Adélie Land Bottom Water

The general circulation and time evolution of wintertime water masses in the Adélie Depression is described from hydrographic observations taken during the Mertz Polynya Experiment in July-September 1999. Sea-ice growth was estimated and found to vary from 4.8±1.7,4.1± 1.5,8.4±1.7 cm day−1 respectively over a three week period. The average growth rate of sea-ice in the Mertz Glacier polynya is 5.8 cm day−1. Near the Mertz Glacier polynya the ocean heat flux is between 15±6 and 43±11 W m−2, with an average of 30 W m−2 and the latent heat budget is between 125±45 to 254±52 W m−2, with an average of 174 W m−2. These estimates agree with ship-based measurements of sea-ice observed during the experiment. It is concluded that during winter the Mertz polynya is primarily a latent heat polynya. The seasonal evolution of dense shelf waters in the Adélie Depression is described from time series of salinity and temperature from moored hydrographic instruments. Velocity measurements in the Adélie Sill define the total production of dense shelf waters from the Adélie Depression and the annual export of shelf water with sufficient density to become Adélie Land Bottom Water (ALBW), i.e., greater than sq = 27.85 kg m−3, is estimated to be 0.3 ± 0.2 Sv. The downslope mixing of cold, dense shelf waters that escapes the Adélie Depression through the Adélie Sill is investigated using a kinematic gravity current model. ADCP currents on the continental slope at 1150m show northward and bottom-intensified flow which increases during the winter months. Further downslope at 3200m, cold and relatively fresh ALBW is detected in the deepest microcat record. The entrainment of ambient slope waters into a downslope gravity current of cold dense shelf water significantly increases the volume of the final water mass by 200-300%. The total production rate of ALBW into the abyssal layer (> 3500 m) during 1998 to 1999 is estimated to be 1.15 ± 0.5 Sv. This represents between 6-23% of the 8-12 Sv of total Antarctic Bottom Water production reported in the literature and is broadly in agreement with the 25% volume fraction of ALBW suggested by Rintoul (1998). The overall production of ALBW from the Adélie Depression is sensitive to small-scale changes to the air/sea/ice interaction within the depression, and large-scale changes to the supply of circumpolar deep waters across the slope front. It is likely that the unique combination of factors present in the Adélie Depression such as sufficient residence volume (depression), sill depth, intense sea-ice production (formation and removal), the blocking effect of the Mertz Glacier Tongue and export pathway through the Adélie Sill make this the largest source of ALBW to the Australian-Antarctic Basin.