Synoptic and regional-scale conditions associated with precipitation events leeward the Andes in Argentina are studied. Anomaly patterns of mid-tropospheric circulation in conjunction with local precipitation are analysed from Tartagal (about 20oS) to Ushuaia (about 55oS) mainly along the longitude 70oW east of the Andes Cordillera in order to examine the anomaly circulation behaviour and seasonality, and also to detect spatial coherence and geographical differences in anomaly patterns having place in precipitating situations. A number of studies have described the circulation features in subtropical and midlatitudes around South America and associated anomalies with precipitation (Aceituno, 1988; Minetti and Sierra, 1989) Garreaud and Rutllant, 1996). In this study, the circulation features in the South American troposphere associated with precipitation occurrence events east of the Andes are analysed using biserial-composite fields prepared from ECWMF operational numerical analyses. Emphasis has been placed on the geopotential anomalies in the middle troposphere in the austral cold and warm seasons and the distinct circulation patterns that appear in the north–south direction. A variable wavetrain pattern of anomalies extends from Western South Pacific Ocean to the north or northeast depending on the specific site examined. It is worth noting that each one of the regional synoptic map-pattern classifications investigated is appropriate or of exclusive use for the corresponding surface station. There exist separate classification systems for each in situ precipitation, thereby, different responses for each station. Notwithstanding, north of Malargüe (about 40oS) there seem to be regional anomaly patterns in connection with precipitation occurrence substantially different, and perhaps independent from an

In the context of climate change it is useful to downscale low-resolution large-scale features of models towards the high resolution small-scale features of environment and ecosystems which are modulated and driven by weather and climate. In this work, we address the problem of deriving local weather in terms of precipitation and the sensitivity to be well-described in different areas of Argentina, through a fairly simple transfer function. The performance of the dynamicalstatistical approach derived, that is based on a synthesis of significant anomaly circulation features linked to precipitation occurrence, is assessed by means of examining frequency distributions of results with two purposes: validate the diagnostic study, and give a representation of model uncertainty in geographic terms through examining the spatial variability as well as the interseasonal variability. From these results, western and southern areas are better predictable than the humid pampas and the Chaco region. Therefore, the latter may be considered as more vulnerable from the point of view of a downscaling approach or forecast. In seasonal terms, wintertime and summertime have a very similar level of predictability. The downscaling transfer functions developed in this study may be also considered able enough to carry out future studies of precipitation vulnerability in view of potential scenarios coming from hypothetic climatic changes.

A synoptic climatological study of precipitation in some regions of Argentina based on 500-hPa relative vorticity fields derived from geopotential heights produced at the National Meteorological Service of Argentina is performed. Some climatic-synoptic inferences concerning the association between large and synoptic-scale behavior of the upper-level flow and the occurrence of precipitation at a given place through the construction of composite/biserial correlation fields are drawn. In general, the results indicate that the correlation between vorticity and precipitation is nearly null in the vicinity of each one of the stations examined, and becomes increasingly negative to the west (cyclonic circulation associated with precipitation), and increasingly positive to the east (anticyclonic circulation). Then, almost all region of Argentina is affected by the westerly wind regime with precipitation usually associated with cyclonic vorticity advection. However, each locality exhibits some proper features that condition the occurrence of precipitation as well as a marked regional variation. Geographical and seasonal inferences are highlighted. For central and eastern Argentina vorticity anomalies are well defined: cyclonic anomalies to the west and anticyclonic anomalies to the east, generally more significant to the west sector. For the subtropical northeastern region the inter-relationship between vorticity and precipitation is something smaller. The importance of cyclonic vorticity maxima increases southwards. The northwestern region fits a different spatial configuration of maxima and minima vorticity with the anomaly axis in the north–south direction as a consequence of the deep alterations of the synoptic-scale systems induced by the Andes. For moderate to heavy precipitation the dynamics of the midtroposphere is still a decisive factor in all of the regions of Argentina. It is emphasized that the vorticity anomaly patterns as a manifestation of large and synoptic-scale upper-level circulation capture a considerable amount of information that allows us to classify precipitation data.

This work concerns the examination of a methodology of synoptic climatology, the biserial correlation technique, which allows studying the relationship between atmospheric circulation and precipitation. The physical meaning of biserial correlation fields between variables representing synoptic-scale circulation, particularly vorticity fields, and local precipitation is explored. One purpose is to examine this approach used to link the large-scale circulation and the smaller-scale surface environment, which seems to be simple, efficient and easy to interpret. An analysis based on biserial correlation configurations between 500 hPa vorticity and precipitation takes into account anomalous vorticity gradients including curvature and shear effects to describe some mechanisms favoring the occurrence of rainfall. It is shown that anomalies in the curvature of synoptic systems are largely causing precipitation. Daily precipitation at Cordoba, Argentina is used as an example to illustrate the results. The position of the cyclonic and anticyclonic anomaly centers and the position of the jet streams in association with precipitation may be clearly identified. The analysis is made extensive to heavier rainfall.

