Disentangling the interference between Indian and Pacific Ocean teleconnections with Southern Africa during austral early summer

This study investigates the interference between Indian and Pacific Ocean teleconnections with Southern Africa (SA) during the austral early summer (November-December) of El Niño-Southern Oscillation (ENSO) events. During 1979-2024, reanalysis and observational datasets suggest that ENSO events drive below-average rainfall conditions over SA.

Partial regression analysis shows that the Indian Ocean teleconnection accounts for 38% of the ENSO-related rainfall variability over SA, while the Pacific teleconnection accounts for 62%, evidencing an interference of distinct teleconnection pathways. We reveal two mechanisms driving these anomalies: a) a Matsuno-Gill-like response over the Indian Ocean, related to the Indian Ocean teleconnection; and b) a South Atlantic zonal wavenumber-4 Rossby wave train, associated with the Pacific teleconnection.

The wave train originates from the La Plata sector, where rainfall anomalies generate vorticity tendency through vortex stretching, forming a Rossby wave source. By implementing a Rossby wave ray tracing algorithm, we show that the South Atlantic Rossby wave undergoes wave splitting, with shorter waves refracted toward SA, and longer waves continuing toward the Southern Ocean. A further analysis using a large ensemble of CMIP5/CMIP6 models suggest that the La Plata Rossby wave source is positively correlated to subsidence over SA. Furthermore, models failing to simulate the ENSO-La Plata teleconnection also do not reproduce the South Atlantic Rossby wave train, reinforcing the role of the ENSO-La Plata rainfall link in promoting subsidence over SA. 

We further corroborate the observational evidence through ad-hoc sensitivity experiments using the SINTEX-F2 general circulation model. Two experiments are performed: a tropical Pacific experiment, in which Sea Surface Temperatures (SSTs) are allowed to freely evolve over the tropical Pacific, and a tropical Indian Ocean–Maritime Continent experiment, in which SSTs are free over the tropical Indian Ocean and Maritime Continent. Outside these regions, SSTs are strongly nudged toward the control climatology. The model results confirm the potential interference of teleconnections originating from the Pacific and Indian Oceans in driving dry anomalies over SA. In particular, Pacific forcing induces a Southern Atlantic Rossby wave train, whereas Indian Ocean forcing produces a Matsuno–Gill-type response, further conforming observational results.