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Different rainfall parameters are analysed in the present work in order to describe precipitation in three Argentine reference stations. These locations represent their precipitation climate region through a centenary data record. The selected parameters are annual accumulated precipitation, number of days with precipitation and precipitation intensity. All three stations indicate a positive trend in the first two variables over the studied period; moreover, because of a proportional increase, the precipitation intensity parameter has no significant variability. From the analysis of these variables and applying statistical methods, a climatic jump is found in each station, around the 1950s. A second focus of this work is the analysis of rain spells. To this end, a cutoff value for each station is found and used so as to establish the relationship between the accumulated precipitation and the frequency of rain spells per year. The total number of rain spells follows an exponential decay function for their relative frequency, considering the number of days that constitutes them. Rain spell characterisation allows for the understanding of their contribution to precipitation, according with its duration.

The impact of climate variability on exportable surpluses of maize and soya beans in producing/exporting countries is well known. This situation necessitates the study of climate variability and yields in some of the major producer countries (Argentina, Brazil and the United States). This study aims to characterize the climate regimes of each region, seeking the highest degree of regional homogeneity and representativeness of yields and climate variables. The series variability is analysed according to various effects (different stages of crop development and conditioning of yield). The temperatures and rainfall occurring during the flowering stage exert greater conditioning on yield at the end of the crop cycle. Different theoretical fit models of yield are obtained for both crops with climate information from their core areas of high production.

The main goal of this work was to conduct an intraseasonal climate variability analysis using wavelet and principal component analysis over a southeastern South American daily maximum and minimum temperature series from the end of the nineteenth until the beginning of the twenty-first century. The analysis showed that there is a definite and coherent signal in the intraseasonal maximum and minimum temperatures. The most noticeable signal was observed during the winter months. The frequency of the intraseasonal signal was more complex for the maximum temperature, and in some stations, it displayed a bimodal distribution. A defined pattern that described a coherent variability between 30 and 60 days throughout the entire region was observed. This pattern potentially allows classification of the regional variability and adjustments to the temperature forecasting models on a daily basis.

An important goal of this work is to study the variability of corn and soya bean crop yields in four countries with large production and substantial commercial trade in these commodities. This problem can be investigated in terms of the role that these two crops play in food programmes and in terms of the use of both crops for energy production. Four countries were chosen and divided into six production areas. A climatic summary was made of the annual cycles of extreme temperatures and precipitation. Their assessment in agriculture programmes was likewise summarized. It is seen that the variability range of the temperatures and precipitation are broad and different for each region. This finding indicates the high adaptability of these crops. This concept of adaptability is used to compare the coefficients for precipitation and crop yield. Results of the study show that corn crops show less year-to-year variability than do soya bean crops. The United States and the northern part of China are the regions that best use the rain supply with respect to crop yield. Soya bean crops show a greater year-to-year variability in the ratio of precipitation to crop yields. Argentina, the United States and northern China are the areas that best use the rain supply. To compare crop yields with climatic variables in the different regions, three types of regression model were used. The best fit is obtained by using the maximum temperatures and accumulated precipitation for each growth stage over the growing season. Copyright © 2011 Royal Meteorological Society

Important features of extreme precipitation in the La Plata Basin are studied on daily, seasonal, and annual time scales, using information from 1861 to 2005 and a common period from 1959 to 1998. A relation between the daily precipitation annual maximum and different time scales is developed. The points that make up part of the association field, the location of the maximum annual frequencies, the maximum annual precipitation totals, and the years in which they occur in the basin region are also presented. Owing to the increase in precipitation documented in the region during the last decades of the twentieth century, this work will attempt to estimate the trends during the time periods studied in each of the basin stations prior to analysing the trend estimators calculated for different periods. Conditioning over the physical inference of the trends is related to the previous estimation results. Indeed, trend estimators may identify a long wavelength with a small amplitude at a physical level or the presence of waves during the calculation period with wavelengths longer than the period itself. Due to the fluctuations present in the selection of the daily precipitation maximum, the three annual maxima and the mean of the three absolute maxima for each year are analysed. This procedure is sound, whereby the frequency distribution models are the same for the individual maxima as well as for the mean values. The most suitable models for the adjustment of extreme precipitation events in the basin are the GEV and Gamma distributions. Copyright © 2010 Royal Meteorological Society

