Dry conditions over Argentina and the related monthly circulation patterns

This paper presents an analysis of the frequency and spatial distribution of droughts in Argentina during the 20th century. Special attention is given to the mean monthly atmospheric circulation associated with dry conditions in the Pampas during the second half of the century. A reduction in the number of dry cases is found throughout the century, especially during the warm season (October to March). Droughts occurred more often in only one or two regions simultaneously and only a few covered a large territory. Many of the dry months in the Pampas occurred under, or were preceded by, neutral ENSO conditions. During the warm season the ENSO cold phase preceded some of the dry months. Circulation patterns at 1000 hPa and 500 hPa and their temporal evolution are determined using Principal Component Analysis. A coupled 1000-500 hPa level analysis is performed to study the vertical coherence associated with dry conditions. Different patterns related to the warm season are clearly identified using this methodology. The principal feature are positive anomalies over the continent (principally at low levels), high circulation index and an enhancement of the westerlies. Patterns related to the cold season reflect high pressure at the surface, but some differences preand post1970 have been observed. In this season, some dry cases were found to have high correlations with opposites modes of PC scores. In these cases, a water deficit acted as the principal causes of the dry conditions.