A Refined Perspective on Indian Ocean Dipole–Driven Surface Salinity Changes and the Salinity Dipole Index

The Indian Ocean Dipole (IOD) is a dominant coupled ocean–atmosphere mode in the tropical Indian Ocean, characterized by opposing sea surface temperature anomalies (SSTAs) between the western and southeastern regions. In addition to its well-established climatic and biological impacts, the IOD strongly modulates sea surface salinity (SSS), a key variable governing ocean circulation and air–sea interactions. However, the nature of IOD-induced salinity changes and the appropriate formulation of a salinity dipole index (SDI) remain debated, particularly across different IOD phases. Previous studies have proposed contrasting definitions of salinity dipole regions: one, based on high-resolution eddy-resolving model simulations, identifies the dipole between the Central Equatorial Indian Ocean (CEIO: 70°E–90°E, 5°S–5°N) and the Sumatra–Java Coast (SJC: 100°E–110°E, 13°S–3°S); while another, derived from satellite-based SSS observations, aligns with the conventional IOD regions. Here, we revisit the IOD–SSS relationship and reassess the SDI using long-term observational data and historical simulations from a Coupled Model Intercomparison Project Phase 6 (CMIP6) model datasets. Our results show that the IOD-induced salinity dipole emerges most clearly during boreal autumn but is not co-located with the conventional IOD regions. Instead, it consistently develops between the CEIO and SJC, particularly during strong positive IOD events, exhibiting low salinity in the CEIO and high salinity along the SJC. This dipole structure is primarily controlled by IOD-driven surface water advection and freshwater flux anomalies. Based on these findings, we propose a refined definition of the salinity dipole regions and SDI, which provides a more robust and consistent representation of salinity variability during IOD events.