500 hpa Vorticity Analyses over Argentina: Their Climatology and Capacity to Distinguish Synoptic-Scale Precipitation

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1 Theor. Appl. Climatol. 60, 77±92 (1998) Departamento de Ciencias de la AtmoÂsfera, Universidad de Buenos Aires, Argentina 500 hpa Vorticity Analyses over Argentina: Their Climatology and Capacity to Distinguish Synoptic-Scale Precipitation N. E. Ruiz and W. M. Vargas With 13 Figures Received April 17, 1997 Summary The Southern South America climatological 500 hpa relative vorticity mean state was examined using regional objective analyses of 500 hpa geopotential heights provided by the Servicio MeteoroloÂgico Nacional of Argentina. The dataset, covering the period June 1983 to July 1987, was strati ed into two samples: the cold and warm seasons. Mean cyclonic vorticity south of 40 S results in a climatological trough over Patagonia with a northwest-southeast tilt. North of this latitude, mean anticyclonic circulation dominates with the exception of a centre of cyclonic vorticity over the RõÂo de la Plata (35 S, 56 W). Seasonal changes appear to be small. Relative vorticity frequency distributions were also analysed. The association between precipitation and synoptic-scale features of the mid-troposphere circulation was investigated through vorticity elds. A particular distribution of vorticity anomalies associated with daily precipitation in Buenos Aires is revealed by biserial correlation coef cient elds. In winter, the strongest relationships are found between 35 S and 40 S over the Andes Mountains (minimum signi cant correlation coef cients indicating a cyclonic vorticity anomaly), and in the south of Brazil and east of Buenos Aires over the Atlantic Ocean down to a latitude of 40 S (maximum correlation coef cients related to anomalously anticyclonic circulation). This shows the preferential position of troughs and ridges that produce precipitation in Buenos Aires on the time scale of a day. In summer, centres of anomalously cyclonic and anticyclonic vorticity associated with precipitation shift slightly southward. For moderate or intense precipitation in Buenos Aires, advection of warm and wet air southwards appears to be more important in winter, while in summer the strong anomalous vorticity gradient north of the negative centre over the Andes Cordillera favours rainfall in Buenos Aires. 1. Introduction The relationship between the upper air circulation and precipitation is considered to be important for regional climatological diagnoses and/or for forecast purposes. Following this line, the examination of some characteristics of the circulation in the middle troposphere over Argentina is performed. For the Southern Hemisphere, some description concerning 500 hpa height data, circulation, and meteorological and climatological features has been given in van Loon et al. (1972), and particularly for South America in Schwerdtfeger (1976). More recently, studies based mainly on hemispheric analyses, dealing with different aspects of circulation and storm tracks in the Southern Hemisphere have been carried out by Trenberth (1979, 1981, 1982 and 1991). In regional terms, atmospheric circulation and synoptic aspects of rainfall in Argentina are hardly found in the literature. With the Southern Hemisphere being mainly oceanic, data are scarce. Besides this limitation, the Andes Cordillera introduces another dif culty in the analyses, not only in the large scale but in the regional and synoptic scale as well. The Andes Cordillera, a high, narrow mountain range with a north-south direction in the subtropical and middle latitudes, greatly modi es the atmospheric circulation in the southern part of South America. Unfortunately, data are also

