Extended Precipitation Forecast in central Chile-SA

The subtropical west coast of South America is particularly suitable to detect intraseasonal variations (Vergara and Fuenzalida, 1993). These variations affect the whole troposphere. Climatic conditions, such as subtropical subsidence, extreme temperatures and precipitations are dependent on such oscillations. Due to the low frequencies they might be used for an extended forecast.

In this paper an index is introduced, calculated by the mean circulation on the anticylonic domain in the Southern Pacific. This 'intraseasonal anticyclonic circulation index' (IACI) follows the intraseasonal variations of this region and can therefore be used as a tool for an extended precipitation forecast. The oscillation of IACI was compared with precipitation anomalies in winter time (April - October) for the last 19 years (1980-1998).

According to the data considered twicw as much of the positive precipitation anomalies occur when the index shows a negative tendency compared to positive anomalies occuring during positive tendency. It is also shown that events of high precipitations are more frequent during periods of negative tendency.

The observed results demonstrate the expected intraseasonal frequency of IACI and a relation to precipitaion anomalies. It proves that IACI can be forecasted with a benfit of 50% against a climatological forecast in the first month. This fact and the diagnostic properties of IACI therefore clearly presents it as a potential tool for the operative extended precipitation forecast.