The Hydrologic Effects of Synchronous El Nino Southern Oscillation and Subtropical Indian Ocean Dipole Events over Southern Africa

Southern Africa precipitation during the December-March rainy season is closely related with variations of the El Nino-Southern Oscillation (ENSO) and the Subtropical Indian Ocean Dipole (SIOD) modes of climate variability.  However, despite the importance of ENSO and SIOD phasing on Southern Africa floods and droughts, research has yet to be conducted on whether the SIOD modifies the relationship between Southern Africa precipitation and ENSO and the subsequent land-surface hydrology of the region.  Here, a large ensemble of atmospheric model simulations and a large-scale hydrologic model are used to test the SIOD modifications of the ENSO and Southern Africa precipitation relationship and the impacts on the regional land-surface hydrology, respectively.

When ENSO and SIOD are in opposing phases (e.g. El Nino and a negative SIOD or La Nina and a positive SIOD), the ENSO and SIOD atmospheric responses are complimentary, and result in strong Southern Africa climate impacts relative to what typically occurs during ENSO alone.  The strong regional precipitation and near-surface air temperature anomalies lead to significant soil moisture and evapotranspiration anomalies in the year following the ENSO event.  By contrast, when ENSO and SIOD are in the same phase (e.g. El Nino and a positive SIOD or La Nina and a negative SIOD), then the ENSO and SIOD atmospheric responses interfere with one another, resulting weaker Southern Africa impacts relative to what typically occurs during ENSO alone.  These weak regional precipitation anomalies do not lead to lasting effects on the land-surface hydrology in the year following the ENSO event.