Abstract
AbstractThe influence of each phase of the Indian Ocean dipole (IOD) on the large-scale circulation in South America is investigated using rainfall observations, fully coupled, large-ensemble, historical simulations (LENS), and forced experiments using the coupled model’s atmospheric component. IOD events often occur when El Niño–Southern Oscillation (ENSO), the largest source of interannual variability of precipitation in South America, is active. To distinguish from effects of ENSO, only cases during neutral ENSO conditions are analyzed in LENS and observations. During the positive IOD polarity, a perturbation in the local Walker circulation leads to increased convection over equatorial South America, resulting in wet anomalies in the Amazon basin. This signal is the opposite of what is expected during El Niño events. Tropical convection anomalies in the Indian Ocean also force an extratropical Rossby wave train that reaches subtropical South America. During positive IOD, the moisture flux from the Amazon to central and southeastern Brazil weakens, resulting in a drying of the area associated with the South Atlantic convergence zone. Meanwhile, the South Atlantic subtropical high strengthens, contributing to a drying in southeastern Brazil. During negative IOD, the induced wave train from the Indian Ocean leads to increased moisture transport to the La Plata basin, leading to wet anomalies in the region.
Highlights
The Indian Ocean dipole (IOD) consists of a dipole in sea surface temperature (SST) anomalies in the Indian Ocean
Some studies suggest that the IOD develops not as an independent process, but as a consequence of the wind anomalies in the eastern sector of the Indian Ocean generated by El Niño–Southern Oscillation (ENSO) (Stuecker et al 2017; Zhao and Nigam 2015)
In a previous study (Sena and Magnusdottir 2020) we found that LENS captures the evolution of the South American monsoon system including the seasonal distribution of rainfall anomalies, in spite of a mean rainfall bias in the model
Summary
The Indian Ocean dipole (IOD) consists of a dipole in sea surface temperature (SST) anomalies in the Indian Ocean. Warm SST anomalies are observed in the western part of the Indian Ocean, while cool SST anomalies occur in the eastern sector. IOD events often occur when El Niño–Southern Oscillation (ENSO) is active. Some studies suggest that the IOD develops not as an independent process, but as a consequence of the wind anomalies in the eastern sector of the Indian Ocean generated by ENSO (Stuecker et al 2017; Zhao and Nigam 2015). Wang et al (2016) argue that IOD can be generated even in the absence of ENSO as a result of anomalies in Indonesian convection amplified by surface wind response to the precipitation anomalies and subsurface conditions.
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