Combined Effects of the Tropical and Extratropical Modes of Variability on Precipitation in Southeastern South America
Authors: Barreiro Marcelo 
Published: December 2024

Precipitation over Southeastern South America (SESA), the region located to the east of the Andes between 20°S and 40°S, shows large year-to-year variations, which are the result of the collective impact of several modes of variability and regional processes. Climate modes refer to recurrent spatial structures with defined seasonality and time scales and can have an atmospheric or oceanic origin, as well as may result from the interaction between these two media. The El Niño–Southern Oscillation (ENSO) is a prime example as well as the main driver of rainfall variability over SESA through the establishment of atmospheric teleconnections that modify the regional circulation, and its impact depends on the season and the spatial structure and intensity of associated sea surface temperature anomalies. Moreover, other climate modes of variability such as the Indian Ocean Dipole (IOD) or the Southern Annular Mode (SAM) can influence SESA rainfall and interact with ENSO constructively or destructively resulting in the observed interannual variability. El Niño induces maximum positive rainfall anomalies in spring through the establishment of stationary Rossby wave trains that propagate in an arch-like fashion from the Pacific toward South America. La Niña decreases SESA rainfall, and feedback between the tropically forced wave and the transient eddies is instrumental to generate the time mean response. ENSO interacts with the IOD and SAM during spring. For example, a combination of El Niño and a positive IOD induces stronger rainfall anomalies over SESA, while the presence of a positive SAM phase during La Niña strengthens the upper level and low-level circulation anomalies, intensifying the negative rainfall response. In high summer, the tropical–extratropical teleconnection from the Pacific weakens because the atmospheric basic state is not favorable to propagation, as the upper level jet shifts toward higher latitudes, and thus, the strength of ENSO becomes very important. In addition, other tropical oceans such as the Atlantic can modulate SESA rainfall. After the peak in early summer, ENSO tends to decay in fall, but the ENSO signal over SESA strengthens. At the same time, fall is the season when SAM has the strongest impact over SESA and is such that a negative phase induces positive rainfall anomalies due to higher frontogenetic activity and a northward shift of the cyclone track. Moreover, the occurrence of SAM in the absence of ENSO has a much stronger impact on rainfall over SESA than when SAM occurs under any ENSO phase, suggesting that rainfall predictability in this season is relatively small given that SAM has poor predictability on seasonal time scales. On longer time scales, the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation can also modulate the ENSO impact. Climate change is altering the background conditions under which the modes of variability develop and the related atmospheric teleconnections are established, and therefore, it is likely that the influence of the different modes on SESA rainfall will change in the future, making seasonal climate prediction more challenging.