Ventilation of the Southern Ocean Pycnocline

Abstract

Ocean ventilation is the transfer of tracers and young water from the surface down into the ocean interior. The tracers that can be transported to depth include anthropogenic heat and carbon, both of which are critical to understanding future climate trajectories. Ventilation occurs in both high- and midlatitude regions, but it is the southern midlatitudes that are responsible for the largest fraction of anthropogenic heat and carbon uptake; such Southern Ocean ventilation is the focus of this review. Southern Ocean ventilation occurs through a chain of interconnected mechanisms, including the zonally averaged meridional overturning circulation, localized subduction, eddy-driven mixing along isopycnals, and lateral transport by subtropical gyres. To unravel the complex pathways of ventilation and reconcile conflicting results, here we assess the relative contribution of each of thesemechanisms, emphasizing the three-dimensional and temporally varying nature of the ventilation of the Southern Ocean pycnocline. We conclude that Southern Ocean ventilation depends on multiple processes and that simplified frameworks that explain ventilation changes through a single process are insufficient.

Subtropical Contribution to Sub‐Antarctic Mode Waters

“…The subduction of SAMW and AAIW compensates for the upwelling of deeper, southward‐flowing Circumpolar Deep Waters along the sloping density surfaces of the Antarctic Circumpolar Current (ACC) (Marshall & Speer, 2012; Sloyan & Rintoul, 2001). SAMW ventilate the lower pycnocline of the subtropical gyres (Jones et al., 2016; McCartney, 1982; Morrison et al., 2022), playing a major role in a series of important climate‐related processes. SAMW are responsible for a substantial fraction of global uptake and storage of anthropogenic heat (Roemmich et al., 2015) and carbon (Gruber et al., 2019; Sabine et al., 2004) and, in returning nutrients from the Southern Ocean to the northern ocean basins, sustain biological production and carbon export there (Sarmiento et al., 2004).…”

Enhanced Ventilation in Energetic Regions of the Antarctic Circumpolar Current

“…Through both lateral (Abernathey & Marshall, 2013; Roach et al., 2018) and vertical (Adams et al., 2017; Klein & Lapeyre, 2009) motions, mesoscale and submesoscale eddies contribute significantly to ventilation in the ACC. Throughout this work, we refer to “ventilation” as any process or combination of processes that work to transfer surface waters and tracers into the pycnocline, which as described above, can occur on a variety of temporal and spatial scales (Morrison et al., 2022). Additionally, stirring refers to the advection of tracers by an eddying velocity field, while mixing is an irreversible process that removes tracer variance; only the former contributes directly to ventilation although mixing influences the interpretation of ventilation from tracer distributions (Villermaux, 2019).…”

Circumpolar variations in the chaotic nature of Southern Ocean eddy dynamics

“…The ACC acts to connect the other major basins and thereby regulates climate and nutrients. The Southern Ocean region is also a place where mid-and high-latitude ventilation of the oceans occurs, leading to carbon and heat uptake and controlling deep ocean stratification (Morrison et al, 2022;Rousselet et al, 2021). However, the Southern Ocean is also poorly observed (compared with other ocean basins) and its unique properties mean that improved understanding of the dynamics of this region will have important implications (with the aim of predicting future responses to climate change).…”