im Table 7. Frequency of swelling (Cd) with I > 0 or blocking and swelling with I < 0 at 100 W, 70 Wand40 W. Years Block. Or I > 0 I < 0 Frec. of Cd Total Table 8. Frequency of blocking and I > 0 on swelling days at each selected longitude. B100 B70 B40 I100 I70 I Table 9. Frequency of swelling days with simultaneous blocking and I>0 occurrence. In brackets, only one condition. Bq and I > 0/ Bq and I > 0 B70 B40 I100 I70 I40 B100 (6) B70 (21) B40 (1) I100 (7) 4 2 I70 (26) 3 I40 (6) Table 9 shows the frequency of two of the six possible conditions (B100, B70, B40, I100, I70 and I40) taking place simultaneously on a given day. The number of cases with only one condition causing floods is shown in brackets in the first column. The greatest number of floods is associated with the combination of blockings or I > 0at70 W. There are six cases in which three conditions occur simultaneously and they include one day with blocking action B70 or an I70 > 0. These cases are: two with B100, B70 and B40; one with B100, B70 and I40; one with B70, B40 and I100; and two with B100, I70 and I40. The composite field of Figure 8a was obtained by averaging those days on which B100 and B70 are observed simultaneously. The average of flood occurrences with B100 and I70 (eight cases) forms the pattern shown in Figure 8b. Finally, the six cases in which only B100 was involved are shown in Figure 8c. These figures show that the index at 100 W detects high pressure systems located to the east of that longitude which are also detected by I at 70 W. This means that the two indices (I100 and I70) are not independent. Moreover, the index at 100 W detects high pressure systems that are located further west of the position 131

10 A. P. Alessandro 60W (b) (b) (c) Figure 8. Composite daily 1000 hpa field for (a) RP swelling taking place on a day with blocking at both 100 Wand70 W; (b) as for (a) but one day with blocking at 100 W and circulation index I > 0at70 W; and (c) as for (a) but a blocking at 100 W alone. Table 10. Frequency of days with h 2.3 m (Cd), and relative percentages of these days with blocking (%B70), I > 0 (%I70) and both at 70 W. Station Winter Spring Summer Autumn Frec. of Cd % B % I %B70orI of those high pressure systems that, according to the mean fields previously found, can favour overflow in the RP on that day. This is why index I100 is not an appropriate discriminating indicator, which agrees with the results obtained with the Student s t test. Given the values in Table 9 and the six cases mentioned above, the probability of flood occurrence with a blocking action or I > 0at70 W is The probability of flooding is 0.16 if a blocking action or a positive index is detected at 40 W. Given the higher frequency of days with swelling and positive I at 70 W, and in order to identify the seasonal influence of these anomalies, total and relative seasonal frequencies were estimated at this longitude. The results (Table 10) reveal that the greatest influence of blocking on overflow occurs in winter and autumn, whereas the influence of I70 > 0 prevails in spring and summer. Blocking action at the selected longitudes is lowest in summer (Alessandro 2003a), which explains the low frequency of swelling days associated with blocking action in this season. The greater influence of I70 on RP overflow in summer can be explained from a climatological viewpoint, as migratory anticyclones move south (north) of the RP estuary in summer (winter) generating southeasterly (northwesterly) winds. A daybyday analysis of synoptic situations reveals that the highest percentage of floods is related to moving anticyclones detected at 70 W in accordance with the mean anomaly field of Figure 3c. The remaining critical days are related to cyclogenesis over northeastern Argentina and Uruguay or the presence of a low pressure system over the Atlantic near the province 132 of Buenos Aires. On a seasonal basis, probability of flooding during blocking episodes or positive circulation indices at 40 W is in autumn, winter, and spring, 4. Conclusions The daily composite and anomaly fields at 1000 hpa associated with overflows of the Río de la Plata (R P) (water level at Buenos Aires above 2.3 m) show the incursion of high pressure systems over the continent at latitudes higher than normal. This situation gives rise to strong S and SE winds over the Atlantic north of 40 S and the intensification of high pressure systems over Patagonia. According to the mean fields for the days with floods in the RP, the prevailing wind directions at Aeroparque station, in the city of Buenos Aires, and over the nearby sea are SE and SSW. However, the frequency of SE winds is greater at