Role of Extreme Indian Ocean Dipole in Regulating Three‐Dimensional Freshwater Content in the Southeast Indian Ocean

The tropical Southeast Indian Ocean (SEIO) is a key area linking the global freshwater and heat exchanges. The Indian Ocean Dipole (IOD) fundamentally modulates the Indian Ocean circulation and thus regulates the basin‐wide freshwater balance. However, our knowledge of this effect remains limited. Using observational‐based data sets, this study suggests that extreme positive IOD events have notable signatures on the three‐dimensional freshwater content of the SEIO, leading to the vertically opposite salinity anomalies in the surface and subsurface layers. The wind changes led to the northwestward extension of the South Equatorial Current and intensified Sumatra‐Java upwelling. The changing horizontal and vertical currents jointly result in the complicated salinity anomalies. The Equatorial Undercurrent serves as the conduit for water exchange between the equator and the SEIO. This work highlights a strong coupling between the equatorial circulation and the three‐dimensional freshwater inventory of the SEIO within the framework of the IOD. Plain Language Summary The tropical Southeast Indian Ocean (SEIO) connects the three oceans and contributes to the global mass and heat exchanges. The changing freshwater storage and heat content in the SEIO impact regional and global climates. Positive Indian Ocean Dipole (pIOD) is a pattern of internal variability with anomalously low sea surface temperature off Sumatra and high sea surface temperature in the western Indian Ocean, with accompanying wind and precipitation anomalies. The IOD‐related wind anomalies drive the ocean circulation and thus regulate the freshwater content. However, the related process is not clear. This study suggests a framework of IOD regulating the three‐dimensional freshwater content in the SEIO. During the extreme pIOD phase, the abnormal horizontal and vertical currents jointly modulate mass and material distributions in the SEIO, leading to the salting anomalies in the surface layer but the freshening anomalies in the subsurface layer. This study also provides evidence for water exchange between the subsurface layer of the Indian Ocean equator and the surface layer of the SEIO. These findings are a crucial step toward fully understanding the three‐dimensional freshwater content in this critical area. Key Points Extreme positive Indian Ocean Dipole has notable signatures on the three‐dimensional freshwater content in the Southeast Indian Ocean (SEIO) The combined effect of horizontal and v