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) × 10 −3 for WWSP‐86 and (1.45 ± 0.09) × 10 −3 for WWGS‐89. Neutral drag coefficient values are C N10 = 1.68 × 10 −3 for WWSP‐86 and 1.44 × 10 −3 for WWGS‐89. The slightly lower values for WWGS‐89 reflect a smaller surface roughness ( z 0 ) attributed to the thicker snow cover present in the 1989 study region (median 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) × 10 −3 , and the ice‐water turning angle . 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 ∼33%.