2 78 N. E. Ruiz and W. M. Vargas insuf cient in this sector of the continent. Hence, regional research and papers about a subject of major interest for Argentina, such as those midtropospheric circulations yielding precipitation on the lee side of the Andes, are few. In spite of the need and importance of research relating weather to circulation, it has not yet been ascertained to what extent and how dynamical variables in uence precipitation in different regions of Argentina, especially in Buenos Aires, on a daily time scale. As is well known, midtropospheric variables such as 500 hpa relative vorticity may be considered a sensitive indicator of synoptic-scale vertical motion in the middle latitudes. The presence of the Andes Cordillera affects the westerly winds and the synoptic migratory systems embedded in them; therefore, the analysis of relative vorticity spatial distribution in this region may be required. The aim of the present paper is to examine the climatological aspects and descriptive statistics of 500 hpa geostrophic vorticity, and to determine objectively synoptic-scale relationships between this variables and local precipitation in Buenos Aires area, using the regional objective analyses made at the Servicio MeteoroloÂgico Nacional (SMN, National Meteorological Service) of Argentina. The period examined corresponds to the rst dataset of regional objective elds available for this kind of study. Objective analyses of 1000 hpa are not available for that period. With the southern part of South America being a sparse-data region surrounded by oceans, it is not easy to perform reliable either wind or humidity objective analyses. Other mid-troposphere variables, such as geostrophic wind components and vorticity advection and tendency, and their association with daily local precipitation for one-year data have been examined in a pilot study (Ruiz et al., 1996). Relative vorticity appears to be the parameter which keeps the most information about precipitation. Synoptic climatology techniques and procedures to relate the atmospheric circulation and the surface environment of a region have been extensively described in Yarnal (1993). He states that scale linkage is one of the most dif cult tasks in science, and the synoptic climatology's deductive, statistical approach to the problem may help to develop a better understanding of regional climate and of point-to-synoptic or large scale relationships. In this context, the synoptic classi cation method used in the present work to identify the vorticity patterns associated with precipitation is the construction of biserial correlation elds. This methodology is, on the one hand, analogue to compositing, and, on the other one, it allows for the precipitation to be considered as a dichotomous variable, occurrence or non-occurrence of the event. As a synopticclimatological tool, this approach provides a means to examine the atmospheric circulation patterns associated with speci c environmental conditions at the surface, such as rainy events. Another objective of this work is to contribute for the knowledge of actual circulation patterns in the middle troposphere of this particular area for diagnostic climatological purposes, and, eventually, for Global Climate Model (GCM) validation. General circulation model outputs may also be compared with these results to test their accuracy in this region. When downscaling, attempts are made to determine local or regional scale parameters in terms of larger scale atmospheric circulation (Hewitson, 1995). With this in mind, it is desirable to study rst the relation between the variable of interest at the local scale, such as precipitation, and the atmospheric forcing to derive physically-based empirical linkages. The relationships obtained could be important when dealing with GCM validation (Jones and Conway, 1995). Conway et al. (1996) analyse the relationships between daily precipitation time series in the British lsles and some indices of air ow; they also examine their capacity for use in GCM downscaling. Vorticity, calculated from gridpoint sea level pressure data, is one of the indices utilised by these authors, as it exerts a strong control over daily precipitation conditions (Conway et al., 1996). They also present a model for generating daily precipitation series from vorticity, based on empirical relationships between these two variables. In this context, the study of 500 hpa vorticity elds over Argentina as representative of the larger scale circulation and their potential to distinguish single site precipitation may be of aid, not only for regional climate knowledge, but for empirical downscaling purposes. The paper is organised as follows. Section 2 describes the data used and the domain of study.

3 500 hpa Vorticity Analyses over Argentina 79 The methodology employed, the basic statistics, frequency distributions and biserial correlation techniques, and the results obtained are presented in section 3. Finally, some ndings and conclusions are given in section Description of Data The data used are daily 500 hpa geopotential height elds at 1200 UTC developed at the National Meteorological Service of Argentina (Hordij and Ciappesoni, 1975). These objective analyses were constructed following the ideas of Bergthorsson and DoÈoÈs (1955) and Cressman (1959), and cover the period June 1983 to July 1987 (analyses at 0000 UTC are not available). Radiosonde stations used for the grid point analyses are: Salta ( S, W), Resistencia ( S, W), CoÂrdoba ( S, W), Mendoza ( S, W), Ezeiza ( S, W), Santa Rosa ( S, W), Cdte. Espora ( S, Fig. 1. Region of analysis and grid used. Grid points are marked by dots, radiosonde stations are marked by triangles. Inset gure shows a close view of Buenos Aires region with three meteorological stations in the area: Aeroparque, Observatorio Central Buenos Aires and Ezeiza

