Generation of the Internal Pycnocline in the Subpolar Southern Ocean by Wintertime Sea Ice Melting

• Published in: Journal of geophysical research. Oceans 2023-03, Vol.128 (3), p.n/a
• Main Authors: Klocker, A., Naveira Garabato, A. C., Roquet, F., Lavergne, C., Rintoul, S. R.
• Format: Article
• Language: English
• Subjects: Antarctic Circumpolar Current; Antarctic front; Continental slope; dynamics; Ecology; Ekologi; Fronts; Geophysics; Ice cover; Ice edge; Ice melting; ice-ocean interaction; Latitude; Melting; Mixed layer; Ocean circulation; Ocean currents; Ocean models; Ocean, Atmosphere; Oceanography; Oceans; overturning circulation; Pycnocline; Pycnoclines; Salinity; Salinity effects; Satellite observation; Sciences of the Universe; Sea ice; Southern Hemisphere; Southern Ocean; stability; Stratification; Temperature; thermocline; transport; Upper ocean; ventilation; Vertical profiles; water; Water circulation; Winter
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2022JC019113

The ocean's internal pycnocline is a layer of elevated stratification that separates the well‐ventilated upper ocean from the more slowly renewed deep ocean. Despite its pivotal role in organizing ocean circulation, the processes governing the formation of the internal pycnocline remain little understood. Classical theories on pycnocline formation have been couched in terms of temperature and it is not clear how the theory applies in the high‐latitude Southern Ocean, where stratification is dominated by salinity. Here we assess the mechanisms generating the internal pycnocline at southern high latitudes through the analysis of a high‐resolution, realistic, global sea ice–ocean model. We show evidence suggesting that the internal pycnocline's formation is associated with sea ice‐ocean interactions in two distinct ice‐covered regions, fringing the Antarctic continental slope and the winter sea‐ice edge. In both areas, winter‐persistent sea‐ice melt creates strong, salinity‐based stratification at the base of the winter mixed layer. The resulting sheets of high stratification subsequently descend into the ocean interior at fronts of the Antarctic Circumpolar Current, and connect seamlessly to the internal pycnocline in areas further north in which pycnocline stratification is determined by temperature. Our findings thus suggest an important role of localized sea ice‐ocean interactions in configuring the vertical structure of the Southern Ocean. Plain Language Summary Satellite observations have revealed significant trends in Antarctic sea‐ice concentration over recent decades. While the science community is starting to unravel the causes of the observed changes in sea‐ice extent, our understanding of how these ice changes are influencing ocean circulation remains rudimentary. Here we take a step toward addressing this important gap by analyzing relationships between sea ice and ocean density structure in a state‐of‐the‐art, realistic sea ice‐ocean model. We find that localized sea ice‐ocean interactions in the Southern Ocean, in particular the counter‐intuitive melting of sea ice in winter, contribute to shape the vertical structure of the Southern Hemisphere oceans. Key Points The internal pycnocline in the high‐latitude Southern Ocean is generated by winter‐persistent sea ice melting Sea‐ice melt persists in winter due to ice drift and warm‐water entrainment, thus maintaining salinity‐based stratification at the base of the winter mixed layer The subpolar i