This study addresses the dry spells observed in the La Plata Basin using daily data from 94 observation stations during sampling periods from 1900 to 2005. Dry days were defined as having less than 0.3 mm of accumulated precipitation. This definition allowed for the assessment of the dryness in the La Plata Basin and a comparison with other regions. The main purpose of this study was to analyse dry spells, especially extreme cases (meteorological droughts), and assess them on a daily basis. Trends and low frequency of droughts were analysed using a general framework to detect and compare properties of dry states based on daily and annual time scales. The trends were estimated using two different methods. Overall, the trends showed a decrease, especially in the eastern basin region during the period of 1972-1996. The results showed sporadic decreases in dry events and events of extreme dryness (droughts). Spectral structure permits an inference of low-frequency maxima and confirmed an inter-annual 2- to 3-year period of variability in drought occurrence for most of the basin. Furthermore, probabilistic distribution functions of dry spells at basin stations were analysed to confirm that they followed a geometric-binomial distribution. Additional tests were used to determine whether there was a second threshold, using the Weibull and gamma adjustment models. In order to study spatial homogeneity, the field of dry spell maxima in the basin was generated using a vector array based on the occurrence date and length of the maximum spell. Due to the dependence of spell length on the annual cycle, the longest spells were observed from April to the beginning of winter in the Argentine northwest region and in the northern and western regions of the basin. The intensity of droughts decreased in the Pampas and Mesopotamia regions. The drought of 1988 was considered to be the longest dry spell in the basin. The water deficits from this drought resulted in Argentinean economic losses of more than four billion US dollars during 1988. Fil: Vargas, Walter Mario. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Naumann, Gustavo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Minetti, Juan Leonidas. Universidad Nacional de Tucumán; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

From a regional inventory of monthly droughts, which was evaluated in six regions of southern South America, the seasonal occurrence of the phenomenon and its persistence together with the duration of monthly and annual sequences, extreme events, and other statistical estimates of the proposed index have been studied. In a primary analysis, it is possible to observe that regions present different behaviors regarding the duration of dry sequences, with more persistence in the Argentinean continental region. Temporal behaviors of the annual indexes have also been analyzed, in the attempt to determine any aspects of the impact of global warming. Through this analysis, the presence of long favorable tendencies regarding precipitations or the inverse of droughts occurrence are confirmed for the eastern Andes Mountains in Argentina (ARG) with its five sub-regions (Northwest Argentina-NWA, Northeast Argentina-NEA, Humid Pampa-HP, West-Centre Provinces-WC and Patagonia-PAT, and the inverse over the central region of Chile (CHI). Other climatic great-scale changes are interdecadales variations and variances explained according to ENSO.

In the past various authors have investigated climatic change (CC) by using the latitude of the subtropical ridge in the mean sea-level pressure (MSLP) for a range of geographic regions. In this paper the index L is defined as the latitudinal position of the subtropical anticyclone along the Chilean coast. This L index on the South American coast of the South Pacific is analysed using an instrumental record, covering the 1901-2004 period. An objective methodology is proposed for estimating L, and the physical consistency is assessed with the SOI and pressure at sea level in Valdivia, Chile. The analysis of a shorter period is compared with other authors’ results. The L variable can be useful in CC studies or in the analysis of low-frequency fluctuations in climate variability, and it is analysed as a dependent and independent time series. L was displaced towards the south over the last century but not to a statistically significant extent. Natural fluctuations of 50 years and another, shorter, of 16-22 years were also observed. In this paper, the greatest persistence and distant associations between anomalies of monthly L are shown; these associations can be useful for climate forecasting.

This paper analyses the soya bean yield variability in the Argentine Pampas in relation to the atmospheric circulation patterns in southern South America. The region studied is the main soya bean production area in Argentina and contributes a high percentage to world trade in soya beans and their by-products. Two datasets were used: the soya bean yield series of 58 provincial districts and daily average fields of 1000 and 500 hPa geopotential height corresponding to the NCEP_Reanalysis 2 in the period 1979–2001. Adverse impacts on final yield may be mainly associated to weather types (WTs) related to intense rainfall during harvest time (April and May) and to structures which favour stability and warm advection in the summer at lower levels. The thermal effect on soya bean may be observed in the WTs whose configuration of anomalies favours the incursion of cold air at the lower levels contributing to low temperatures during seed germination and early vegetative growth stages. Positive impacts may be associated with WTs that reduce thermal stress during flowering and pod set stages (summer) and WTs which favour stability at harvest time (April). In general terms, atmospheric circulation structures have been identified as elements which may be added to the set of climate predictors to explain a part of interannual yield variability. The structures found have direct implications for short and middle term forecasts. They constitute a primary element for the monitoring of the growing season of the soya bean crop and for planning strategies. Copyright © 2009 Royal Meteorological Society