4 80 N. E. Ruiz and W. M. Vargas W), NeuqueÂn ( S, W) and Com. Rivadavia ( S, W) in Argentina (measurements are only made at 1200 UTC); Antofagasta (23.5 S, 70.5 W), Quintero (32.5 S, 71.5 W), Puerto Montt (42 S, 73 W) and Punta Arenas (53 S, 72 W) in Chile, and Porto Alegre (30 S, 51 W) and Campo Grande (20.5 S, 54.5 W) in Brazil. Radiosonde data have been object of regional climatological studies and statistical control (Bischoff, 1988). The location of the stations and the grid spacing are illustrated in Fig. 1. The average distance between stations over the continent is about 550 km and a grid step, at the latitude of Buenos Aires 35 S, is approximately 250 km. The radius of in uence of the present Cressman analysis method is 6 grid steps. Wind data, considered to be more reliable, are doubly weighted with respect to geopotential height data. The dataset consists of 1356 days, owing to gaps in the series. The domain corresponds to a polar stereographic projection of 3735 grid points centered at longitude 65 W. In this work, a subset of this grid covering Argentina, approximately from latitudes 20 S to 60 S and from longitudes 80 W to 45 W as shown in Fig. 1, is used. It should be noted that daily regional analyses include all radiosonde data available for a given day, as the cut-hour of the model is rather long and errors are corrected, if possible. In contrast, ECMWF analyses, for example, not always contain all aerological data as southamerican data are not of high quality, and the model often rejects most of them. So, objective analyses made at the SMN are considered to be the most realistic ones for the region, at least for the period under consideration. As for synoptic-scale motions, the relative vorticity can be closely approximated by its geostrophic value (Holton, 1979). The geostrophic vorticity elds were obtained by g ˆ g r 2 z f 0 where: g is the relative geostrophic vorticity; z, the geopotential height; g, the gravity acceleration; and f 0 is the Coriolis parameter at 42 S, 65 W, representing the local region of interest. The Laplacian is approximated by a 9 grid point nite difference scheme. Precipitation data for three meteorological stations for the greater Buenos Aires area (Fig. 1) have been used: Ezeiza ( S, W), Aeroparque ( S, W) and Observatorio Central Buenos Aires ( S, W). 3. Results 3.1 Spatial Dependence of the Relative Vorticity Firstly, it was of interest to determine the area of in uence of the grid point values of relative vorticity. With this purpose, the point to point correlation matrix of the vorticity elds was calculated for the subregion shown in Fig. 2. Correlation coef cients with an absolute value higher than 0.07 are statistically signi cant to the 99% level. Grid points around the reference one are highly correlated (a grid step is about 250 km). Correlation coef cients decrease rapidly and are negligible beyond two grid steps. This may be visualized in Fig. 2, where the grid point nearest Buenos Aires (34.3 S, 57 W) is considered as a reference. Since synoptic systems at midlatitudes move with an eastward component, associations at grid points toward the west are examined in Fig. 3, where the grid extends in the west-southwest direction. Correlation coef - Fig. 2. Correlation coef cients between 500 hpa relative vorticity at the grid point nearer Buenos Aires and the surrounding points

