Impacts of the IOD and its combined action with the ENSO on the springtime climate in South America

ANDRIAN, LUCIANO G.; MARISOL OSMAN; VERA, CAROLINA S.

Conferencia; WCRP Open Science Conference; 2023

The impacts of the El Niño-Southern Oscillation (ENSO) on the springtime climate in South America (SA) and their predictability  are well known to the scientific community (Cai et al 2020, Osman and Vera 2016). On the other hand, the Indian Ocean Dipole (IOD) can also influence SA and is associated with positive precipitation anomalies in the La Plata Basin and negative ones in central Brazil during its positive phase (Chan et al. 2008). Although IOD influence on SA climate has not received that much attention yet. Moreover, in the extratropical region of SA, both ENSO and IDO exert their influence through very similar Rossby wave trains emanating from the tropical Pacific and Indian Ocean, respectively. Although ENSO and IOD often occur simultaneously, the changes in the signal and the predictability resulting from the combination of ENSO and IOD is not known yet. In this work, we assess the individual impacts on precipitation of the IOD and how it can modify the signal and predictability of ENSO in SA using reanalysis, observational data sets, and the CFSv2 model.The results indicate that in the observational record only positive IOD has a significant impact on the large-scale circulation in the Southern Hemisphere, capable of modulating the precipitation in SA on seasonal timescales. Positive IOD events lead to positive precipitation anomalies in southeastern SA (SESA) and negative ones in central-eastern Brazil. On the other hand, negative IOD events do not show a coherent response as the positive ones, and the precipitation signal over SA during these events is non significant. The results obtained with the CFSv2 model suggest that these differences between the two IOD phases may be due to the fact that observed positive IOD events are associated with a stronger SST anomaly dipole than the negative ones. The latter is essential to determine the amplitude threshold of the IOD events that can affect the climate of SA. We also conclude that a positive IOD can strengthen the El Niño-induced regional anomalous circulation and thus intensify El Niño impacts in SA. This leads to more localized and intense positive precipitation anomalies over SESA and central Chile, and more widespread and intense negative anomalies over central-northern SA. On the other hand, negative IOD alone does not have a significant impact on SA, but in combination with La Niña events it can modulate the position of anomalous circulation centers in the vicinity of the continent and modify the typical La Niña impacts. The response of ENSO-IOD events then varies depending on their phase, suggesting non-linear impacts.