Water resources management and agriculture planning models require a statistical synthesis of some rainfall features, in particular those representing dry atmospheric conditions. The bigger the basin, the more important these features become, as is the case of the La Plata Basin (LPB). This paper focuses on the precipitation variability in the large LPB in South America, analysing the number of months per year with low rainfall and the sequences of months with low rainfall, their theoretical distributions and stability, which are needed as input for the models mentioned above. Long time series are used to analyse the low-frequency variability and the relative importance of decadal variability. Changes are evident in the number of months per year with low rainfall, with a decrease of about 20% in the period after 1970. Theoretical distribution models (binomial and geometric) are fitted to these empirical distributions, and the regional variability of the fitting parameters is shown. In practically the entire region, the goodness-of-fit of the two theoretical models considered is statistically satisfactory. The temporal variability of the parameters of the theoretical binomial (p) and geometric (α) distributions is analysed, in excluding sub-periods of 10 and 5 years, respectively. The results show low-frequency variability overlapped on a decadal variability, with low homogeneous regional behaviour. The distribution models have proven to be efficient for frequency adjustments of the rainfall properties studied. These results are an acceptable and necessary input to decision models in LPB. They also make it possible to infer effects of climate change. Copyright © 2008 Royal Meteorological Society

Se identifican las sequías climáticas ocurridas en el espacio y tiempo durante el siglo pasado para la Pampa Húmeda. Haciendo uso de índices de sequía mensual, trimestral y anual propuestos se analizan algunas de las propiedades estadísticas como la persistencia, marcha temporal y distribución espacial, entre otras. También se muestran las potenciales consecuencias de las sequías sobre la productividad de un cultivo de importancia regional como el maíz, muy sensible a la misma. Para ello se elabora y evalúa un modelo de diagnóstico entre este estado del clima y la productividad del maíz para analizar los impactos.

This study analyses some synoptic situations in which extreme temperatures occur. Cases of temperature-anomaly spells from ten stations in Argentina are studied. The duration and the maximum deviation in each case are considered. The Lund correlation method or manual classification was used to analyse the synoptic situations. The cold spells in winter during the period 1971/80 occurred in three kinds of synoptic situation. Most of the days have a high-pressure system to the west and a trough in the centre of the country, which is associated with south-westerly winds. The coldest days have south-easterly winds to the north (from the Atlantic), which blow around a high-pressure system over the southern part of the country. There is a difference between this synoptic situation and the coldest synoptic situation in summer. The coldest conditions in summer take place in south-westerlies when three or more cold fronts have passed over the area. The warmest spell in summer (January 1972) occurred with a cold front coming from the south, a low-pressure centre in the middle of the country (between two high-pressure systems), with a flow from the north over Argentina. In winter the warmest spells occur in similar situations, but there are also warm fronts present over the northern part of the country. Lund's method is appropriate for finding the synoptic situations connected with some special cases.

This paper examines the temporal and spatial variations of annual and monthly extreme precipitation for ten stations (1890–1980) in the central-east of Argentina. Extremes of yearly and monthly rainfall are statistically defined. The time series of the annual and monthly rainfall anomalies are analysed. The dry years are concentrated in the beginning of the period of observation (before 1940) and the wet years are distributed throughout the whole period. The probability of having a wet or dry month is the same, and the probability of having two or more months with the same extreme condition is low. Harmonic analysis is applied to the monthly rainfall of the driest and wettest years to describe and analyse their different cycles. The annual cycle, for the driest years, tends to dominate over the whole region. The most important harmonic for the wettest years is the second one, associated with the advection of humidity.

Climate variability is examined and discussed in this work, emphasizing its influence over the fluctuation of soybean yield in the Pampas (central-eastern Argentina). Monthly data of rainfall, maximum and minimum temperatures, thermal range and seasonal rainfall were analysed jointly with the soybean yield in the period 1973-2000. Low-frequency variability was significant only in the minimum temperature during November in almost all the stations. This situation is favourable to the crop since during this month, seed germination, a growth stage sensitive to low temperatures, takes place. In the crop's core production region, 72% of the series of soybean yield presented a positive trend. Except in years with extreme rainfall situations, interannual variability of the soybean yield is in phase with the seasonal rainfall interannual variability. During these years, losses in the soybean crop occurred, with yield negative anomalies greater than one standard deviation. Soybean yield showed spatial coherence at the local scale, except in the crop's core zone. The association between each climate variable and yield did not show a defined regional pattern. Summer high temperature and rainfall excesses during the period of maturity and harvest have the greatest negative impact on the crop, whilst higher minimum temperatures during the growing season favour high yields. The joint effect of climate variables over yield was studied with multivariate statistical models, assuming that the effect of other factors (such as soil, technology, pests) is contained in the residuals. The regression models represent the estimates of the yield satisfactorily (high percentage of explained variance) and can be used to assess expected anomalies of mean soybean yield for a particular year. However, the predictor variables of the yield depend on the region. Copyright © 2007 Royal Meteorological Society.

