Ice-ocean-atmosphere interactions in the western Weddell Sea: Coastal polynyas, cyclones and bottom water formation
Final Report Abstract
The main goal of the project was an improved understanding of polynja formation processes and the quantification of the bottom water formation and ice production associated with coastal polynyas in the Weddell Sea area. High-resolution (5km) simulations have been performed using the non-hydrostatic atmospheric model COSMO for the winter 2008. Adjustments of COSMO have been made within the project for the Antarctic region, particularly with respect to a new parameterization of the roughness length, soil/ice parameterization and the implementation of a thermodynamic sea ice model. COSMO data is used for detailed studies of polynya dynamics in the Weddell Sea region and as forcing for the sea-ice/ocean model FESOM. The COSMO results show that polynya formation in the Weddell Sea region is dominated by wind forcing. The Luitpold coast is the only area, where katabatic winds control polynya formation. Although coastal polynyas at the east side of the Antarctic Peninsula are often considered to be a result of katabatic winds, this is not confirmed by our high-resolution simulations. In contrast to our simulations, coarse-resolution models such as atmospheric reanalyses allow for katabatic winds to reach the coastline of the Antarctic Peninsula because of their smoothed topography. The tracking algorithm TRACK was implemented for the Antarctic for the use of COSMO, GME and ERA-Interim data, which are band-pass filtered in order to select mesocyclones. Furthermore, additional filtering was applied to eliminate other mesoscale features such as fronts. The FESOM simulations show that the atmospheric heat loss from coastal polynyas is dominated by the turbulent flux of sensible heat. In the surrounding pack ice, the longwave radiation is the biggest component of the surface energy budget, but interannual variability is still dominated by the sensible heat flux. Oceanic heat flux is found to be surprisingly high with typical winter values between 50 and 150 W/m2 for the coastal polynyas, clearly indicating substantial intrusions of warm deep water on the Weddell Sea continental shelves. While the polynyas' reputation of being the "ice factories" of the Southern Ocean was confirmed in a sense that sea ice production per unit area was found to exceed the values in the surrounding ice-covered ocean by a factor of ten, our results also indicate that the total sea ice volume production on a basin scale (here: for the whole Weddell Sea) still originates from the offshore seasonally ice-covered ocean. Even in the pack ice in winter, typically 1- 3% of the area remain ice-free due to the non-uniform sea ice drift; on a scale of the southwestern Weddell Sea these leads add up to a "fractal polynya" that exceeds the size of coastal polynyas by a factor of five. Leads within the moving pack ice, however, are highly transient features so that the signature of brine release is smoothed very rapidly. Only the spatial persistence of coastal polynyas allows for a salt enrichment intense enough to exceed the threshold salinity for the formation of High Salinity Shelf Water, one of the necessary ingredients to the Antarctic Bottom Water that covers the biggest part of the World Ocean abyss.
Publications
- 2011: Impact of Laptev Sea flaw polynyas on the atmospheric boundary layer and ice production using idealized mesoscale simulations. Polar Research 30, 7210
Ebner, L., D. Schröder and G. Heinemann
(See online at https://doi.org/10.3402/polar.v30i0.7210) - 2012: Dense shelf water formation at coastal polynyas in the Weddell Sea. SCAR Open Science Conference, Portland, Oregon, USA, 17-19 July 2012
Haid, V., Timmermann, R., Heinemann, G. and Ebner, L.
- 2013: A Workshop on Polar Lows. Bull. Am. Meteorol. Soc.
Heinemann, G., Saetra, Ø.
(See online at https://dx.doi.org/10.1175/BAMS-D-12-00190.1) - 2013: Simulated heat flux and sea ice production at coastal polynyas in the southwestern Weddell Sea. J. Geophys. Res. Oceans 118
Haid, V., and R. Timmermann
(See online at https://doi.org/10.1002/jgrc.20133)