5 500 hpa Vorticity Analyses over Argentina 81 Fig. 3. Same as in Fig. 2, but for a larger grid extending westward cients loose signi cance, or at the most, they are slightly negative to the west side of the grid. Therefore, it is suggested that relative vorticity information is restricted to a region of about two grid steps or less around the examined point hpa Relative Vorticity Climatology Secondly, a basic climatological analysis of 500 hpa relative vorticity has been performed through the leading statistical moments and frequency distributions. The selected years do not exhibit any particularity for the climate and they may be considered as quite normal for South America and the adjacent oceans (no important consequences caused by El NinÄo or no-el NinÄo phenomenon). Mean 500 hpa relative vorticity elds were calculated for the cold semester (May to October), as can be seen in Fig. 4. Units are given in (10 5 s 1 ). Mean cyclonic vorticity (negative for the Southern Hemisphere) appears over Patagonia extending towards the south-east over the Atlantic Ocean. This may be related to migratory surface cyclones which come from the Paci c Ocean and weaken when they enter the continent, but they keep cyclonic vorticity maxima in 500 hpa troughs and they sometimes redevelop over the Atlantic Ocean, as may be also observed by other authors (Taljaard, 1972; Yasunari, 1977; Necco, 1982b). Mean anticyclonic circulation is observed north of 38 S, with the exception of the cyclonic centre over the RõÂo de la Plata (35 S, 57 W). Another anticyclonic region occurs over the south-west of the continent (55 S, 75 W). During the warm semester (November to April), mean cyclonic circulation is evident south of 55 S, as illustrated in Fig. 5. North of this latitude, anticyclonic vorticity predominates, though mean vorticity is near zero over the north of the Patagonia. Anticyclonic vorticity appears to be more intense than in winter north of 35 S over the west and north of Argentina. This perhaps is a re ection of the summer Bolivian Anticyclone in the upper troposphere and its large scale circulation pattern induced by the thermal in uence of the Altiplano (Peruvian-Bolivian Highland) as a heating surface and the precipitation processes, as explained in Kreuels et al. (1975). The cyclonic centre over the RõÂo de la Plata still remains. Seasonal changes are found to be small, as also mentioned in Trenberth (1982). The vorticity minimum near the mouth of RõÂo de la Plata is possibly related to a cyclogenetic area (mainly in winter) in the Argentine Litoral, at the boundary between Argentina and Uruguay near 35 S and in the adjacent sea, as also mentioned by other authors (Streten and Troup, 1973; Carleton, 1979; Necco, 1982a; Necco, 1982b). As shown in Necco (1982b) using NMC analyses for the FGGE year, moderate or intense surface vortices exist which have their origins over this region and then follow a south-eastern trajectory over the Atlantic Ocean. Seasonal averages maps of 500 hpa relative vorticity obtained from 1200 UTC ECWMF geopotential height elds for the region between 15 S and 70 S, and between 120 W and 20 W for the period January 1983 to December 1985 were examined in Ruiz and Vargas (1983). Just for comparison, mean vorticity elds are displayed in Fig. 6a and 6b for both semesters, respectively. The cyclonic vorticity pattern close to Buenos Aires is not observed. However, when analysing the skewness of the vorticity distributions, it is possible to appreciate a region negatively skewed over Uruguay for both seasons, as shown in Fig. 7a and 7b. These values are somehow more intense than those found through the regional analyses (Table 1). Frequency distributions of vorticity at different points for one year of data are presented in Fig. 8.

6 82 N. E. Ruiz and W. M. Vargas Fig. 4. Mean 500 hpa relative vorticity eld (10 5 s 1 ) for the cold semester (May to October) Mean, standard deviation, skewness and kurtosis (Panofsky and Brier, 1965) are given in Table 1. In general, standard deviations cover the range from about s 1 for lower latitudes increasing to about s 1 for higher latitudes, showing the great variability present at middle latitudes which is proportional to the eddy activity. These values are somewhat larger than those found by Trenberth (1991) when using ECMWF ltered data retaining uctuations with 2- to 8-day periods, presented as zonal-mean cross sections. Skewness (whose value is zero for normal distributions) appears to be negative for every latitude. This indicates that cyclonic systems are more intense than anticyclonic ones, which is consistent with the dynamic condition restricting the intensity of anticyclonic relative vorticity by the Coriolis parameter. Kurtosis

