Changes in Spreading of Southeast Indian Subantarctic Mode Water During Argo Period.

Subantarctic Mode Water (SAMW) is one of the most important water masses for the global ocean uptake and storage of heat and carbon. Based on Argo observations, this study focus on the Southeast Indian Subantarctic mode water (SEISAMW), and investigates the changes of SEISAMW spreading and associated mechanisms. SEISAMW is formed through air‐sea interaction as low potential vorticity water in late winter and subducts into permanent thermocline between outcrop lines of 26.6 σθ and 26.9 σθ in Southern Indian Ocean (SIO). After subduction, the SEISAMW spreads northwestward into subtropical gyre and southeastward into Antarctic Circumpolar Current (ACC) separately. During Argo period, the percentage of SEISAMW volume spreading into SIO subtropics decreases at light layer (26.6–26.7 σθ) while increases at medium (26.7–26.8 σθ) and dense (26.8–26.9 σθ) layer. These changes are attributed to the meridional shifts of outcrop lines. The outcrop lines of 26.6 σθ and 26.7 σθ in the central of SIO (75°–90°E), where light SEISAMW forms, shift poleward. The poleward shifts of outcrop lines result in less light SEISAMW spreading into subtropical gyre. In contrast, outcrop lines of 26.7 σθ and 26.8 σθ south of Australia shift equatorward and favor more medium and dense SEISAMWs spreading into subtropical gyre. The shifts of outcrop lines are closely related to the enhancement of Southern Annular Mode. Plain Language Summary: Subantarctic mode water (SAMW), a water mass with vertically homogeneous properties, forms at the base of deep mixed layer during austral winter in the Subantarctic Zone, providing subsurface oceanic reservoirs of carbon and heat that can affect the global climate significantly. After subduction, SAMW that forms in Southern Indian Ocean (called SEISAMW) spreads into subtropical gyre and Antarctic Circumpolar Current (ACC). We find that the spreading of SEISAMW changes in different isopycnal layers during Argo period. The percentage of light SEISAMW volume spreading into subtropical gyre decreases, while the percentage of medium and dense SEISAMW volume spreading into subtropical gyre increases. The meridional shift of mode water formation region controls the change in spreading of SEISAMW here. The mode water formation regions shift oppositely in the central of SIO (where light mode water forms) and south of Australia (where the medium and dense mode water forms), and the opposite shifts of the formation regions result in the opposite changes in spreading of SEISAMW in the two isopycnal layers. This study reveals that the change of SEISAMW spreading is layer‐dependent, which would possibly improve our understanding of the heat redistribution process in the Southern Ocean. Key Points: The spreading of Southeast Indian subantarctic mode water (SEISAMW) changes during Argo periodThe changes in the spreading of light, medium and dense SEISAMWs are differentThe change of SEISAMW spreading is associated with change of formation region due to meridional shift of outcrop line [ABSTRACT FROM AUTHOR]