How does Subantarctic Mode Water ventilate the Southern Hemisphere subtropics?

Abstract

In several regions north of the Antarctic Circumpolar Current (ACC), deep wintertime convection refreshes pools of weakly stratified subsurface water collectively referred to as Subantarctic Mode Water (SAMW). SAMW ventilates the subtropical thermocline on decadal timescales, providing nutrients for low‐latitude productivity and potentially trapping anthropogenic carbon in the deep ocean interior for centuries. In this work, we investigate the spatial structure and timescales of mode water export and associated thermocline ventilation. We use passive tracers in an eddy‐permitting, observationally‐informed Southern Ocean model to identify the pathways followed by mode waters between their formation regions and the areas where they first enter the subtropics. We find that the pathways followed by the mode water tracers are largely set by the mean geostrophic circulation. Export from the Indian and Central Pacific mode water pools is primarily driven by large‐scale gyre circulation, whereas export from the Australian and Atlantic pools is heavily influenced by the ACC. Export from the Eastern Pacific mode water pool is driven by a combination of deep boundary currents and subtropical gyre circulation. More than 50% of each mode water tracer reaches the subtropical thermocline within 50 years, with significant variability between pools. The Eastern Pacific pathway is especially efficient, with roughly 80% entering the subtropical thermocline within 50 years. The time required for 50% of the mode water tracers to leave the Southern Ocean domain varies significantly between mode water pools, from 9 years for the Indian mode water pool to roughly 40 years for the Central Pacific mode water pool.

The Fate of Carbon and Nutrients Exported Out of the Southern Ocean

“…Our time scales are also somewhat shorter because our experiment includes perturbed carbon and nutrient concentrations in the surface transport and not only mode and released passive particles in a Lagrangian trajectory analysis and applied also a stricter definition of the equatorial upwelling region only in the Pacific. The estimated time of re-emergence is also in line with the estimate that 50% of each mode water tracer reaches the subtropical thermocline in 50 years (Jones, Meijers, et al, 2016).…”

“…Furthermore, nutrient concentrations show signals of variability or change in multiple regional studies from surface to depth (Ayers & Strutton, 2013;Hoppema et al, 2015;Iida et al, 2013;Panassa et al, 2018;Pardo et al, 2017). Given that mode and intermediate water masses reach the subtropical thermocline on a time scale of a few decades (Jones, Meijers, et al, 2016;Rodgers et al, 2003), it is very likely that these changes may reappear in the low latitudes on a time scale relevant to human anthropogenic CO 2 emissions and emission reduction endeavors (Palter et al, 2010;Pasquier & Holzer, 2016).…”

Freshwater flux from ice sheet melting and iceberg calving in the Southern Ocean

“…The model setup (called BASSOON) is described in Jones et al . (). When forced with the previously available freshwater flux field, BASSOON developed large, open ocean polynyas associated with deep convection within 4–6 years in both the Weddell Sea and Ross Sea.…”

Evidence of Jet‐Scale Overturning Ocean Circulations in Argo Float Trajectories

“…Much of the SAMW formed will continue along the ACC path. Given the relatively long residence time SAMW is able to mix out of the jet core and move equatorward to ventilate the subtropics (Jones et al, ). These results may help explain why SAMW is found to ventilate the subtropics as a broad water mass even though the source is a local filamented input.…”

“…The Southern Ocean regulates climate by transporting and sequestering greenhouse gases and heat (Frölicher et al, 2015;Sabine et al, 2004;Sallée et al, 2012;Sarmiento et al, 2004). Transport across the base of the mixed layer is the pathway to sequestration (Jones et al, 2016;Sallée et al, 2012;Sarmiento et al, 2004). Many processes have been proposed as possible candidates for the ventilation mechanisms, such as wind-driven Ekman transport (Dong et al, 2007(Dong et al, , 2008Downes et al, 2011;Holte et al, 2012;Rintoul & England, 2002;Sallée et al, 2006), air-sea buoyancy flux (Hogg, 2010;Sallée et al, 2010), and ocean eddy heat transport (Kuo et al, 2005;Phillips & Rintoul, 2000;Schmitz, 1996;Treguier et al, 1997;Weller et al, 2004), turbulent mixing (Belcher et al, 2012;Wenegrat & McPhaden, 2016), and vertical and lateral entrainment (Close & Goosse, 2013;Dong et al, 2007).…”