Climatology of 1000 hPa weather type circulations in southern South America. Association with rainfall episodes.

BETTOLLI, MARIA LAURA; PENALBA, OLGA C; VARGAS, WALTER M

CLARIS News

CLARIS Project

Año: 2006 vol. 5 p. 2 - 7

1669-3264

This work aims to identify weather type circulations in the South American sector and analyze their frequency, distribution and temporal variability. Daily averages of 1000 hPa geopotential heights for the period 1979-2001 were used in order to perform the weather type (WT) classification. The series of daily maps was obtained from the NCEP Reanalysis 2 data provided by the NOAA-CIRES Climate Diagnostics Center. The months analyzed were those when major precipitations in central-eastern Argentina occur (October to May). This period also corresponds to the growing season of the summer crops in this area. Using k-means clustering method on the most relevant T-mode principal components, a classification into 7 fundamental circulations emerged, based on daily anomalies of geopotential over southern South America region: WT1, negative anomalies over the SW Atlantic and SW flow over the continent; WT2, trough over the SE Pacific and ridge over the SW Atlantic; WT3, positive anomalies over the southern tip of the continent; WT4, ridge over the E of the continent; WT5, ridge over the SE Pacific and S of the continent and trough downstream; WT6, low-pressure system centred 45S-55W; WT7, negative anomalies over the southern tip of the continent. The temporal series of seasonal frequencies of the 7 WT were studied in order to find main characteristics. Major interannual variabilities were found for those groups with enhanced westerlies (WT1 and WT7) while the opposite behavior was found for those days characterized by a ridge at the east of the continent (WT4) inducing subsidence at low levels. It was observed a progressive diminution in time of cold air advection situations (WT5). This result is coincident with the increase of summer minimum temperature values observed in recent decades over the studied area. Meanwhile an increase in seasonal frequencies of enhanced mid-latitude westerlies and disturbances migration over Patagonia (WT7) was detected. This group exhibited pronounced low frequencies in the strongest El Niño years (1982/83, 1991/92 and 1997/98) showing changes in low level circulation and in westerlies intensity during these years. The analysis of the monthly frequencies for each group displayed that WT1 presents the highest frequencies in December in agreement with the maximum in the meridional circulation index (intense South Pacific high). Situations with SE flow over central Argentina (WT3) reach their maximum during spring and fall, this is coincident with the monthly distribution of the SE windstorm (sudestada) in the Rio de la Plata estuary. The persistence of each group was also analyzed. The less persistent groups were WT4 and WT6 which represent opposite situations from the dynamical point of view (positive and negative anomalies at the east of the continent respectively). While WT2 and WT7 were the most persistent groups. Daily precipitation station data for the Pampas region was used in order to link the WT with the regional climate. It was found that widespread rainy days (more than 10 mm in all stations) were related to WT3, WT6 and WT7. On the other hand, days with no precipitation over the whole area tended to be associated with WT4.