334 / 2024-09-14 12:56:37

The oceanographic processes on the Ross Sea continental shelf based on BGC-Argo analysis: autumn/winter

Ross Sea, mixed layers, salinity, nitrate, remineralization

Research cruises to Antarctic continental shelves are necessary restricted to ice-free periods due to the challenges of working in extreme environments. The deployment of BGC-Argo floats on the Ross Sea continental shelf in 2022-23 has provided unprecedented information about changes in the water column in autumn/winter.  Mixed layers began to deepen immediately upon ice formation, and increases in mixed layer depths were most rapid in the first month after ice formation. Differences in the initiation of ice formation were noted, with ice forming first in the southern locations and were delayed by ca. one month in the northern locations.  Maximum mixed layer depths occurred in the south (up to 500 m). Coincident with the deepening mixed layers, mixed layer chlorophyll and particulate organic carbon (POC) were both diluted and remineralized in the water column, utilizing dissolved oxygen and increasing dissolved inorganic carbon concentrations. Chlorophyll disappeared more rapidly than POC, and at all locations reached low (≤0.1 µg L^-1) concentrations. Short-term events (storms) occurred in winter that opened ice cover, and induced melting by the warmer water underneath and created localized ice formation, both of which altered the mixed layers for short (days to weeks) periods. Using Argo float data, the extent of the spatial variations in mixing layer depths in autumn/winter is determined, and appears to be a function of the frequency and duration of winds, as well as bathymetry. Winter processes are a poorly known portion of the annual pattern of any Antarctic continental shelf, but BGC-Argo data are now providing unique insights into a critical aspect of Antarctic annual cycles.