The Parana-Plata basin is the second largest hydrological basin in South America and is of great importance for the countries of the region (Argentina, Bolivia, Brazil, Paraguay and Uruguay). The present study focuses on the long-term trends in basin-scale precipitation with special emphasis on the role of distribution changes in extreme large-scale precipitation events and on the characteristics and evolution of ENSO teleconnections over the last 50 years. First, we defined a Parana-Plata basin total precipitation index (PTPI) as the precipitations spatially averaged over the hydrological basin. On interannual time scales, such an index is mainly representative of anomalous monsoon precipitations in the northern part of the basin and large convective precipitation anomalies in the center of the basin (Paraguay-southern Brazil-Uruguay-northern Argentina) typical of the canonical ENSO teleconnection pattern. Our major findings clearly highlight a positive trend in yearly averaged PTPI mainly from the late 1960s to the early 1980s with a strong dependence from month-to-month. The largest precipitation increase is observed from November to May in southern Brazil and Argentina. A close examination of PTPI distributions during the two halves of the period 1950–2001 shows that the changes in the mean state from 1950–1975 to 1976–2001 result from significant changes in each calendar month mean state and in the tails of the PTPI anomaly distributions in May with lesser and weaker large-scale dry events and stronger large-scale wet events. Further studies will be needed to assess whether the observed trend in large-scale extreme precipitation conditions can be related to natural climate variability or anthropogenic activities and whether it is associated to changes in local/regional extreme events. The stronger wet conditions in different months seem to be associated to changes in ENSO characteristics (amplitude, propagation, spatial structure, ...) since the 1982–1983 El Nino. Indeed, spatial ENSO teleconnections (stronger in November and April–May) have greatly evolved from 1950–1975 to 1976–2001. Moreover, we demonstrate that there is a strong modulation and displacement of the teleconnection patterns from one event to another, impeding the definition of robust statistical relationship between ENSO and precipitation in the Parana-Plata basin (except maybe over a very limited area near the common border between Paraguay, Argentina and Brazil). Finally, the non-antisymmetrical patterns of precipitation between El Nino and La Nina conditions and the non-linear relationship between precipitation and either Nino3.4 or Nino1+2 sea surface temperature indices show that linear statistical forecast systems are actually of very limited use for impact predictions on society on a local or regional scale.

Long, annual precipitation time series from central-northeastern Argentina are analysed with special attention to interdecadal and interannual variations. The results show a variability that is highly nonstationary. The most outstanding feature is the difference in annual precipitation before and after the 1950s. The interdecadal variability is particularly well defined in the west, and there is a significant linear trend in the study area. Negative anomalies of the areal-averaged annual precipitation can be observed in all the areas around 1910 and 1930 or 1940. In the western zone there are two better-defined negative periods around 1950 and 1970, and a gradual increase can be observed in the eastern zones starting in the 1950s. The interdecadal and interannual variations affect the behaviour of extreme precipitation on the annual scale and during the months with maximum precipitation in the region. In support of these conclusions, different representation methods are used (running means, power spectrum analysis, and wavelet transforms). The variations in the annual cycle of extreme precipitation under different conditions are analysed by applying harmonic analysis. The wettest years are represented by an annual and semi-annual cycle, whereas dry years need the input of lower frequency cycles. Copyright © 2004 Royal Meteorological Society