7 500 hpa Vorticity Analyses over Argentina 83 Fig. 5. Mean 500 hpa relative vorticity eld (10 5 s 1 ) for the warm semester (November to April) (whose value is 3 for normal distributions) shows a progressive change from leptokurtic (> 3) to platykurtic (< 3) distributions with latitude. In the tropics, mean 500 hpa relative vorticity is slightly negative (cyclonic, s 1 ) to the lee side of the Andes in the northwest of Argentina about 65 W, and slightly positive (anticyclonic, s 1 ) about 55 in the northeast of the country and south of Brazil. Variance and skewness are low, as intense cyclonic systems do not usually reach these latitudes, and, in terms of kurtosis, distributions are leptokurtic (> 3), though nearly gaussian. In the subtropical latitudes, mean 500 hpa relative vorticity is positive and higher than s 1, particularly for Santiago de Chile to the windward side of the Andes. Skewness increases to more negative values, with maxima in this region. Distributions are markedly leptokurtic, showing the great persistence of systems

8 84 N. E. Ruiz and W. M. Vargas Fig. 6. Mean 500 hpa relative vorticity eld (10 5 s 1 ) from ECMWF analyses, a) for the cold semester, b) for the warm semester. Interval: 0.5 Fig. 7. Skewness eld of 500 hpa relative vorticity from ECMWF analyses, a) for the cold semester, b) for the warm semester. Interval: 1 Table 1. Mean, Standard Deviation, Skewness and Kurtosis of 500 hpa Relative Vorticity Distributions at Selected Points over Southern South America Position Mean (10 5 s 1 ) St. dev (10 5 s 1 ) Skew Kurt. 24 S, 65 W S, 56 W S, 73 W S, 60 W S, 57 W S, 68 W S, 62 W S, 62 W S, 65 W S, 65 W with weak vorticity values affecting this region (slightly anticyclonic for Santiago de Chile and Entre RõÂos (32 S), and slightly cyclonic for Buenos Aires). About 40 S, mean 500 hpa relative vorticity is still positive (anticyclonic) over the Atlantic Ocean with kurtosis approximating the normal one. Over the continent in the north of Patagonia, the mean is slightly negative which is evident from the northern part of the climatological trough to the lee of the Andes. Nevertheless, the frequency distribution still maintains subtropical characteristics. Between middle and subpolar latitudes, mean 500 hpa relative vorticity is negative and lower than s 1 with high variability (double

9 500 hpa Vorticity Analyses over Argentina 85 Fig. 8. Frequency distributions of 500 hpa relative vorticity at selected points of Southern South America that of lower latitudes), since midlatitude synoptic perturbations move with high speed and intensity. Distributions are negatively skewed, though fairly symmetrical as at these latitudes cyclonic and anticyclonic disturbances are nearly of same intensity. Frequency distributions are platykurtic (< 3) as may be clearly seen by the histograms appearing at. 3.3 Vorticity and Precipitation Relationships One of the purposes of this work is to determine the association between 500 hpa relative vorticity and daily precipitation focused around Buenos Aires. First, linear correlation coef cients between vorticity at 1200 UTC of day i and accumulated precipitation in the 24 hours from

