Interannual variations and predictability of intraseasonal convective events over South America

Strong convection over the Amazon basin is associated with the arrival of eastward-propagating negative sea level pressure anomalies generated by intraseasonal oscillations over the Indian and Western Pacific Oceans. On interannual timescales, previous studies show that Pacific SST anomalies appear to be related to the onset of the rainy season over Amazon. However, most of the rainfall variance over the region is not explained by ENSO alone. In this study, we explore the hypothesis that ENSO-modified intraseasonal oscillations induce a distinct behavior of large-scale convection over South America. An analysis of intraseasonal convection over tropical South America using NOAA's OLR and NCEP/NCAR Reanalysis data reveals that El Niños are characterized by lower intraseasonal activity during winter (April-September). Furthermore, the first intraseasonal convective event of an El Niño summer (October-Mar) tends to occur in late December or early January, one month later than it occurs during a La Niña summer. Cold SST anomalies in the Eastern Pacific Ocean are linked to a higher number of summertime intraseasonal events when compared to warm ENSO episodes. Reanalysis data are also used to describe the dynamical impact of ENSO in the intraseasonal cycle of convection in South America. Finally, these findings are combined to seasonal diagnostics and used to assess the predictability of extreme convection over South America associated with tropical intraseasonal oscillations.