Variability of the Subantarctic Mode Water Volume in the South Indian Ocean During 2004–2018.

An analysis of Argo data reveals that the subantarctic mode water (SAMW) in the South Indian Ocean (SIO), characterized by a vertical potential vorticity (PV) minimum, decreases by 10% in volume from 2004 to 2018. Most of this decrease occurs at the 26.8–26.9 kg m−3 density range which forms southwest of Australia, while a slight volume increase occurs at 26.6–26.8 kg m−3. Further analysis indicates that the weakening of the Mascarene High and westerly winds in the SIO reduces the evaporation‐precipitation, surface heat flux, and Ekman pumping and shoals the mixed layer southwest of Australia, which leads to a volume decrease at 26.8–26.9 kg m−3 in approximately 3 years. West of 90°E, the parameters exhibit the opposite change, leading to a volume increase at 26.6–26.8 kg m−3. This result suggests that surface winds play an important role in the variability of the SIO SAMW volume during the Argo period. Plain Language Summary: During austral winter in the subantarctic zone, a water mass called the subantarctic mode water with homogeneous properties (temperature, salinity) forms, sinks, and moves to the interior of the ocean. The formation and transport of the mode water are crucial for the climate, as the water takes up heat, freshwater, carbon, oxygen, and nutrients into the ocean interior. We find a distinct volume loss of the mode water in the South Indian Ocean from 2004 to 2018. Although this mode water forms in broad density ranges in different regions, the volume loss only occurs in the dense mode water south of Australia. Moreover, the volume of the light mode water, which forms in the center of the South Indian Ocean, slightly increases. We find that surface winds control the surface heat flux and water convergence, which determine the volume change here. These winds tend to vary oppositely in the two different mode water formation regions and make the volume vary in opposition phases. This study reveals the layer dependence of the mode water volume variation and reinforces the importance of atmospheric forcing, which would possibly help interpret the heat uptake in the Southern Ocean. Key Points: Argo data reveal a subantarctic mode water volume loss in the South Indian Ocean over the period 2004–2018Most of the volume loss occurs in the density range of 26.8–26.9 kg m−3, while a volume increase occurs at 26.6–26.8 kg m−3Changes in the SAMW volume are controlled by surface forcing, which is closely related to the Mascarene High variation [ABSTRACT FROM AUTHOR]