Over the last decade, the Southern Ocean has experienced episodes of severe sea ice area decline. Abrupt events of sea ice loss are challenging to predict, in part due to incomplete understanding of processes occurring at the scale of individual ice floes. Here, we use high-resolution altimetry (ICESat-2) to quantify the seasonal life cycle of floes in the perennial sea ice pack of the Weddell Sea. The evolution of the floe chord distribution (FCD) shows an increase in the proportion of smaller floes between November and February, which coincides with the asymmetric melt–freeze cycle of the pack. The freeboard ice thickness distribution (fITD) suggests mirrored seasonality between the western and southern sections of the Weddell Sea ice cover, with an increasing proportion of thicker floes between October and March in the south and the opposite in the west. Throughout the seasonal cycle, there is a positive correlation between the mean chord length of floes and their average freeboard thickness. Composited floe profiles reveal that smaller floes are more vertically round than larger floes and that the mean roundness of floes increases during the melt season. These results show that regional differences in ice concentration and type at larger scales occur in conjunction with different behaviors at the small scale. We therefore suggest that floe-derived metrics obtained from altimetry could provide useful diagnostics for floe-aware models and improve our understanding of sea ice processes across scales.

Marginal ice zones are composed of individual sea ice floes, whose breakage and melt influence its dynamical behavior. These processes are not well represented by global or regional climate models due to the continuum approximations and uncertainties regarding forcing, data resolution and parameterizations used for sea ice. Here, we use a Discrete Element Model (DEM) coupled to a slab thermodynamic ocean to investigate how breakage and melt processes impact the decay of summer sea ice. The DEM is calibrated using MODIS satellite imagery and reanalysis data within the Arctic Ocean's Baffin Bay during June–July 2018. The sensitivity of the sea ice decay is evaluated by varying the solar heating, the ice/ocean heat exchange parameter, and a prescribed floe breakage rate. For the parameter regime that best fits observations, the ratio of mass loss of resolved floes (diameter (Formula presented.) 2 km) due to breakage versus melt is 0.47, and oceanic versus solar melt is 0.46. The rate at which resolved floes lose mass is most sensitive to the breakage rate, as compared to the solar and oceanic melt parameters. The number decay of the largest floes (D (Formula presented.) 21 km) is controlled by breakage, whereas the decay of smaller floes (D = 2–21 km) depends strongly on lateral and basal melt. Inferences from this exploration of the parameter space may help motivate more accurate parameterizations of the floe size distribution evolution in climate models.

The marginal ice zone represents the periphery of the sea ice cover. In this region, the macroscale behaviour of the sea ice results from collisions and enduring contact between ice floes. This configuration closely resembles that of dense granular flows, which have been modelled successfully with the rheology. Here, we present a continuum model based on the rheology that treats sea ice as a compressible fluid, with the local sea ice concentration given by a dilatancy function. We infer expressions for and by nonlinear regression using data produced with a discrete element method (DEM) that considers polygon-shaped ice floes. We do this by driving the sea ice with a one-dimensional shearing ocean current. The resulting continuum model is a nonlinear system of equations with the sea ice velocity, local concentration and pressure as unknowns. The rheology is given by the sum of a plastic term and a viscous term. In the context of a periodic patch of ocean, which is effectively a one-dimensional problem, and under steady conditions, we prove this system to be well-posed, present a numerical algorithm for solving it, and compare its solutions to those of the DEM. These comparisons demonstrate the continuum model's ability to capture most of the DEM results accurately. The continuum model is particularly accurate for ocean currents faster than 0.25 m s; however, for low concentrations and slow ocean currents, the continuum model is less effective in capturing the DEM results. In the latter case, the lack of accuracy of the continuum model is found to be accompanied by the breakdown of a balance between the average shear stress and the integrated ocean drag extracted from the DEM. Since this balance is expected to hold independently of our choice of rheology, this finding indicates that continuum models might not be able to describe sea ice dynamics for low concentrations and slow ocean currents.

Ocean heat exchanges at the marginal ice zone (MIZ) play an important role in melting sea ice. Mixed-layer eddies transport heat and ice floes across the MIZ, facilitating the pack's access to warm waters. This study explores these frontal dynamics using disk-shaped floes coupled to an upper-ocean model simulating the sea ice edge. Numerical experiments reveal that small floes respond more strongly to fine-scale ocean currents, which favors higher dispersion rates and weakens sea ice drag onto the underlying ocean. Floes with radii smaller than resolved turbulent filaments (∼2–4 km) result in a wider and more energetic MIZ, by a factor of 70% each, compared to larger floes. We hypothesize that this floe size dependency may affect sea ice break-up by controlling oceanic energy propagation into the MIZ and modulate the sea ice pack's melt rate by regulating lateral heat transport toward the sea ice cover.