Evolution of the Seasonal Surface Mixed Layer of the Ross Sea, Antarctica, Observed With Autonomous Profiling Floats

Oceanographic conditions on the continental shelf of the Ross Sea, Antarctica, affect sea ice production, Antarctic Bottom Water formation, mass loss from the Ross Ice Shelf, and ecosystems. Since ship access to the Ross Sea is restricted by sea ice in winter, most upper ocean measurements have been acquired in summer. We report the first multiyear time series of temperature and salinity throughout the water column, obtained with autonomous profiling floats. Seven Apex floats were deployed in 2013 on the midcontinental shelf, and six Air-Launched Autonomous Micro Observer floats were deployed in late 2016, mostly near the ice shelf front. Between profiles, most floats were parked on the seabed to minimize lateral motion. Surface mixed layer temperatures, salinities, and depths, in winter were -1.8 degrees C, 34.34, and 250-500 m, respectively. Freshwater from sea ice melt in early December formed a shallow (20 m) surface mixed layer, which deepened to 50-80 m and usually warmed to above -0.5 degrees C by late January. Upper-ocean freshening continued throughout the summer, especially in the eastern Ross Sea and along the ice shelf front. This freshening requires substantial lateral advection that is dominated by inflow from melting of sea ice and ice shelves in the Amundsen Sea and by inputs from the Ross Ice Shelf. Changes in upper-ocean freshwater and heat content along the ice shelf front in summer affect cross-ice front advection, ice shelf melting, and calving processes that determine the rate of mass loss from the grounded Antarctic Ice Sheet in this sector. Plain Language Summary Measurements of temperature and salinity in coastal Antarctic waters are generally restricted to summer when sea ice disappears so that ships can operate there. Moored sensors can collect data through winter but are usually deployed below 200-300 m to minimize risk of damage from drifting icebergs. We describe a novel approach, using autonomous profilers deployed by ship and aircraft, to collect data from the seabed to the ocean surface throughout the year. We deployed 13 profilers in the Ross Sea, Antarctica. Most profilers were programmed to sit on the seabed between profiles to minimize drift and to continue profiling even when ice cover prevented their ascent to the surface. As expected, annual changes in upper-ocean temperature and salinity were closely related to seasonal changes in sea ice. However, the upper ocean continued to freshen even after the sea ice had all melted, suggesting that substantial amounts of freshwater must be coming from ice melting in the adjacent Amundsen Sea. Increasing our understanding of sources of freshwater in the Ross Sea will improve our predictions of this region's changing role in Antarctic sea ice formation, ice loss from the Antarctic Ice Sheet, and Southern Ocean ecosystems.