Winter Arctic Sea Ice Surface Form Drag During 1999-2021: Satellite Retrieval and Spatiotemporal Variability

The neutral form drag coefficient is an important parameter when estimating the surface turbulent flux over Arctic sea ice. The form drag caused by the surface features (C-dn,C-fr) dominates the total drag in the winter, but long-term basin-scale records of are still lacking for Arctic sea ice. In this study, we first developed an improved surface feature detection algorithm and characterized the surface features (including height and spacing) over Arctic sea ice during the late winter of 2009-2019 using the full-scan laser altimeter data obtained in the Operation IceBridge mission. C-dn,C-fr was then estimated using an existing parameterization scheme. This was followed by applying a satellite-derived backscatter coefficient (sigma(0)(vv) to the C-dn,C-fr regression model to extrapolate, for the first time, C-dn,C-fr to the pan-Arctic scale for the entire winter season (from 1999 to 2021). We found that the surface features have a larger height and smaller spacing over multi-year ice (1.15 +/- 0.21 m and 142 +/- 49 m) than over first-year ice (0.90 +/- 0.16 m and 241 +/- 129 m). The monthly mean C-dn,C-fr increases through the winter, from 0.2 x 10(-3) in November to 0.4-0.5 x 10(-3) in April. The central Arctic has the largest C-dn,C-fr (up to 2 x 10(-3)), but experienced a drop of similar to 50% during the winters of 2001/2002-2008/2009. The interannual fluctuations in C-dn,C-fr are strongly linked to the variability of sea ice thickness and sea ice deformation, of which the latter has become increasingly important for C-dn,C-fr since 2009.