IOD-driven seesaw mode of upper-ocean salinity variability in the central and eastern tropical Indian Ocean

Abstract Understanding the mechanisms behind interannual salinity variability in the tropical Indian Ocean (TIO) is crucial for interpreting its role in regional and global climates. While localized salinity anomalies—particularly in the central and eastern TIO (CTIO and ETIO)—are well-documented, their basin-wide connection under the influence of the Indian Ocean Dipole (IOD) remains unclear. This study advances the salt diagnostics based on leading mode and associated salt budget to robustly reveal the IOD-driven mechanisms on salinity’s basin-wide connections using observation and simulation. Results indicate that a coherent seesaw mode connects CTIO and Western Bay of Bengal (BoB) salinity anomalies during the IOD mature phase (September–November), with its principal component and associated localized salinity anomaly strongly correlated with the Dipole Mode Index. Composite analysis and wave diagnostics reveal two key mechanisms. First, anomalous cyclonic/anticyclonic circulations co-develop in CTIO and BoB during IOD events, driving salt advection aligned with mean salinity gradients and shaping the leading salinity pattern. Second, IOD-induced oceanic waves—both remote and local—jointly generate current anomalies that enhance salinity tendencies. In particular, equatorial easterlies during positive IOD events force upwelling Kelvin waves that propagate into the BoB, reinforcing anticyclonic salt advection and producing high salinity anomalies. Concurrently, local wind over the CTIO drives equatorial upwelling Kelvin and downwelling Rossby waves to constructively strengthen westward equatorial current and southern anticlockwise circulation, leading to anomalous freshening. These findings improve our understanding of IOD-induced hydrological variability and support better predictability of coupled ocean-atmosphere dynamics in the TIO.