10 86 N. E. Ruiz and W. M. Vargas Fig. 9. Linear correlation eld between 500 hpa relative vorticity at 1200 UTC and daily precipitation at Ezeiza ( S, W). Critical correlation value, 0.14, is dashed 1200 UTC of day i to 1200 UTC of day i 1 at Ezeiza (Buenos Aires) were calculated (Fig. 9) as a preliminary examination. In spite of the fact that daily precipitation was considered here as a continuous variable, it may be observed that some signi cant negative correlation coef cients exist toward the south-west of the region about 40 S over the Andes mountain range (coef cients higher than 0.14 are statistically signi cant at the 99% level). It should be taken into account that one is dealing with daily precipitation and not with monthly means, so that the total values explained by this relation is not too high; however, this reveals that 500 hpa cyclonic vorticity maxima in that area are related to precipitation at Ezeiza in the following 24 hours. Second, taking into account that vorticity could be thought as a potential predictor of precipitation, frequency distributions of this variable with and without occurrence of precipitation were analysed. The vorticity series inspected were those at points suspected of having some relation with rainfall, as will be con rmed later. Occurrence is considered if precipitation at Ezeiza is 0.1 mm, and non-occurrence, otherwise. Relative frequency distributions of 500 hpa relative vorticity at grid points 39 S, 71 W (West of NeuqueÂn), 33.5 S, 52 W (East of Uruguay), and 34.5 S, 57 W (Buenos Aires) are shown in Fig. 10a, b, and c, respectively, for Fig. 10. Relative frequency distribution of 500 hpa relative vorticity at a ) west of NeuqueÂn (39 S, 71 W), b) east of Uruguay (33.5 S, 52 W), and c) Buenos Aires (34.5 S, 57 W) for occurrence and non-occurrence of precipitation at Ezeiza ( S, W) occurrence and non-occurrence of precipitation. As the separation of the means (yes/no precipitation) is the fundamental signal detection index of discrimination, their difference is tested by means of Student-t statistic (critical value: 2.58, to 99% con dence level). The values obtained are: 5.65 for Fig. 10a (the ``yes'' distribution is to the left of the ``no'' one), 5.10 for Fig. 10b (the ``yes'' distribution is to the right of the ``no''

11 500 hpa Vorticity Analyses over Argentina 87 one), and 0.01 for Fig. 10c (both distributions completely overlap). Therefore, vorticity distributions at points to the south west and to the east of Buenos Aires are statistically different and reveal a certain degree of discrimination for the precipitation event at Ezeiza, while the difference is not signi cant at Buenos Aires. This agrees with the development theory concepts (Petterssen, 1956) in the sense that mid-troposphere vorticity advections are to some extent responsible of synoptic vertical motion and eventually precipitation, and not vorticity in itself. As precipitation may be better considered as a dichotomous variable, occurrence or non-occurrence of precipitation given a critical value, biserial correlation coef cients between 500 hpa relative vorticity at 1200 UTC day i and precipitation as a binary variable were evaluated. To provide a somewhat spatially homogeneous series of daily precipitation, data at three stations in Buenos Aires (Ezeiza, Aeroparque and Observatorio Central Buenos Aires) were used. The precipitation event was de ned as the occurrence of 0.1 mm of average precipitation at the three stations in a 24-h period (from 1200 UTC day i to 1200 UTC day i 1). The biserial correlation coef cient (Panofsky and Brier, 1965) is given by: r bis ˆ x 1 x 0 S p q z where x 1 is the mean of x when the binary variable is 1 (occurrence), x 0 is the mean of x when the binary variable is 0 (non-occurrence), S is the standard deviation of x for all the data, p and q are the empirical probabilities of precipitation and no precipitation, respectively, and z is the ordinate on the Gaussian curve at which the total area is divided into p and q. Biserial correlation elds may be regarded as analogous to composite-difference maps, and also, as in this case, to anomalous vorticity patterns associated with local precipitation. In addition, these correlation coef cients take into account the climatological probabilities of each group as weighting factors. Composite 500 hpa vorticity charts for dry days and for rainy days are not shown as biserial correlation- eld patterns summarize the existing anomalies. The biserial correlation eld during the cold period ( p ˆ 0.29, q ˆ 0.71) is displayed in Fig. 11. Biserial correlation coef cients with absolute values higher than 0.15 and 0.17 are statistically signi cant at the con dence level of 95% and 99%, respectively. Two regions with signi cant coef cients of same magnitude appear clearly, but inverse signs. One of them, centered between 35 S and 40 S over the Andes Mountains, indicates that a negative (cyclonic) vorticity anomaly affecting this zone is associated with precipitation in Buenos Aires in the following 24 hours. The other one shows that anomalously anticyclonic circulation in the south of Brazil and east of Buenos Aires over the Atlantic Ocean down to a latitude of 40 S occurs also in connection with rainfall. This points out the preferential position of troughs and ridges that produces precipitation. It may be seen that not only a mid-tropospheric disturbance or an upper air low is necessary to favour precipitation, but a high pressure perturbation moving away as well, as also suggested in Trenberth (1991). The distance between these two centres is about 2000 km which manifests the synoptic-scale wave-length related to precipitation, and the nearly eastward propagation. The position of the anomalous baroclinicity associated with bad weather is evidenced by the west-east gradient in the correlation isolines around Buenos Aires. During the warm semester (p ˆ 0.31, q ˆ 0.69), as shown in Fig. 12, centres of anomalously cyclonic and anticyclonic vorticity associated with precipitation shift slightly southward. The positive nucleus to the east of Buenos Aires over the Atlantic is less intense, since the entrainment of moisture from low latitudes is not as important as in winter, as in the warm months moisture is climatologically present. The anticyclonic circulation in the north of Chile reinforces. An interesting feature is the anomalous vorticity gradient north of the negative centre over the Andes related to anticyclonic wind shear equatorward, and cyclonic poleward. Positive coef cients insinuate to the south of the continent. As revealed by these gures, the anomalous vorticity patterns associated with precipitation in Buenos Aires are remarkably persistent throughout the year. In both semesters, the zero-correlation isoline passes over Buenos Aires, which is consistent with the results obtained in terms of vorticity frequency distributions for the occurrence or non-occurrence of precipitation in