Different circulation types for southern South America are derived from the circulation anomaly maps at 500 hPa corresponding to a 9 year period. The series of daily maps was obtained from the ECWMF reanalysis. These data are particularly useful, since real data, especially the radiosonde network, are sparse in this region. The properties to be studied are selected in such a way that they describe different flow conditions over the region, in order to obtain a statistical diagnosis useful for modelling an objective forecast. The circulation types were obtained by a correlation–map-based pattern classification technique. Lund's method is used in this paper to identify the most frequent circulation patterns. The classification method was applied to daily Z anomalies at 500 hPa over the whole record (1980–88). This allows one to analyse the evolution and presence of a particular type over different months and years. The most outstanding circulation-type structures, represented by only eight types, explain about 63% of the total number of cases in the sample. The most frequent type, Type 1 (Z), shows an almost zonal circulation with a strong meridional gradient, associated with a trough in the west of the region. Type 2 (LCE) is represented by a low-pressure system in the centre of the region (approximately at 37° S, 65° W). In general, this type is related to the occurrence of blocking situations in the South Atlantic Ocean and the passage of cold fronts over the region. Types 3 (SW) and 4 (WNW) show a SW and WNW atmospheric circulation over the whole region with a meridional gradient lower than in Type 1. Type 5 (TNS) shows an NW–SE trough axis over the continent. Type 6 (WW) has an intense SW flow over the southern part of the continent. Type 7 (NW) shows a very deep trough to the west of the continent, located over the Pacific Ocean at 80° W and 34° S. The continent is affected by a NW atmospheric circulation. Type 8 (R) represents a ridge over the continent with an intense NW flow in the south. The frequency of Types 1 to 8 is about 63%. None of the circulation types mentioned accounts for more than 12% of the sample. There is a considerable variability in monthly circulation type distributions; the first six types only account for approximately 50% of the variability. This suggests that the use of probability models related to circulation type occurrence in the middle troposphere should be analysed on a monthly basis. The results attempt to summarize the region's synoptic regime through a small number of circulation types at 500 hPa and the corresponding 1000 hPa maps. The frequency distribution of each type during warm, neutral and cold phases in the equatorial Pacific Ocean was analysed. These results partially explain the best dynamic conditions (cyclonic vorticity), for the Argentine humid and semi-humid region, southern Brazil, Paraguay and Uruguay, that lead to the harvest precipitation events in spring (October–December 1982) with SOI < 0. In winter these circulation types are more important during cold phases with advection of cold and dry air (July–September 1982). The circulation types at 500 hPa and the corresponding maps at 1000 hPa associated with mean maximum and minimum monthly temperature and precipitation in the region are studied. The monthly maximum and minimum precipitation recorded in stations near the Uruguay River basin in southeastern South America and the mean monthly maximum and minimum temperatures from three stations in Argentina during 1980–88 are used in the analysis. Some of the daily patterns are strongly associated with the occurrence of maximum and minimum monthly temperature and precipitation in different places of the country. In reference to the maximum and minimum precipitation records, the behaviour of the variables under study is observed to be physically consistent with the associated circulation patterns. Copyright © 2003 Royal Meteorological Society

The existence of low-frequency variability in climatic-hydrological-oceanic variables may be useful for long-term forecasting and climate modelling. By using long time series this paper attempts to identify large-scale quasi-cycles in the precipitation regimes of Northern Argentina, moisture advection from the Atlantic Ocean and the streamflow of the Parana River. This work also shows the presence of coherent waves with long periodicity between the three series. As the three variables are estimated over different time intervals, the presence of waves in each variable is studied separately, to show they respond to the same process. The three variables show significant interdecadal variability at low frequency (22–26 years), which might be related to the ENSO cycle modulation and to the intensification or weakening of the South American Low Level Jet (SALLJ) and South Atlantic Convergence Zone (SACZ).

The main purpose of this work is to provide a broad assessment of the influence of the El Ni˜ no–southern oscillation (ENSO) on the frequency of extreme temperature events throughout the year in Argentina. The cold and warm events, defined by the persistence of significant anomalies arising from the maximum and minimum temperatures, are more intense in the subtropical latitudes, north of Argentina (northeast for cold events and northwest for warm events), during the coldest months, and in the south (higher latitudes) in the warmest months. The persistence of the air masses is always greater in the north. The warm (cold) events have a preferential region of dissipation in the northwest (northeast) of Argentina, close to the Andes. The effect of the ENSO phases on the probability of occurrences of these events has a large inter-monthly variability. The different La Ni˜ na episodes are more homogeneous in relation to their effects and feasibility for the prediction of extreme temperatures within the region, especially for cold events. During La Ni˜ na episodes, all 2 month periods (except August–September) present a higher occurrence probability of very persistent and intense cold events, compared with Neutral months in different regions of the country. Conversely, the different El Ni˜ no episodes are more heterogeneous and, therefore, their effects on temperature are more varied and so less predictable. However, El Ni˜ no episodes exhibit a stronger signal, which is associated with the tropical air incursions that generate extreme warm events, particularly in winter. These events have a higher occurrence probability in the north and a lower one in the south during the months of June to November. Cold events have a lower occurrence probability in different

