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

Abstract

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 2004-2018 • Most 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 −3 • Changes in the SAMW volume are controlled by surface forcing, which is closely related to the Mascarene High variation Supporting Information: • Supporting Information S1 Correspondence to: Y. Du,

Southeast Indian Subantarctic Mode Water in the CMIP6 Coupled Models

“…By comparing statistical characteristics of SEISAMW in 12 CMIP6 models to observations, we assess the capability of the CMIP6 models in simulating the SEISAMW. In Argo observations, the SEISAMW shows similar subduction rate and properties to previous studies (Figures 1 and 2; Hong et al., 2020; Qu et al., 2020). The SEISAMW subducts during winter and moves along 26.6–26.9 kg m −3 isopycnals to reach around 650 m (Figure 3).…”

“…According to the PV distribution, the SEISAMW is identified as water masses with PV values less than 5 × 10 −11 m −1 s −1 in Argo, which is in agreement with Hong et al. (2020). The PV minima in the CMIP6 climate models are generally different with each other, attributing to systematic differences among the CMIP6 models and Argo (Table 1).…”

“…In addition, Hong et al. (2020) suggest decrease of the SEISAMW in volume by 10% during the Argo period, which are in contrast to the increase of SAMW formation as revealed by Gao et al. (2018) and Qu et al.…”

Variability of the Sub‐Antarctic Mode Water Subduction Rate During the Argo Period

“…We therefore conclude that across the Southern Hemisphere ocean the trend of increasing wind stress curl associated with the strengthening westerly winds is primarily responsible for the deepening MLDs in the SAMW subduction regions. Consequently, the SAMW subduction rates across the Southern Hemisphere ocean have increased and should continue to increase, despite large differences between the three southern subtropical oceans (e.g., Hong et al, 2020; Lu et al, 2018; Meijers et al, 2019). The enhanced subduction of warmer surface water may directly contribute to the previously reported subsurface heat gain in the middle‐ to high‐latitude Southern Hemisphere ocean.…”

Determining the Distribution of Fluorescent Organic Matter in the Indian Ocean Using in situ Fluorometry

“…Mixed layer depth was defined as the depth where a density increase from the sea surface value exceeds 0.125σ θ . SAMW was considered to have a PV minimum lower than 50 (×10 −12 m −1 s −1 ) within the density range (26.6 < σ θ < 26.9) according to Hong et al (2020). 4 O was extracted from the austral summer WOA18 data according to the method of Tsubouchi et al (2016).…”