The basic statistics of aerological variables obtained from radiosonde data at Ezeiza, Buenos Aires, Argentina, for 1986 are examined, and their relationships with occurrence and non‐occurrence of daily local precipitation are analysed. The resulting samples are statistically different for nearly all the variables. Nevertheless, thermodynamic variables at the 700 mb level and the K‐Whiting instability index yield the best correlations with rainfall, though it is noted that they are diagnostic and not prognostic variables. 500 mb relative vorticity at selected gridpoints is included in the analysis, given the effectiveness of this dynamic field to recognise favourable patterns for daily precipitation occurrence. Some experiments using these potential predictors to specify occurrence or non‐occurrence of the event within a 24‐hour period are presented. Copyright © 1998 Royal Meteorological Society

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 Meteorológico Nacional of Argentina. The dataset, covering the period June 1983 to July 1987, was stratified 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 Río de la Plata (35° S, 56° W). Seasonal changes appear to be small. Relative vorticity frequency distributions were also analysed.

El proposito primario de esta investigacion es estudiar algunos aspectos basicos del comportamiento de la circulacion en los niveles medios de la atmosfera y su climatologia sinoptica con relacion a la ocurrencia de precipitacion diaria. Otro objetivo importante es desarrollar y aplicar metodos estadisticos para derivar ecuaciones empiricas con el fin de obtener pronosticos objetivos de probabilidad de precipitacion. La informacion utilizada consiste en campos diarios de analisis objetivos de alturas geopotenciales de 500 mb elaborados en el Servicio Meteorologico Nacional que cubren el cono sur de Sudamerica, datos aerologicos de Ezeiza y de precipitacion diaria de localidades argentinas. Se presentan resultados concernientes a variables dinamicas de 500 mb, tales como alturas geopotenciales, vorticidad geostrofica relativa y absoluta y sus advecciones, tendencia de vorticidad y componentes geostroficas del viento zonal y meridional, y su relacion con la precipitacion diaria en Buenos Aires. De los campos de correlacion se detectan las posiciones preferenciales de las anomalias de cada variable que favorecen la ocurrencia de precipitacion. Las anomalias mas significativas, en terminos estadisticos, se manifiestan en la componente meridional del viento y en la vorticidad relativa, no asi en las advecciones. En particular, se analiza la distribucion espacial y el desfasaje temporal de las anomalias de vorticidad relativa en 500 mb donde se muestra la localizacion y movimiento relativo de los centros anomalos ciclonicos y anticiclonicos que aumentan la probabilidad de precipitacion en Buenos Aires. La asociacion entre la precipitacion y las caracteristicas de escala sinoptica de la circulacion en la troposfera media se investiga a traves de los campos de vorticidad relativa en 500 mb, por lo que previamente se examinan los aspectos climatologicos basicos de esta variable por medio de sus momentos estadisticos y distribuciones de frecuencia. Se deriva una climatologia sinoptica de la precipitacion basada en la vorticidad, mediante la construccion de campos de correlacion biserial entre este parametro de altura y la precipitacion local diaria. Las configuraciones o patrones de anomalias de vorticidad, que sugieren la posicion relativa optima de los maximos de vorticidad ciclonica y anticiclonica con relacion a la ocurrencia de lluvia, representan los rasgos principales de una clasificacion climatico-sinoptica de las precipitaciones en distintas localidades del pais. Del analisis empirico de estos datos meteorologicos se hallan relaciones climatologicas y de pronostico entre la precipitacion local y la circulacion de gran escala de altura. Estos resultados contribuyen a la interpretacion objetiva de los productos del analisis y la prediccion numerica en terminos de fenomenos de tiempo meteorologico en superficie por intermedio de la aplicacion de metodos estadisticos de pronostico. Bajo la formulacion ‘’perfect prog’’ se derivan ecuaciones de regresion para la obtencion de la probabilidad de precipitacion diaria en distintos puntos del territorio argentino. Las tecnicas estadisticas utilizadas son Analisis Discriminante Multiple y Estimacion de la Regresion de las Probabilidades de un Evento. Los resultados de la verificacion indican que los porcentajes de acierto oscilan entre 60 por ciento y 70 por ciento y que, ademas, este esquema estadistico posee un mayor poder predictivo que la climatologia. Por estas razones, es util el desarrollo y la implementacion de metodos estadisticos objetivos de diagnostico y/o pronostico de probabilidad de precipitacion en el Servicio Meteorologico Nacional, que pueden resultar en una ayuda o guia en el momento de formular los pronosticos.