Source: AGU Advances
Every year as the austral winter sets in, frigid Antarctic air blasts the Southern Ocean. The chill dissipates the warmth of the ocean’s surface water, and cold, dense layers form in the sea’s upper reaches. Known as Subantarctic Mode Water (SAMW), the cold water body amasses north of the Antarctic Circumpolar Current before sliding north into the ocean interior.
SAMW is an essential component of the global climate system: It is critical to our understanding of climate change. As the ocean’s global conveyor belt assimilates the wintry water mass, the well-mixed water is transported into the Pacific and Indian Oceans, bringing nutrients, oxygen, and carbon—lots and lots of carbon. The Southern Ocean takes up 50% of the carbon absorbed by the world’s oceans, and models and observations show that SAMW has accumulated 20% of the ocean’s anthropogenic carbon inventory and more than half of its anthropogenic heat uptake.
In a new study in AGU Advances, Bushinsky and Cerovečki characterize SAMW using novel data collected by the Biogeochemical Argo (BGC-Argo) float array. By characterizing the water mass’s role in nutrient export and carbon uptake, scientists can better interpret ocean measurements and models.
The array’s floating robots measure oxygen, nitrate, and pH up to 2,000 meters below the sea surface. Over 7 years, the researchers observed SAMW across its entire wintertime formation area.
The authors found that SAMW that forms in the Pacific Ocean sector is colder, fresher, and higher in oxygen, nitrate, and dissolved inorganic carbon than its Indian Ocean sector counterpart. The Pacific SAMW also displayed a seesaw pattern of variability across the years, which linked to the Southern Annual Mode and the El Niño–Southern Oscillation—the dominant climate modes of variability in this region.
In both Pacific and Indian SAMW, the biogeochemical properties depend on the density of the newly formed water. Additionally, when SAMW initially forms, it is undersaturated in oxygen. The authors believe this is related to cooling and entrainment of deep water depleted in oxygen opposing the injection of oxygen from the atmosphere.
Last, the results showed that the partial pressure of carbon dioxide was near or above atmospheric levels during SAMW formation. This revelation suggests that SAMW formation does not directly drive ocean uptake of present-day carbon dioxide; however, it may still be an overall sink of anthropogenic carbon.
SAMW plays a critical role in global climate and carbon systems. The authors note that this improved understanding of its formation and fundamental properties will have ramifications for subsurface ocean models, interpretations of ocean measurements, and, ultimately, global climate and biogeochemical models. (AGU Advances, https://doi.org/10.1029/2022AV000722, 2023)
—Aaron Sidder, Science Writer