Southeast Indian Subantarctic Mode Water in the CMIP6 Coupled Models

Abstract

This study assesses the capability of 12 models in the Phase 6 of the Coupled Models Intercomparison Project (CMIP6) in simulating the Southeast Indian Subantarctic mode water (SEISAMW) by comparing to Argo observations. The results show that all of the analyzed CMIP6 coupled models can reproduce the SEISAMW and its seasonal cycle, albeit with discrepancies of formation region and properties among the models. The SEISAMW subduction rate shows significant interannual variability, which is primarily induced by lateral induction in these models, in agreement with observations. Furthermore, the longterm trends of the SEISAMW show volume loss in accordance with the descending trend of the subduction rate, which are co‐occurred with the ascending trend of the Southern Annular Mode (SAM) indices in most of the analyzed CMIP6 models, similar to those in the CMIP3 and CMIP5 coupled models. Meanwhile, the potential density of the SEISAMWs show decreasing trends in these volume loss models, which could be explained by the warming (SAM0‐UNICON, CESM2‐WACCM, CAS‐ESM2‐0 and CIESM), the freshening (IPSL‐CM6A‐LR, CAMS‐CSM1‐0, MRI‐ESM2‐0, and FGOALS‐f3‐L) or the both (FIO‐ESM‐2‐0, E3SM‐1‐0, and CanESM5) trends of the SEISAMWs in different CMIP6 coupled models. Decreases in the projected subduction rate and volume of SEISAMW imply a slowdown of Southern Indian Ocean circulation in the future, reducing the heat and carbon transport from atmosphere to ocean interior contributed by SEISAMW.

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

“…Compared to the planetary vorticity, the relative vorticity is usually negligible, and thus PV is given by $$$Q=(f/g){N}^{2}$$$ where f is the Coriolis parameter, as the planetary vorticity; N 2 is the buoyancy frequency squared, which is a measure of vertical stratification, and is calculated as follows: $$${N}^{2}=-\frac{\mathrm{g}}{\rho }\frac{\partial \rho }{\partial z}$$$ where g is the acceleration of gravity, and ρ is the potential density. In this study, the SEISAMW is identified as the water mass of PV value less than 5 × 10 −11 m −1 s −1 , which is consistent with previous studies (Hong et al., 2020; Qiu et al., 2021; Zhang et al., 2021).…”

“…where g is the acceleration of gravity, and ρ is the potential density. In this study, the SEISAMW is identified as the water mass of PV value less than 5 × 10 11 m 1 s 1 , which is consistent with previous studies (Hong et al, 2020;Qiu et al, 2021;Zhang et al, 2021).…”

“…Following de Boyer Montégut et al. (2004), the mixed layer depth (MLD) in this study is calculated by a 0.03 kg m −3 criterion using potential density referenced to the ocean surface (10‐m depth), which is suitable and has been widely used for high latitudes in the Southern Hemisphere (de Boyer Montégut et al., 2004; Hong et al., 2020; Qiu et al., 2021; Sallée et al., 2013).…”

“…Argo floats have continuously been employed in global upper oceans since 2000, and distributed over the global oceans at an average of 3‐degree spacing (Argo, 2023). Argo gridded monthly data sets have been widely used in previous studies of SAMW recently (Hong et al., 2020; Qiu et al., 2021; Qu et al., 2020; Wang et al., 2022; Zhang et al., 2021). In this study, the Argo gridded data of temperature and salinity is provided by the Asia‐Pacific Data‐Research Center (APDRC), with a horizontal resolution of 1° × 1° and 27 (standard) vertical levels from 0 to 2,000 m during 2005–2019.…”