Enhanced influences of ENSO on the subtropical Indian Ocean dipole since the early 1990s

Investigating the impact of El Niño-Southern Oscillation (ENSO) on subtropical Indian Ocean dipole (SIOD) facilitates understanding their combined effects on African and Australian climate. We found the correlation between ENSO in austral summer and SIOD in following autumn is enhanced after the early 1990s. Before the early 1990s, ENSO only changes the southeastern Indian Ocean (SEIO) temperature by modulating the Walker circulation. In contrast, after the early 1990s, El Niño induces positive sea surface temperature (SST) anomalies in the SEIO and negative SST anomalies in the southwestern Indian Ocean (SWIO), causing the enhanced ENSO-SIOD correlation. This increasing influence of ENSO on the SWIO closely links to the ENSO-related atmospheric teleconnection in the midlatitude Southern Hemisphere (SH). Before the early 1990s, the El Niño-related teleconnection coincides with a low-pressure anomaly to the south of southern Africa. Anomalous westerlies on the northern flank of this low-pressure warm the SWIO SST by increasing the solar radiation and reducing the mixed-layer thickness but cool the SWIO SST by changing the oceanic heat advection. These opposite effects cause a weak ENSO-SWIO connection. After the early 1990s, the low-pressure related to El Niño shifts to the SWIO in January–February–March, causing anomalous local moisture divergence. The increased air–sea humidity difference enhances the latent heat release and then deepens the mixed-layer thickness, cooling the SWIO SST. The interdecadal differences in the ENSO-related SH teleconnection are probably attributed to the westward-shifted ENSO pattern and the reduced response of the Tasman Sea SST to ENSO after the early 1990s.