Journal Article

Controls on Wintertime Ventilation in Southern Drake Passage

Flexas, M. Mar, Viglione, Giuliana A., Sprintall, Janet, Thompson, Andrew F., Dove, Lilian A., Cason, Taylor R.

2023

Overview

Drake Passage is a key region for transport between the surface and interior ocean, but a mechanistic understanding of this exchange remains immature. Here, we present wintertime, submesoscale‐resolving hydrographic transects spanning the southern boundary of the Antarctic Circumpolar Current and the Polar Front (PF). Despite the strong surface wind and buoyancy forcing, a freshwater lens suppresses surface‐interior exchange south of the PF; ventilation is instead localized to the PF. Multiple lines of the analysis suggest submesoscale processes contribute to ventilation at the PF, including small‐scale, O(10 km), frontal structure in water mass properties below the mixed layer and modulation of a surface eddy diffusivity at sub‐50 km scales. These results show that ventilation is sensitive to both submesoscale properties near fronts and non‐local processes, for example, sea‐ice melt, that set stratification and mixed layer properties. This highlights the need for adaptive observing strategies to constrain Southern Ocean heat and carbon budgets. Plain Language Summary Drake Passage is a region of the Southern Ocean between the southern tip of South America and the Antarctic Peninsula. Due to its relative accessibility as compared to the rest of the polar ocean, it is the most frequently occupied region of the Southern Ocean. Most occupations by ships in Drake Passage acquire measurements at 20–100 km spacing or “mesoscale” resolution. Here, we present data collected by piloted robotic underwater vehicles that sampled across the southern section of Drake Passage with submesoscale, or 1–10 km, resolution in wintertime. These novel observations indicate that while the southernmost region of Drake Passage is strongly stratified in density, the Polar Front (PF), one of the major dynamical features of the Southern Ocean, is more weakly stratified. The reduced stratification at the PF presents a pathway for the localized exchange of water between the surface and interior ocean. In addition, this study finds that the PF is eddy‐suppressing, meaning that the mean flow of the PF can transport oceanic properties away before they can be stirred. These findings have implications for the estimation of carbon fluxes between the atmosphere and the Southern Ocean, a vital part of the climate system. Key Points High‐resolution hydrographic sections across the Drake Passage provide insight into spatial variability in surface‐interior exchange Wintertime observations suggest ventilation is spatially localized to the Polar Front and influenced by submesoscale processes A mixing length estimate shows modulation at submesoscales and mixing suppression in the upper layers of the Polar Front

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Antarctic Circumpolar Current / Antarctic front / Carbon / Carbon budget / Climate / Drake Passage / Eddy diffusion / Eddy diffusivity / eddy suppression / Freshwater / Frontal structure / Ice melting / Inland water environment / Mixed layer / Ocean circulation / Oceans / Polar Front / Polar fronts / Salinity / Stratification / submesoscale / Surface wind / Ventilation / Water mass properties / Water masses