The Combined and Isolated Impacts of El Niño and Positive Indian Ocean Dipole Events on South American Precipitation During Austral Winter and Spring Depending on the Atlantic Multidecadal Oscillation Phases

Abstract

The isolated/combined impacts of the positive Indian Ocean Dipole (pIOD) and El Niño (EN) events on precipitation in South America (SA) were investigated during austral winter and spring for the 1901–2012 period, considering both Atlantic Multidecadal Oscillation (AMO) phases. Under the warm phase (WAMO), EN events are well characterised in winter and, in spring, are accompanied by anomalous warming of the Tropical North Atlantic (TNA); thus, variations in the Walker circulation and the northward shifted Intertropical Convergence Zone (ITCZ) reduce precipitation over northern SA. In the cold AMO phase (CAMO), EN events are weak in winter. At the same time, an intense cooling in the equatorial North Atlantic, favoured by the CAMO, enhances moisture transport from the Amazon to central and southeastern Brazil, increasing precipitation in the South Atlantic Convergence Zone region. As EN develops in spring, the anticyclone off the east coast of SA associated with the Pacific-South American (PSA) pattern decreases (increases) precipitation in central (southeastern) SA. The pIOD events predominantly occur during WAMO phase, when warming in the TNA is favoured by AMO. In winter, the moisture transport to northern SA is weakened, and the ITCZ remains northward shifted, inhibiting the precipitation over northeastern Brazil and southeastern Amazon. In spring, pIOD intensified ascending motions in equatorial Atlantic, increasing precipitation over northeastern Brazil. A wave train from the Indian Ocean strengthens the South Atlantic subtropical high, suppressing precipitation in central and eastern SA. EN-pIOD events are well established in both AMO phases, though the sea surface temperature anomalies in the TNA depend on the AMO. During WAMO, reduced precipitation in western Amazon and northeastern SA is influenced by the Walker circulation, while in CAMO, TNA cooling enhances moisture transport from the Amazon into southeastern SA, where the PSA pattern and wave train from the Indian Ocean increase precipitation.