SPRINGTIME ICE MOTION IN THE WESTERN ANTARCTIC PENINSULA REGION
Cathleen Geiger and Don Perovich
Oscillatory motion of sea ice is examined using two ice drifting buoys separated by one degree latitude around 66ºS during the winter to spring transition in the Marguerite Bay region west of the Antarctic Peninsula. The buoy’s motion exhibits spectrally distinct periods (12.87 ± 0.04 and 13.03 ± 0.04 hours, respectively) despite highly correlated motion between them (r2 is 0.62 and 0.81 for u and v, respectively). The periods shift with latitude and nearly match the local inertial periods (13.00 and 13.10, respectively). The oscillations are further examined with respect to the kinematics involved in the breakup process of sea ice. These include hourly resolved manifestation of circular trajectories, semi-circular oscillations with compressed trajectory cusps, and “accordion-like” compressions along straight line trajectories. Oscillations are found in all trajectory types over the lifetime of both buoys (several months). Traditional circular and semi-circular oscillations are particularly prominent during two episodes, one of which is preceded by strong wind events and a substantial decrease in ice thickness and concentration. These episodes combine with seasonally warming temperatures to break up and melt the sea ice cover. We discuss potential relationships between the degradation of the ice pack during spring breakup and the increase in energy at near-inertial frequencies including a non-linear cascade of energy within the ice from the low frequencies (commensurate with storms and fortnightly tides) to semi-diurnal frequencies. We further comment on the implications this type of high-frequency motion has on local biological ecosystems. Specifically we find that sea ice semi-diurnal oscillations are at their peak during the final decay of sea ice just before springtime primary productivity begins. Hence the oscillatory motion of sea ice not only serves as an effective mixing agent within the ice-ocean mixed layer, but also serves as an effective seeding platform for distributing phyto- and zooplankton that have over-wintered within and around the ice floes. This type of bio-physical coupling is very important from the modeling perspective, bio-physical process, and biomass productivity.