Spatial hierarchy in Arctic sea ice dynamics

We define a new classification for Arctic sea ice dynamics based on a spatial and temporal scale: floe, multifloe, aggregate, coherent, sub-basin and seasonal. The classification is supported by remote sensing and in situ observations of ice motions at scales of 1–700 km, as found in the existing scientific literature. The first significant change in sea ice behavior appears as an “emergent” property of the sea ice at the transition from the multifloe scale (2–10 km/<1 d) to the aggregate scale (10–75 km/1–3 d). This emergent behavior establishes a statistical mechanical length where sea ice can be considered a plastic continuum. A second important, or coherent scale occurs at 75–300 km and 3–7 d, where the spatial/temporal processes of sea ice dynamics best match the scales of the wind forcing, i.e., winds of this duration and fetch are necessary to fully load the internal stress field. At scales smaller than the coherent scale, the spatial dimension is important because the sea ice motions on the coherent scale provide non-local forcing to the aggregate scale. At dimensions larger than the coherent scale, including the sub-basin and seasonal scales, spatial and temporal averaging occurs, which smooths discontinuities. To understand and model sea ice dynamics at each of these scales requires an understanding of the detail at the next smallest level. Proper understanding and representation of sea ice dynamics at all scales is critical to devising a sound strategy for data assimilation into sea ice models.