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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.

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 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

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.

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 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

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 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.