Physical attributes of sea-ice kinematics during spring 2007 off East Antarctica

In austral spring 2007, two meso-scale (of the order of 10–100 km) drifting buoy arrays were deployed to investigate the sea-ice kinematics off East Antarctica during the Sea Ice Physics and Ecosystem eXperiment (SIPEX). Deployment locations were within the one oceanic surface-drift regime with similarly high ice concentrations, the two arrays were about 440 km apart. Deployments of the two arrays were separated by 18 days. During spring 2007 the sea ice in the region was under the influence of enhanced atmospheric forcing, associated with an unprecedented seasonal increase in cyclone intensity. This atmospheric anomaly also led to repeated winter breakouts of the fast ice around much of East Antarctica (112–134°E), which, in turn, enabled the southward advection of pack ice into zones of quasi-stationary ice. Consequently, synoptic-scale atmospheric activity strongly influenced the characteristics of the regional fast-ice zone. Despite the intensified atmospheric forcing, only 73% of the variability in sea-ice motion is explained by direct atmospheric forcing, compared to 85% derived in previous studies for the same region and time of year. This reduction of synoptic-scale (multi-daily period) variability agrees with relatively high levels of semi-diurnal motion variance (compared to previous studies). Non-linear ice mechanics enable this energy cascade to higher frequencies. Both arrays experienced significant divergence, which enlarged their area by 230% (eastern array, 28 days) and 267% (western array, 30 days) without significant decrease of ice concentration within the enclosed area, indicating that active ice growth was still prevalent in the region. Divergence and shear deformation dominated the deformation parameters.