Drift of Pancake Ice Floes in the Winter Antarctic Marginal Ice Zone During Polar Cyclones

High temporal resolution in situ measurements of pancake ice drift are presented, from a pair of buoys deployed on floes in the Antarctic marginal ice zone during the winter sea ice expansion, over 9 days in which the region was impacted by four polar cyclones. Concomitant measurements of wave-in-ice activity from the buoys are used to infer that the ice remained unconsolidated, and pancake ice conditions were maintained over at least the first 7 days. Analysis of the data shows (i) the fastest reported ice drift speeds in the Southern Ocean; (ii) high correlation of drift velocities with the surface wind velocities, indicating absence of internal ice stresses >100km from the ice edge where remotely sensed ice concentration is 100%; and (iii) presence of a strong inertial signature with a 13hr period. A Lagrangian free drift model is developed, including a term for geostrophic currents that reproduce the 13hr period signature in the ice motion. The calibrated model provides accurate predictions of the ice drift for up to 2days, and the calibrated parameters provide estimates of wind and ocean drag for pancake floes under storm conditions.Plain Language Summary During the Antarctic winter, small pancake ice floes, which form rapidly in wavy conditions, dominate new ice growth and create a dynamic environment. However, there are only a handful of local observations of pancake ice drift, particularly during the intense polar cyclones that frequently reshape the ice cover. More observations are needed to generate better understanding and modeling of pancake ice response to winds, waves, and currents. We describe a set of pancake ice drift and wave-in-ice measurements over 9 days in which four polar cyclones affected the region, from buoys deployed on pancake floes 100km from the ice edge. We also develop an ice drift model.The data show how the cyclones affect ice drift and contain the fastest reported ice speed in the Southern Ocean (0.75ms(-1)). The instantaneous drift speed closely correlates with the wind speed, and the ice also displays a 13hr period rotational motion that we reproduce in the model with forcing from ocean currents. We show that pancake ice is in free drift, despite sea ice covering the entire ocean surface in the measurement region and that the model predicts drift accurately over 2 days with calibration of only two parameters.Key PointsIce speed (0.75 m s(-1)) measured during intense polar cyclones is the fastest recorded in the Southern Ocean Mean ice drift follows the wind, but with a strong inertial component, period approximate to 13 hr Pancake ice drift agrees with the free drift approximation (no internal ice stresses), even though remotely sensed concentration is 100%