12 88 N. E. Ruiz and W. M. Vargas Fig. 11. Biserial correlation eld between 500 hpa relative vorticity and precipitation 0.1 mm in Buenos Aires, during the cold semester (May-October) Buenos Aires (Fig. 10c) where these curves cannot be differentiated. In order to show the inter-relationship between circulation and heavier precipitation, a similar analysis was made considering occurrence of the event when the average precipitation was 10 mm. For the cold semester ( p ˆ 0.08, q ˆ 0.92), the con guration of vorticity anomalies (not shown) is quite similar as in Fig. 11, except that the cyclonic vorticity region appears partially in the west of the Andes Cordillera and with less intensity, while the positive anticyclonic centre over the Atlantic still persists. This means that the advection of warm and moist air southwards is more important than 500 hpa perturbations when dealing with moderate or intense precipitation, at least in winter. For the warm semester ( p ˆ 0.11, q ˆ 0.89), all the

13 500 hpa Vorticity Analyses over Argentina 89 Fig. 12. Same as in Fig. 11, except during the warm semester (November-April) vorticity anomalies are well-marked (Fig. 13). It is worth noting the gradient in the correlation eld located between 28 S and about 33 S over Chile. As the gradient in the correlation eld between a variable and precipitation may be regarded as proportional to the correlation between the gradient of that variable and precipitation, provided that standard deviations do not change too much (Stidd, 1954), it is observed that a relationship exists between the gradient of anomalous vorticity at that place and precipitation in Buenos Aires. This implies the presence of a strong anomalous wind shear at that latitude, probably coming from anomalous westerlies associated with the jet stream, in connection with precipitation. In both semesters, the zerocorrelation line passes about 400 km west of Buenos Aires, which re ects the existence of

14 90 N. E. Ruiz and W. M. Vargas Fig. 13. Biserial correlation eld between 500 hpa relative vorticity and precipitation 10 mm in Buenos Aires, during the warm semester (November-April) slightly anticyclonic circulation in the middle troposphere over Buenos Aires, perhaps with the persistence of several previous days to moderate to intense precipitation, as synoptic-empirical evidence indicates. Some aspects of the synoptic climatology of precipitation based on 500 hpa vorticity elds over Argentina were described above. When dealing with the validation of GCM output, comparison of control simulations with the vorticity con gurations in terms precipitation for this region of South America could be useful. Additionally, in global change analysis, the circulation-local climate relationships are derived from observational data and then applied to GCM circulation data set to derive local-scale information consistent with the synoptic-scale forcing of GCM (Hewitson and Crane, 1996). So, the results found in the present work may serve to derive quantitative relations between larger

