Abstract
The present study focuses on the leading interannual mode of continental-scale atmospheric variability over South America, which is characterized by an equivalent barotropic vortex (referred to as VOSA in the text) centered over the eastern subtropical coast of the continent. The principal aim is to determine whether and in what season VOSA is the downstream extension of the leading Pacific South American mode (PSA1). Another objective is to examine the extent to which VOSA and PSA1 are forced by El Niño Southern Oscillation (ENSO). The research is based on examination of reanalysis data and output of experiments with an atmospheric general circulation model. The emphasis is on the southern spring, summer and fall seasons, during which VOSA modulates the interannual precipitation variability over the continent. A similar relationship is not found during the southern winter. It is found that VOSA is an integral part of PSA1 during spring and fall. In these seasons, PSA1/VOSA is originated primarily by large-scale atmospheric
internal
variability with the forcing by ENSO accounting for 14 and 8% of the total variance, respectively. During the southern summer season, when ENSO peaks, PSA1 is not a dominant mode of atmospheric variability, and VOSA primarily results from continental-scale internal variability.