Summer rainfall variability (October to March) shows inter-annual to multi-decadal fluctuations over a vast area of subtropical Argentina between 28°S–38°S and 65°W–70°W. Statistically significant oscillations of quasi-period in the bands of 18–21, 6, 4 and 2 years can be found throughout the region and intra-regionally, though the latter are variable. The lower frequency variation produces alternating episodes of above and below normal rainfall each lasting roughly 9 years. This quasi-fluctuation appears to be shared with the summer rainfall region of South Africa and were in-phase related one another until mid-1970s. The teleconnection between both subtropical regions could be generated by an atmospheric-oceanic bridge through the global sea surface temperatures (SSTs), particularly those of the equatorial-tropical South Atlantic. From mid-1970s, the alternating wet and dry pattern has been interrupted in the Argentine region producing the longest, as yet unfinished, wet spell of the century. Thus, a significant change of the long-term variation was observed around 1977 toward lower frequencies. Since then the statistical model that explains more than 89% of the variance of the series until 1977, diverges from the observed values in the 1980s and 1990s. In addition the Yamamoto statistical index, employed to detect a climatic jump, reaches its major value in 1973 at the beginning of the current long wet spell. Therefore the change could be located between 1973 and 1977. Application of the t-student's test gives significant differences of mean values for pre-1977 and post-1977 sub-samples from both individual time series and the regional index series. The spectral analysis also shows changes in energy bands in concordance with the features of the change that occurred from mid-1970s. The change gives rise to a significant increment of more than 20% in average of normal rainfall over the region. Conversely, a drought between mid-1980s and the 1990s has been observed in the South African counterpart with severe characteristics, thereby continuing the quasi-18-year oscillation. Consequently, the low-frequency coherent behaviour between both the Argentine and South African regions is lost from the mid-1970s. The analysis of association of wet/dry spells and warm/cold, El Nino/La Nina episodes appears to be not significant at scales of year-to-year variability although at decadal to multi-decadal scales the association could be relevant. More than one process of multi-decadal variability of global SSTs could influence the Argentine summer rainfall region and the former bi-decadal teleconnection. Finally, potential hypothetical factors of change are discussed, such as the strengthening of direct and indirect mechanisms of moisture flux transport associated with global warming, low-level atmospheric circulation changes and/or to SSTs mean condition long-term variations over tropical and subtropical South Atlantic and South Pacific oceans.

This paper describes the meteorological variables directly affecting precipitation in the southern region of South America. Results show the relationships between daily precipitation at Ezeiza, Buenos Aires, and 500 mb dynamic variables (e. g. geopotential heights, relative geostrophic vorticity and its advection, vorticity tendency and zonal and meridional geostrophic wind components) using regional synoptic analyses from the Servicio Meteorologico Nacional of Argentina for 1986. The distribution of significant biserial correlation coefficients reveals the preferred positions for the anomalous wind components which favour the occurrence of rainfall at Ezeiza. In the same way, spatial distributions and temporal lag relations of 500 mb relative vorticity anomalies in relation to precipitation show the positions and relative movement of centres of anomalously cyclonic and anticyclonic vorticity which increase the probability of precipitation at Buenos Aires. Vorticity advection does not yield higher correlations, at least in a daily temporal scale. The geographical distribution of temporal means and standard deviations of geopotential heights and zonal and meridional wind components at the 500 mb level are also shown. Copyright © 1999 Royal Meteorological Society

The main source of the Cuyo rivers' water volume is the melting of winter accumulated snow over the Andes subtropical high mountains. Particularly between 30 and 40°S, Andean winter precipitation (April–September) shows both spatial coherence and significant correlations with the Santiago precipitation. That is why this zone, which corresponds to the Cuyo rivers' basin areas, is homogeneous with respect to the streamflow's inter-annual variability. The streamflows of the rivers in this region are highly correlated to each other. Therefore, the Mendoza river, which is one of the Cuyo rivers is taken as reference for all these rivers to study the summer streamflow inter-annual variability (October–March) and the association with ENSO events. This river exhibits behaviour associated with equatorial Pacific Ocean anomalies. Above average streamflow is more likely to occur during a mature phase of El Nino event in the southern summer. This implies abundant and frequent snowfalls during the previous winter over the high subtropical Andes. Abundant and frequent snowfalls in the winter after an El Nino year, and the subsequent high flows during the following summer, are less likely to occur. This teleconnectivity is not absolute since some warm events were recorded and simultaneous flows remained below average. Instead, values under the mean flow are more likely during cold event years, with exceptions being quite rare. This atmospheric circulation study involves principal component analysis of daily surface pressure fields from 1972 to 1983. It reveals identifiable characteristics for Andean winters with above average precipitations. Wet winters are charecterized by both less explained variance by the first principal component, whose pattern corresponds to a high zonal flow component, and more explained variance of those patterns matching low pressure systems and cold frontal passages which have a high meridional flow component. The inverse occurs in dry winters. Generally, patterns corresponding to post-frontal anticyclones show no significant correlation with the precipitation over the high subtropical Andes. © 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 Meteorologico 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 Rio de la Plata (35° S, 56° W). Seasonal changes appear to be small. Relative vorticity frequency distributions were also analysed.

