Weddell Sea Moorings

DESCRIPTION: The world's deep oceans are filled with water masses formed at the continental margins of Antarctica, with the Weddell Sea a major source. Relatively warm, salty water originating in the North Atlantic enters the Weddell Gyre to the east of the Greenwich Meridian as Circumpolar Deep Water (CDW). As it traverses the gyre, the CDW cools and freshens, mixing with Antarctic waters, feeding bottom water-forming processes on the continental shelves, and interacting with floating ice shelves to produce Weddell Deep and Bottom water types. These formation processes include heat exchange with the atmosphere and ice shelves, so the properties of the water masses formed carry an imprint of any recent changes in atmospheric and shelf ice characteristics, including temperature, distribution of shelf and sea ice, and shifts in large scale wind stress patterns such as those associated with the Southern Annular Mode (SAM) and El Nino/Southern Oscillation (ENSO).
The project involves an array of oceanographic moorings south of the South Orkney Islands in the Northwest Weddell Sea to provide a time series of the properties of the combined outflow of Antarctic Deep and Bottom Water drawn from various sites within the Weddell Sea. The goal of this project is to observe the properties of the Weddell deep and bottom waters as they exit the Weddell Gyre system. The time series obtained since 1999 provides insight to deep water formation and long term changes in ocean circulation and their relation to the climate system.
The moorings sites are visited approximately every 2 years, under the auspices of an Agreement of Cooperation between Lamont-Doherty Earth Observatory of Columbia University (LDEO) and the British Antarctic Survey (BAS). The agreement with BAS provides for sharing of equipment, personnel and data between LDEO and BAS, with BAS providing the ship time. By sharing material resources with BAS, we have been able to expand the mooring array to encompass the Orkney Passage to the east of the Orkney Plateau, a site of potential escape of Weddell Deep Water into the Southern Ocean

OUTCOMES: Recent results (September 2019): The export of Weddell Sea Bottom Water (WSBW) from February 1999 to February 2019 reveals a distinct seasonal cycle as well as interannual variability. From 2014 through 2017, while the seasonality remains, an anomalous salinity decrease was recorded. We hypothesize that this is derived from injection of winter surface water, rather than to changes in the shelf water T/S characteristics. The surface water is drawn from the apex of V-shaped feature formed by a double front along the shelf/slope regime of the Weddell Gyre's western boundary current. Water within the apex of the V-shaped feature is a blend of winter surface water, shelf water and offshore warm Weddell Deep Water (WDW). We estimate that when the V-shaped apex attains a depth is about 700m, the thermobaric effect (greater compressibility of cold water) allows for descend of the cold water within V-shaped into the WSBW. We propose that this occurred during anomalous strong cyclonic wind stress curl over the Weddell Gyre from 2014 into 2017, which increased the intensity of the gyre and its western boundary current, deepening the V-shape feature leading to the observed freshening of the WSBW. The persistent polynya in the vicinity of Maud Rise the austral spring of 2016 and 2017 is likely to also be a manifestations of the Weddell Gyre spin-up, which lifts the pycnocline within the gyre interior.