Recent recovery of Antarctic Bottom Water formation in the Ross Sea driven by climate anomalies

Abstract

Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation, ventilates the abyssal ocean and sequesters heat and carbon on multidecadal to millennial timescales. AABW originates on the Antarctic continental shelf, where strong winter cooling and brine released during sea ice formation produce Dense Shelf Water, which sinks to the deep ocean. The salinity, density and volume of AABW have decreased over the last 50 years, with the most marked changes observed in the Ross Sea. These changes have been attributed to increased melting of the Antarctic Ice Sheet. Here we use in situ observations to document a recovery in the salinity, density and thickness (that is, depth range) of AABW formed in the Ross Sea, with properties in 2018-2019 similar to those observed in the 1990s. The recovery was caused by increased sea ice formation on the continental shelf. Increased sea ice formation was triggered by anomalous wind forcing associated with the unusual combination of positive Southern Annular Mode and extreme El Niño conditions between 2015 and 2018. Our study highlights the sensitivity of AABW formation to remote forcing and shows that climate anomalies can drive episodic increases in local sea ice formation that counter the tendency for increased ice-sheet melt to reduce AABW formation.

Spatial Variability of Antarctic Bottom Water in the Australian Antarctic Basin From 2018–2020 Captured by Deep Argo

“…The easternmost float, located in the southeast corner of the basin at ~150°E, captured the youngest RSBW in the basin that flows in from the east around Cape Adare (Figure 1; float 12008, red). Consistent with Silvano et al, 2020, the February 2018 occupation of the 150°E line showed that the deep waters here had warmed by 0.1°C and increased in salinity by 0.03 since the last occupation in 2011. The float data in 2019 also show the densest water between 46.16 and 46.18 kg/m 3 σ 4 have increased salinity by 0.04 compared to the 2011 hydrographic data, a rebound not seen in waters of similar density at the western end of the basin (Figure 2c; float 12008, red).…”

“…Decadal occupations of hydrographic sections gridding the deep ocean conducted by the Global Ocean Ship‐based Hydrographic Investigations Program (GO‐SHIP; Talley et al, 2016) have shown that the abyssal ocean has warmed significantly throughout the globe (Desbruyères et al, 2016; Fukasawa et al, 2004; Kawano et al, 2006; Kouketsu et al, 2011; Purkey & Johnson, 2010). This deep warming is likely driven by changes in the salinity of the dense shelf waters around Antarctica that produce AABW (e.g., Aoki et al, 2005, 2013; Kobayashi, 2018; Menezes et al, 2017; Purkey & Johnson, 2012, 2013; Rintoul, 2007; Shimada et al, 2012; van Wijk & Rintoul, 2014), which have undergone short‐ and long‐term variability owing to increased glacial runoff and interannual variability in sea ice production and export (Castagno et al, 2019; Jacobs et al, 2002; Jacobs & Giulivi, 2010; Silvano et al, 2020). Furthermore, high resolution models have shown a similar climate signal of abyssal warming, driven by a combination of decreased AABW production from anthropogenic freshening and warming on the shelves near Antarctica and from warmer waters being advected through the bottom limb of the MOC (Boé et al, 2009; Bryan et al, 2014; Newsom et al, 2016).…”

“…Furthermore, high resolution models have shown a similar climate signal of abyssal warming, driven by a combination of decreased AABW production from anthropogenic freshening and warming on the shelves near Antarctica and from warmer waters being advected through the bottom limb of the MOC (Boé et al, 2009; Bryan et al, 2014; Newsom et al, 2016). Most recently, a long‐term freshening trend on the Ross Shelf, one of the primary formation regions of AABW, reversed in 2014 (Castagno et al, 2019; Silvano et al, 2020). Here, we use the new Deep Argo data in 2018–2020 to show the rebound in high salinity shelf water is detectable in the AABW in the eastern portion of the Australian Antarctic Basin (AAB).…”

The role of tides in bottom water export from the western Ross Sea

“…The CALM observations show a slight decrease of salinity of 0.007/year 11 , also consistent with the observations in the Ross Sea. The recent salinity increase has been linked to increase in sea ice production in the Ross Sea 32 .…”

Response of the Pacific Sector of the Southern Ocean to Wind Stress Variability From 1995 to 2017

“…The connections between the interannual variability of the wind stress and large‐scale indices and of regional parameters have been extensively investigated in the literature, defining how the interaction between SAM index and ENSO can influence the general SO circulation (Cerrone, Fusco, Cotroneo, et al, 2017), its sea ice cover (Cerrone, Fusco, Simmonds, et al, 2017; Cerrone & Fusco, 2018), its mixed layer depth (Buongiorno Nardelli et al, 2017), and even the surface heat fluxes at regional scales (Fusco et al, 2018). Recently, Silvano et al (2020) demonstrated how the combined effect of the ENSO and the SAM state can affect the sea ice formation in the Ross Sea, explaining the recent reversal of the salinity trend observed in the Ross Sea (Castagno et al, 2019). The ability of El Niño to mitigate the effect of the strong SAM is already described by Morrow et al (2010), Fogt et al (2011), and Langlais et al (2015).…”