Streamflows have a direct dependence on precipitation and these are directly linked to the climate. Then, in this paper the temporal climatic variability in the ‘Rio de la Plata’ Basin is analysed through the changes in the river's discharges. These are the reflection of the climatic inputs areally integrated, and in consequence, contain more information on climate variability than that provided by the scarce punctual records of precipitation and temperature. The time series of streamflows correspond to monthly and annual means in stations selected in the basin for the period 1931–1992. However, in the present paper, the period 1901–1992 was considered in all cases whenever possible. The following changes and tendencies in the flow series were detected: 1. An important change of tendency between 1970 and 1972, and another not so significant before that date were detected in 1917–1918 and 1943–1944. 2. The jumps in the means in several sub-periods were detected using different methods. They showed jumps mainly in the period 1970–1972 in the annual streamflows series. The jumps in the annual streamflow series consist of an abrupt change in climatic variables affecting temporarily the averages of such variables during a certain period of time (years). The results are consistent with the conclusions obtained by other authors for the same region, both in precipitation and in the general circulation of the atmosphere. Keeping in mind this analysis of the series of streamflows, indicators of normal variability of tendencies relative to natural regional causes were detected, although the local causes were not anthropogenically analysed, and so no other manifestations of randomless in the zone of the Basin under study because of the lacking of data.

This paper studies the climatic conditions of warm and cold dry months in the humid and semi-humid Argentine region and some aspects of the regional circulation related to these cases. The climatic analysis of warm (temperatures above percentile 80) and cold (temperatures below percentile 20) dry months is based on precipitation and temperature data registered at reference stations over a period of at least 70 years, while the associated circulation is derived from daily data of geopotential height at 500 hPa from NCEP-DOE Reanalysis 2 database. The reference station for the center of the country registered a greater number of warm dry months during both the warm season (October–March) and the cold season (April–September), whereas the reference stations in the north-east and center-east showed differences depending on the time of the year, with more cold dry months during the April–September season and more warm dry months in the October–March season. A classification of daily fields of geopotential height anomalies at 500 hPa was used to analyze the atmospheric circulation related to warm and cold dry months. The circulation patterns were obtained by applying principal component analysis and cluster analysis. Findings show that some mid-level circulation patterns occur with a significant different frequency during the warm dry months or the cold dry months studied. Finally, cases of spatially extended precipitation-deficit conditions (hereinafter generalized droughts) were studied, noting dominant patterns that are coherent with the previous results. Fil: Barrucand, Mariana Graciela. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Vargas, Walter Mario. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Bettolli, Maria Laura. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

Recently daily series of meteorological variables have been studied according to signal-noise patterns by different authors. Later on, the persistence phenomenon was modeled as an autoregressive structure of Markovian red noise. In series of daily anomalies obtained from filtered annual wave, a particular memory remains that is seasonally dependent. The method presented in this paper allows the modeling of (a) seasonal and intraseasonal variation of persistence, and (b) temporal distant action between perturbations. Data are showing that temporal distant action between perturbations would result from slow changes in border conditions, such as ENSO. Four examples of previous changes in the structure of daily extreme temperature series of San Miguel de Tucuman, Argentina, known as “season structure,” are shown. One of them corresponds to the El Nino/no El Nino dichotomic phenomenon.

The diurnal and seasonal patterns of the dry and wet bulb temperatures over Argentina are studied. The wet bulb temperature frequency distributions have negative skewness over all the country. The higher values are in the lower latitude stations, which are under a subtropical regime. The dry bulb temperature frequency distributions change their skewness from north (negative values) to south, and the highest positive value occurs at the most southerly station, which is under a middle latitude regime. Three stations in the Buenos Aires area are analysed to study the effects of the river and the city on temperature. The effects of the city and the river on the dry bulb temperature and the wet bulb temperature depend on the wind direction and the time of day. The effect of the river also depends on the month. These effects are also present in the anomalies. If it is necessary to represent extreme climatic or synoptic conditions over a region then the city stations are more appropriate than coastal ones.