Aeroparque than over the ocean. Such winds cause water to accumulate at the mouth of the RP, causing a subsequent water level rise in the estuary. A southerly wind component is present simultaneously at both reference points on about 82% of days with floods. At both points, when estimated on all swelling days, wind direction and mean speed were SSE and 5.8 m/s, respectively. Daily 1000 hpa fields associated with floods can be represented by a linear combination of the first six rotated PCs. The first PC, which accounts for 71.5% of the overall explained variance, synthesises the structure that is common to most of the situations. Absolute values of factor loadings of this PC are equal to or greater than 0.8 in 77% of the cases and its structure is similar to that of the mean field at 1000 hpa. The five remaining PCs are much less representative as most of the factor loadings are between 0.2 and 0.2. From the analysis of the composite fields constructed using PCA and the mean and anomaly fields at 1000 hpa, two prevailing causes of floods in the RP were identified: (1) the presence of a surface anticyclone, located

11 Critical water level rises in the Río de la Plata south of the Buenos Aires Province and over northern Patagonia, and (2) cyclogenesis over northeastern Argentina and over Uruguay. Very often both cases take place simultaneously. Flood frequency in the RP estuary is highest in summer (54 cases), and lowest in winter (23 cases approximately half of the summer frequency), while its values in spring (37 cases) and autumn (31 cases) are intermediate. This is related to the trajectory of the migratory anticyclones, which in summer (winter) generate southeasterly (northwesterly) winds. The probability of the occurrence of blocking action or positive index I at 70 W and flooding on the same day is Probability of flooding under these conditions for summer, autumn, winter and spring is 0.48, 0.72, 0.78 and 0.73, respectively. The probability of flooding under blocking or I > 0 conditions at 40 W is only When analysed seasonally, this probability decreases to 0.03 in spring, winter, and autumn. Based on these results and using weather charts obtained from different numerical models, it is found that the population in the affected areas can be warned about the possibility of RP estuary overflow five days in advance. Acknowledgements I would like to think Dr Eric Lichtenstein for his highly esteemed suggestions. This study was supported by the 01/X102 grant from the Universided de Buenos Aires. References Alessandro, Adelia P. (1995) Trayectorias de anticiclones en Sudamérica. Meterologica 20: Alessandro, Adelia P. (2003a) Blocking action situations in the South America during the 1990s. Meteorologica 28: Alessandro, Adelia P. (2003b) The influence of blocking events on temperature and precipitation in Argentina during the 1990s. Meteorologica 28: Berbery, E. H. & Lozano, Alfaro (1991) Características Regionales de alturas persistentes en los Oceanos Atlántico ypacíficosur. Anales Congremet VI. Catell, R. (1978) The scientific use of factor analysis. In Behavioral and Life Sciences, New York and London: Plenum Press, Carleton, A. M. (1979) A synoptic climatology of satelliteobserved extratropical cyclone activity for the Southern Hemisphere:winter.Arch. Met. Geoph. Viokl. Ser. 27: Ciappesoni, H. & Salio, P. (1997) Pronóstico de sudestada en el Río de la Plata. Meteorologica 22(2): Ekman, V. W (1905) On the influence of the earth s rotation on ocean current. Arch. Math. Astron. Phys. 2: Gan, A. P. & Rao, B. V. (1991) Surface cyclogenesis over South America. Mon. Wea. Rev. 119(5): Grandoso,H. N. & Nuñez, J. (1955) Análisis de una situación de bloqueo en la parte austral de América del Sur. 1 & 2, Green, P. E. (1978) Analysing Multivariate Data. Illinois: Dryden Press, 519pp. Harman, H. (1976) Modern Factor Analysis. Chicago: University of Chicago. Press, Hessling, N. A. (1923) Ciclones y anticiclones en la República Argentina. Boletín mensual de la Oficina Meteorológica 7: Kaiser, H. F. (1958) The Varimax criterion for analytic rotation in factor analysis. Psychometrika 23: North, G. R., Bell, T. L. & Cahalan, R. F. (1982) Sampling errors in the empirical orthogonal functions. Mon. Wea. Rev. 110: Richman, M. (1986) Rotation of principal components. J. Climatol. 6: Richman, M., Angel, J. R. & Gong, X. (1992) Determination of dimensionality in Eigenanalysis. Fifth International Meeting on Statistical Climatology.: Toronto. Atmospheric Environmental Service of Canada Seluchi, Marcelo E. (1993) Estudio del comportamiento de los sistemas sinópticos migratorios en la Argentina. PhD thesis, Universidad de Buenos Aires. 133