Atmospheric drivers of a winter-to-spring Lagrangian sea-ice drift in the Eastern Antarctic marginal ice zone

Abstract

Sea-ice drift in the Antarctic marginal ice zone (MIZ) is discussed using data from a 4-month-long drift of a buoy deployed on a pancake ice floe during the winter sea-ice expansion. We demonstrate increased meandering and drift speeds, and changes in the dynamical regimes of the absolute dispersion during cyclone activity, together with high correlations between drift velocities and wind from atmospheric reanalyses. This indicates a dominant physical control of wind forcing on ice drift and the persistence of free-drift conditions. These conditions occurred despite the buoy remaining largely in >80% ice concentrations and at distances >200 km from the estimated ice edge. The drift is additionally characterised by a strong inertial signature at 13.47 h, which appears initiated by passing cyclones. A wavelet analysis of the buoy's velocity confirms that the momentum transfer from winds at the multi-day frequencies is due to atmospheric forcing, while the initiation of inertial oscillations of sea ice has been identified as the secondary effect. Propagating storm-generated waves may initiate inertial oscillations by increasing the mobility of floes and enhance the drag of the inertial current. This analysis indicates that the Antarctic MIZ in the Indian Ocean sector remains much wider and mobile, during austral winter-to-spring, than defined by sea-ice concentration.

An indicator of sea ice variability for the Antarctic marginal ice zone

“…10, but the shaded field indicates that SIC has been more variable in the interior of the sea ice where the buoy drifted, as well as in the outer edge in December when the buoy sank. The MIZ was not homogeneous in July and August, and although this variability did not show in the SIC values at the buoy location, the spots of high σ SIA values indicate the presence of synoptic activity at the margin (Vichi et al, 2019;Womack et al, 2022) that resemble the trajectory of the buoy. September and October were quieter, although we still observe high intensity at the margin that coincides with the meandering of the trajectory.…”

“…This leads to divergence and lowers the chances of rafting and ridging, which are still considered the main thickening mechanisms in the Southern Ocean (Worby et al, 1996). An analysis of one icetethered buoy deployed in the East Antarctic sector revealed a large MIZ band of almost 300 km that persisted throughout the winter expansion until early December (Womack et al, 2022), with satellite-retrieved ice cover permanently above 100 %. In this region there would be no exchange through the ice between the ocean and the atmosphere, largely underestimating the possible fluxes.…”

“…The atmospheric and oceanic drivers that are more active in these regions modify the sea ice area and extent and should not be considered absent in regions with 100 % cover. There is growing evidence in the Southern Ocean that (1) extended regions with mixed types of sea ice in a fully covered ocean show MIZ characteristics from austral winter to spring (Al-berello et al, 2019;Vichi et al, 2019;Alberello et al, 2020;Womack et al, 2022), (2) waves penetrate deep into the pack ice throughout the seasons (Kohout et al, 2014(Kohout et al, , 2015Stopa et al, 2018;Massom et al, 2018;Kohout et al, 2020), and (3) extended regions of high variability in sea ice concentration and drift can be found in correspondence to largescale synoptic events (Vichi et al, 2019;de Jager and Vichi, 2022). Figure 1 gives an example of the complex conditions observed in the Antarctic MIZ.…”

“…I offer two examples to demonstrate the advantages of this diagnostic. Womack et al (2022) analysed the trajectory of an ice-tethered, non-floating buoy that drifted through the marginal zone in the East Antarctic sector for more than 5 months from July to the beginning of December 2017. The study indicated that the sea ice was permanently in free-drift conditions with SIC close to 100 %, showing a high correlation between the sea ice drift and the wind direction, as well as various loops in the trajectory in correspondence with the passage of extratropical cyclones.…”

Wave propagation in the marginal ice zone: connections and feedback mechanisms within the air–ice–ocean system

“…More often, analysis suggests that combined atmospheric and oceanic stresses cause ice loss events [ 70 ]. In some cases, the ice loss from a given region is mostly a wind and wave transport process [ 71 , 72 ]. In other cases, waves break up sea ice and the ice is lost via enhanced melting [ 16 ].…”