Indian Ocean Subtropical Underwater and the Interannual Variability in Its Annual Subduction Rate Associated with the Southern Annular Mode

In this study, the Indian Ocean Subtropical Underwater (IOSTUW) was investigated as a subsurface salinity maximum using Argo floats (2000–20) for the first time. It has mean salinity, potential temperature, and potential density values of 35.54 ± 0.29 psu, 17.91° ± 1.66°C, and 25.56 ± 0.35 kg m−3, respectively, and mainly extends between 10° and 30°S along the isopycnal surface in the subtropical south Indian Ocean. The annual subduction rate of the IOSTUW during the period of 2004–19 was investigated based on a gridded Argo dataset. The results revealed a mean value of 4.39 Sv (1 Sv ≡ 106 m3 s−1) with an interannual variability that is closely related to the southern annular mode (SAM). The variation in the annual subduction rate of the IOSTUW is dominated by the lateral induction term, which largely depends on the winter mixed layer depth (MLD) in the sea surface salinity (SSS) maximum region. The anomalies of winter MLD are primarily determined by SAM-related air–sea heat flux and zonal wind anomalies through modulation of the buoyancy. As a result, the annual subduction rate of the IOSTUW generally increased when the SAM index showed negative anomalies and decreased when the SAM index showed positive anomalies. Exceptional cases occurred when the wind anomaly within the SSS maximum region was weak or was dominated by its meridional component. Significance Statement The Indian Ocean Subtropical Underwater (IOSTUW) is characterized as a subsurface salinity maximum in the subtropical south Indian Ocean, and it has potential contribution to regional sea level change and interoceanic salinity exchange. The purpose of this work is to give a more precise description of the IOSTUW and to better understand the formation process of this water mass. Our results shine new light on hydrological characteristics of the IOSTUW based on an observational dataset collected during the past two decades. In addition, the formation of the IOSTUW was found to be closely related to the leading mode of variability in the atmospheric circulation of the Southern Hemisphere though air–sea heat flux and surface wind anomaly.