Low-level circulation patterns and precipitation in the Argentine Pampas. Association with soybean yield.

The study of atmospheric circulation patterns and the analysis of their frequency, distribution and temporal variability result a useful tool for climate diagnostics. Furthermore, in regions with an important world agricultural production it is necessary to study the climate events and related meteorological conditions that have a strong impact on the economy. Argentine Pampas (central-eastern Argentina) is one of these regions, therefore and in this context, this work aims to identify characteristics of the atmospheric circulation at low levels over southern South America in order to relate these types to precipitation amounts and soybean yields in the Pampas region.

Daily averages of 1000 hPa geopotential heights for the period 1979-2001 were used. The series of daily maps was obtained from the NCEP/NCAR reanalysis II. The period of months chosen was from October to May since major precipitations in central-eastern Argentina occur in this period and due to the importance of agriculture within the study area (third-largest producer region of soybean crop in the world). The areal average of ‘total precipitation' (precipitation from October to May) in the Pampas region was included in the analysis jointly with soybean yield. These datasets were obtained from government records.

Principal component analysis (PCA) was carried out on daily maps to determine the main synoptic types and their frequency. The PCA results using the T-mode approach and the correlation matrix show that the first six principal components (PC) explain more than 80 per cent of the total variance. The 12 patterns obtained (direct and inverse modes of the first six PC scores patterns) fit very well with the main synoptic types recognized by forecasters. A 0.7 factor loading value was used as a threshold to retain the daily maps that are represented fairly well by these 12 synoptic patterns and the series of seasonal frequencies were calculated for each pattern.

The most outstanding features are that the major interannual variability of seasonal frequencies was found for the synoptic situations with an intense/weak zonal flow in mid- and high latitudes (maps associated with direct and inverse modes of the second PC pattern) mostly in the second half of the record. The opposite behaviour was observed in the interannual variability of situations with intense S-SW flow over the southern part of the continent (inverse mode of the third PC pattern) at the same time as the seasonal frequency of N-NE intense flow situations over this area presented a weak positive trend (direct and inverse modes of the third and fourth PCs respectively).

The seasonal frequency of daily maps for the 12 synoptic patterns associated with the areal average of the total precipitation in Argentine Pampas was also analyzed. It was found that the maximum total precipitation in central-eastern Argentina is mainly associated with the occurrence of inverse modes of the second and fourth PCs. These patterns result in intensified N-NE flow (warm and humid air advection in the lower troposphere) over the Pampas and weaker zonal flow situations. Total minimum precipitation is linked to high frequencies of circulation patterns associated with the direct mode of the first PC. Even tough these synoptic types cause a warm and humid N-NE advection over the whole region, the South Atlantic High is strengthened inhibiting ascending air motions and obstructing disturbances migration toward lower latitudes.

Finally, the annual frequency of provincial departments in the Pampas region where the soybean yield was classified as a minimum was related to the averaged total precipitation series and the circulation types. It was found that higher frequencies of minimum soybean yield are well associated with minimum averaged seasonal precipitation. Moreover, higher (lower) frequencies of circulation types associated with the direct (inverse) mode of the first (third) PC can be linked to higher frequencies of minimum soybean yield condition. These synoptic types induce, on one hand, warm and humid anomalous advection without ascending air motions resulting in high air temperature and no rainfall and on the other hand, cold southwestesterly flow anomalies in the Pampas region resulting in below normal air temperature or probably in late frosts. Both effects impact negatively on soybean yield. In addition, in this kind of analysis it is important to study the patterns monthly distribution.