Drag Coefficients for Winter Antarctic Pack Ice

This paper presents air-ice and ice-water drag coefficients referenced to 10-m-height winds for winter Antarctic pack ice based on measurements made from R/V Polarstern during the Winter Weddell Sea Project, 1986 (WWSP-86), and from R/V Akademik Fedorov during the Winter Weddell Gyre Study, 1989 (WWGS-89). The optimal values of the air-ice drag coefficients, made from turbulent flux measurements, are C-10 = (1.79 +/- 0.06) x 10(-3) for WWSP-86 and (1.45 +/- 0.09) x 10(-3) for WWGS-89. Neutral drag coefficient values are C(N10) = 1.68 x 10(-3) for WWSP-86 and 1.44 x 10(-3) for WWGS-89. The slightly lower values for WWGS-89 reflect a smaller surface roughness (z0) attributed to the thicker snow cover present in the 1989 study region (median z0BAR = 0.47 mm for WWSP-86 and 0.27 mm for WWGS-89). These values are consistent with Arctic measurements for 80-100% concentration of sea ice and with those of Andreas et al. (this issue) for the Antarctic. A single (average) ice-water drag coefficient for both WWSP-86 and WWGS-89, estimated from periods of ice drift thought to represent free-drift conditions (air-ice stress balanced by ice-water drag and Coriolis force), is (1.13 +/- 0.26) x 10(-3), and the ice-water turning angle betaBAR = 18 +/- 18-degrees. This drag value is significantly lower than Arctic values for thick multiyear ice, but it is similar to the values obtained by Langleben (1982) for first-year Arctic ice. Consistent with previous findings for WWSP-86, the free-drift form of the momentum balance can be used to describe the observed WWGS-89 ice drift observations by using an ''effective'' drag coefficient and turning angle that subsume the influence of ice-ice interaction. For a typical Antarctic winter pack ice cover, it appears that the ice cover reduces the momentum flux from the atmosphere to the ocean by approximately 33%.