Synoptic weather types in the south of South America and their relationship to daily rainfall in the core crop-producing region in Argentina

Abstract

This paper classifies daily geopotential height anomalies at the 1000 hPa and 500 hPa levels for the period 1979-2001, and examines the capability of these atmospheric patterns to discriminate between the occurrence and non-occurrence of rainfall in the core crop-producing region located in the Argentine Humid Pampas. The resulting optimal classification has seven and five weather-type categories for the 1000 hPa and 500 hPa levels, respectively. The analysis focusses on the season of highest rainfall in the region, which coincides with the growing season of the main summer crops (October-May). These crops are critically dependent on rainfall and its spatial and temporal distribution, since irrigation is rarely employed in the region. The synoptic structures identified in this work can be related to daily rainfall in the region under study. Dry days are significantly favoured by structures with negative geopotential height anomalies at low levels toward the south of the South Atlantic Ocean, inducing an anomalous flow from the southwest over the south of the continent. They are also favoured by positive geopotential height anomalies centred to the east of the continent, inducing stability at low levels. At the 500 hPa level, a positive anomaly in the southwest of the continent centred over the Pacific Ocean, intensifying ridges west of the Andes, favours dry days. On the other hand, rainy days are favoured by patterns with positive geopotential height anomalies at the 1000 hPa level over the south of the continent, enhancing an anomalous flow from the eastsoutheast in the central region of Argentina and its corresponding moisture advection at low levels. Rainy days are also favoured by the patterns characterised by a cyclonic anomaly to the east of the continent, affecting the whole region. At the 500 hPa level, local rainfall can be related to patterns characterised by negative geopotential height anomalies centred over the Falkland Islands, which induce instability. More regional rainfalls are favoured by structures which weaken the westerlies over Patagonia at the 1000 hPa level and by structures which induce northwest flow over Patagonia and northeast flow over the northeast of the country at the 500 hPa level. The seasonal frequencies of the 1000 and 500 hPa synoptic weather types are consistent with the interannual rainfall variability documented for the region. The influence of the El Niño-Southern Oscillation was also identified in the daily atmospheric circulation anomalies in the south of South America.