Monthly and annual climatic variabilities of precipitation were analyzed from a 60 years record of rainfall data at 10 measuring stations in the humid region of Argentina.

The objective of this research is to explore the relationship between maximum and minimum temperatures, daily precipitation, and the Madden–Julian oscillation (MJO). It was found that the different phases of the MJO show a consistent signal on winter temperature variability and precipitation in southeastern South America. Additionally, this paper explores the spatial–temporal variations of mutual information and joint entropy between temperature and the MJO. A defined spatial pattern was observed with an increased signal in northeastern Argentina and southern Brazil. In the local mutual information analysis, periods in which the mutual information doubled the average values were observed over the entire region. These results indicate that these connections can be used to forecast winter temperatures with a better skill score in situations where both variables covary. Fil: Naumann, Gustavo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Vargas, Walter Mario. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

[1] A Singular Value Decomposition (SVD) analysis was performed jointly on extreme temperature events in Argentina and sea surface temperature (SST) in the South Atlantic and South Pacific. Sea level pressure (SLP) patterns associated with the first SVD coupled mode were also analyzed. Winter is the season of the year that is best represented by the first mode, accounting for up to 70% of the winter covariance between temperature events and SST. The warm and cold events in Argentina are essentially a consequence of the creation of meridional atmospheric circulations over the continent. Such atmospheric patterns result from displacements and intensity changes of the subtropical anticyclones over the oceans and of the continental low-pressure center in northwestern Argentina. The temperature events in southern Argentina are also closely related to the warming and cooling of the coastal waters in the South Atlantic and South Pacific. The analysis suggests that in summer and winter, high (low) occurrence of warm events and low (high) occurrence of cold events are related to similar oceanic and atmospheric circulation situations. The temperature events in Argentina show higher correlation with the Atlantic than with the Pacific, which reflects the importance of the “orographic barrier” of the Andes Mountains in driving the atmospheric circulation. The only exception to this rule concerns the warm events in spring, for which the warming of the equatorial Pacific (the ENSO pattern) appears as the dominant mode. The temporal patterns of the temperature events in Argentina exhibit significant interannual variability in fall, winter, and spring, with periods of 3 to 5 years. The summer patterns suggest a very low-frequency variation with a period longer than 20 years.

Se analizan las estructuras de las autocovarianzas y autocorrelaciones de las temperaturas máximas y mínimas en estaciones de referencia en el sudeste de Sudamérica. A través de estas propiedades se observa que existen oscilaciones de períodos entre 18 y 25 años en las series bajo estudio, aunque estas periodicidades presentan cambios con el tiempo, especialmente entre los años 1950-1970. La excepción es Río Gallegos donde la periodicidad de 18 años se observa durante todo el registro. La persistencia definida mediante el primer coeficiente de autocorrelación es función de la cantidad de términos de autocovarianza negativas. No obstante, durante algunos años es dependiente de la magnitud de los términos positivos dados por las irrupciones cálidas o frías intensas. Entre los años 1950 y 1970 se ha observado un aumento de la variabilidad con una alta alternancia entre períodos de mayor y menor persistencia. Estos cambios analizados en la persistencia sugieren una variación en la frecuencia de distintos tipos de circulación que tienen un impacto directo en la estructura térmica regional. Así mismo, variaciones de este tipo pueden tener serios impactos socio-económicos ya que estos afectarían directamente la frecuencia y duración de los eventos extremos. Esto tiene importancia también en el diseño de métodos de control y consistencia ya que en ellos es posible considerar como outlier o información errónea los cambios producidos por las irrupciones de escala sinóptica mencionadas anteriormente. Eight reference stations in south-eastern South America are analysed to derive the auto-covariance and auto-correlations of maximum and minimum temperature. Through the analysis are observed oscillations of periods between 18 and 25 years in the series studied. These periodicities show changes over time, especially in the 1950-1970 period. The exception is Río Gallegos where a periodicity of 18 years is observed throughout the entire record. Persistence is defined as the first correlation coefficient and is a function of the number of negative terms in the auto-covariance. However for some years it is mainly driven by the magnitude of the positive terms given by extreme warm or cold outbreaks. Between 1950 and 1970 it was observed an increased variability in the analysed properties. These changes suggest a variation in the frequency of different circulation patterns having a direct impact on the regional thermal structure. Likewise, this kind of variations can have serious socio-economic impacts as these directly affect the frequency and duration of extreme events. This is relevant in the design of consistency and quality control methods to detect outliers or systematic errors. Fil: Naumann, Gustavo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Vargas, Walter Mario. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina