The Observed Spatiotemporal Variability of Antarctic Winter Water

• Published in: Journal of geophysical research. Oceans Vol. 129; no. 10
• Main Authors: Spira, T., Swart, S., Giddy, I., Plessis, M.
• Format: Journal Article
• Language: English
• Published: 01.10.2024
• Subjects: Antarctic Winter Water; sea ice; Southern Ocean
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2024JC021017

The Southern Ocean (SO) is central to the global overturning circulation. South of the Antarctic Polar Front, Antarctic Winter Water (WW) forms in the wintertime mixed layer (ML) and becomes a subsurface layer following summertime restratification of the ML, overlaying upwelled deep waters. Model simulations show that WW acts as a conduit to seasonally transform upwelled deep waters into intermediate waters. Yet, there remains little observational evidence of the distribution and seasonal characteristics of WW. Using 18 years of in situ observations, we show seasonal climatologies of WW thickness, depth, core temperature, and salinity, revealing a distinct regionality and seasonality of WW. The seasonal cycle of WW characteristics is tied to the annual sea ice evolution, whereas the spatial distribution is impacted by the main topographic features in the SO driving an equatorward flux of WW. Through the identification of these localized northward export regions of WW, this study provides further evidence suggesting an alternative view from the conventional “zonal mean” perspective of the overturning circulation. We show that specific overturning pathways connecting the subpolar ocean to the global ocean can be explained by ocean‐topography interactions. Plain Language Summary The Southern Ocean (SO) around Antarctica is central to the global ocean circulation system. The cold wintertime atmosphere drives ocean cooling and sea ice formation, which causes surface waters to become denser, mixing with deep waters that rise to the surface from the deep ocean. As the ocean surface layer warms in summer, there remains a cold layer below the surface known as Antarctic Winter Water (WW). This layer warms throughout the summer, thinning the WW layer. However, the properties of WW (temperature, salinity, thickness, and depth) vary in space and in erosion rate around the SO. By compiling 18 years of ocean observations, we investigate the physical dynamics that determine how WW changes in space and over the average annual cycle. We find that there are localized regions across the SO where WW properties are transported northward as part of the ocean circulation system, which typically aligns with large topographic features and acts to connect SO water masses to the global ocean. Key Points Seasonal climatologies of Antarctic Winter Water (WW) and its properties are mapped using 18 years of in situ observations Observations reveal the distinct seasonal and regional characteristics of WW and their potential link to processes such as sea ice formation Localized redistribution of WW properties equatorward is steered by large topographic features