Responses of Subantarctic mode water spreading to Southern Annular Mode in Southern Indian Ocean

Abstract

The Subantarctic Mode Water (SAMW) is a voluminous water mass with relatively uniform physical properties in the Southern Ocean and plays an important role in ocean heat uptake, transport, and storage. Based on Argo observations, this study investigates how the spreading of Southeast Indian Subantarctic Mode Water (SEISAMW) responds to the Southern Annular Mode (SAM). The results show that the meridional shift of the outcrop line, which controls the variations of SEISAMW spreading, is closely linked to the SAM index. This shift is dominated by the SAM-related Ekman transport and surface flux. During positive phase of SAM, in response to the weakening of westerlies in the central of Southern Indian Ocean, where lighter (26.6–26.7 σ_θ) SEISAMW forms, the latent heat loss is reduced and Ekman heat transport exhibits positive anomalies, leading to the increase of sea surface temperature (SST). As a result, the outcrop line shifts poleward, which favors less lighter SEISAMW spreading into subtropical gyre. Meanwhile, the southwest wind anomaly south of Australia, where denser (26.7–26.9 σ_θ) SEISAMW forms, brings the cold dry air from higher latitudes and leads to the enhanced evaporation, combined with reduced precipitation, resulting in the increased sea surface salinity (SSS). Consequently, the outcrop line shifts equatorward, which favors more denser SEISAMW spreading into subtropical gyre. The opposite occurs during negative phase of SAM.