Buoyancy and Wind Driven Changes in Subantarctic Mode Water During 2004–2019

Abstract

The formation and transport of the Subantarctic Mode Water (SAMW) are part of the upper branch of the global meridional overturning circulation (Hanawa & Talley, 2001;McCartney, 1997). The SAMW accounted for more than half of the heat gain in the upper 2,000 m of the Southern Ocean during (Roemmich et al., 2015 and contributes significantly to the global ocean heat capacity (Morrison et al., 2016;Rathore et al., 2020). It is also the largest contributor to the uptake and storage of anthropogenic carbon dioxide in the Southern Ocean (Raupach et al., 2007;Sallée et al., 2012). This study investigates the changes in the SAMW via observations and the drivers of that change using models.Prior to the early 2000s, there were few hydrographic sections in the Southern Ocean, thereby limiting the observations of SAMW variability. The SAMW shows warming and freshening trends between the 1960s and the 1990s (Bindoff & Church, 1992;Johnson & Orsi, 1997), which has been linked to warming due to the greenhouse effect (Banks et al., 2002;Bindoff et al., 2007;Downes et al., 2010). However, conflicting views exist. Bryden et al. (2003) observe that, in 2002, the salinity in the Indian Ocean recovered to the values of the 1960s. The temperature and salinity of the SAMW are reported to have interannual to interdecadal variability (e.g., Naveira-Garabato et al., 2009). However, Herraiz-Borreguero and Rintoul (2010) caution that based on infrequently repeated sections, researchers may misinterpret the eddy advection of an anomalous SAMW from another region as interannual variability.The Argo program, which launched in the early 2000s, has dramatically increased the frequency and coverage of global observations of ocean temperature and salinity in the top 2,000 m (Gould et al., 2004). Gao

Southeast Indian Subantarctic Mode Water in the CMIP6 Coupled Models

“…In this study, although both Argo observations and CMIP6 models do not have sufficient resolution to identify eddies, they are capable of capturing the characteristics of SEISAMW and are comparable overall. Meanwhile, the long-term trend of SEISAMW simulated by CMIP6 models is similar to that of SAMW simulated by an eddy-resolving ocean model, which also has become warmer, fresher, lighter, and weaker in recent decades (Xu et al, 2021). In addition, there may be ensemble variability within a single CMIP model, because we used one ensemble member "r1i1p1f1" for CMIP6 models.…”

“…It is interesting that the SAMWs also show volume loss in the previous CMIP3 and CMIP5 coupled models under anthropogenic forcing (Downes et al, 2010;Sallée et al, 2013b;Xia et al, 2021). The mechanism is explained primarily by the reduction of buoyancy gain in the SAMW formation region, while the increasing zonal wind stress induced Ekman transport enhancement plays the secondary role (Downes et al, 2009;Sallée et al, 2010aSallée et al, , 2010b2013a;Xia et al, 2021;Xu et al, 2021). In addition, the SEISAMWs show lighter tendency of potential density in most of the analyzed CMIP6 coupled models (Figure 14).…”

“…PV minima (×10 −11 m −1 s −1 ) (a) NESM3 1° × 0.65°3.5 definition may affect the value of lateral induction and thus subduction rate slightly, but would not affect the climatological properties and long-term trend of SEISAMW significantly. In addition, most references adopted the approach of using a density threshold of 0.03 kg m−3 from surface to calculate MLD to investigate the SAMW (de Boyer Montégut et al, 2004;Hong et al, 2020;Sallée et al, 2013a;Xia et al, 2021;Xu et al, 2021).…”

Subantarctic Mode Water Variations in the Three Southern Hemisphere Ocean Basins During 2004–2019

“…Xu et al. (2021) found that the SAMW is weakening during 2004–2019 based on the SAMW core layer (PV minimum layer) variation, which is mainly influenced by surface buoyancy gain due to climate warming. A recent study also indicated that the SAMW seasonal variations are governed by buoyancy flux changes (Li et al., 2021).…”

“…Hong et al (2020) indicated that the Indian Ocean SAMW volume changes are controlled by surface forcing, which is related to the Mascarene High. Some earlier studies considered SAMW in the Indian, Pacific and Atlantic Oceans as a whole when discussing the SAMW variations (Gao et al, 2018;Li et al, 2021; S. R. Rintoul & England, 2002;Xu et al, 2021) and summarized the overall characteristics of the SAMW variations. However, previous study found that the characteristics of SAMW properties are different in each ocean: SAMW is cooler and thicker in the Pacific and eastern Indian Oceans but warmer and thinner in the Atlantic and western Indian Oceans (Herraiz-Borreguero & Rintoul., 2011;McCartney, 1982).…”