15 500 hpa Vorticity Analyses over Argentina 91 scale circulation and local climate at the surface environment in some form of transfer function. On the other hand, daily vorticity values which were proved to be a valuable indicator of the potential for incorporating air ow parameters in daily precipitation diagnosis could be used in models with a view to GCM downscaling (Conway et al., 1996). 4. Summary and Conclusions The Andes Cordillera introduces a singularity in the atmospheric ow of subtropical and middle latitudes of the southern part of South America. The climatology of 500 hpa relative vorticity elds for this area based on Argentinean regional analyses has been examined. Spatial dependence of this variable was limited to no larger than two grid steps. As result of this study, it may be seen that mean anticyclonic circulation dominates north of 40 S during the cold semester. This is even more evident during summer, and, in addition, mean anticyclonic vorticity extends towards the south over the Atlantic Ocean. Nevertheless, seasonal changes are small. A climatological trough appears all over the year over Patagonia with a northwest-southeast tilt, probably indicating poleward momentum transport. An interesting feature is the mean cyclonic vorticity area appearing over RõÂo de la Plata (35 S, 56 W). With respect to relative vorticity frequency distributions, they change with latitude remarkably and progressively from peak distributions in the subtropical region to at ones in the extratropics. A comparative analysis of frequency distributions of 500 hpa relative vorticity at given points for rainy days in Buenos Aires on the one hand, and for dry days on the other, showed that distributions were statistically different at grid points situated to the west of NeuqueÂn and to the east of Uruguay, but not at Buenos Aires. Although the potential importance of 500 hpa vorticity for rainfall is clear, it was important to study the vorticity con gurations that favour precipitation at a speci c location; that is to say, which transient disturbances are more likely to yield rainfall at Buenos Aires. As far as we know, this type of study has not yet been performed for the region. The analysis revealed that some signi cant biserial correlation coef cients emerge between 500 hpa relative vorticity and precipitation at Buenos Aires. A particular distribution of vorticity anomalies, therefore, is associated with local precipitation on a time scale of one day. The passage of a midtropospheric trough with its anomalously cyclonic centre over the Chilean coast between 35 S and 40 S and penetrating the continent, and anomalously anticyclonic vorticity to the east of Uruguay and to the north of Chile results in enhanced probability of occurrence of precipitation at Buenos Aries. When heavier precipitation at Buenos Aires was considered, advection of warm and moist air southwards appears to be more important in winter, while in summer it is worth noting the strong anomalous vorticity gradient north of the negative centre over the Andes Cordillera associated with anomalous westerlies and strong anomalous shear at that latitude. Con gurations and conclusions showing inverse signs may be, of course, related to dry days. The patterns produced in both cold and warm seasons bear qualitative resemblance. Consequently, this dynamical mechanism may be regarded as responsible for a signi cant fraction of precipitation in Buenos Aires, although some mechanism other than this is required in order to explain the observed variance. These results could be also used to assess the consistency between realistic and GCM-generated circulation patterns over Argentina. Acknowledgements We want to thank the Servicio MeteoroloÂgico Nacional of Argentina for providing all the data and computational facilities. We acknowledge Dr. P. D. Jones for reading the manuscript, and Drs. H. H. Ciappesoni and J. H. Hordij for their kind help and suggestions. This research was partially supported by the Universidad de Buenos Aires under Grant EX274, and by CONICET under PIP4557. References Bergthorsson, P., DoÈoÈs, B., 1955: Numerical weather map analysis. Tellus, 7, 329±340. Bischoff, S., 1988: AnaÂlisis espectral de perturbaciones sinoâpticas troposfeâricas sobre aâreas argentinas, PhD thesis, Departamento de MeteorologõÂa, Universidad de Buenos Aires.

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