Atmospheric drivers of ice drift in the Antarctic marginal ice zone

Sea-ice drift in the Antarctic marginal ice zone (MIZ) was investigated using an array of five drifting ice buoys, deployed during the winter sea-ice expansion, in July 2017. An initial 15- day analysis of pancake ice drift is presented, using the cluster of buoys, which shows: (1) exceptionally fast ice drift speeds and increased meandering of the buoys during cyclone activity; (2) high correlation of drift velocities with the surface wind velocities, even at 100% remotely sensed ice concentration, indicating free drift conditions where ice drift is primarily governed by wind; and (3) the presence of a clear energy peak (»13.5 hour period), which is suggested to be excited by the passage of cyclones through the transfer of momentum from wind. Additionally, one of the buoys (buoy U1) drifted for approximately four months from the South Atlantic sector to the Indian Ocean sector of the Southern Ocean. The analysis of this buoy revealed that it remained within the MIZ even during the winter ice expansion, as the mixed pancake-frazil field was maintained. This allowed for a continued assumption of free drift conditions for buoy U1's full drift, where it continued to respond linearly to the momentum transfer from surface winds. The analysis of buoy U1 also indicated a strong inertial signature at a period of 13.47 hours however, the wavelet analysis indicated majority of the power remained within the lower frequencies. This strong influence at the lower (multiday) frequencies has therefore been identified as the primary effect of atmospheric forcing. When these lower frequencies were filtered out using the Butterworth high-pass filter it allowed the inertial oscillations to become more significant within the wavelet power spectrum, where it can be seen that these inertial oscillations were often triggered by the passage of cyclones. The initiation of inertial oscillations of sea ice has therefore been identified as the secondary effect of atmospheric forcing, which dominates ice drift at sub-daily timescales and results in the deviation of ice drift from a straight-line path. This comprehensive analysis suggests that the general concentration-based definition of the MIZ is not enough to describe the sea-ice cover, and that the MIZ, where ice is in free drift and under the influence of cyclone induced inertial motion, and presumably waves, can extend up to »200 km