Seasonal variations of the upper ocean in the Scotia Sea and West of the Antartic Peninsula as measured from instrumented seals

Abstract

In the Southern Ocean, the Scotia Sea and Bransfield Strait represent two major exits through which water masses driven by the western boundary current system of the Weddell Sea gyre leave the basin and feed the global thermohaline circulation. Furthermore, the South Shetland Islands and the Antarctic Peninsula (AP) are the locations where the relatively warm and strong Antarctic Circumpolar Current (ACC) flows closest to Antarctica, where some of the largest freshwater reservoirs on the planet can be found. However, due to the local hazardous weather conditions and sea-ice coverage that prevail through the fall and winter seasons, a year-round description of the regional hydrography has been traditionally hampered, especially in areas shallower than 1000 m where the standard parking depth of the freely drifting Argo floats prevent them to enter. The year-round hydrography of these shallower areas, with a greater influence in coastal scenarios, is of key interest given the role that the ocean plays as a thermal forcing to glacier retreat in polar regions. In the upper ocean of the study area (0-400 m), five water masses govern the hydrography: Antarctic Surface Water (AASW), Winter Water (WW), Transitional Zonal Water with Bellingshausen Sea influence (TBW), Transitional Zonal Water with Weddell Sea influence (TWW) and modified Circumpolar Deep Water (mCDW). To characterize the seasonal variations these water masses may experience, we use a set of historical data based on observations from instrumented seals. These seals migrate every year, during the warm seasons, from the South Georgia Island and South Orkney Islands towards the south along the west Antarctic Peninsula, performing the reverse route as the colder seasons evolve. Available observations enable the construction of a series of transects of temperature and salinity down to 400 m depth, which cover the year-round variability of the regional hydrography. These transects extend over 2000 km, sampled during for almost two months. Notably, on two occasions, the seals remained over nearly the same area in the Scotia Sea for several months, acting like a ‘living mooring’ and recording the time-varying temperature and salinity properties of local water masses through different seasons. Results in this study show the temperature and salinity variations governing the seasonal water mass transformation of the upper ocean in the Scotia Sea and West Antarctic Peninsula.