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

Abstract

Both a quasi‐biennial variability and an overall linearly increasing trend are identified in the Sub‐Antarctic Mode Water (SAMW) subduction rate across the Southern Hemisphere ocean, using the Argo data during 2005–2019. The quasi‐biennial variability is mainly due to variability of the mixed layer depth. Variability of wind stress curl in the SAMW formation regions associated with the Southern Annular Mode plays a critical role in generating the quasi‐biennial variability of the mixed layer depth and consequently the SAMW subduction rates. The SAMW subduction rate across the Southern Hemisphere ocean, long‐term mean totaling 56 Sv, has increased at 0.73 ± 0.65 Sv year−1 over the past 15 years. The increase has directly contributed to the observed increase in the total SAMW volume. Much of this increasing trend can be explained by the deepening mixed layers, which in turn are primarily forced by the strengthening westerly winds under an increasing Southern Annular Mode.

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).…”

“…The SAM is believed as the most dominant mode that has widely impact on the Southern Ocean variability. The enhanced SAM in positive phase could induce increase of both the wind stress curl in the SAMW subduction regions and northward Ekman transport across the subantarctic front, resulting in increase of SAMW volume and thickness, which in turn favor the increase in heat storage in the Southern Hemisphere oceans during the Argo period (Gao et al., 2018; Qu et al., 2020). However, the SEISAMW is found to lose volume from 2004 to 2018, suggesting different mechanism of SEISAMW variability in response to SAM enhancement (Hong et al., 2020).…”

“…The Southern Annular Mode (SAM) is believed as the most dominant mode that has widely impact on the Southern Ocean variability, and has shown a significant increasing trend over the past decades (Marshall, 2003; Thompson & Solomon, 2002). This enhanced SAM in its positive phase, which is associated with an increase of both the wind stress curl in the SAMW subduction regions and the northward Ekman transport across the SAF, and has contributed to the increase of SAMW volume and thickness during the Argo period (Gao et al., 2018; Hall & Visbeck, 2002; Qu et al., 2020; Saenko et al., 2005). Meanwhile, the heat storage is expected to increase in accordance with the accelerated formation of SAMW in the southern hemisphere oceans (Gao et al., 2018; Kolodziejczyk et al., 2019; Rintoul, 2018).…”

Evolution of the Oceanic 13C Suess Effect in the Southeastern Indian Ocean Between 1994 and 2018

“…Conversely, the subsequent increase in oceanic carbon sequestration since the mid‐2000s has been tied to a proposed slowdown in upper ocean overturning rates within the Southern Ocean (DeVries et al., 2017). However, recent analysis of Southern Hemisphere ARGO float data spanning 2005–2017 demonstrate that Ekman pumping within the region of SAMW formation has actually increased since the mid‐2000s (Gao et al., 2018; Qu et al., 2020), leading to an increase in SAMW production. It therefore appears that the large increase in the magnitude of the oceanic Suess effect between 2008 and 2018 identified here cannot be accounted for by a slowdown in Ekman divergence, as suggested previously.…”

Recent Shift in the Warming of the Southern Oceans Modulated by Decadal Climate Variability

“…One important driver of the Southern Ocean warming is the strengthening and poleward shift of the Southern Hemisphere westerly winds (Swart & Fyfe, 2012) associated with a positive trend in the Southern Annular Mode (SAM; Marshall, 2003). Over the relatively short Argo period, a key process in the Southern Ocean warming is the wind‐driven thickening of the Subantarctic Mode Water (SAMW) (Gao et al., 2018) due to the increased subduction rate (Qu et al., 2020). Changing winds also lead to the spin‐up of the Southern Hemisphere subtropical ocean gyres (Qu et al., 2019; Roemmich et al., 2016).…”