Abstract: Hydrographic data collected during the early Antarctic winter 2011 by CTD tags fitted to five elephant seals were analyzed to study the evolution of the upper water layer in the Prydz Bay polynya adjacent to the Amery ice shelf. The evolution of upper ocean structure could be divided into three stages. First, temperature shifted from a stratified profile to homogeneity. Warm water was still present in the subsurface in late March. This warm water gradually disappeared with cooling of the surface water, so the temperature profile evolved from temperature inversion to homogeneity at temperatures close to freezing. Second, salinity shifted from a stratified profile to homogeneity. Salinity increased with the freezing of seawater, which strengthened vertical convection and resulted in homogeneous salinity throughout the upper water layer. Third, seawater continued to cool and salinity continued to increase. Stratified profiles of temperature and salinity were reestablished. With increased depth, salinity decreased and temperature increased. Air–sea heat fluxes calculated from temperature and salinity profiles during the first, second, and third stages were -90.93 W m^-2, -82.20 W m^-2, and -43.44 W m^-2, respectively. The increase in salinity was mainly the result of sea ice formation, and the sea ice formation rates were 5.4 cm d^-1, 4.9 cm d^-1, and 2.5 cm d^-1, respectively. In summary, the present study demonstrated that during the early Antarctic winter, as temperatures in the upper water layer continue to decrease and salinity increases, the heat flux released from ocean to atmosphere and the sea ice formation rate gradually decrease.

Key words: Prydz Bay, polynya, elephant seal, CTD tag, evolution of water sturcture, ice formation rate